This document summarizes an interview with Terry Orr-Weaver, a biologist who studies chromosome partitioning during cell division and DNA replication. Some key points: - Orr-Weaver switched from yeast to fruit flies as her model organism to study how cell division and pattern formation are coordinated during development. - Her research group uses genetics, biochemistry, and cell biology approaches like microscopy to study these processes directly. For example, they tagged a protein involved in DNA replication to see where it localizes in cells. - Meiosis reduces the chromosome number from diploid to haploid by having an extra round of chromosome separation where maternal and paternal chromosomes are separated. This ensures sperm and eggs have one copy of

1. AN INTERVIEW WITH basis of inheritance, the structures that carried A big and useful surprise in this gene-
the genes. Decades later, the master regulatory discovery enterprise has been how many genes
Terry l. Orr-Weaver genes that set up the animal body plan in em- have been 'conserved" through evolutionary
How the daughter cells resulting from cell bryonic development were discovered in history and are still verysimiJar in organisms as
division end up with equal numbers of chro- Drosoph.ila. These genes determine, for exam- distantly related as fruit flies and humans. It's
mosomes is the focus of much of Terry Orr- ple, where the fly's head is and where its legs very hard to figure out directly what human
Weaver's research; she also studies how cells are. Then, to the amazement of biologists. it genes do. But when we discover a new gene in
control the DNA replication that precedes turned out that exactly the same genes control Drosophila, invariably it turns out thai there's a
cell division. Her research group has identi· how the human body plan is set up! $0 similar gene in humans that does the same
fied a number of proteins involved in these Drosophila has a really rich heritage. When I thing. So Drosophila turns out to be an even
processes, A professor of biology at MIT and started working on Drosophila, it had a 70- better model organism than we had guessed.
the first woman to become a member of the year-long history of use in genetics, prOViding
Whitehead Institute for Biomedical Research, an incredible array of knowledge and methods. Besides genetics, what other approaches
Dr. Orr-Weaver has an undergraduate de- and methods do you use?
gree in chemistry from UC San Diego and a How do geneticists approach biological A combination ofgenetics, biochemistry, and cell
Ph.D. in biological chemistry from Harvard. questions? biology turns out to be incredibly powerful for
She is a member of the U.S. National Acad- Geneticists want to discover the genes that are our research. For instance, with the microscope,
emy of SCiences and a past president of the involved in a biological process and then figure a tool ofcell biology. we can literally watch the
Genetics Society of America. out what the genes do. First. you decide what chromosomes as they undergo mitosis or meio-
process you're interested in-in our case, how sis.Ifwe have a mutation causing a defect in one
After receiving your Ph.D., you switched chromosomes gel partitioned during cell divi- ofthose processes, we can look directly at how
from using single-celled yeast as your modl'l sion. Next, you figure out what you would see the chromosomes behave in mutant cells. And in
organism to thl' multicelled fruit fly if you had a mutant in which that process was our research on DNA replication, we can look di-
Drosophila melanogaster. Why? perturbed-how would you recognize it? Then rectly at proteins that attach to the DNA during
[ was excited by the possibility of working at you generate mutations, usually with the help DNA replication. In this lab, we try to do things
the interface of two fields. At that time, the cell of high-energy radiation or a chemical, hoping as directly as possible!
cycle field was exploding from work with sin- that one or more of the resulting mutants are Here's an example from our work on DNA
gle cells-mainly yeasts and mammalian cells affected in the process of interest. vt1at you're replication in Drosophila. Using a genetic ap-
grown in culture-and biochemical experi- doing by making a mutation is generating a proach. we discovered a mutant that couldn't
ments using extracts. Meanwhile, develop- disease state in your organism. vt1en you've carry out DNA replication-although such a
mental biology was entering a new era with found a mutant you're interested in. you can mutant can live and grow for a while by using
the discovery of pattern formation genes. [ re- find out what gene has been made defective, proteins its mother stockpiled in the egg. It
alized there was a key question at the interface and you know that that gene has to play an im- looked like the gene affected might code for a
of these two fields that was being ignored: If an portant role in the process you're interested in. really important protein. But we wanted to find
organism starts as a single cell and ends up as a Now that we have the genome sequences of out if it was a protein that played a direct role in
multicellular entity, how are pattern and cell many organisms, including the fruit fly and the DNA replication, and we couldn't tell that from
divisions coordinated? And I realized there human, you might think that all their genes are the genetics alone. However, by labeling the
had to be intrinsic regulation from the cell cy- known-so what's to discover? But it turned out normal version of the protein with a fluores-
cle components but also extrinsic develop- that we didn't know the functions of many of the cent tag, we could use the microscope to see
mental control feeding into that. This was genes that showed up in genome sequences. where the protein was located in cells, and we
clearly going to be an important area of study. Even in an organism as simple and well-studied saw that the protein got on the DNA right at a
as yeast, at the publication of the genome se- place where DNA synthesis starts. That estab-
In choosing a multicellular model organism, quence we didn't know what 70% ofthe genes did. lished that this protein was directly involved.
why use Drosophila? And we still don't have a clue about halfofthe
A lot of fundamental discoveries in biology human genes. So having genome sequences pro- How is meiosis different from mitosis?
have been made with fruit nil's. For example, it vides a foundation, but we must still find out Let me first review mitosis. After a cell dupli-
was research with DroS<Jph.i1a that established what all the genes do, and we geneticists feel cates its DNA, mitosis ensures that each dupli-
in 1916 that chromosomes were the physical the best way to go about this is 10 use genetics. cated chromosome gets partitioned to the
,%

2. daughter cells so that you end up with two brings the homologous chromosomes to- In fact, errors in meiosis are the leading cause
cells that have exactly the same DNA content gether. Given the relatively gigantic volume of of menial retardation in the United States.
and chromosome number. That's what hap- the nucleus and the huge mass of chromatin in Some scientists argue that, in evolutionary
pens in normal cell division. a eukaryotic cell, how do the right chromo- terms, the human species is able to cope with
But what about making a sperm or an egg? somes find each other? That's the number one such a high rate of meiotic errors because most
In fertilization, a sperm and egg are going to mystery about meiosis. And [would say the of the pregnancies are lost very early,
fuse and give rise to a new progeny. [fthe or- second big mystery is why humans are so un-
ganism is diploid-that is, has two similar believably bad at carrying out meiosis. Whal do you like besl about research?
copies of each chromosome, as nil'S and hu- What's so great about research is that you get
mans do-and the progeny is going to be What makes you say thaB Are we worse to unwrap presents all the time! 'ifhen you
diploid, then you've got to make sperm and than fruit flies? find an interesting mutant, it's like a beautifully
eggs that have only one copy of each chromo- We're about a thousand times worse. Here are wrapped present. And then you unwrap it, and
some, so that when those sperm and egg come some amazing statistics: Twenty percent of it can be a big surprise to learn which gene is
together you restore the right chromosome recognized pregnancies end in spontaneous affected and what its protein product is. Some-
number. And to produce sperm and eggs with miscarriage, and out of those at least half are times you unwrap it and it's beautiful and sat-
only half the diploid chromosome number, you due to a mistake during meiosis. And for every isfying and completely makes sense; other
need a special kind of cell division-meiosis. pregnancy that proceeds far enough to be rec- times you don't at first understand what the
There has to be a way to bring together the ognized, there have been many that ended gift is, and then you have to figure that out, [
two similar copies of each chromosome and without being recognized. So about 10-20% of want to tell students how wonderful it is get-
then pull them apart, with each going to a dif- the time meiosis doesn't work properly in hu- ting these presents to unwrap. But they also
ferent daughter cell. [n meiosis, there is an mans. In fruit flies, meiosis seems to occur in- need to realize that it might take three years to
extra round of chromosome partitioning accurately only 0.01-0.05% of the time. Even get the ribbon and all the tape off. It can take
where what we call the homologous pairs of mice, which are mammals like ourselves, do some real patience to unwrap the present!
chromosomes-the chromosome that initially much better than humans. So I would call this
came from dad and the similar chromosome the second big mystery regarding meiosis: In your view, how important is il for
that came from mom-get separated from Why are humans so bad at it? scientists to reach out beyond the scientific
each other. To sum up, in mitosis you're sepa- community?
rating identical copies of each chromosome, Tell us more about what happens when I think scientists do have an obligation to edu-
whereas in meiosis you have an extra round of meiosis doesn't work correctly. cate the public. It's especially critical for peo-
division stuck in there, where the copy ofeach Most of the time the result is early death of the ple to understand the importance of basic
chromosome from dad and the copy from embryo. In humans, if any chromosome other research and the study of model organisms,
mom get separated from each other. than X or Y is present in only one copy or Because if I'll' shortchange basic research, in
more than two copies, the embryo dies very the long run I'll' won't be able to benefit from
What imporlanl questions about meiosis early. The only exceptions are for chromo- medical applications. The reason we've done
still need to be answeredl somes number 13, 18, and 21: When oneof so well in this country is our willingness to in-
What we don't understand at all is how the these is present in three copies, the embyo usu- vest in basic research science.
chromosomes of a homologous pair find each ally dies at an early stage, but it may survive. The challenge is figuring out how to convey a
other. That pairing is unique to meiosis; it Even in the case of Down syndrome, a rela- message like this in an era ofSQundbites. The
doesn't happen in mitosis. The two strands of tively common condition where a person has media and the public want to hear that a piece of
the DNA don't come apart, so although the three copies of chromosome 21, only a minor- research is directly going to cure a disease. So
homologous chromosomes have very similar ity of fetuses survive to term, And when the in- how can we get a message across that requires an
DNA sequences, it's not base-pairing that dividual does survive, there are serious effects, explanation more complex than a catchphrase?
There are many stories I'll' can tell where discov-
eries in basic research have led to very important
applications, but these stories take at lea;,t a
couple of paragraphs to communicate. Also,
there's a real need for scientists to try to build a
bridge between science and the public.
It's especially important to educate our rep-
resentatives in government. A coalition of sev-
eral biology societies, including the Genetics
Society of America, has a Joint Steering Com-
mittee for Public Policy, which has been ter-
rific in educating members of Congress and
their staffs. They bring in scientists to explain
their research and why investing in research is
important. Maybe that's a good starting point.
Learn about an experiment by Terry Orr-
Weaver in Inquiry Figure 16,22 on page 322,
Jane Reece and Terry Orr-Weaver
247

3. Mei"",,-, an
Sexual ife
Cycles
.... Figure 13.1 What accounts for family resemblance?
KEY CONCEPTS
13.1 Offspring acquire genes from parents by Genetics is the scientific study of heredity and hereditary
inheriting chromosomes variation. In this unit, you will learn about genetics at multiple
13.2 Fertilization and meiosis alternate in sexual life levels, from organisms to cells to molecules. On the practical
cycles side, you will see how genetics continues to revolutionize med-
13.3 Meiosis reduces the number of chromosome icineand agriculture, and you will be asked to consider some so-
sets from diploid to haploid cial and ethical questions raised by our ability to manipulate
13.4 Genetic variation produced in sexual life cycles DNA, the genetic material. At the end of the unit, you will be
contributes to evolution able to stand back and consider the whole genome, an organ-
ism's entire complement of DNA. The rapid accumulation of
the genome sequences of many species, including our own, has
taught us a great deal about evolution on the molecular level-
in other words, evolution of the genome itself. In fact, genetic
methods and discoveries are catalyzing progress in all areas of
biology, from cell biology to physiology, developmental biology,
ost people who send out birth announcements
M mention the sex of the baby, but they don't feel the
need to specify that their offspring is a human be-
ing! One of the characteristics of life is the ability of organ-
behavior, and even ecology.
We begin our study of genetics in this chapter by examin-
ing how chromosomes pass from parents to offspring in sexu-
ally reproducing organisms. The processes of meiosis (a
isms to reproduce their own kind-elephants produce little
special type of cell division) and fertilization (the fusion of
elephants, and oak trees generate oak saplings. Exceptions
sperm and egg) maintain a species' chromosome count during
to this rule show up only as sensational but highly suspect
the sexual life cycle. We will describe the cellular mechanics of
stories in tabloid newspapers.
meiosis and how this process differs from mitosis. Finally, we
Another rule often taken for granted is that offspring resem-
will consider how both meiosis and fertilization contribute to
ble their parents more than they do Wlrelated individuals. If you
genetic variation, such as the variation obvious in the family
examine the famUy members shown in Figure 13.1-Sissy
shown in Figure 13.1.
Spacek and Jack Fisk with daughters Madison and Schuyler
Fisk-you can pick out some similar features among them. The
~7;::~~g~'~ire genes
transmission oftraits from one generation to the next is called in-
heritance, or heredity (from the Latin heres, heir). However, sons
and daughters are not identical copies ofeither parent or oftheir
siblings. Along with inherited similarity, there is also variation. from parents by inheriting
Farmers have exploited the principles of heredity and variation chromosomes
for thousands of years, breeding plants and animals for desired
traits. But what are the biological mechanisms leading to the Family friends may tell you that you have your mother's freck-
hereditary similarity and variation that we call a "family resem- les or your father's eyes. However, parents do not, in any lit-
blance"? This question eluded biologists until the development of eral sense, give their children freckles, eyes, hair, or any other
genetics in the 20th century. traits. What, then, is actually inherited?
248

4. Inheritance of Genes
Parents endow their offspring with coded information in the
form of hereditary units called genes. The genes we inherit
from our mothers and fathers are our genetic link to our par-
ents, and they account for family resemblances such as shared
eye color or freckles. OUf genes program the specific traits
that emerge as we develop from fertilized eggs into adults.
The genetic program is written in the language of DNA,
the polymer of four different nuc1eotides you learned about
in Chapters 1 and 5. Inherited information is passed on in
the form of each gene's specific sequence of DNA nu-
deotides, much as printed information is communicated in
the form of meaningful sequences of letters. In both cases,
the language is symbolic. Just as your brain translates the
word apple into a mental image of the fruit, cells translate
genes into freckles and other features. Most genes program (a) Hydra (b) Redwoods
cells to synthesize specific enzymes and other proteins, ... Figure 13.2 Asexual reproduction in two multicellular
whose cumulative action produces an organism's inherited organisms. (a) This relatively simple animal. a hydra. reproduces by
traits. The programming of these traits in the form of DNA budding. The bud. a localized mass of mitotically dividing cells. develops
into a small hydra. whICh detaches from the parent (LM). (b) Each tree in
is one of the unifying themes of biology. this circle of redwoods grew from a single parent tree. whose stump is in
The transmission of hereditary traits has its molecular ba- the center of the circle.
sis in the precise replication of DNA, which produces copies
ofgenes that can be passed along from parents to offspring. In capable of reproducing asexually (Figure 13.2). Because the
animals and plants, reproductive cells called gametes are the cells of the offspring are derived by mitosis in the parent, the
vehicles that transmit genes from one generation to the next. ~chip off the old block" is usually genetically identical to its
During fertilization, male and female gametes (sperm and parent. An individual that reproduces asexually gives rise to a
eggs) unite, thereby passing on genes of both parents to their done, a group ofgenetically identical individuals. Genetic dif-
offspring. ferences occasionally arise in asexually reproducing organ-
Except for small amounts of DNA in mitochondria and isms as a result of changes in the DNA called mutations,
chloroplasts, the DNA of a eukaryotic cell is packaged into which we will discuss in Chapter 17.
chromosomes within the nucleus. Every living species has a In sexual reproduction, two parents give rise to offspring
characteristic number of chromosomes. For example, hu- that have unique combinations ofgenes inherited from the m'o
mans have 46 chromosomes in almost all of their cells. Each parents. In contrast to a clone, offspring ofsexual reproduction
chromosome consists of a single long DNA molecule elab- vary genetically from their siblings and both parents: They are
orately coiled in association with various proteins. One variations on a common theme of family resemblance, not ex-
chromosome includes several hundred to a few thousand act replicas. Genetic variation like that shown in Figure 13.1 is
genes, each of which is a specific sequence of nuc1eotides an important consequence of sexual reproduction. What
within the DNA molecule. A gene's specific location along mechanisms generate this genetic variation? The key is the be-
the length of a chromosome is called the gene's locus (from havior of chromosomes during the sexual life cycle.
n
the Latin, meaning "place ; plural, loci). Our genetic en-
dowment consists of the genes carried on the chromosomes CONCEPT CHECK 13.1
we inherited from our parents.
I. How are the traits of parents (such as hair color)
transmitted to their offspring?
Comparison of Asexual
2. Explain how asexually reproducing organisms pro-
and Sexual Reproduction
duce offspring that are genetically identical to each
Only organisms that reproduce asexually produce offspring other and to their parents.
that are exact copies ofthemselves. In asexual reproduction, 3. •i,'!lfU1jM A horticulturalist breeds orchids, trying
a single individual is the sole parent and passes copies of all its to obtain a plant with a unique combination of desir-
genes to its offspring. For example, single-celled eukaryotic able traits. After many years, she finally succeeds. To
organisms can reproduce asexually by mitotic cell division, in produce more plants like this one, should she breed it
which DNA is copied and allocated equally to two daughter or clone it? Why?
cells. The genomes of the offspring are virtually exact copies For suggested answers, see Appendix A.
ofthe parent's genome. Some multicellular organisms are also
(HAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 249

5. r;:~~~I~::i~~'~nd
13.3
•
meiosis Preparing a karyotype
alternate in sexual life cycles APPLICATION A karyotype is a display of condensed chromo·
somes arranged in pairs. Karyotyping can be used to screen for
A life cycle is the generation-ta-generation sequence of abnormal numbers of chromosomes or defective chromosomes
stages in the reproductive history of an organism, from con- associated with certain congenital disorders, such as Down
syndrome.
ception to production of its own offspring. In this section, we
use humans as an example to track the behavior of chromo-
somes through sexual life cycles. We begin by considering the
chromosome count in human somatic cells and gametes; we
will then explore how the behavior of chromosomes relates to
the human life cycle and other types of sexual life cycles.
Sets of Chromosomes in Human Cells
In humans, each somatic cell-any cell other than those in-
volved in gamete formation-has 46 chromosomes. During
mitosis, the chromosomes become condensed enough to be
visible in a light microscope. Because chromosomes differ in
size, in the positions of their centromeres, and in the pattern
of colored bands produced by certain stains, they can be dis~
tinguished from one another by microscopic examination
when sufficiently condensed. TECH Nt QUE Karyotypes are prepared from isolated somatic
cells, which are treated with a drug to stimulate mitosis and then
Careful examination of a micrograph of the 46 human grown in culture for several days. Cells arrested in metaphase are
chromosomes from a single cell in mitosis reveals that there stained and then viewed with a microscope equipped with a digi-
are two chromosomes of each of 23 types. This becomes tal camera. A photograph of the chromosomes is displayed on a
computer monitor, and the images of the chromosomes are
dear when images of the chromosomes are arranged in
arranged Into pairs according to size and shape.
pairs, starting with the longest chromosomes. The resulting
Pair of homologous
ordered display is called a karyotype (Figure 13.3). The two replicated chromosomes
chromosomes composing a pair have the same length, cen- ~
tromere position, and staining pattern: These are called
Centromere
homologous chromosomes, or homologs. Both chromo-
somes of each pair carry genes controlling the same inher-
[illk '. , .
ited characters. For example, if a gene for eye color is
I ,,' Of
situated at a particular locus on a certain chromosome, then
•• ~ • • "
.- '
the homolog of that chromosome will also have a gene spec-
,,, Ii"
•
' ,
.. .. H
,
ifying eye color at the equivalent locus.
The two distinct chromosomes referred to as X and Yare
an important exception to the general pattern of homolo-
T Sister
chromatids
Metaphase
'-"
, ii
,,~
I ;
#i l(
,
gous chromosomes in human somatic cells. Human females chromosome
· , .
·, E ••
'.
have a homologous pair of X chromosomes (XX), but males
have one Xand one Ychromosome (XY). Only small parts of ",
II ii
,
~
" " "
the X and Yare homologous. Most of the genes carried on 0' H II !I
the X chromosome do not have counterparts on the tiny Y, " D•
and the Y chromosome has genes lacking on the X. Because
they determine an individual's sex, the X and Y chromo-
".•
•
.,
"
"
II
•
•
" ..
H
•
somes are called sex chromosomes. The other chromo-
RESULTS This karyotype shows the chromosomes from
somes are called autosomcs.
a normal human male. The size of the chromosome, position of
The occurrence of homologous pairs of chromosomes in the centromere, and pattern of stained bands help identify spe-
each human somatic cell is a consequence ofour sexual origins. cific chromosomes. Although difficult to discern in the karyotype,
each metaphase chromosome consists of two closely attached
We inherit one chromosome of each pair from each parent
sister chromatids (see the diagram of a pair of homologous
Thus, the 46 chromosomes in our somatic cells are achlally two replicated chromosomes).
sets of 23 chromosomes-a maternal set (from our mother)
250 UNIT THREE Genetics

6. and a paternal set (from our father). The number of chromo- Behavior of Chromosome Sets
somes in a single set is represented by n. Any cell with two in the Human Life Cycle
chromosome sets is called a diploid cell and has a diploid
number of chromosomes, abbreviated 2n. For humans, the The human life cycle begins when a haploid sperm from the
diploid number is 46 (2n = 46), the number of chromosomes father fuses with a haploid egg from the mother. This union
in our somatic cells. In a cell in which DNA synthesis has oc- of gametes, culminating in fusion of their nuclei, is called
curred, all the chromosomes are replicated, and therefore each fertilization. The resulting fertilized egg, or zygote, is
consists oftwo identical sister chromatids, associated closely at diploid because it contains two haploid sets of chromosomes
the centromere and along the arms. Figure 13.4 helps clarify bearing genes representing the maternal and paternal family
the various terms that we use in describing replicated chromo- lines. As a human develops into a sexually mature adult, mitosis
somes in a diploid ceiL Study this figure so that you understand of the zygote and its descendants generates all the somatic ceUs
the differences between homologous chromosomes, sister of the body. Both chromosome sets in the zygote and all the
chromatids, nonsister chromatids, and chromosome sets. genes they carry are passed with precision to the somatic cells.
Unlike somatic cells, gametes (sperm and eggs) contain a The only cells of the human body not produced by mitosis
single chromosome set. Such cells are called haploid cells, are the gametes, which de'elop from specialized reUs called
and each has a haploid number of chromosomes (n). For hu- germ cells in the gonads-ovaries in females and testes in
mans, the haploid number is 23 (n = 23). The set of 23 con- males (Figure 13.5). Imagine what would happen if human
sists of the 22 autosomes plus a single sex chromosome. An gametes were made by mitosis: They would be diploid like
unfertilized egg contains an X chromosome, but a sperm may the somatic cells. At the next round offertilization, when two
contain an X or a Y chromosome. gametes fused, the normal chromosome number of 46 would
Note that each sexually reproducing species has a charac- double to 92, and each subsequent generation would double the
teristic diploid number and haploid number. For example, the Key Haploid gametes (n " 23)
fruit ny, DrosoplJila melanogaster, has a diploid number of 8 Haploid (n)
Egg (n)
and a haploid number of 4, while dogs have a diploid number DiplOId (2n)
of78 and a haploid numberof39.
Now that you have learned the concepts ofdiploid and hap-
loid numbers of chromosomes, let's consider chromosome
behavior during sexual life cycles. We11 use the human life cy-
cle as an example.
Koy
• Maternal set of
2n =6 chromosomes (n = 3)
{ Paternal set of
• chromosomes (n = 3)
Two sister chromatids
of one replicated Diploid
chromosome zygote
Centromere (2n = 46)
Mitosis and
development
Two nonSlSter ~""::.l,,--.tl .., ,L-_7'_palf of homologous
duomalldsm !!I chromosomes
a homologous pair (one from each set)
Mulllcellular diploid
adults (2n = 46)
• Figure 13.4 Desaibing chromosomes. A cell with a dipbd
number of 6 (2n = 6) is depicted here followang chromosome replicatJOn • Figure 13.5 The human life cycle. In each generation, the
and condensa1JOfl. Each of the SIX replicated chromosomes conSISts of number of chromosome sets doubles at fertilization. but IS halved
two SiSler chromatids assoaated dosefy along their lengths. Each dunnq meIOSIS. For humans, the number of chromosomes in a haplOId
homologous pair IS composed of one chromosome from the maternal cell is 23. COOSlSllnq of ooe set (n = 23); the number of chromosomes
set (~ and one from the paternal set (blue). Each set is made up of In the diploid lY90te and all somatIC cells ansmg from It is 46,
thr~ chromosomes Ir'I thIS example. NonSlSter chromatids are any two Consrstlnq of two sets (2n = 46).
chromatids In a pa... of homologous chromosomes that a~ not SISter
ThIS figure IntrodllCe5 a color code that wia be IlSf'C:f for other life
"""""""
n What is the haploid number of this celf? ~ a
. . chromosomes haploid Of diploid?
~set~ of
cydes later In thIS book. The teal arrows highlight haploid stages of a
tife~, and the betge arrows highlight dIploid stages.
CHAHII THIRTUN Meiosis il.nd Sexua.l Life Cycles 251

7. number of chromosomes yet again. This does not happen, Plants and some spe<ies of algae exhibit a second type oflife
however, because in sexually reproducing organisms, the gam- cycle called alternation of generations. This type includes
etes are formed by a modified type of cell division called both diploid and haploid stages that are multicellular. The mul-
meiosis. This type ofcell division reduces the number ofsets of ticellular diploid stage is called the sporophyte. Meiosis in the
chromosomes from two to one in the gametes, counterbalanc- sporophyte produces haploid cells called spores. Unlike a gam-
ing the doubling that occurs at fertilization. In animals, meiosis ete, a haploid spore doesn't fuse with another cell but divides
occurs only in the ovaries or testes. As a result of meiosis, each mitotically, generating a multicellular haploid stage called the
human sperm and egg is haploid (n = 23). Fertilization restores gametophyte. Cells of the gametophyte give rise to gametes by
the diploid condition by combining two haploid sets of chro- mitosis. Fusion of two haploid gametes at fertilization results
mosomes, and the human life cycle is repeated, generation af- in a diploid zygote, which develops into the next sporophyte
ter generation (see Figure 13.5). You will learn more about the generation. Therefore, in this type ofHfe cycle, the sporophyte
production of sperm and eggs in Chapter 46. generation produces a gametophyte as its offspring, and the
In general, the steps of the human life cycle are typical of gametophyte generation produces the next sporophyte gener-
many sexually reproducing animals. Indeed, the processes ation (Figure 13.6b). Clearly, the term alternation of
of fertilization and meiosis are the unique trademarks of generations is a fitting name for this type of life cycle.
sexual reproduction, in plants as well as animals. Fertiliza- A third type oflife cycle occurs in most fungi and some pro-
tion and meiosis alternate in sexual life cycles, maintaining tists, including some algae. After gametes fuse and form a
a constant number of chromosomes in each species from diploid zygote, meiosis occurs without a multicellular diploid
one generation to the next. offspring developing. Meiosis produces not gametes but hap-
loid cells that then divide by mitosis and give rise to either uni-
The Variety of Sexual Life Cycles
cellular descendants or a haploid multicellular adult organism.
Although the alternation of meiosis and fertilization is com- Subsequently, the haploid organism carries out further mi-
mon to all organisms that reproduce sexually, the timing of toses, producing the cells that develop into gametes. The only
these two events in the life cycle varies, depending on the diploid stage found in these species is the single-celled zygote
species. These variations can be grouped into three main (Figure 13.6c).
types of life cycles. In the type that occurs in humans and most Note that either haploid or diploid cells can divide by mito-
other animals, gametes are the only haploid cells. Meiosis oc- sis, depending on the type oftife cycle. Only diploid cells, how-
curs in germ cells during the production of gametes, which ever, can undergo meiosis because haploid cells have a single
undergo no further cell division prior to fertilization. After set of chromosomes that cannot be further reduced. Though
fertilization, the diploid zygote divides by mitosis, producing the three types ofsexual life cycles differ in the timing of meio-
a multicellular organism that is diploid (Figure 13.6a). sis and fertilization, they share a fundamental result: genetic
K.y
Haploid (n)
Haploid unicellular or
• Diploid (2n) multICellular organism
Gametes n
o
n n
FERTILIZATION n
Gametes
20
DiPIOid~~-::~::;::::::
multicellular
Diploid
organism multicellular J.~ Mitosis
organism / ~ Zygote
(sporophyte)
(a) Animals (b) Plants and some algae (c) Most fungi and some protists
.. Figure 13.6 Three types of sexual life cycles. The common feature of all three cycles is the alternation of
meiosis and fertilization, key events that contribute to genetic variation among offspring. The cycles differ in the liming
of these two key events.
252 UNIT THREE Genetics

8. variation among offspring. A closer look at meiosis will reveal
the sources of this variation.
CONCEPT CHECK 13.Z
I. How does the karyotype of a human female differ
io diploid P'A'
Homologous pair of chromosomes
~ ~
from that of a human male?
2. How does the alternation of meiosis and fertilization
in the life cycles of sexually reproducing organisms
maintain the normal chromosome count for each
)
species?
1J
Chromosomes
3. Each sperm of a pea plant contains seven chromo- replicate
somes. What are the haploid and diploid numbers for
Homologous pair of replicated chromosomes
peas?
4. • i,'llfu1iM A certain eukaryote lives as a unicellu-
lar organism, but during environmental stress, its cells /
produce gametes. The gametes fuse, and the resulting
zygote undergoes meiosis, generating new single cells.
What type of organism could this be?
Si"" m
chrom~
)
Diploid cell with
replicated
For suggested answers, see Appendix A. chromosomes
r:·~~::~s·r::~~es the number ; 0 X
of chromosome sets from
diploid to haploid
X Homolog,",
chromosomes
separate
Haploid cells with
replicated chromosomes
Many of the steps of meiosis closely resemble corresponding
steps in mitosis. Meiosis, like mitosis, is preceded by the repli-
cation of chromosomes. However, this single replication is 8 Sister chromatids
D'P'"' c~
followed by not one but two consecutive cell divisions, called
meiosis I and meiosis II. These two divisions result in four
daughter cells (rather than the two daughter cells of mitosis),
each with only half as many chromosomes as the parent cell.
Haploid cells with unrepllCated chromosomes
The Stages of Meiosis
The overview of meiosis in Figure 13.7 shows that both mem- ... Figure 13.7 Overview of meiosis: how meiosis reduces
bers of a single homologous pair of chromosomes in a diploid chromosome number. After the chromosomes replicate in
interphase. the diploid cell divides twice. yielding four haploid
cell are replicated and that the copies are then sorted into four daughter cells This overview tracks just one pair of homologous
haploid daughter cells. Recall that sister chromatids are m'O chromosomes, which for the sake of simplicity are drawn in the
copies of one chromosome, closely associated all along their condensed state throughout (they would not normally be condensed
during interphase), The red chromosome was inherited from the
lengths; this association is called sister chromatid cohesion. To- female parent, the blue chromosome from the male parent.
gether, the sister chromatids make up one replicated chromo- ••l.f.WIII Redraw the cells in this figure using a simple DNA
some (see Figure 13.4). In contrast, the two chromosomes of a double helix (0 represent each DNA molecule
homologous pair are individual chromosomes that were inher-
ited from different parents. Homologs appear alike in the
microscope. but they may have different versions of genes, Figure 13.8, on the next rn'o pages, describes in detail the
called alleles, at corresponding loci (for example, an allele for stages of the two divisions of meiosis for an animal cell whose
freckles on one chromosome and an allele for the absence of diploid number is 6. Meiosis halves the total number of chro-
freckles at the same locus on the homolog). Homologs are not mosomes in a very specific way, reducing the number of sets
associated with each other except during meiosis, as you will from two to one. with each daughter cell receiving one set of
soon see. chromosomes. Study Figure 13.8 thoroughly before going on.
CHAPTE~ THI~HEN Meiosis and Sexual Life Cycles 253

9. • FIguro 13.1
@:j[;mmim·TheMeioticDivisionofanAnimaICell
..........MEIOSiS I: Separates homologous chromosomes"
Prophase I Metaphase I Anaphase I Telophase I and
Cytokinesis
SISter chromatids
Centrosome remdlll attached
(WIth centnole pair)
Sister ChlilSlTlilla Centromere
chromattds (WIth kinetochore)
Metaphase
p"~
Homologous
"omologoo,
chromosomes
Fragments
of nuclear
chromosomes
separate
envelope Each pair of homologous Two haploid cells
Replkated homologous Microtubule chromosomes separates form; each chromosome
chromosomes (red and blue) attached to still consists of two
pair and exchange segments; kinetochore sister chromatids
2n '" 6 in this example
Chromosomes line up
by homologous pairs
Prophase I • Centrosome movement, spindle Anaphase I Telophase I and
• Chromosomes begin to formation, and nuclear envelope • Breakdown 01 proteins Cytokinesis
condense, and homologs loosely b/eakdown occur as in mitosis. responsible for sister chromatid • At the beginning of telophase I,
pair along their lengths. aligned • In late prophase I(after the stage cohesion along chromatid arms each half of the cell has a
gene by gene. shown), microtubules from one allows homologs to separate. complete haploid set of
replicated chromosomes. Each
• Crossing over (the exchange of pole or the other attach to the • The homologs move toward
corresponding seglTl(!nts of DNA two kinetochores, protein opposite poles. guided by the chromosome is composed of two
mole<ules by nonsisler chromatids) structures at the centromeres of spindle apparatus. sister chromatids; one or both
is completed while homologs ale the two homologs. The chromatids include regions of
• Sister chromatid cohesion persists nonsister chromatid DNA.
in synapsis, held tightly together homologous pairs then move at the centromere. causing
by proteins along their lengths toward the metaphase plate. chromatids to move as a unit • Cytokinesis (division of the
(before the stage shown). toward the same pole. cytoplasm) usually occurs
• Synapsis ends in mid·prophase. Metaphase I simultaneously with telophase I.
and the chromosomes in each forming two haploid daughter ceI~
• Pairs of homologous
pair move apart slightly, as chromoso~ are now arranged • In animal cells. a cleavage furrow
shown above. on the metaphase plate, with IOlms.. (In plant cells. a cell plate
one chromosome in each pair IOlms..)
• Each homologous pair has one 01
more chiasmata, points where facing each pole. • In some spe<ies. chromoso~
Gossing over has occuued and • Bolh dwomatids of all! tonoIog are deconr:lense aoo the nudear
the homologs are still associated attad'led 10 kinetochore envtlope re-forms..
due 10 cohesion betweef1 sister iOOouDJIes frrm all! jXIIe; those of • No replication occurs between
dllomalids (sister chromatid the other hlmolog are attathed 10 meiosis Iand meiosis II.
cohesion). iOOoniluIes frrm the oppo5ite pole.
254 UNIT TUff Genetics

10. .................. MEIOSIS II: Separates sister chromatids .....
Prophase 11 Metaphase n-l,.--1111 Anaphase 11 Telophase II and
Cytokinesis
During another round of (ell division, the sister dlromatids finally separate:
four haploid daughter cells result., containing unrepficated chromosomes
SISler chromatids Haploid daughtef cells
separate fOfml"9
Prophase II Metaphase II Anaphase II Telophase II
• Aspindle apparatus forms. • The chromosomes are positioned • Breakdown of proteins holding and Cytokinesis
• In late prophase II (nOI shown on the metaphase plate as in the sister chromatids togethel at • Nuclei form, the chlOmosomes
here), chromosomes, each still mitosis. the centromere allows the begin de(Ondensing, and
composed of two chromatids • Because of crossing over in
chromatids to separate, The cytokinesis occurs.
asso<iated <lIthe centromere, meiosis I, the two sister chromatids move toward
• The meiotic division of one
move toward the metaphase II chromatids of each chromosome opposite poles as individual parent cell produces four
plale. are not genetically identical. chromosomes. daughter cells, each with a
• The kinetochores of sisler haploid set of (unreplicatedl
chromatids afe attached to chlOmosomes.
microtubules extl'nding from • Each of the fOUl daughter cells is
opposite poles, geflelically distinct hom the
olhel daughter cells and flOm
the parent cell.
BioFlix Vlsn the Study Area
at www.masteringbio.(om
fOf the 8IoAix 3-D Ammation on
"""'"
(HAHUI THIRTUH Meiosis and Sexual Life C)"des 255

11. MITOSIS MEIOSIS
Parent cell Chiasma (site of
crossing over) MEIOSIS I
(before chromosome replication)
Prophase
Chromosome
.Ai
=- t
,LcOh-,-,m-,,-,-m-,-(
Prophase I
Homologous chromosome
replication replication pair held together by
ReplICated chromosome chiasma and sister
(two sister chromatids) 2n= 6 chromatid cohesion
Chromosomes Chromosomes line
Metaphase line up Individu- up by homologous Metaphase I
ally at the pairs at the
metaphase plate metaphase plate
Anaphase Sister chromatids Homologs Anaphase I
Telophase separate during separate Telophase I
anaphase during
-- .....--
anaphase I; Haploid
sister n=3
®
"
..... -
20
@ .....
20
chromatids
remain
attached at
centromere
Sister
chroma-
MEIOSIS II
Daughter cells tids
of mitosis separate o
dunng Daughter cells of meiosis II
anaphase II
SUMMARY
Property MitOSIS MeIOSIS
DNA Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins
replication
Number of One, including prophase, metaphase, Two, each including prophase, metaphase, anaphase, and
divisions anaphase, and telophase telophase
Synapsis of Does not occur Occurs during prophase I along with crOSSing over
homologous between nonsister chromatids; resulting chiasmata
chromosomes hold pairs together due to sister chromatid cohesion
Number of Two, each diploid (2n) and genetically Four, each haploid (n), containing half as many chromosomes
daughter cells identical to the parent cell as the parent cell; genetically different from the parent
and genetic cell and from each other
composition
Role in the Enables multicellular adult to arise from Produces gametes; reduces number of chromosomes by half
animal body zygote; produces cells for growth, repair, and, and introduces genetic variability among the gametes
in some species, asexual reproduction
... Figure 13.9 A comparison of mitosis and meiosis in diploid cells.
••I;t-W'I' Could any other combinations of chromosomes be generated during meiosis II from the specific cells
shown in telophase P Explain. (Hint: Draw fhe cells as they would appear in metaphase II.)
256 UNIT THREE Genetics

12. AComparison of Mitosis and Meiosis
Figure 13.9 summarizes the key differences between meiosis What prevents the separation of sister
and mitosis in diploid cells. Basically, meiosis reduces the chromatids at anaphase I of meiosis?
number of chromosome sets from two (diploid) to one (hap-
EXPERIMENT Yoshinori Watanabe and colleagues knew that
[oid), whereas mitosis conserves the number of chromosome
during anaphase I. the protein shugoshin is present only around
sets. Therefore, meiosis produces cells that differ genetically centromeres, They wondered whether it protects cohesins there
from their parent cell and from each other, whereas mitosis from degradation in meiosis I. ensuring that chromatids stay to-
produces daughter cells that are genetically identical to their gether while homologs separate. To test this hypothesis. they used
a species of yeast in which meiosis produces haploid spores lined
parent cell and to each other. up in a specific order inSide a spore case To follow the movement
Three events unique to meiosis occur during meiosis I: of chromosomes. they fluorescent/y labeled a region near the cen-
tromere of both chromatids in one homolog, leaving the other ho-
1. Synapsis and crossing over. During prophase I, repli- molog unlabeled. They then disabled the gene coding for
cated homologs pair up and be<ome physically connected shugoshin and compared this yeast strain (shugoshin ) with nor-
along their lengths by a zipper-like protein structure, the mal yeast cells (shugoshin +), The researchers expected that the two
labeled chromosomes ariSing from the labeled chromatids in nor-
synaptonemal complex; this process is called synapsis. mal cells would end up in separate spores at one end of the spore
Genetic rearrangement between nonsister chromatids, case. They further predicted that if shugoshin does protect co-
known as crossing over, is completed during this stage. hesins from cleavage at the centromere at anaphase I, then the la-
beled chromosomes in shugoshin- cells would separate randomly
Following disassembly of the synaptonemal complex in in meiosis II, sometimes ending up in the same spore.
late prophase, the hvo homologs pull apart slightly but re-
main connected by at least one X-shaped region called a Shugoshin+ (normal) Shugoshin-
chiasma (plural, chiasmata). A chiasma is the physical Spore case Fluorescent label
~tL
manifestation of crossing over; it appears as a cross be-
cause sister chromatid cohesion still holds the m'o origi-
nal sister chromatids together, even in regions where one
Metaphase I --
of them is now part of the other homolog. Synapsis and
crossing over normally do not occur during mitosis. -)('-- t
Anaphase I -1/ ~.--
2, Homologs on the metaphase plate, At metaphase I of
meiosis, chromosomes are positioned on the metaphase
plate as pairs of homologs, rather than individual chro- k
t
Metaphase II II l.
mosomes, as in metaphase of mitosis.
3. Separation of homologs. At anaphase I of meiosis, the
-I -•
.... t
Anaphase II ,II' ""-
OR
,II' ""-
replicated chromosomes ofeach homologous pair move
toward opposite poles, but the sister chromatids of each t
~
replicated chromosome remain attached. In anaphase of Mature
spores
mitosis, by contrast, sister chromatids separate.
Spore Two of three possible arrange-
How do sister chromatids stay together through meiosis I
ments of labeled chromosomes
but separate from each other in meiosis II and mitosis? Sister
chromatids are attached along their lengths by protein com- # 100
RESULTS In shug-
plexes called cohesins. In mitosis, this attachment lasts until the oshin +cells, the two labeled 80
end of metaphase, when enzymes cleave the cohesins, freeing chromosomes ended up in ~ 60
the sister chromatids to move to opposite poles of the cell. In different spores in almost all ~ 40
meiosis, sister chromatid cohesion is released in two steps. In
cases, In shugoshin- cells. &. 20
they were in the same spore ~ 0 .t,2~::-;;c
metaphase I, homologs are held together by cohesion between in about half the cases. Shugoshin+ Shugoshin-
sister chromatid arms in regions where DNA has been ex-
CONCLUSION The researchers concluded that shugoshin pro·
changed. At anaphase I, cohesins are cleaved along the arms,
tects cohesins at the centromere at anaphase I. thus maintaining
allowing homologs to separate. At anaphase II, cohesins are the attachment between sister chromatids and ensuring that they
cleaved at the centromeres, allowing chromatids to separate. separate properly during meiosis II,
Figure 13.10 shows one of a series of experiments carried SOURCE T. S, KitaJ'III<l, S. A. KiJWa'>hlma, and Y. Watanabe, The
out by Yoshinori Watanabe and colleagues at the University of conserved konetochore proteon shugoshin protects centromeric coheSIon during
Tokyo. They knew that similar proteins were present in co- meIOsis. N~lure 427510-517 aOO4)
hesin complexes during mitosis and meiosis, and they won-
dered what was responsible for preventing cohesin cleavage at
N'mu". Draw a graph showing what you expect happened to
the chromatids of the unlabeled chromosome in both strains of cells,
the centromere while it was occurring along sister chromatid
CHAPTER THIRTEEN Meiosis and Sexual Life Cycles 257

13. arms at the end of metaphase I. They found a protein they Origins of Genetic Variation Among Offspring
named shugoshin (Japanese for "guardian spirit") that protects
cohesins from cleavage at the centromere during meiosis I. In species that reproduce sexually, the behavior of chromo-
Shugoshin is similar to a fruit fly protein identified lOyears ear~ somes during meiosis and fertilization is responsible for most
lier by Terry Orr-Weaver, this unit's interviewee. of the variation that arises each generation. Let's examine
three mechanisms that contribute to the genetic variation
Meiosis I is called the reductional division because it
arising from sexual reproduction: independent assortment of
halves the number of chromosome sets per cell-a reduc·
tion from two sets (the diploid state) to one set (the haploid chromosomes, crossing over, and random fertilization.
state). During the second meiotic division, meiosis II (some-
times called the equational division), the sister chromatids Independent Assortment of Chromosomes
separate, producing haploid daughter cells. The mechanism One aspect ofsexual reproduction that generates genetic vari-
for separating sister chromatids is virtually identical in ation is the random orientation of homologous pairs of chro-
meiosis II and mitosis. The molecular basis of chromosome mosomes at metaphase of meiosis L At metaphase I, the
behavior during meiosis continues to be a focus of intense homologous pairs, each consisting of one maternal and one
research interest. paternal chromosome, are situated on the metaphase plate.
(Note that the terms maternal and paternal refer, respectively,
CONCEPT CHECI( 13.3 to the mother and father of the individual whose cells are un·
I. How are the chromosomes in a cell at metaphase of dergoing meiosis.) Each pair may orient with either its mater·
mitosis similar to and different from the chromo- nal or paternal homolog closer to a given pole-its orientation
somes in a cell at metaphase of meiosis II? is as random as the flip of a coin. Thus, there is a 50% chance
2. -'M"'I. Given that the synaptonemal complex
disappears by the end of prophase, how would the
that a particular daughter cell of meiosis I will get the mater-
nal chromosome of a certain homologous pair and a 50%
two homologs be associated if crossing over did not chance that it will get the paternal chromosome.
occur? What effect might this ultimately have on Because each homologous pair of chromosomes is posi-
gamete formation? tioned independently ofthe other pairs at metaphase I, the first
For suggested answers, see Appendi~ A. meiotic division results in each pair sorting its maternal and
paternal homologs into daughter cells independently of every
other pair. This is called independent assortment. Each daugh·
ter cell represents one outcome of all possible combina·
tions of maternal and paternal chromosomes. As shown in
~:~':t~;v~~i~~on produced Figure 13.11, the number of combinations possible for
daughter cells formed by meiosis of a diploid cell with two
homologous pairs of chromosomes is four (two possible
in sexual life cycles contributes arrangements for the first pair times two possible arrange-
to evolution ments for the second pair). Note that only two of the four
How do we account for the genetic varia-
tion illustrated in Figure 13.1? As you will Possibility 1 Possibility 2
learn in more detail in later chapters, mu-
tations are the original source of genetic Two equally probable
arrangements of
diversity. These changes in an organism's chromosomes at
DNA create the different versions of metaphase I
genes known as alleles. Once these differ~
ences arise, reshuffling of the alleles dur-
ing sexual reproduction produces the
variation that results in each member of a
species having its own unique combina- Metaphase II
tion of traits.
Daughter
.... Figure 13.11 The independent cells
assortment of homologous
chromosomes in meiosis. Combination 3 Combination 4
258 UNIT THREE Genetics

14. combinations of daughter cells shown in the figure would result ing meiosis II further increases the number of genetic types of
from meiosis of a single diploid cell, because a single parent cell daughter cells that can result from meiosis.
would have one or the other possible chromosomal arrangement You will learn more about crossing over in Chapter 15. The
at metaphase I, but not both. However, the population of daugh- important point for now is that crossing over, by combining
ter cells resulting from meiosis of a large number of diploid cells DNA inherited from two parents into asinglechromosome, is
contains aU four types in approximatelyequal numbers. In the case an important source of genetic variation in sexual life cycles.
of n = 3, eight combinations of chromosomes are possible for
daughter cells. More generally, the number of possible combina- Random Fertilization
n
tions when chromosomes sort independently during meiosis is 2 ,
The random nature of fertilization adds to the genetic vari·
where n is the haploid number of the organism.
ation arising from meiosis. In humans, each male and fe-
In the case of humans (n = 23), the number of possible 23
male gamete represents one of about 8.4 million (2 )
combinations of maternal and paternal chromosomes in the
possible chromosome combinations due to independent as-
resulting gametes is 223 , or about 8.4 million. Each gamete that
sortment. The fusion of a male gamete with a female gamete
you produce in your lifetime contains one of roughly 8.4 mil-
during fertilization will produce a zygote with any of about
lion possible combinations of chromosomes.
70 trillion (2 23 x 223 ) diploid combinations. If we factor in
Crossing Over the variation brought about by crossing over, the number of
possibilities is truly astronomical. It may sound trite, but
As a consequence of the independent assortment of chro- you really are unique.
mosomes during meiosis, each of us produces a collection of
gametes differing greatly in their combinations of the chro-
mosomes we inherited from our two parents. Figure 13.11 Nonsist~r chromatids
Prophas~ I
suggests that each individual chromosome in a gamete is ex- h~ld tog~ther
of m~iosis
clusively maternal or paternal in origin. In fact, this is not during synapsis
the case, because crossing over produces recombinant
chromosomes, individual chromosomes that carry genes
(DNA) derived from two different parents (Figure 13.12). --''-'';:0-/
Pair of o In crossingtover
and
prophas~ synapsis
homologs
In meiosis in humans, an average of one to three crossover occur; then homologs
events occur per chromosome pair, depending on the size of mov~ apart slightly.
Chiasma.
the chromosomes and the position oftheir centromeres. site of f.) Chiasmata and
Crossing over begins very early in prophase I, as homolo- crossing atlachm~nts between
gous chromosomes pair loosely along their lengths. Each gene over sister chromatids hold
on one homolog is aligned precisely with the corresponding homologs together;
they move to the
gene on the other homolog. In a single crossover event, spe- Centromere metaphase I plate,
cific proteins orchestrate an exchange of corresponding seg-
ments of two nonsister chromatids-one maternal and one
IEM
o Breakdown ofchroma·
holding sister
prot~ins
paternal chromatid ofa homologous pair. In this way, crossing Anaphase [ tid arms together
over produces chromosomes with new combinations of ma- allows homologs with
ternal and paternal alleles (see Figure 13. I2). recombinant chroma-
tids to separate,
In humans and most other organisms studied so far, cross-
ing over also plays an essential role in the lining up ofhomolo-
gous chromosomes during metaphase I. As seen in Figure 13.8,
a chiasma forms as the result of a crossover occurring while
Anaphase II
sister chromatid cohesion is present along the arms. Chias-
mata hold homologs together as the spindle forms for the first
meiotic division. During anaphase I, the release of cohesion
along sister chromatid arms allows homologs to separate.
During anaphase II, the release ofsister chromatid cohesion at
the centromeres allows the sister chromatids to separate. Daughter
cells
At metaphase II, chromosomes that contain one or more re-
combinantchromatids can be oriented in two alternative, non-
equivalent ways with respect to other chromosomes, because Recombinant
chromosomes
their sister chromatids are no longer identical. The different
possible arrangements of nonidentical sister chromatids dur- ... Figure 13.12 The results of crossing over during meiosis.
CHAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 259

15. The Evolutionary Significance of Genetic Although Darwin realized that heritable variation is what
Variation Within Populations makes evolution possible, he could not explain why offspring
resemble-but are not identical to-their parents. Ironically,
Now that you've learned how new combinations of genes Gregor Mendel, a contemporary ofDarwin, published a theory
arise among offspring in a sexually reproducing popula- of inheritance that helps explain genetic variation, but his dis-
tion, let's see how the genetic variation in a population re- coveries had no impact on biologists until 1900, more than 15
lates to evolution. Darwin recognized that a population years after Darwin (1809-1882) and Mendel (l822-1884) had
evolves through the differential reproductive success of its
died. In the next chapter, you will learn how Mendel discovered
variant members. On average, those individuals best the basic rules governing the inheritance of specific traits.
suited to the local environment leave the most offspring,
thus transmitting their genes. This natural selection re·
suits in the accumulation of those genetic variations fa- CONCEPT CHECK 13.4
vored by the environment. As the environment changes, 1. What is the original source of all the different aneles
the population may survive if, in each generation, at least of a gene?
some of its members can cope effectively with the new 2. The diploid number for fruit flies is 8, while that for
conditions. Different combinations of alleles may work grasshoppers is 46. If no crossing over took place,
better than those that previously prevailed. Mutations are would the genetic variation among offspring from a
the original source of different alleles, which are then given pair of parents be greater in fruit flies or
mixed and matched during meiosis. In this chapter, we grasshoppers? Explain.
have seen how sexual reproduction greatly increases the 3. -Q@i1 IM Under what circumstances would
genetic variation present in a population. In fact, the abil- crossing over during meiosis not contribute to genetic
ity of sexual reproduction to generate genetic variation is variation among daughter cells?
one of the most commonly proposed explanations for the For $ugg~sted an$w~rs, see Appendix A.
persistence of sexual reproduction.
-W ]f.- Go to the Study Area at www.masteringbio.(om for BioFlix of two sets of23-one set from each parent. In human diploid
3-D Animations. MP3 Tuto~. Videos, Pr3ctke Tests. an eBook, and mor~_ cells, there are 22 homologous pairs of autosomes, each with
a maternal and a paternal homolog. The 23rd pair, the sex
chromosomes, determines whether the person is female (XX)
or male (XY).
SUMMARY OF KEY CONCEPTS
.. Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity, ovaries and testes (the gonads) produce
.i.III1I'_ 13.1 haploid gametes by meiosis, each gamete containing a Single
Offspring acquire genes from parents by inheriting set of 23 chromosomes (n == 23). During fertilization, an
egg and sperm unite, forming a diploid (2n = 46) single-
chromosomes (pp. 248-249) celled zygote, which develops into a multicellular organism
.. Inheritance of Genes Each gene in an organism's DNA exists by mitosis.
at a specific locus on a certain chromosome. We inherit one set .. The Variety of Sexual life Cycles Sexual life cycles differ
of chromosomes from our mother and one set from our father.
in the timing of meiosis relative to fertilization and in the
.. Comparison of Asexual and Sexual Reproduction In point(s) of the cycle at which a multicellular organism is
asexual reproduction, a single parent produces genetically produced by mitosis.
identical offspring by mitosis. Sexual reproduction combines
sets of genes from two different parents. forming genetically
diverse offspring. .ill"I'-13.3
Meiosis reduces the number of chromosome sets from
Actiity Asexual and Sexual Life Cycles diploid 10 haploid (pp. 253-258)
.. The Stages of Meiosis The two cell divisions of meiosis pro-
.i.IIIII'-13.2 duce four haploid daughter cells. The number of chromosome
sets is reduced from two (diploid) to one (haploid) during
Fertilization and meiosis alternate in sexual life cycles meiosis I, the reductional division.
(pp.250-253) .. A Comparison of Mitosis and Meiosis Meiosis is distin-
.. Sets of Chromosomes in Human Cells Normal human so- guished from mitosis by three events of meiosis I, as shown
matic cells are diploid. They have 46 chromosomes made up on the next page:
260 UNIT THREE Genetics