9.2 Meiosis and genetic variation

Question 1

What are the two types of cell division

What do they each produce

Answer 1

Mitosis:
Produces two identical daughter cells with same number of chromosomes as the parent cell and as each other

Meiosis:
Produces 4 genetically different haploid daughter cells, each with half the chromosomes as the parent cell (so 23 each)

Question 2

What happens in sexual reproduction

Answer 2

Two gametes (eg sperm cell and egg cell) fuse to give rise to new offspring .

Each gamete has half the number of chromosomes as a diploid cell (so they have 23 each) so that when they fuse it makes cells with 46 each.

This is important because if each gamete had 46, cells, 92 would be produced and so there will be double the amount of chromosomes in any future cells made.

Question 3

How are gametes made in animals vs plants

Answer 3

gametes in animals are made my meiosis whereas in plants they are made via mitosis
eg with spores

Question 4

What is a diploid cell of an organism

How does this impact meiosis

Answer 4

It has 46 chromosomes :
So two complete sets, one set from each parent.

In meiosis, homologous pairs of chromosomes separate , so only one of each pair enters a daughter cell (haploid number is 23)

Question 5

How many nuclear divisions are there in meiosis, what are they called

Answer 5

There are 2 divisions

Meiosis 1 (first division)
Meiosis 2 (second division)

Question 6

How does crossing over occur in meiosis 1
to produce genetic variation
Summary

Answer 6

. First division:

Homologous chromosomes pair up opposite each other along equator of cell.

Non sister chromatids on homologous pairs of chromosomes wrap around each other until parts break off.
Equivalent portions of these chromatids are exchanged/swapped in a process called ‘crossing over’.

So by the end of this division, the homologous pairs have separated, with one chromosome from each pair going into one of the two daughter cells

Question 7

What happens in meiosis 2 summary

Answer 7

Where the second meiotic division occurs so the chromatids move apart.
So eventually makes 4 daughter cells

Question 8

What are homologous pairs of chromosomes

Answer 8

Two chromosomes which have exactly the same genes on them but different alleles
Maternal and paternal versions of the chromosome

Question 9

Meiosis produces genetic variation among the offspring which may lead to improved adaptations that improve survival chances.

What two ways does meiosis bring about genetic variation

Answer 9

. Independent segregation of homologous chromosomes
. New combinations of maternal and paternal alleles by CROSSING OVER

Question 10

What is a…
Gene
Locus
Allele

Answer 10

Gene is a length of DNA found on a chromosome, that codes for a polypeptide

Locus is the position of the gene on a chromosome or DNA molecule

Allele is the alternative form of a particular gene

Question 11

How does independent segregation occur
How does it produce genetic variation

Answer 11

. During meiosis 1
Each chromosome lines up along the equator of the cell next to its homologous pair (the 46 chromosomes have 23 from mother and 23 from father already)
(In humans this means 23 pairs lying side by side)

They arrange themselves in this line at random, so you don’t know which side of the equator they will line up.
Eg all paternal chromosomes on one side, and all maternal on the other.
Or just some maternal some paternal on each side.

So when it divides, the chromosomes on the right side and left side split off, so there is a random combination of which ones go to each daughter cells.

Question 12

If homologous chromosomes have the same genes, why is there variation when they’re split off into the 4 daughter cells in meiosis

Answer 12

Although homologous pairs have the same genes, the alleles of these genes may differ.
So they have the same genes that code for the same type of thing eg blood group, but the alleles (whether type A or B) is different

So this random assortment of chromosomes produces new genetic combinations

Question 13

Describe the stages of genetic recombination by CROSSING OVER AND RECOMBINATION

How does this lead to genetic variation

Answer 13

. The chromosomes line up alongside their homologous partners

. The chromatids of each pair twist around each other
. During this twisting, tension is created so portions of chromatids break off

. These portions rejoin with the chromatids of their homologous partner
Eg equivalent portions are exchanged

. This makes new genetic combinations of maternal and paternal alleles

Question 14

What is the area where chromatids snap off and recombine called

Answer 14

Chiazma

Question 15

What would happen if there was no recombination by crossing over in meiosis

Answer 15

. Only two different types of cell would be produced, even though there would still be 4 cells

Question 16

How do you calculate the genetic variation possibilities using number of homologous chromosomes

Answer 16

2 to the power of n

n is the number of pairs of homologous chromosomes

Eg humans with 23 homologous pairs can produce 2^23 so over 8 million different combinations of chromosomes of maternal and paternal origin in its daughter cells

Use number 2 because there are 2 chromosomes in each pair

Question 17

How do you calculate genetic variation possibilities of offspring using gametes

Answer 17

Random pairing of male and female gametes:

As any egg could fuse with any sperm cells

When gametes come from different parents, two different genetic complements with different alleles are combined.

(2n)^2 where n is number of homologous chromosomes.

But this calculation is before crossing over

Question 18

Apart from crossing over and independent segregation, what is another way genetic variation in a species is increased

Answer 18

. Random fusion of gametes

As each gamete contains alleles, there will be a new combination of alleles made when they fuse