Genetic Information and Variation - Meiosis

Question 1

What is mitosis?

Answer 1

Mitosis produces two daughter cells with the same number of chromosomes as the parent cell and as each other.

Question 2

What is meiosis?

Answer 2

Meiosis usually produces four daughter cells, each genetically different and with half the number of chromosomes as the parent cell.

Question 3

What is the importance of meiosis?

Answer 3

In sexual reproduction, two gametes fuse to give rise to new offspring. If each gamete had a full set of chromosomes (diploid number) then the cell that they produce has double this number. In humans, the diploid number of chromosomes is 46, which means that this cell would have 92 chromosomes. This doubling of the number of chromosomes would continue at each generation.

It follows that, in order to maintain a constant number of chromosomes in the adults of a species, the number of chromosomes must be halved at some stage in the life cycle. This halving occurs as a result of meiosis.

Question 4

What does meiosis occur in the formation of?

Answer 4

In most animals, meiosis occurs in the formation of gametes. In some plants such as ferns, however, gametes are produced by mitosis. In the fern life cycle, meiosis occurs in the formation of spores.

Question 5

What is the haploid number of chromosomes?

Answer 5

Every diploid cell of an organism has two complete sets of chromosomes: one set provided by each parent. During meiosis, homologous pairs of chromosomes separate, so that only one chromosome from each pair enters a daughter cell. This is known as the haploid number of chromosomes which, in humans, is 23. When two haploid gametes fuse at fertilisation, the diploid number of chromosomes is restored.

Question 6

What happens in the first division of meiosis?

Answer 6

In the first division (meiosis 1), homologous chromosomes pair up and their chromatids wrap around each other (crossing over at the chiasmata takes place). Equivalent portions of these chromatids may be exchanged in a process called crossing over. By the end of this division, the homologous pairs have separated, with one chromosome from each homologous pair going into one of the two daughter cells.

Question 7

What happens in the second division of meiosis?

Answer 7

In the second meiotic division (meiosis 2), the chromatids move apart. At the end of meiosis 2, four cells have usually been formed. In humans, each of these cells contains 23 chromosomes.

Question 8

How does meiosis bring about genetic variation?

Answer 8

In addition to halving the number of chromosomes, meiosis also produces genetic variation among the offspring, which may lead to adaptations that improve survival chances. Meiosis brings about this genetic variation by:

• independent segregation of homologous chromosomes
• new combinations of maternal and paternal alleles by crossing over

Question 9

What is a gene?

Answer 9

a length of DNA that codes for a polypeptide

Question 10

What is a locus?

Answer 10

the position of a gene on a chromosome or DNA molecule

Question 11

What is an allele?

Answer 11

one of the different forms of a particular gene

Question 12

What are homologous chromosomes?

Answer 12

a pair of chromosomes, one maternal and one paternal, that have the same gene loci but may exist as different alleles

Question 13

What is independent segregation (independent assortment of chromosomes)?

Answer 13

There are various combinations of chromosome arrangement. During meiosis 1, each chromosome lines up alongside its homologous partner. In humans, for example, this means that there will be 23 homologous pairs of chromosomes lying side by side. When these homologous pairs arrange themselves in this line, they do so at random. One of each pair will pass to each daughter cell. Which one of the pair goes into the daughter cell, and with which one of any of the other pairs, depends on how the pairs are lined up in the parent cell. Since the pairs are lined up at random, the combination of chromosomes of maternal and paternal origin that go into the daughter cell at meiosis 1 is also a matter of chance. This is called independent segregation.

Question 14

How does the independent assortment of chromosomes produce new genetic combinations?

Answer 14

Each member of a homologous pair of chromosomes has exactly the same genes and therefore determines the same characteristics. However, the alleles of these genes may differ. The independent assortment of these chromosomes therefore produces new genetic combinations.

Question 15

What are the stages of independent segregation?

Answer 15

Stage 1:
There are two possible arrangements of the two chromosomes at the start of meiosis.

Stage 2:
At the end of meiosis 1, the homologous chromosomes have segregated into two separate cells.

Stage 3:
At the end of meiosis 2, the chromosomes have segregated into chromatids producing four gametes for each arrangement. The actual gametes are different, depending on the original arrangement of the chromosomes.

Question 16

What happens if the cells produced in meiosis are gametes?

Answer 16

Where the cells produced in meiosis are gametes, these will be genetically different as a result of the different combinations of the maternal and paternal chromosomes/alleles they contain.

These haploid gametes fuse randomly at fertilisation. The haploid gametes produced by meiosis fuse to restore the diploid state. Each gamete has a different make-up and their random fusion therefore produces variety in the offspring.

Where the gametes come from different parents, two different genetic make-ups are combined and even more variety results.

Question 17

What is the process of genetic recombination?

Answer 17

• During meiosis 1, each chromosome lines up alongside its homologous partner.
• The chromatids of each pair become twisted around one another at the chiasma.
• During this twisting process, tensions are created and portions of the chromatids break off.
• These broken portions might then rejoin with the chromatids of its homologous partner.
• Usually it is the equivalent portions of homologous chromosomes that are exchanged.
• In this way, new genetic combinations of maternal and paternal alleles are produced (four cells with a different genetic composition).

Question 18

Why is it called crossing over and recombination?

Answer 18

The chromatids cross over one another many times and so the process is known as crossing over. Crossing over increases genetic variety.

The broken-off portions of chromatid recombine with another chromatid, so this process is called recombination.

Question 19

How many combinations of chromosomes are there in any daughter cell?

Answer 19

Homologous pairs of chromosomes line up at the equator of a cell during meiosis 1. Either one of a pair can pass into each daughter cell (independent segregation) and so there are a large number of possible combinations of chromosomes in any daughter cell.

Question 20

What is the formula to determine the number of possible combinations of chromosomes for each daughter cell?

Answer 20

2^n where n = the number of pairs of homologous chromosomes

Question 21

How is variety further increased?

Answer 21

Variety is further increased through the random pairing of male and female gametes. Where the gametes come from different parents, two different genetic complements with different alleles are combined, providing yet more variety.

(2^n)^2 where n = the number of pairs of homologous chromosomes

Question 22

What do these calculations assume?

Answer 22

That the chromosomes stay intact throughout meiosis. In practice, crossing over between chromatids during meiosis 1 exchanges sections of chromosomes between homologous pairs in the process called recombination. As recombination occurs each time gametes are made, it will greatly increase the number of possible chromosome combinations in the gametes.

Question 23

What does meiosis need to do?

Answer 23

• halve the number of chromosomes (produce haploid cells) to maintain chromosome number at each generation
• ensure each gamete has one chromosome from each homologous pair
• increase variation in a population