AS Diversity, Classification and Variation

Question 1

Describe Gametes

Answer 1

Gametes are sex cells that join at fertilisation to form a zygote, which divides to develop into a new organism

Question 2

Explain what is Meant by Diploid Number of Chromosomes

Answer 2

• This means each cell contains 2 of each chromosome, 1 from each parent
• Gametes have a haploid number of chromosomes
• In fertilisation, two haploid gametes fuse to form a diploid zygote

Question 3

Explain how Random Fertilisation Increases Genetic Diversity

Answer 3

Random fertilisation produces zygotes with different combinations of chromosomes to parents. Mixing of genetic material increases genetic diversity within a species

Question 4

Describe the Process to Prepare for Meiosis 1

Answer 4

• DNA unravels and replicates - forms two copies of each chromosome, chromatids
• DNA condenses - forms double-armed chromosomes, made from two sister chromatids joined in the middle by a centromere

Question 5

Describe the Process of Meiosis 1 (First Division)

Answer 5

• Chromosomes arranged into homologous pairs

- Homologous pairs separated, halving chromosome number

Question 6

Describe the Process of Meiosis 2 (Second Division)

Answer 6

• Pairs of sister chromatids separated, centromere divided

- Four haploid cells (gametes) that are genetically different are produced

Question 7

Describe Crossing Over of Chromatids in Meiosis 1

Answer 7

• Homologous pairs of chromosomes ‘pair up’
• Non-sister chromatids twist around each other, bits of chromatids swap over
• Chromatids contain same genes but different combination of alleles

Question 8

Explain how Crossing Over Leads to Genetic Variation

Answer 8

• After the chromatids cross over in Meiosis 1, the first division occurs so one chromosome from each homologous pair ends up in each cell
• Then in Meiosis 2, each haploid cell has a different chromatid and therefore a different set of alleles
• This increases genetic variation

Question 9

Explain how Independent Segregation of Chromosomes Leads to Genetic Variation

Answer 9

• Homologous pairs are made up of one chromosome from each parent
• Pairs are separated randomly in Meiosis 1 - random which chromosome ends up in which daughter cell
• Four daughter cells have different combinations of maternal and paternal chromosomes - called independent segregation of chromosomes
• ‘Shuffling’ of chromosomes leads to genetic variation in any potential offspring

Question 10

Compare the Outcomes of Mitosis and Meiosis

Answer 10

Mitosis - produces cells with the same number of chromosomes as the parent cell
Meiosis - produces cells with half the number of chromosomes as the parent cell

Mitosis - daughter cells genetically identical to each other and parent cell
Meiosis - daughter cells genetically different to each other and parent cell

Mitosis - produces two daughter cells
Meiosis - produces four daughter cells