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OCR Biology F215 > Meiosis > Flashcards

Meiosis Flashcards

0
Q

Describe what happens during meiosis 1 (prophase 1)

A
  • the chromatin condenses and undergoes supercoiling so that chromosomes shorten and thicken
  • the chromosomes come together in their homologous pairs to form a bivalent
  • the non-sister chromatids wrap around each other and attach at points called chiasmata
  • they may cross over and swap sections with each other (crossing over)
  • the nucleolus disappears and the nuclear envelope breaks down
  • a spindle forms
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1
Q

What do homologous chromosomes have in common?

A
  • similar size/shape
  • same bonding pattern when stained
  • centromeres in same position
  • 1 maternal, 1 paternal
  • same genes is same loci
  • pair up to form bivalent in meiosis
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2
Q

Describe what happens during meiosis 1 (metaphase 1)

A
  • bivalents line up across the equator of the spindle, attached to spindle fibres at the centromeres
  • the bivalents are arranged randomly (random assortment) with each member of the homologous pair facing opposite poles
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3
Q

Describe what happens during meiosis 1 (anaphase 1)

A
  • spindle fibres contract and the bivalent is pulled apart
  • so chromosomes are pulled to opposite poles
  • the chiasmata separate and the lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached
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4
Q

Describe what happens during meiosis 1 (telophase 1)

A

ANIMAL CELLS ONLY
• a nuclear envelope forms around each group of chromosomes and the cytoplasm divides (cytokinesis) so there are now two haploid daughter cells
• there is a grief interphase and chromosomes uncoil

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5
Q

Describe what happens during meiosis 2 (prophase 2)

A
  • chromosomes condense
  • centrioles migrate to opposite poles
  • spindle forms
  • nuclear envelope breaks down
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6
Q

Describe what happens during meiosis 2 (metaphase 2)

A
  • chromosomes align along the equator
  • they attach to spindle fibre at centromere
  • chromatids or each chromosome arranged in a random assortment
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7
Q

Describe what happens during meiosis 2 (anaphase 2)

A
  • spindle contracts
  • centromeres divide and sister chromatids are pulled to opposite poles by spindle fibres
  • the chromatids randomly segregate
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8
Q

Describe what happens during meiosis 2 (telophase 2)

A
  • nuclear envelope reforms around the haploid nuclei
  • in animals the two cells divide by cytokinesis to give 4 daughter cells
  • in plants a tetrad of 4 haploid cells is formed
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9
Q

Define the term allele

A
  • alternative/different version of a gene

* has a difference in the DNA base sequence that is expressed as a slightly different polypeptide

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10
Q

Define the term locus

A

The position of a gene on a chromosome

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11
Q

Define the term crossing over

A

When non-sister chromatids exchange alleles during prophase 1 of meiosis

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12
Q

What factors can lead to meiosis and fertilisation leading to genetic variation?

A
  • crossing over
  • reassortment of chromosomes in meiosis 1
  • reassortment of chromatids in meiosis 2
  • fertilisation
  • mutation
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13
Q

What happens during crossing over?

A
  • during prophase 1 homologous chromosomes pair up and come together to form bivalents
  • in each pair one chromosome is maternal and one is paternal
  • the non-sister chromatids wrap around each other tightly and attach at points called a chiasmata
  • the chromosomes can break at these points and the broken ends rejoin to the ends of non-sister chromatids in the same bivalent but they have been swapped
  • the chromosomes now have different combinations of alleles
  • this means that each of the 4 daughter cells contains chromatids with a different combinations of alleles
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14
Q

What happens during the reassortment of chromosomes in meiosis 1?

A
  • each gamete has a random assortment of maternal and paternal chromosomes
  • during meiosis 1 different combinations of maternal and paternal chromosomes go into each cell so each cell ends up with a different combination of alleles
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15
Q

What happens during the reassortment of chromatids in meiosis 2?

A
  • this is the result of the random distribution of sister chromatids on the spindle equator
  • due to crossing over the sister chromatids are no longer genetically identical
  • how they align at metaphase 2 determines how they segregate at anaphase 2
  • different combinations of chromatids go into each daughter cell so each cell ends up with a different combination of alleles
16
Q

How does fertilisation lead to variation?

A

Any egg can fuse with any sperm to form a zygote

17
Q

How does a mutation lead to variation?

A
  • chromosome mutations may occur which increases genetic variation
  • if the mutation occurs in the sperm of egg that are used in fertilisation then the mutated gene will be present in every cell of the offspring

OCR Biology F215 (44 decks)

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