module 2 - 6.3 meiosis

Question 1

what are homologous chromosomes?

Answer 1

a pair of ‘identical’ chromosomes, one from dad and one from mum
- homologus chromosomes do not have the same base sequence (copying with DNA polymerase isn’t 100% accurate)

Question 2

when is a homologous pair of chromosomes replicated?

Answer 2

in S phase

Question 3

what is a gene?

Answer 3

a section of DNA that codes for a polypeptide

Question 4

what is an allele?

Answer 4

a variation of A gene

Question 5

where are ALL alleles for A gene?

Answer 5

at the same place on the same chromosome, this is called LOCUS

Question 6

what happens during meiosis?

Answer 6

• reduces chromosome number from diploid to haploid
• has 2 stages: meiosis 1 and meiosis 2
• produces 4 daughter cells
• DNA replicated in S phase
• generates genetic variation in gametes by independent assortment and crossing over

Question 7

what is meiosis sometimes called?

Answer 7

reduction division

Question 8

what happens during prophase 1?

Answer 8

• chrom. have been replicated during S phase
• homologous chrom. pair up to form a BIVALENT
• centrioles begin to form spindle fibres that bind to centromeres, drags bivalents onto metaphase plate
• nuclear envelope disintegrates
• crossing over happens

Question 9

why is the nuclear envelope presented as a dotted line?

Answer 9

it is disintegrating

Question 10

why does the nuclear envelope disintegrate?

Answer 10

to allow more space in the cell

Question 11

what happens during metaphase 1?

Answer 11

• nuclear envelope has gone
• spindle fibres bound to centromeres and arranges bivalents on metaphase plate
• the orientation of each bivalent is dependent on other bivalents (INDEPENDENT ASSORTMENT)
• independent assortment shuffles alleles (not genes - every human has the same

Question 12

what is a bivalent?

Answer 12

homologous chromosomes associated in pairs

Question 13

what happens during anaphase 1?

Answer 13

• bivalents are pulled apart by contracting spindle fibres - centromeres remain intact!
• homologous chromosomes pulled to opposite poles of the cell
• effects of ‘crossing over’ become apparent

Question 14

what happens in telophase 1?

Answer 14

• new nuclear envelopes form around each chromosome
• new cleavage furrow forms and cytokinesis happens
• each nucleus is now haploid and reduction of diploid number is complete

Question 15

what happens during prophase 2?

Answer 15

• nuclear envelops disintegrates
• centrioles move to poles of each cell and spindle fibres form
• spindle fibres bind to centromeres and drag chromosomes onto metaphase plate

Question 16

what happens during metaphase 2?

Answer 16

spindle fibres have arranged the chromosomes on the metaphase plate
- due to crossing over, chromatids aren’t identical, independent assortment happens again

Question 17

what happens in anaphase 2?

Answer 17

• spindle fibres contract and separate the centromeres
• the chromatids now become chromosomes and are pulled to the poles of the cell

Question 18

what happens during telophase 2?

Answer 18

• new nuclear envelope forms
• chromosomes will uncondense and form chromatin to fill the volume of the nucleus
• a new cleavage furrow forms and 4 daughter cells are made, each gamete is haploid

Question 19

what is crossing over?

Answer 19

there are breaks in 1 chromatid arm (breaks are random) and form a chiasma
- ends rejoin but with other sister chromatid

Question 20

what does crossing over create?

Answer 20

creates genetic variation by shuffling alleles