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Bio Y12 > 3.4.3 Meiosis and Diversity (C9) > Flashcards

3.4.3 Meiosis and Diversity (C9) Flashcards

1
Q

Give 2 possible causes of variation that result from meiosis during gamete formation

A
  • Random segregation of homologous chromosomes
  • Crossing over and recombination of chromosomes
  • (Mutation less common)
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2
Q

Describe prophase I

A
  1. Chromatin condenses
  2. Homologus pairs form a bivalent
  3. Nucleolus disappears
  4. Spindle forms
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3
Q

Describe metaphase I

A

Bivalents line up on equator of cell

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4
Q

Describe anaphase I

A

Homologus chromosome in each bivalent are pulled to opposite poles

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5
Q

Describe telophase I

A
  1. X2 new nuclear envelopes form
  2. Cell divides by cytokinesis
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6
Q

Describe prophase II

A
  1. Nucleous disappears
  2. Chromosomes condense
  3. Spindle forms
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7
Q

Describe metaphase II

A
  1. Chromosomes arrange themselves at equator
  2. Attached by centromere to spindle fibres
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8
Q

Describe anaphase II

A
  1. Centromere divides
  2. Chromatids pulled apart to opposite poles
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9
Q

Describe telophase II

A
  1. Nuclear envlopes reform around haploid nuclei
  2. Cell divides by cytokinesis
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10
Q

Explain what happens to chromosomes in meiosis

A
  • Chromosomes condense
  • Chromosomes undergo independent segregation and arrange themselves in homologous pairs
  • Crossing over and recombination of sections of chromosomes (this is rare)
  • Chromosomes join to spindles at the middle of the cell joined by the centromere
  • Homologous chromosomes are pulled apart to opposite poles (meiosis 1)
  • The pairs or chromosomes are then separated at the centromere in meiosis 2 producing 4 genetically unique gametes.
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11
Q

Describe how meiosis causes variation and explain the advantage of variation to the species

A
  • Crossing over
  • Independent assortment/segregation (Random assortment)

This generates new combinations of alleles

  • Some members of the species will be better adapted so may survive if the environment changes
  • So will be able to reproduce and pass on the advantageous alleles
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12
Q

What is the biological importance of reducing the chromosome number when the cell divides by meiosis?

A

As the diploid number is restored during fertilisation

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13
Q

State ways in which meiosis and mitosis are different

A
  • Meiosis reduces the chromosome number where as mitosis maintains the chromosome number
  • In meiosis chromosomes associate in pairs where as in mitosis chromosomes do not pair
  • In meiosis there are 2 nuclear divisions that produce 4 gametes where as in mitosis there is 1 nuclear division producing 2 body cells
  • Meiosis results in genetically unique daughter cells where as in mitosis the daughter cells are all genetically identical
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14
Q

Describe the role of the centromere in mitosis

A

It holds the chromatids together
It attaches the chromatids to the spindle and then splits the chromatids apart pulling them to separate poles of the cell

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15
Q

Homologous chromosomes carry the same genes but are not genetically identical. Why?

A

Homologous chromosomes have different alleles

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16
Q

Meiosis results in cells that have the haploid number of chromosomes and show genetic variation. How?

A
  • Homologous chromosomes pair up
  • Crossing over produces new combinations of alleles
  • Chromosomes separate at random
  • This produces varying combinations of chromosomes
  • Chromatids then separate at meiosis 2
17
Q

Explain the role of independent segregation in meiosis

A
  • To provide genetic variation.
  • Allows different combinations of maternal and paternal chromosomes.
  • Produces haploid cells.
18
Q

Identify the 2 ways that meiosis produces variation

A

crossing over

independent segregation

19
Q

Name an event that occurs during division 1 of meiosis that does not occur in division 2

A

pairing up of homologous pairs, crossing over followed by their separation to either pole

20
Q

How is mitosis similar to the second division of meiosis?

A

separation of sister chromosomes

21
Q

Define mutation.

A

Any change to the quantity of bases or the base sequence of DNA in an organism.

22
Q

Identify the 3 types of mutation that can occur.

A
  1. Base substitutions
  2. Base deletions
  3. Chromosome mutations
23
Q

Describe how DNA nucleotide substitutions can lead to a non-functional protein.

A
  1. A DNA nucleotide of a gene is replaced by a different nucleotide with a different base.
  2. This change alters the DNA triplet and its resulting mRNA codon.
  3. The altered codon translates into a different amino acid.
  4. This results in a different primary structure.
  5. Different hydrogen, ionic, disulphide bonds will form a new tertiary structure.
  6. Protein with different shape results with different function/non-functional.
24
Q

Describe how DNA nucleotide deletions can lead to a non-functional protein.

A
  1. A DNA nucleotide is lost from a normal DNA sequence.
  2. This causes a ‘frame shift’ - each triplet after the deletion is shifted to the left by one base.
  3. This alters the resulting mRNA codons.
  4. The altered codons translate into different amino acids.
  5. This results in a different primary structure.
  6. Different hydrogen, ionic, disulphide bonds will form a new tertiary structure.
  7. Protein with different shape results with different function/non-functional.
25
Q

Describe how DNA nucleotide substitution can lead to production of a functional protein.

A
  1. A DNA nucleotide is swapped from a normal DNA sequence.
  2. This alters the resulting mRNA codons.
  3. But they still code for the same amino acid.
  4. Primary structure is not altered, and the tertiary structure remains the same

or

  1. A DNA nucleotide is swapped from a normal DNA sequence.
  2. This alters the resulting mRNA codons.
  3. But they are in an intron
  4. So are removed by splicing
  5. Correct protein is still formed
26
Q

What is a chromosome mutation?

A

Changes in the structure or number of whole chromosomes.

27
Q

What is ‘non-disjunction’?

A
  1. Where homologous chromosomes fail to separate during meiosis 1.
  2. This results in a gamete having too many or too few chromosomes e.g. Down’s Syndrome sufferers have an extra chromosome.
28
Q

What is the equation for calculating the possible chromosome combinations as a result of independent segregation during meiosis.

A

No. of chromosome combinations = 2^n

where n = the number of PAIRS of chromosomes

29
Q

What is the equation for calculating the possible chromosome combinations as a result of independent segregation during meiosis AND random fertilisation of gametes.

A

No. of chromosome combinations = (2^n)^2

where n = the number of PAIRS of chromosomes

30
Q

explain the difference in a cell that has under gone meiosis 1 and meiosis 2

A

meiosis 1 - only 2 nuclei/cells - only homologous chromosomes separated
meiosis 2 - 4 nuclei/cells - separation of sister chromosomes (haploid cells)

31
Q

How is chromosome number halved in meiosis

A

separation of homologous pairs

one of each pair ends up in the daughter cells

32
Q

In down syndrome, people have an extra chromosome (trisomy) - how does this happen?

A

In meiosis

homologous chromosomes/sister chormatids fail to separate

33
Q

define proteome

A

full range of proteins that a cell can produce/that the genome can code for

34
Q

Describe a brief overview of the events in meiosis

A

DNA is replicated in interphase before meiosis begins

In the first division there is a separation of homologous chromosomes, halving the chromosome number

In the second division there is separation of the sister chromatids

This produced four genetically different daughter cells

35
Q

How does meiosis lead to variation?

A

Mutations can occur changing the base sequence leading to the formation of new alleles

In the first division of meiosis there is crossing over where homologous chromosomes swap DNA producing new combinations of alleles

In meiosis homologous chromosomes may be independently segregated, separating into different daughter cells producing new combinations of alleles

During fertilisation there is random fusion of gametes this produces new combinations of alleles

36
Q

Define Genome

A

All the DNA in a cell

37
Q

Describe how a DNA mutation may lead to a shorter (truncated) protein being produced

A

1.Mutation changes the base sequence
2. One triplet is changed to a STOP CODON
3. Stop codon stops translation and the protein isn’t complete

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