3.1.9 Genetic Diversity may Arise by Meiosis

Question 1

What are the gametes in plants?

Answer 1

Ovum and pollen

Question 2

Why do diploid cells (ones that divide into gametes) have to half their number of chromosomes?

Answer 2

Gametes have to fuse with another gamete to make a full set of genes to code for all the proteins

Question 3

Explain how sexual reproduction increases genetic diversity within a species?

Answer 3

Fertilisation of gametes is random & produces zygotes with different combinations of chromosomes to both parents

Question 4

Where does meiosis take place in?

Answer 4

Reproductive organs

Question 5

What happens before meiosis starts?

Answer 5

DNA unravels and replicates, forming 2 copies of each chromosomes = called chromatids

Question 6

How are sister chromatids joined?

Answer 6

By centromere in the middle

Question 7

Describe what happens in meiosis 1

Answer 7

1. Prophase 1
   
   • Chromosomes condense and super coil
2. Metaphase 1
   
   • Chromosomes line up along the centre of nucleus with their homologous pair (bivalents)
3. Anaphase 1
   
   • One of each homologous pair get pulled to either side of nucleus
4. Telophase 1
   
   • Nuclear envelope reforms to make 2 separate nuclei

Question 8

Describe what happens in meiosis 2

Answer 8

1. Prophase 2
   
   • Chromosomes condense and super coil
2. Metaphase 2
   
   • Chromosomes line up along centre of nucleus still attached at centromere
3. Anaphase 2
   
   • Each chromatid gets pulled to either side of nucleus (centromere is divided)
4. Telophase 2
   
   • Nuclear envelope reforms to make 4 operate nuclei each with single copy of each chromosome
5. End up with 4 haploid non-identical daughter cells (n)

Question 9

What is genetic variation?

Answer 9

Differences in the genes of individual organisms

Question 10

Describe what is happening in this diagram & what this means

Answer 10

1. Crossing over
2. Between chromatids of homologous chromosomes
3. (Chromatid twist around each other & bits of chromatids swap over)
4. Chromatids still contain same gene but now have different combination of alleles

Question 11

Name 2 ways how meiosis produces cells that are genetically different

Answer 11

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

Question 12

Describe how crossing over of chromatids increases genetic variation

Answer 12

Each of 4 daughter cells formed from meiosis contains a different chromatid with a different set of alleles

Question 13

Describe how independent segregation increases genetic variation

Answer 13

• We don’t know which maternal or paternal chromosomes will get pulled to each side
  
  • i.e. we don’t know which chromosome (from each pair) ends up in which daughter cell
• ∴ 4 daughter cells produced have different combinations of those maternal and paternal chromosomes
• (Leads to genetic variation in any potential offspring)

Question 14

What is chromosome mutation?

Answer 14

When cell produced in meiosis contains variation in numbers of whole chromosomes or parts of chromosomes

(caused by errors during meiosis)

Question 15

Describe and explain the appearance of this chromosome

Answer 15

• Chromsome is formed of 2 chromatids
• ∵ of DNA replication
• Sister chromatids are held together by centromere

Question 16

What is the role of the centromere (2x)?

Answer 16

• Holds chromatids together
• Allows chromatids to be separated

Question 17

This cell produces gametes by meiosis. Draw a diagram to show the chromosomes in one of the gametes.

Answer 17

3 chromosomes - one from each homologous pair

Question 18

Crossing over of chromatids is ________

Answer 18

infrequent/rare

Question 19

Name 3 advantages of variation (caused by meiosis) to species

Answer 19

• Different adaptations
• Passing on alleles/genes
• Allows for changing environment

Question 20

Explain why Plant A (has 42 chromosomes) is fertile while Plant B (has 21 chromosome) is not

Answer 20

1. Plant A’s cells are diploid - have 2 copies of each chromosome
2. Plant A’s chromosomes can produce haploid gametes
3. Plant A’s gametes receive a copy of every chromosome

Question 21

Explain the role of independent segregation in meiosis

Answer 21

• To provide genetic variation
• Allows different combinations of maternal and paternal chromosomes

Question 22

How do you calculate the number of different types of gametes that can be produced from a diploid cell?

Answer 22

Use this formula:

2ⁿ

n = number of gene pairs

Question 23

Calculate the number of different types of gametes that can be produced in a species with a diploid number of 24

Answer 23

2¹² = 4096

Question 24

Assuming random fertilisation, calculate the number of different combinations of maternal and paternal chromosomes in the zygotes of this species (with a diploid number of 24)

Answer 24

4096 x 4096 = 16 777 216

Question 25

Unit 5

During meiosis, when the X and Y chromosomes pair up, they do not form a typical bivalent as do other chromosomes. Explain why. (2)

Answer 25

• X and Y chromosomes are different sizes / shapes
• Chromatids unable to line up and form bivalent / only short pairing region / most of length not homologous