Meiosis

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How does sexual reproduction increase genetic variation?
Increases genetic variation because it involves combining genetic material from two unrelated individuals.
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What is the name of this process?
Fertilisation.
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Why is it important for there to be genetic variation in a population?
Increases the species' chances of survival as some individuals will have characteristics that enable them to be better adapted to the change.
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What is the name for cells that have two sets of chromosomes?
Diploid.
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What must happen to diploid cells in order for sexual reproduction?
They must produce haploid gametes.
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What is the translation of meiosis?
'Reduction'
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What is the name given to the organs that hold the cells undergoing meiosis?
Gonads
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What phase are the cells in before undergoing meiosis?
Interphase
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How many chromosomes are in a human cell?
46
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How many of these chromosomes come from your mother and how many come from your father?
23 from each.
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What are homologous chromosomes?
Matching pairs of chromosomes - one from your mother and one from your father. These chromosomes contain the same genes at the same loci.
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When does the S phase of interphase occur in relation to meiosis?
Before meiosis
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What happens in the S phase of interphase?
Each chromosome is duplicated when it's DNA replicates.
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Then, each chromosome consists of ...?
Two sister chromatids.
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How many divisions are there in meiosis?
2
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How many stages are there in each division?
4
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What are the four stages in the first meiotic division?
Prophase 1, Metaphase 1, Anaphase 1, Telophase 1
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What happens in between divisions?
A short interphase
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What are the four stages of the second meiotic division?
Prophase 2, Metaphase 2, Anaphase 2, Telophase 2
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Where does the second meiotic division take place?
In a plane at right angles to that of meiosis 1.
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What may occur at the end of the second division?
Cytokinesis
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What happens to the chromatin and chromosomes in prophase 1?
The chromatin condenses and each chromosome supercoils.
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What can you do with the chromosomes in this state?
They take up stains and you can see them under a light microscope
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What happens to the nuclear envelope in prophase 1?
It breaks down
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Where do the spindle fibre form from in prophase 1?
The centriole
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What are the spindle threads made from?
Tubulin protein
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What is the second thing to happen to the chromosomes in meiosis?
The chromosomes come together in their homologous pairs.
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Where does crossing over occur?
Where non-sister chromatids wrap around each other and may swap sections so that alleles are shuffled.
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What happens to the homologous chromosomes in Metaphase 1?
They attach along the equator of the spindle, still in their crossed over state.
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How does the chromosome attach to the spindle fibre?
Each attaches by its centromere.
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How are the homologous pairs arranged?
Randomly, with members of each pair facing the opposite poles of the cell.
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What is the name for this arrangement?
Independent assortment.
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Why is the way the chromosomes line up important?
Because this determines how they will segregate in Anaphase
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How are the members of each pair pulled apart in anaphase?
By motor proteins that drag them along the tubulin threads of the spindle.
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What happens to the crossed over areas when the chromosomes are pulled apart?
They separate from each other.
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What does this result in?
Swapped areas of chromosome and allele shuffling.
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In Telophase 1, what forms around each set of chromosomes?
Two new nuclear envelopes.
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What happens to the cell when the envelopes have formed?
The cell divides by cytokinesis.
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What happens during the short interphase?
The chromosomes uncoil.
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What is special about each new nucleus?
It contains half the original number of chromosomes, but each chromosome consists of two chromatids.
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However, what happens in most plant cells?
The cell goes straight from anaphase 1 into prophase 2.
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In prophase 2, what breaks down?
The nuclear envelopes (if they've formed.)
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What happens to the chromosomes in prophase 2?
They coil an condense
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What does each chromosome consists of?
Two chromatids.
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What is different about these chromatids?
They are no longer identical, because of the crossing over in prophase 1.
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What forms?
Spindles.
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In metaphase 2, what do the chromosomes do?
They attach to the the centre of the spindle by their centromere.
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What happens to the chromatids of each chromosome in this stage?
They are randomly arranged.
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Describe the three things that happen during Anaphase 2.
The centromeres divide, the chromatids of each chromosome are pulled apart by motor proteins towards opposite poles, the chromatids are randomly segregated.
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In telophase 2, nuclear enveloped form where?
Around each of the four haloid nuclei.
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In animals, what then happens to these two cells?
They divide to give four haploid cells.
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In plants, what happens to the cell?
A tetrad (group of 4) of four haploid cells is produced.
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What four things mean that meiosis ensures genetic variation?
Crossing over during prophase 1 shuffles alleles. Independent assortment in anaphase 1 = random distribution of maternal/paternal chromosome, independent assortment in anaphase 2, haploid gametes produced which can undergo fusion with other gametes.
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Fertilisation.

Card 3

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Card 4

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What is the name for cells that have two sets of chromosomes?

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Card 5

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What must happen to diploid cells in order for sexual reproduction?

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