3.4.3 meiosis and genetic diversity

Question 1

what is the use of meiosis

Answer 1

• In sexual reproduction two gametes fuse to give rise to new offspring
• To maintain the chromosome number in the adults of a species the number must be halved at some stage in the life cycle
• Meiotic division halves the chromosome number

Question 2

Where does meiosis occur

Answer 2

ovaries and testes

Question 3

prophase I

Answer 3

• chromosomes condense, nuclear envelope disappears
• Homologous pairs - bivalents formed, chromosome from both parent cells join by overlapping
• where the chromosomes overlap is called crossing over where you get an exchange of allelles between mother and fathers genes, the chromatids break and swap over
• there can be between 0-3 crossovers on each chromosome
• This creates chiasma - different genetic material in one structure
• start of genetic variation

Question 4

metaphase I

Answer 4

• Pairs of homologous chromosomes move to the equator of the cell
• spindle fibres produced by the centriole attach to the centromere

Question 5

anaphase I

Answer 5

• homologous chromosomes move to the opposite pole of the cell
• the centromere does not split
• Independent segregation of chromosomes - random assortment of chromosomes so one of each homologous pair ends up in each daughter cell, second chance for genetic variation, completely random

Question 6

telophase I

Answer 6

• Chromosomes unwind
• Nuclear envelope reappears
• cytoplasm and organelles split
• folding of the cell membrane

Question 7

cytokinesis I

Answer 7

-cell divides into two, genetically different but still diploid cells
- they stay close together

Question 8

prophase II

Answer 8

• chromatin shortens and thickens
• nuclear envelope disappears

Question 9

metaphase II

Answer 9

• chromosomes line up at the equator
• centrioles produce spindle fibres which attach to the centromere

Question 10

anaphase II

Answer 10

• spindles contract, centromere splits and one of each chromatid goes to either pole

Question 11

telophase II

Answer 11

• chromatids get longer and thinner, chromatin appears
• nuclear envelopes appears
• organelles and cytoplasm split
• membrane folds in

Question 12

Cytokinesis II

Answer 12

• 4 daughter cells called tetrad
• genetically different from each other and the parent cell
• haploid cells, one of each chromosome

Question 13

what stages does the genetic variation occur in meiosis?

Answer 13

prophase I
anaphase I

Question 14

When does interphase occur in meiosis

Answer 14

Once at the beginning before prophase I

Question 15

Meiosis in pollen

Answer 15

• Undergoes meiosis to form genetically different tetrads
• Theses genetically different cells undergo mitosis
• They are able to mass produce cells while also having genetic variation

Question 16

non-disjunction

Answer 16

• Failure of chromosomes to separate and segregate into daughter cells
• Can occur during meiosis I or II
• monosomic - successeful, 1 chromatid of each pair in each cell
• disomic, 2 chromatids of one pair in one cell
• nullsomic, no chromatids of a specific pair in the cell

Question 17

What is the cause of mutation during meiosis

Answer 17

weak spindle fibres

Question 18

Name the two mehcanisms which cause variation to occur during meiosis

Answer 18

1. Independent segregation of homologous chromosomes
2. Crossing over between homologous chromosomes
   they both occur in meiosis I

Question 19

formula for the number of different variations during independent segregation

Answer 19

2^n
n= number of homologous pairs
In humans 2^23 = over 8 million different variations in gametes

Question 20

How else is genetic variation acheived outside the cycle of mitosis

Answer 20

Random fertilisation between sperm and egg