Activity 4: Understanding the basics of meiosis Mitosis and meiosis both lead to new daughter cells, but meiosis sets up organisms for sexual reproduction. Meiosis produces cells (sperm and egg in humans) with only one set of chromosomes so that when fertilization occurs, it results in a new cell with two sets of chromosomes (one from the egg and one from the sperm). This is how there is genetic recombination of DNA resulting in unique individuals. Before we talk about what can go wrong in meiosis, you need to be clear about the process itself and how genetic recombination in particular occurs. To this end, complete the following set of questions. Questions 1. In the following diagram, draw what a cell with 1 chromosome would look like in the stages of meiosis. Prophase 1 is filled in for you and includes replicated homologous chromosomes; the black replicated chromosome is from the reproducing male’s mom and the grey replicated chromosome is from dad. 2. In your diagram from question 1: a. How many tetrads are formed? _______________ b. How many chromosomes are in the sperm? _______________ 3. In what stage(s) of meiosis: a. Are tetrads formed? ________________________________ b. Does crossing over occur? ________________________________ c. Do the chromosomes move to the poles? ________________________________ d. Do replicated chromosomes separate? ________________________________ e. Does the cytoplasm divide? ________________________________ 4. What processes in meiosis result in genetically unique daughter cells? When do these processes occur? (Note: There are two main processes; discuss both). 5. Compare and contrast meiosis with mitosis to complete the following table. Table 2. Comparison of key characteristics between meiosis and mitosis. Characteristics Mitosis Meiosis Type of organisms it occurs in # of chromosomes in human parent cell Number of times chromosomes replicate Number of cell divisions Crossing over occurs? (Y/N) Type of daughter cells produced Number of daughter cells produced Daughter cells identical to parent cell? (Y/N) Daughter cells are: 1n or 2n? # of chromosomes in human daughter cells Activity 3: Chromosomes in mitosis and meiosis In the nucleus of the cell are the chromosomes that are composed of the hereditary material DNA. In every somatic (body) cell of a human there are 46 chromosomes. Each species may have a different number of chromosomes than another species. Since each somatic cell of an organism contains the same number of chromosomes, there must be a duplication of material before the nucleus divides during mitosis. In each somatic cell, there are two sets of chromosomes; this is referred to as the 2n (diploid) number, in which n means number of chromosomes. In humans, 2n = 46 chromosomes. In each gamete (sex) cell, there is only one set of chromosomes; this is referred to as the 1n (haploid) number. In humans, 1n = 23 chromosomes. This means the.

1. Activity 4: Understanding the basics of meiosis
Mitosis and meiosis both lead to new daughter cells, but meiosis
sets up organisms for sexual
reproduction. Meiosis produces cells (sperm and egg in humans)
with only one set of chromosomes so that when fertilization
occurs, it results in a new cell with two sets of chromosomes
(one from the egg and one from the sperm). This is how there is
genetic recombination of DNA resulting in unique individuals.
Before we talk about what can go wrong in meiosis, you need to
be clear about the process itself and how genetic recombination
in particular occurs. To this end, complete the following set of
questions.
Questions
1. In the following diagram, draw what a cell with 1
chromosome would look like in the stages of meiosis. Prophase
1 is filled in for you and includes replicated homologous
chromosomes; the black replicated chromosome is from the
reproducing male’s mom and the grey replicated chromosome is
from dad.
2. In your diagram from question 1:
a. How many tetrads are formed?
_______________
b. How many chromosomes are in the sperm?
_______________
3. In what stage(s) of meiosis:
a. Are tetrads formed?
________________________________
b. Does crossing over occur?

2. ________________________________
c. Do the chromosomes move to the poles?
________________________________
d. Do replicated chromosomes separate?
________________________________
e. Does the cytoplasm divide?
________________________________
4. What processes in meiosis result in genetically unique
daughter cells? When do these processes occur? (Note: There
are two main processes; discuss both).
5. Compare and contrast meiosis with mitosis to complete the
following table.
Table 2. Comparison of key characteristics between meiosis and
mitosis.
Characteristics
Mitosis
Meiosis
Type of organisms it occurs in
# of chromosomes in human parent cell
Number of times chromosomes replicate

3. Number of cell divisions
Crossing over occurs? (Y/N)
Type of daughter cells produced
Number of daughter cells produced
Daughter cells identical to parent cell? (Y/N)
Daughter cells are: 1n or 2n?
# of chromosomes in human daughter cells
Activity 3: Chromosomes in mitosis and meiosis
In the nucleus of the cell are the chromosomes that are
composed of the hereditary material DNA. In every somatic

4. (body) cell of a human there are 46 chromosomes. Each species
may have a different number of chromosomes than another
species.
Since each somatic cell of an organism contains the same
number of chromosomes, there must be a duplication of material
before the nucleus divides during mitosis. In each somatic cell,
there are two sets of chromosomes; this is referred to as the 2n
(diploid) number, in which n means number of chromosomes. In
humans, 2n = 46 chromosomes.
In each gamete (sex) cell, there is only one set of chromosomes;
this is referred to as the 1n (haploid) number. In humans, 1n =
23 chromosomes. This means there are 23 different types of
chromosomes in the nucleus of a human cell. The autosomes
(non-sex chromosomes; i.e., everything but X and Y) are
numbered from 1-22 according to their length and centromere
position (part of the chromosome that links sister chromatids);
the sex chromosomes (i.e., X and Y) are number 23.
Karyotypes are used to examine the number
and appearance of chromosomes. In the
adjacent karyotype, the chromosomes are laid
out in order by size for the autosomes
followed by the sex chromosomes. There are
two chromosomes of each type (2n); one
came from this person’s mother through the
egg (1n) and the other from her father
through the sperm (1n) during fertilization.
This is the karyotype of a somatic cell that is
beginning mitosis; all chromosomes have
been replicated. In this karyotype, the sex
chromosomes are XX, so these chromosomes
belong to a female (males would have XY).
Karyotypes are a key tool used to detect

5. chromosomal abnormalities. They can be
performed on a variety of tissues, including
amniotic fluid (amniocentesis), placenta
(chorionic villus sampling), blood
(venipuncture), or bone marrow (biopsy).
Monosomy refers to a condition in which there is one
chromosome missing. It is abbreviated 2n-1. For example,
monosomy X is a condition in which cells have only one X
chromosome. Since they don’t have a Y chromosome, the
individual will be female. Trisomy refers to a condition in
which there is one extra chromosome. It is abbreviated 2n+1.
For example, trisomy X is a condition in which cells have three
X chromosomes; the individual will be female.
Monosomies and trisomies usually result from nondisjunction
during meiosis but can also occur during mitosis. They are more
common in meiosis 1 than meiosis 2. They are generally lethal,
with the exceptions of those involving sex chromosomes,
chromosome 21, or, very rarely, chromosomes 13 and 18.
Affected individuals have a distinctive set of physical and
mental characteristics called a syndrome.
Down syndrome is a developmental disorder generally caused
by an extra copy of chromosome 21. Having an extra copy of
this chromosome means that individuals have three copies of
each of the genes on that chromosome instead of two, making it
difficult for cells to properly control how much protein is made.
Producing too much (or too little) protein contributes to the
various symptoms of Down syndrome. Trisomy 21 is the cause
in approximately 95% of individuals, with 88% coming from
nondisjunction during the development of the mother’s egg, 8%
from nondisjunction in the sperm, and the remainder
from problems in fertilization or mitosis.
Nondisjunction during meiosis 1 in the mother’s gamete results

6. in 67-73% of trisomy 21 humans. The following diagram depicts
what could happen during meiosis 1 to cause Down syndrome.
Only chromosome 21 is represented. The top two eggs would
result in trisomy 21 when fertilized by a normal sperm. The
bottom two eggs would result in monosomy 21 when fertilized
by a normal sperm and would not develop into a viable zygote.
Questions
1. How many chromosomes are in somatic
cells of individuals with Down syndrome?
2. In the adjacent karyotype,
a. What is the individual’s gender?
b. What is the evidence of Down syndrome?
3. Maternal meiosis 2 nondisjunction results in 18-20% of
trisomy 21 humans. In the following diagram, draw what could
happen during meiosis 2 to cause Down syndrome. Prophase 1
is filled in with the replicated chromosomes for chromosome
21; don’t worry about drawing the other chromosomes.
4. People with Down syndrome can reproduce, and frequently
their children do not have Down syndrome. Fill in the following
diagram and explain why this could be. In the diagram,
Prophase 1 shows the black replicated chromosomes 21 were
inherited from the mother and the gray replicated chromosome
21 from the father; don’t worry about the other chromosomes.
Explanation:

7. 6
CBIO Lab: Mitosis and Meiosis