The document describes meiosis and sexual reproduction, detailing the mechanisms of heredity, genetic variation, and the life cycle of living organisms. It emphasizes the differences between meiosis and mitosis, highlighting key stages such as prophase, metaphase, anaphase, and the importance of processes like crossing over and independent assortment. The document also discusses the role of genetic variation in evolution and the implications of sexual reproduction for genetic diversity.

1. Chapter 13
Meiosis and Sexual
Life Cycles
Animation

2. Overview: Hereditary Similarity and
Variation
• Living organisms are distinguished by their
ability to reproduce their own kind
• Heredity is the transmission of traits from one
generation to the next
• Variation shows that offspring differ in
appearance from parents and siblings
• Genetics is the scientific study of heredity and
variation

3. Figure 13.1

4. Inheritance of Genes
• Genes are the units of heredity
• Genes are segments of DNA
• Each gene has a specific locus on a certain
chromosome
• One set of chromosomes is inherited from each
parent
• Reproductive cells called gametes (sperm and
eggs) unite, passing genes to the next
generation

5. 3-17
The physical location of a gene on a
chromosome is called its locus.
Figure 3.3

6. • Gametes are typically haploid
– They contain a single set of chromosomes
• Gametes are 1n, while diploid cells are 2n
– A diploid human cell contains 46 chromosomes
– A human gamete only contains 23 chromosomes
• During meiosis, haploid cells are produced from
diploid cells
– Thus, the chromosomes must be correctly sorted and
distributed to reduce the chromosome number to
half its original value
• In humans, for example, a gamete must receive one
chromosome from each of the 23 pairs
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-43

7. Sets of Chromosomes in Human Cells
• Each human somatic cell (any cell other than a
gamete) has 46 chromosomes arranged in pairs
• A karyotype is an ordered display of the pairs of
chromosomes from a cell
• The two chromosomes in each pair are called
homologous chromosomes, or homologues
• Both chromosomes in a pair carry genes
controlling the same inherited characteristics

8. LE 13-3
5 µmPair of homologous
chromosomes
Sister
chromatids
Centromere

9. • The sex chromosomes are called X and Y
• Human females have a homologous pair of X
chromosomes (XX)
• Human males have one X and one Y
chromosome
• The 22 pairs of chromosomes that do not
determine sex are called autosomes

10. Figure 13.4Key
Maternal set of
chromosomes (n = 3)
Paternal set of
chromosomes (n = 3)
2n = 6
Sister chromatids
of one duplicated
chromosome
Two nonsister
chromatids in
a homologous pair
Centromere
Pair of homologous
chromosomes
(one from each set)

11. 1. Why is it more practical to prepare
karyotypes by viewing somatic diploid cells
rather than haploid gametes?
a. Somatic diploid cells do not contain organelles to
interfere with karyotyping.
b. Both sets of chromosomes, which are present in
somatic diploid cells, need to be examined.
c. DNA in haploid gametes will not stain.
d. The chromosomes are larger in a somatic diploid
cell.
e. Haploid gametes do not have sex chromosomes.

12. 1. Why is it more practical to prepare
karyotypes by viewing somatic diploid cells
rather than haploid gametes?
a. Somatic diploid cells do not contain organelles to
interfere with karyotyping.
b. Both sets of chromosomes, which are present in
somatic diploid cells, need to be examined.
c. DNA in haploid gametes will not stain.
d. The chromosomes are larger in a somatic diploid
cell.
e. Haploid gametes do not have sex chromosomes.

13. Behavior of Chromosome Sets in the Human
Life Cycle
• At sexual maturity, the ovaries and testes
produce haploid gametes
• Gametes are the only types of human cells
produced by meiosis, rather than mitosis
• Meiosis results in one set of chromosomes in
each gamete
• Fertilization, the fusing of gametes, restores the
diploid condition, forming a zygote
• The diploid zygote develops into an adult

14. Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Egg (n)
Sperm (n)
MEIOSIS FERTILIZATION
Testis
Diploid
zygote
(2n = 46)
Ovary
Mitosis and
development
Multicellular diploid
adults (2n = 46)

15. The Variety of Sexual Life Cycles
• The alternation of meiosis and fertilization is
common to all organisms that reproduce sexually
• The three main types of sexual life cycles differ in
the timing of meiosis and fertilization

16. Figure 13.6Key
Haploid (n)
Diploid (2n)
Gametes
Haploid multi-
cellular organism
(gametophyte)
MitosisMitosis Mitosis Mitosis
Haploid unicellular or
multicellular organism
Gametes
FERTILIZATIONMEIOSIS
n
2n
n
n
n
n
nn
n
nn
2n
2n
Zygote
MEIOSIS FERTILIZATION
Spores
Gametes
Mitosis
Diploid
multicellular
organism
(sporophyte)
Mitosis
Diploid
multicellular
organism
(a) Animals (b) Plants and some algae (c) Most fungi and
some protists
2n 2n
MEIOSIS FERTILIZATION
Zygote
n
Zygote
nn

17. • Plants and some algae exhibit an alternation of
generations
• This life cycle includes two multicellular
generations or stages: one diploid and one haploid
• The diploid organism, the sporophyte, makes
haploid spores by meiosis
• Each spore grows by mitosis into a haploid
organism called a gametophyte
• A gametophyte makes haploid gametes by mitosis

18. • In most fungi and some protists, the only diploid
stage is the single-celled zygote; there is no
multicellular diploid stage
• The zygote produces haploid cells by meiosis
• Each haploid cell grows by mitosis into a haploid
multicellular organism
• The haploid adult produces gametes by mitosis

19. • Depending on the type of life cycle, either haploid
or diploid cells can divide by mitosis
• However, only diploid cells can undergo meiosis
• In all three life cycles, chromosome halving and
doubling contribute to genetic variation in offspring

20. MEIOSIS
• Like mitosis, meiosis begins after a cell has progressed
through interphase of the cell cycle
• Unlike mitosis, meiosis involves two successive
divisions
– These are termed Meiosis I and II
– Each of these is subdivided into
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase
• The two cell divisions result in four daughter cells, rather than the
two daughter cells in mitosis
• Each daughter cell has only half as many chromosomes as the parent
cell

21. The Stages of Meiosis
• In the first cell division (meiosis I), homologous
chromosomes separate
• Meiosis I results in two haploid daughter cells
with replicated chromosomes—reduction
division
• In the second cell division (meiosis II), sister
chromatids separate— equatorial division
• Meiosis II results in four haploid daughter cells
with unreplicated chromosomes

22. Figure 13.7
Interphase
Meiosis I
Meiosis II
Pair of
homologous
chromosomes
in diploid
parent cell
Pair of duplicated
homologous
chromosomes
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Sister
chromatids
Homologous
chromosomes
separate
Haploid cells with
duplicated chromosomes
Sister chromatids
separate
Haploid cells with unduplicated chromosomes
1
2

23. • Before Meiosis, chromosomes are replicated in
interphase
• The single centrosome replicates, forming two
centrosomes

24. Figure 13.8MEIOSIS I: Separates
homologous chromosomes
Prophase I Metaphase I Anaphase I
Telophase I
and
Cytokinesis
Prophase II Metaphase II Anaphase II
Telophase II
and
Cytokinesis
Centrosome
(with
centriole
pair)
Centromere
(with
kineto-
chore)
Sister
chromatids
remain
attachedMeta-
phase
plate
Sister
chroma-
tids
Chiasmata
Spindle
Homo-
logous
chromo-
somes Fragments
of nuclear
envelope
Microtubules
attached to kinetochore
Homologous
chromo-
somes
separate
Cleavage
furrow
Sister
chromatids
separate
Haploid
daughter
cells
forming
MEIOSIS II: Separates
sister chromatids

25. Prophase I
• Prophase I typically occupies more than 90% of
the time required for meiosis
• Chromosomes begin to condense
• In synapsis, homologous chromosomes loosely
pair up, aligned gene by gene
• In crossing over, nonsister chromatids exchange
DNA segments
• Each pair of chromosomes forms a tetrad, a
group of four chromatids
• Each tetrad usually has one or more chiasmata,
X-shaped regions where crossing over occurred

26. Crossing Over
• Crossing over produces recombinant
chromosomes, which combine genes inherited
from each parent
• Crossing over begins very early in prophase I, as
homologous chromosomes pair up gene by gene
• In crossing over, homologous portions of two
nonsister chromatids trade places
• Crossing over contributes to genetic variation by
combining DNA from two parents into a single
chromosome

27. LE 13-11
Prophase I
of meiosis
Tetrad
Nonsister
chromatids
Chiasma,
site of
crossing
over
Recombinant
chromosomes
Metaphase I
Metaphase II
Daughter
cells

28. Figure 13.9
DNA
breaks
Cohesins
Centromere
DNA
breaks
Pair of
homologous
chromosomes:
Paternal
sister
chromatids
Maternal
sister
chromatids
Synaptonemal
complex forming
Chiasmata
Crossover Crossover
1
2 4
3

29. Metaphase I
• At metaphase I, tetrads line up at the metaphase
plate, with one chromosome facing each pole
• Microtubules from one pole are attached to the
kinetochore of one chromosome of each tetrad
• Microtubules from the other pole are attached to
the kinetochore of the other chromosome

30. Anaphase I
• In anaphase I, pairs of homologous chromosomes
separate
• One chromosome moves toward each pole, guided
by the spindle apparatus
• Sister chromatids remain attached at the
centromere and move as one unit toward the pole

31. LE 13-8ab
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Microtubule
attached to
kinetochore
Tetrad
MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Pairs of homologous
chromosomes split up
Tetrads line up

32. • At the end of meiosis, there are four daughter
cells, each with a haploid set of unreplicated
chromosomes
• Each daughter cell is genetically distinct from
the others and from the parent cell

33. 2. How and at what stage do
chromosomes undergo independent
assortment?
a. meiosis I pairing of homologs
b. anaphase I separation of homologs
c. meiosis II separation of homologs
d. meiosis I metaphase alignment
e. meiosis I telophase separation

34. 2. How and at what stage do
chromosomes undergo independent
assortment?
a. meiosis I pairing of homologs
b. anaphase I separation of homologs
c. meiosis II separation of homologs
d. meiosis I metaphase alignment
e. meiosis I telophase separation

35. A Comparison of Mitosis and Meiosis
• Mitosis conserves the number of chromosome
sets, producing cells that are genetically
identical to the parent cell
• Meiosis reduces the number of chromosomes
sets from two (diploid) to one (haploid),
producing cells that differ genetically from each
other and from the parent cell
• The mechanism for separating sister chromatids
is virtually identical in meiosis II and mitosis

36. • Three events are unique to meiosis, and all
three occur in meiosis l:
– Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
– At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead of
individual replicated chromosomes
– At anaphase I, it is homologous chromosomes,
instead of sister chromatids, that separate and are
carried to opposite poles of the cell

37. MITOSIS MEIOSIS
Prophase
Duplicated
chromosome
Metaphase
Anaphase
Telophase
2n 2n
Daughter cells
of mitosis
Sister
chromatids
separate.
Individual
chromosomes
line up.
Chromosome
duplication
Chromosome
duplication
2n = 6
Parent cell Chiasma MEIOSIS I
Prophase I
Homologous
chromosome
pair
Metaphase I
Anaphase I
Telophase I
MEIOSIS
II
Pairs of
homologous
chromosomes
line up.
Homologs
separate.
Sister
chroma-
tids
separate.
Daughter cells of meiosis II
Daughter
cells of
meiosis I
n n n n

38. Property Mitosis Meiosis
DNA
replication
During
interphase
During
interphase
Divisions One Two
Synapsis and
crossing over
Do not occur Form tetrads in
prophase I
Daughter cells,
genetic
composition
Two diploid,
identical to
parent cell
Four haploid,
different from
parent cell and
each other
Role in animal
body
Produces cells
for growth and
tissue repair
Produces
gametes

39. Genetic variation produced in sexual life
cycles contributes to evolution
• Mutations (changes in an organism’s DNA) are
the original source of genetic diversity
• Mutations create different versions of genes
• Reshuffling of different versions of genes during
sexual reproduction produces genetic variation
• Three mechanisms contribute to genetic
variation:
– Independent assortment of chromosomes
– Crossing over
– Random fertilization

40. • Sister chromatid cohesion allows sister
chromatids to stay together through meiosis I
• In mitosis, cohesins are cleaved at the end of
metaphase
• In meiosis, cohesins are cleaved along the
chromosome arms in anaphase I (separation
of homologs) and at the centromeres in
anaphase II (separation of sister chromatids)
• Meiosis I is called the reductional division
because it reduces the number of
chromosomes per cell

41. Independent Assortment of
Chromosomes
• Homologous pairs of chromosomes orient
randomly at metaphase I of meiosis
• In independent assortment, each pair of
chromosomes sorts maternal and paternal
homologues into daughter cells independently of
the other pairs
• The number of combinations possible when
chromosomes assort independently into gametes is
2n
, where n is the haploid number
• For humans (n = 23), there are more than 8 million
(223
) possible combinations of chromosomes

42. LE 13-10
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1 Possibility 2
Combination 2Combination 1 Combination 3 Combination 4
Daughter
cells
Metaphase II
Two equally probable
arrangements of
chromosomes at
metaphase I

43. Random Fertilization
• Random fertilization adds to genetic variation
because any sperm can fuse with any ovum
(unfertilized egg)
• The fusion of gametes produces a zygote with
any of about 64 trillion diploid combinations
• Crossing over adds even more variation
• Each zygote has a unique genetic identity

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evolutionary Significance of Genetic Variation
Within Populations
• Natural selection results in accumulation of
genetic variations favored by the environment
• Sexual reproduction contributes to the genetic
variation in a population, which ultimately
results from mutations

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3. Given the fact that 1 fg of DNA = 9.78 × 105
base pairs (on
average), you can estimate the rate of DNA synthesis in
Saccharomyces cerevisiae.
Approximately how many base pairs per minute were
synthesized during the S phase of these yeast cells?
a. 0.19 (1.9 × 10–1
) base pair
per minute
b. 100,000 (1.0 × 105
) base
pairs per minute
c. 200,000 (2.0 × 105
) base
pairs per minute
d. 11,000,000 (11.0 × 106
)
base pairs per minute

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3. Given the fact that 1 fg of DNA = 9.78 × 105
base pairs (on
average), you can estimate the rate of DNA synthesis in
Saccharomyces cerevisiae.
Approximately how many base pairs per minute were
synthesized during the S phase of these yeast cells?
a. 0.19 (1.9 × 10–1
) base pair
per minute
b. 100,000 (1.0 × 105
) base
pairs per minute
c. 200,000 (2.0 × 105
) base
pairs per minute
d. 11,000,000 (11.0 × 106
)
base pairs per minute