Topic 4---B: Diversity and selection- 1. Meiosis and genetic variation

Question 1

What are diploid body cells?

Answer 1

• Normal body cells have the diploid number (2n)

Question 2

What does 2n mean?

Answer 2

Each cell contains 2 of each chromosome
(pair)

Question 3

What is a homologous pair?

Answer 3

The chromosomes that make up the pair are the same size and have the same genes but they could have different alleles

Question 4

What is the diploid number for chromosomes?

Answer 4

46

Question 5

What are gametes?

Answer 5

They are the sperm cells in males and egg cells in females

Question 6

What number do gametes have?

Answer 6

A haploid number of chromosomes meaning they only contain one copy of each chromosome in a homologous pair.

Question 7

What is the haploid number for humans?

Answer 7

23

Question 8

What happens in sexual reproduction?

Answer 8

2 gametes join together at fertilisation to form a zygote which divides and develops into a new organism.
(haploid sperm fuses with a haploid egg making a cell with the normal diploid mumber of chromosomes) (half are from mother and half are from father)

Question 9

What are the cells that divide by meiosis start and end with?

Answer 9

They are diploid to start with and haploid to end with so the chromosome number halves

Question 10

What are the stages of meiosis?

Answer 10

Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2

Question 11

Prophase 1

Answer 11

Chromosomes arrange themselves into homologous pairs

Question 12

Metaphase 1

Answer 12

Homologous chromosomes line up in the middle

Question 13

Anaphase 1

Answer 13

Homologous chromosomes pulled apart

Question 14

Telophase 1

Answer 14

Chromosomes split into 2 nuclei halving the chromosome number

Question 15

Prophase 2

Answer 15

Chromosomes condense getting shorter and fatter
spindle fibres start to form

Question 16

Metaphase 2

Answer 16

Chromosomes each with 2 chromatids line up at the equator of the cell and become attached to the spindle by their centromere

Question 17

Anaphase 2

Answer 17

Spindles contract pulling chromatids to opposite poles

Question 18

Telophase 2

Answer 18

second division
4 haploid cells are made that are genetically different to eachother

Question 19

What are the 2 main events during meiosis that leads to genetic variation?

Answer 19

Crossing over of chromatids
Independant segregation of chromosomes

Question 20

Crossing over of chromatids

Answer 20

• During meiosis 1 (prophase 1) homologous chromosomes come together and pair up.
• The chromatids twist around eachother/ get tangled resulting in sections of the chromatid breaking off and rejoining with the other chromatid and bits of chromatid swap over.
• The chromatids still contain the same genes but they have different combination of alleles

Question 21

Independant segregation

Answer 21

Each homologous pair of chromosomes in your cells is made up of one chromosome from your mum (maternal) and one chromosome from your dad (paternal)
When the homologous pairs are separated in meiosis 1, its completely random which chromosome from each pair ends up in which daughter cell. Leading to variation

Question 22

Outcomes of mitosis and meiosis?

Answer 22

Mitosis produces 2 daughter cells but meiosis produces 4 daughter cells

mitosis daughter cells are genetically identical to eachother and to the parent cell but meiosis daughter cells are genetically different to eachother and the parent cell.

mitosis produces cells with the same number of chromosomes as the parent cell but meiosis produces cells with half the number of chromosomes as the parent cell.