Meiosis and Genetic Variation

Question 1

What are gametes?

Answer 1

The sperm cells in males and the egg cells in females.

They join together at fertilisation to form a zygote, which divides and develops into a new organism.

Question 2

What is the diploid number (2n) of chromosomes?

Answer 2

Normal body cells have the diploid number (2n) of chromosomes- each cell contains two of each chromosome, one from the mum and one from the dad.

Question 3

What is the haploid (n) number of chromosomes?

Answer 3

There’s only one copy of each chromosome.

Question 4

What happens at fertilisation?

Answer 4

A haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. Half these chromosomes are from fathers sperm and half mothers egg. The diploid cell produced by fertilisation is called a zygote.

Question 5

What is meiosis?

Answer 5

A type of cell division. Cells that divide by meiosis are diploid to start with, but the cells that are formed from meiosis (e.g the gametes) are haploid- the chromosome number halves.

Question 6

Explain the 6 steps of meiosis of a diploid cell with 4 chromosomes.

Answer 6

1. The DNA unravels and replicates so there are two copies of each chromosome, called chromotids.
2. The DNA condenses to form double-armed chromosomes, made from two sister chromatids.
3. The chromosomes arrange themselves into homologous pairs.
4. Meiosis I (first division)- the homologous pairs are separated, halving the chromosome number. metaphase anaphase etc
5. Meiosis II (second division)- the pairs of sister chromatids that make up each chromosome are separated.
6. Four haploid cells (gametes) that are genetically different from each other are produced.

Question 7

What are the two main events during meiosis that lead to genetic variation?

Answer 7

1. Crossing over of chromatids

2. Independent segregation of chromosomes.

Question 8

What happens in the first event during meiosis that leads to genetic variation?

Answer 8

Crossing over of chromatids:
During meiosis I, homologous pairs of chromosomes come together and pair up.
The chromatids twist around each other and bits of chromatids swap over.
The chromatids still contain the same genes but now have a different combination of alleles.
The crossing over of chromatids in meiosis I means that each the four daughter cells formed from meiosis II contain chromatids with different alleles.

Question 9

What happens in the second event during meiosis that leads to genetic variation?

Answer 9

Independent segregation of chromosomes:
The four different daughter cells formed from meiosis have completely different combinations of chromosomes.
All your cells have a combination of chromosomes from your parents, half from your mum (called maternal chromosomes) and half from your dad (called paternal chromosomes).
When the gametes are produced, different combinations of those maternal and paternal chromosomes go into each cell.
This is called independent segregation of chromosomes.

Question 10

Are normal body cells haploid or diploid?

Answer 10

Diploid

Question 11

Are gametes haploid or diploid?

Answer 11

Haploid

Question 12

Are zygotes haploid or diploid?

Answer 12

Diploid

Question 13

What happens during meiosis I?

Answer 13

The homologous pairs separate.

Question 14

What happens during meiosis II?

Answer 14

The sister chromatids separate.

Question 15

Other than independent segregation, give one way in which meiosis allows the production of genetically different cells.

Answer 15

Crossing over

Question 16

Homologous chromosomes carry the same genes but they are not genetically identical. Explain why.

Answer 16

Homologous chromosomes carry different alleles.