GI: Meiosis and Genetic Variation

Question 1

Gametes

Answer 1

Sperm cells and egg cells.

Question 2

What happens when gametes join together at fertilisation?

Answer 2

They form a zygote, which divides and develops into a new organism.

Question 3

Zygote

Answer 3

Fertilised egg cell.

Question 4

Diploid number

Answer 4

2n

Question 5

How many chromosomes do body cells have?

Answer 5

Diploid number

Question 6

Haploid

Answer 6

n

Question 7

How many chromosomes do gametes have?

Answer 7

Haploid

Question 8

What happens at fertilisation?

Answer 8

Haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes.

Question 9

Is fertilisation random?

What does this mean?

Answer 9

Yes - produces zygotes with different combinations of chromosomes to both parents.

Means that genetic diversity within a species is increased.

Question 10

Where does meiosis occur?

Answer 10

In the reproductive organs.

Question 11

Describe the process of meiosis:

5 points

Answer 11

1. Before, DNA unravels and replicates so there are two copies of each chromosomes - chromatids.
2. DNA condenses to form double-armed chromosomes, each made from two sister chromatids joined by a centromere.
3. Meiosis I - chromosomes arrange into homologous pairs which are then separated, halving chromosome number.
4. Meiosis II - pairs of sister chromatids are separated and centromere divided.
5. Four genetically different haploid cells are produced.

Question 12

Describe crossing over in meiosis:

Answer 12

In Meiosis I, homologous pairs come together and there chromatids twist and parts swap over.

Chromatids still contain the same genes but now have a different combination of alleles.

Question 13

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

Answer 13

1. Crossing over of chromatids
2. Independent segregation of chromosomes.

Question 14

How does independent segregation of chromosomes lead to increased genetic variation?

Answer 14

1. Homologous paits of chromosomes are made up of 1 maternal and 1 paternal chromosome.
2. When homologous pairs separate in meiosis I, it’s completely randon which chromosome from each pair ends up in which daughter cell.
3. So four daughter cells produced have different combinations of maternal and paternal chromosomes.

Question 15

What are the differences in outcomes between meiosis and mitosis?

Why do these differences exist?

Answer 15

Mitosis - produce 2 daughter cells that have same number of chromosomes as parent cell and are genetically identical to each other and parent cell.

Meiosis - 4 daughter cells with half the number of chromosomes as parent cell and are genetically different from each other and parent.

Mitosis only has one division, whereas meiosis has two. Alsom in mitosis there is no crossing over or independent segregation of chromosomes so daughter cells are identical.

Question 16

What causes chromosomal mutations?

Answer 16

Errors in cell division.

Question 17

What do chromosomal mutations lead to?

Answer 17

Inherited conditions because the errors are present in the gametes.

Question 18

What is an type of chromosome mutation? Give an example of this in humans.

Answer 18

Non-disjunction - failure of chromosomes to separate properly.

Non-disjunction of chromosome 21 during meiosis can lead to Down’s Syndrome.

Question 19

What is Down’s Syndrome caused by?

What does this mean for the resulting zygote?

Answer 19

• Caused by having an extra copy of chromosome 21.
• Non-disjunction means that chromosome 21 fails to separate properly during meiosis, so one cell gets an extra copy and another gets none.
• When gamete with extra copy fuses, zygote will have three copies of chromosome 21.