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A-LEVEL BIOLOGY > 4.3 Meiosis > Flashcards

4.3 Meiosis Flashcards

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1
Q

How many nucleur divisions occur before meiosis

A

2

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2
Q

What is meiosis always preceded by, and what effect does this have

A
  • DNA Replication
  • 2 sister chromatids held together by the centromere
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3
Q

Describe what happens during prophase I of meiosis

A
  • Chromosomes condense and become visible
  • Homologous chromosomes attach to each other at chiasmata, forming bivalents
  • Spindle fibres begin to form
  • Crossing over
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4
Q

When does Crossing-Over occur

A

Prophase I

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5
Q

Describe how crossing-over during Prophase I results in gentic variation

A
  • Bivalents form from homologous chromosomes associating with each other
  • They are joined at the chiasmata
  • Lengths of non-sister chromatids are exchanged
  • They twist and break off
  • They recombine one another non-sister chromatid
  • Results in new combinations of alleles on the chromatids
  • And therefore genetic variation
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6
Q

Describe what happens during metaphase I of meiosis

A
  • Homologous pairs of chromosomes line up at equator of cell
  • Spindle fibres attach to centromeres
  • Independant segregation of homologous chromosomes
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7
Q

When does independant segregation of homologous chromosomes occur

A

Metaphase I

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8
Q

Describe how independant segregation of homologous chromosomes results in genetic variation

A
  • Maternal and paternal chromosomes can be at either side of the equator
    * Maternal and paternal chromosomes are re-shuffled in any order
  • Independant for each pair
    * Different combination of alleles
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9
Q

Describe what happens during anaphase I of meiosis

A
  • Homologous pairs of chromosomes seperate as i ndividual chromosomes are pulled to poles of cell
  • Spindle fibres contract and shortern
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10
Q

Describe what happens during telophase I of meiosis

A
  • Chromosomes uncoil
  • Spindle fibres break down
  • Daughter nuclei are haploid
  • Cytokenisis occurs
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11
Q

Describe what happens during prophase II of meiosis

A
  • Chromosomes condense and become visible
  • Spindle fibres form
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12
Q

Describe what happens in metaphase II of meiosis

A
  • Chromosomes line up at equator of cell
  • Spindle fibres attach to centromeres
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13
Q

Describe what happens during anaphase II of meiosis

A
  • Chromosomes seperate as sister chromatids are pulled to poles of the cell
  • Spindle fibres contract and shortern
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14
Q

Describe what happens in telophase II of meiosis

A
  • Chromosomes uncoil
  • Spindle fibres break down
  • Daughter nuceli have formed
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15
Q

Describe the final stage of meiosis

A
  • Cytokenisis
  • 4 genetically different daughter cells
  • That are haploid
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16
Q

Describe how random fertilisation results in genetic variation

A
  • Each gamete is **genetically varied **
  • In each fertilisation there is an equal probability of any female gamete fusing with any male gamete
17
Q

What is non-disjunction

A

When chromosomes do not seperate during anaphase in meiosis

18
Q

Equation for number of gamete chromosome combinations with independant segregation of homologous chromosomes

A
19
Q

Equation for number of gamete chromosome combinations with independant segregation of homologous chromosomes AND random fertilisation

A
20
Q

Define a mutation

A

The change in the DNA base sequence of a chromosome

21
Q

When do mutations occur

A

Spontaneously during DNA replication

22
Q

What increases the chance of a mutation

A

Exposure to mutagenic agents (ionising radiation, and chemicals)

23
Q

Describe a substitution mutation

A
  • Bases substituted
  • Pinpoint mutation
  • May have no effect, as DNA is degenerate
  • Could change amino-acid produced and therefore tertiary structure
  • Or introduce a stop codon
24
Q

Describe a deletion/addition mutation

A
  • Bases removed or added
  • Frameshift mutation
  • All triplets after mutation are changed
  • Change in amino-acid production and therefore change in tertiary structure
  • As DNA is read in triplets
25
Q

Define a ‘homologous chromosome’

A

2 chromosomes that carry the same genes

A-LEVEL BIOLOGY (34 decks)

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