Understanding the basics of meiosis

CBIO Lab: Mitosis and Meiosis p. 6/10

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.


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? ________________________________

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CBIO Lab: Mitosis and Meiosis p. 7/10

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

CBIO Lab: Mitosis and Meiosis p. 8/10

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.

Species # chromosomes in

somatic cells

Human 46

Bullfrog 22

Lizard 34

Horse 64

Tomato plant 24

Fern 512 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 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

CBIO Lab: Mitosis and Meiosis p. 9/10

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 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.


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?

CBIO Lab: Mitosis and Meiosis p. 10/10

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


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.


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