module 2 - 6.3 meiosis Flashcards

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1
Q

what are homologous chromosomes?

A

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)

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2
Q

when is a homologous pair of chromosomes replicated?

A

in S phase

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3
Q

what is a gene?

A

a section of DNA that codes for a polypeptide

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4
Q

what is an allele?

A

a variation of A gene

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5
Q

where are ALL alleles for A gene?

A

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

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6
Q

what happens during meiosis?

A
  • 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
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7
Q

what is meiosis sometimes called?

A

reduction division

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8
Q

what happens during prophase 1?

A
  • 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
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9
Q

why is the nuclear envelope presented as a dotted line?

A

it is disintegrating

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10
Q

why does the nuclear envelope disintegrate?

A

to allow more space in the cell

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11
Q

what happens during metaphase 1?

A
  • 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
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12
Q

what is a bivalent?

A

homologous chromosomes associated in pairs

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13
Q

what happens during anaphase 1?

A
  • 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
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14
Q

what happens in telophase 1?

A
  • 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
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15
Q

what happens during prophase 2?

A
  • 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
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16
Q

what happens during metaphase 2?

A

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

17
Q

what happens in anaphase 2?

A
  • spindle fibres contract and separate the centromeres
  • the chromatids now become chromosomes and are pulled to the poles of the cell
18
Q

what happens during telophase 2?

A
  • 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
19
Q

what is crossing over?

A

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

20
Q

what does crossing over create?

A

creates genetic variation by shuffling alleles