4B - Meiosis and genetic variation

Question 1

How is DNA passed from one generation to the next?

Answer 1

By gametes.

Question 2

What are gametes?

Answer 2

Sex cells - sperm and egg cells.

Question 3

What do sperm and egg cells join to form?

Answer 3

A zygote.

Question 4

What is a zygote?

Answer 4

What is formed when an egg and sperm cell join at fertilisation.

Question 5

When do an egg and sperm cell join to form a zygote?

Answer 5

At fertilisation.

Question 6

What is the diploid number of normal body cells?

Answer 6

2n

Question 7

Do gametes have a diploid or haploid number of chromosomes?

Answer 7

haploid (n).

Question 8

What happens at fertilisation in mammals?

Answer 8

A haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. Half from the mother and half from the father.

Question 9

Is fertilisation random?

Answer 9

Yes

Question 10

What does random fertilisation produce?

Answer 10

Zygotes with different combinations of chromosomes to both parents.

Question 11

The mixing of genetic material in sexual reproduction does what to genetic diversity within a species?

Answer 11

Increases it

Question 12

By what process are gametes formed?

Answer 12

Meiosis

Question 13

Where does meiosis occur?

Answer 13

In the reproductive organs.

Question 14

Cells that divide meiosis are what to start with?

Answer 14

Diploid

Question 15

Are the cells produced from meiosis diploid or haploid?

Answer 15

Haploid

Question 16

What happens before meiosis starts?

Answer 16

The DNA unravels and replicates so that there are 2 copies of each chromosome called chromatids.

The DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids joined in the middle by a centromere.

Question 17

What makes a chromosome?

Answer 17

The DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids joined in the middle by a centromere.

Question 18

What happens in meiosis 1?

Answer 18

The chromosomes arrange themselves into homologous pairs and align along the equator randomly. These homologous pairs are then pulled to opposite poles (separated), halving the chromosome number.

Question 19

What happens in meiosis 2?

Answer 19

The pairs of sister chromatids that make up each chromosome are separated (the centromere is divided). 4 haploid cells (gametes) that are genetically different from each other are produced.

Question 20

When does crossing over occur?

Answer 20

In meiosis 1 (prophase 1).

Question 21

What is crossing over of chromosomes?

Answer 21

A piece of one chromatid from one chromosome touches a piece of chromosome from the homologous partner.

The chromatids become twisted (crossing over) creating tensions and causing portions to break off.

The broken portions might rejoin with the chromatids of its homologous partner.

Question 22

How does crossing over increase genetic variation?

Answer 22

Because each daughter cell will have a new allelic combination.

Question 23

Why doesn’t crossing over lead to a loss of genetic info?

Answer 23

As it happens at the same gene loci - just replaces each other.

Question 24

What is nondisjunction?

Answer 24

When chromosomes fail to segregate during meiosis, so one cell gets an extra copy and another gets 1.

Question 25

What causes nondisjunction?

Answer 25

Failure of homologous chromosomes to segregate in meiosis 1 or a failure of sister chromatids to segregate in meiosis 2.

Question 26

What might happen to the resulting embryo if the gamete with the incorrect number of chromosomes is fertilised?

Answer 26

It might not be viable and might die or an individual may be born with a genetic disorder.

Question 27

What is trisomy 21?

Answer 27

Down’s Syndrome (3 chromosome 21’s).

Question 28

What is the scientific name for Down’s Syndrome?

Answer 28

Trisomy 21.

Question 29

What are homologous chromosomes?

Answer 29

A pair of chromosomes, 1 maternal and 1 paternal that have the same gene loci.

Question 30

What is an allele?

Answer 30

A form of a gene.

Question 31

What is a gene locus?

Answer 31

A fixed position on a chromosome that is occupied by a gene.

Question 32

Where on chromosomes/chromatids does crossing over happen?

Answer 32

At the same gene loci on each.

Question 33

What is independent segregation of chromosomes?

Answer 33

Each homologous pair of chromosomes contains 1 maternal and 1 paternal chromosome.

When the homologous pairs are separated in meiosis 1, it is completely random which chromosome from each pair ends up in which daughter cell.

So, the 4 daughter cells produced have completely different combinations of those maternal and paternal chromosomes.

This is called independent segregation (separation) of the chromosomes as the way in which the homologous pairs line up is independent of how any other pair aligns.

Question 34

What are the differences in the outcomes of mitosis and meiosis?

Answer 34

Mitosis:

• Produces cells with the same number of chromosomes as the parent cell.
• Daughter cells are genetically identical to each other and 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 from one another and to the parent cell.
• Produces 4 daughter cells.

Question 35

What are chromosome mutations caused by?

Answer 35

Errors in cell division.

Question 36

What can chromosome mutations lead to?

Answer 36

Inherited conditions because the errors are present in the gametes (the hereditary cells).

Question 37

Name a type of chromosome mutation

Answer 37

Nondisjunction.

Question 38

What can nondisjunction of chromosome 21 in humans during meiosis lead to?

Answer 38

Down’s Syndrome.