Meiosis and Genetic Variation 4B

Question 1

Define a gamete

Answer 1

Sex cells (sperm and egg cell in humans)

Question 2

How many chromosomes do gametes have

Answer 2

Haploid number

Question 3

How many chromosomes do normal body cells have

Answer 3

Diploid number

Question 4

What occurs at fertilisation

Answer 4

sperm and egg cell fuse making a cell with a diploid number of chromosomes

Question 5

How are gametes formed

Answer 5

By meiosis

Question 6

What happens before meiosis occurs

Answer 6

The DNA unravels and replicates so that their are two copies of each chromosome, called chromatids, the DNA condenses to form double-armed chromatids each made from two sister chromatids which are joined in the middle by a centromere

Question 7

What happens during meiosis I (first division)

Answer 7

The chromosomes arrange themselves into homologous pairs, which are then separated, halving the chromosome number

Question 8

What happens during meiosis II (second division)

Answer 8

the pairs of sister chromatids that make up each chromosome are separated - the centromere divides

Question 9

What type of cells are formed at the end of meiosis

Answer 9

4 genetically different haploid cells (gametes)

Question 10

Give two ways in which meiosis leads to genetic variation

Answer 10

Crossing over of chromatids and independent assortment/segregation of chromosomes

Question 11

Describe how crossing over of chromatids leads to genetic variation

Answer 11

This means that the four daughter cells formed contain chromosomes with different alleles

Question 12

Define an allele

Answer 12

all the different forms of the same gene

Question 13

Describe how independent assortment/segregation leads to genetic variation

Answer 13

Each homologous pair of chromosomes is made up of one paternal chromosome and one maternal chromosome

When homologous pairs are separated in meiosis I it’s completely random which chromosome from each pair ends up in which daughter cell

So the four daughter cells produced by meiosis have completely different combinations of maternal and paternal chromosomes

The shuffling of these chromosomes is what leads to genetic variation in any potential offspring

Question 14

Define maternal chromosomes

Answer 14

Chromosomes inherited from the mother

Question 15

Define paternal chromosomes

Answer 15

Chromosomes inherited from the father

Question 16

Explain the differences between meiosis and mitosis

Answer 16

Mitosis:
Produces cells with the same number of chromosomes as the parent cell
Daughter cells are genetically identical to the parent cell
Produces 2 daughter cells
Meiosis:
Produces cells with half the number of chromosomes as the parent cell
Daughter cells are genetically different to the parent cell
Produces 4 daughter cells

Question 17

Define chromosome mutation

Answer 17

when cells do not have the correct number of chromosomes due to errors during meiosis

Question 18

Explain one type of chromosome mutation

Answer 18

non-disjunction - a failure of the chromosomes to separate properly in humans non disjunction of chromosome 21 in meiosis can lead to Down’s Syndrome
One zygote will receive three copies of chromosome 21