1. 3.3 Meiosis
Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes

2. One diploid nucleus divides by meiosis to produce four haploid nuclei.
Q: List a similarity and two differences between mitosis and meiosis.

3. Outline the differences between the behaviour of chromosomes in Mitosis and Meiosis
5 marks
Mitosis
Meiosis
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4. Outline the differences between the behaviour of chromosomes in Mitosis and Meiosis
5 marks
Mitosis
Meiosis
One division
Two divisions
Diploid cells produced
Haploid gametes produced
No crossing-over in prophase
Crossing-over in prophase I
No chiasmata formation
Chiasmata form
Homologous pairs do not associate and line up at the equator in metaphase
Homologous pairs associate as bivalents and lined up at the equator in metaphase I
Sister chromatids separate in anaphase
Homologous pairs separate in anaphase I
Sister chromatids separate in anaphase II
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7. The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
Q: Why is halving the chromosome number necessary in sexual reproduction?
Fusion of gametes from different parents promotes genetic variation.
Q: How does sexual reproduction increase diversity?

8. DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
Q: What is the difference between chromosomes and chromatids?

9. An homologous pair of chromosomes…
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10. An homologous pair of chromosomes…
…replicates during S-phase of interphase…
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11. An homologous pair of chromosomes…
…replicates during S-phase of interphase…
centromere
sister chromatids
…giving two pairs of sister chromatids,
each joined at the centromere.
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12. The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
Q: During which stage does crossing-over happen? Draw it using two different colored pencils/pens.
Orientation of pairs of homologous chromosomes prior to separation is random.

13. The homologous pair associates during prophase I, through synapsis…
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14. The homologous pair associates during prophase I, through synapsis…
…making a bivalent.
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15. Crossing-over might take place between non-sister chromatids in prophase I…
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16. …leading to recombination of alleles.
Crossing-over might take place between non-sister chromatids in prophase I…
…leading to recombination of alleles.
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17. This is the reduction division.
In anaphase I, the homologous pair is separated but the sister chromatids remain attached.
This is the reduction division.
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18. Check your language. This image shows…
Four separate chromosomes.
A bivalent.
One pair of sister chromatids.
Non-disjunction.
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19. Check your language. This image shows…
Four separate chromosomes.
A bivalent.
One pair of sister chromatids.
Non-disjunction.
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20. Check your language. This image shows…
Two separate chromosomes.
A bivalent.
One pair of sister chromatids.
Crossing-over.
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21. Check your language. This image shows…
Two separate chromosomes.
A bivalent.
One pair of sister chromatids.
Crossing-over.
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22. Check your language. This image shows…
Two separate chromosomes.
A bivalent.
One pair of sister chromatids.
Homologous chromosomes.
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23. Check your language. This image shows…
Two separate chromosomes.
A bivalent.
One pair of sister chromatids.
Homologous chromosomes.
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24. Check your language. This image shows…
8 separate chromosomes.
Two bivalents.
Two pairs of sister chromatids.
Two homologous chromosomes.
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25. Check your language. This image shows…
8 separate chromosomes.
Two bivalents.
Two pairs of sister chromatids.
Two homologous chromosomes.
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26. Meiosis Is a reduction division from
diploid somatic cells (2n) to produce
haploid gametes (n).
The reduction is in the chromosome number in each nucleus.
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27. Interphase
In the S-phase of the interphase before meiosis begins, DNA replication takes place.
Chromosomes are replicated and these copies are attached to each other at the centromere.
The attached chromosome and its copy are known as sister chromatids.
Following S-phase, further growth and preparation take place for meiosis.
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28. Prophase I
The homologous chromosomes associate with each other, to form bivalents.
The pairs of sister chromatids are joined by the centromere.
Non-sister chromatids are next to each other but not joined.
This bivalent is composed of:
One pair of homologous chromosomes
Which have replicated to form two pairs of sister chromatids.
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29. Prophase I
The homologous chromosomes associate with each other, to form bivalents.
Crossing-over between non-sister chromatids can take place. This results in recombination of alleles and is a source of genetic variation in gametes.
The pairs of sister chromatids are joined by the centromere.
Non-sister chromatids are next to each other but not joined.
The homologous pair is separated in anaphase I. The joined sister chromatids are separated in anaphase II.
This bivalent is composed of:
One pair of homologous chromosomes
Which have replicated to form two pairs of sister chromatids.
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30. Neighbouring non-sister chromatids are cut at the same point.
Crossing-Over
Increases genetic variation through recombination of linked alleles.
Synapsis
Homologous chromosomes associate
Chiasma Formation
Neighbouring non-sister chromatids are cut at the same point.
A Holliday junction forms as the DNA of the cut sections attach to the open end of the opposite non-sister chromatid.
Recombination
As a result, alleles are swapped between non-sister chromatids.
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31. Prophase I The homologous chromosomes associate with each other.
Crossing-over between non-sister chromatids can take place. This results in recombination of alleles and is a source of genetic variation in gametes.
Crossing-over is more likely to occur between genes which are further apart. In this example, there will be more recombination between D and E than between C and D.
During prophase, the nuclear membrane also breaks down and the centrioles migrate to the poles of the cell.
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32. Metaphase I The bivalents line up at the equator.
Random orientation occurs and is a significant source of genetic variation.
There are 2n possible orientations in metaphase I and II. That is 223 in humans – or 8,388,068 different combinations in gametes!
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33. Anaphase I Spindle fibres contract.
Homologous pairs are separated and pulled to opposing poles.
This is the reduction division.
Non-disjunction here will affect the chromosome number of all four gametes.
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34. Telophase I
New nuclei form and the cytoplasm begins to divide by cytokinesis.
The nuclei are no longer diploid.
They each contain one pair of sister chromatids for each of the species’ chromosomes.
If crossing-over and recombination has occurred then the sister chromatids will not be exact copies.
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35. Interphase There is no Synthesis phase in Interphase II.
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36. Prophase II The nuclei break down. No crossing-over occurs.
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37. Metaphase II
Pairs of sister chromatids align at the equator. Spindle fibres form and attach at the centromeres.
Random orientation again contributes to variation in the gametes, though not to such an extent as in metaphase I.
This is because there is only a difference between chromatids where crossing-over has taken place.
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38. Metaphase I vs II: Genetic Variation
Lots of variation in gametes produced
Random orientation of homologous pairs, which may have a great diversity in alleles present
Therefore many possible combinations of alleles could be pulled to each pole
Some variation in gametes produced
Random orientation of sister chromatids
Variation only in regions where crossing over has taken place in prophase I (recombination of alleles)
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39. Anaphase II Spindle fibres contract and the centromeres are broken.
The pairs of sister chromatids are pulled to opposing poles.
Non-disjunction here will lead to two gametes containing the wrong chromosome number.
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40. Telophase II New haploid nuclei are formed.
Cytokinesis begins, splitting the cells.
The end result of meiosis is four haploid gamete cells.
Fertilisation of these haploid gametes will produce a diploid zygote.
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41. Which phase of meiosis is shown? Why?
Interphase
Prophase I
Metaphase I
Metaphase II
Reason:
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42. Which phase of meiosis is shown? Why?
Interphase
Prophase I
Metaphase I
Metaphase II
Reason:
Homologous pairs are aligned (at equator), so must be metaphase.
Crossing-over has already taken place, so must be after prophase I.
Homologous pairs have not yet separated, so must be still in meiosis I (metaphase I).
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43. Which phase of meiosis is shown? Why?
Interphase
Prophase I
Metaphase I
Metaphase II
Reason:
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44. Which phase of meiosis is shown? Why?
Interphase
Prophase I
Metaphase I
Metaphase II
Reason:
Homologous pairs have associated.
Crossing-over has taken place.
Homologous pairs have not aligned at the equator.
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45. Which phase of meiosis is shown? Why?
Interphase
Prophase I
Metaphase I
Metaphase II
Reason:
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46. Which phase of meiosis is shown? Why?
Interphase
Prophase I
Metaphase I
Metaphase II
Reason:
Homologous pairs have not yet associated.
Replication has taken place.
Crossing-over has not yet taken place.
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47. Genetic Variation is almost infinite as a result of meiosis.
Crossing-over in prophase I
Leads to recombination of alleles on the chromosomes.
Random orientation in metaphase I
Huge number of maternal/paternal chromosome combinations possible in the final gametes. There are over 8million possible orientation in humans (223 orientations)
Random orientation in metaphase II
Further genetic variation arises where there are genetic differences between sister chromatids as a result of crossing-over in prophase I.
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48. Genetic Variation is almost infinite as a result of meiosis.
Crossing-over in prophase I
Leads to recombination of alleles on the chromosomes.
Random orientation in metaphase I
Huge number of maternal/paternal chromosome combinations possible in the final gametes. There are over 8million possible orientation in humans (223 orientations)
Random orientation in metaphase II
Further genetic variation arises where there are genetic differences between sister chromatids as a result of crossing-over in prophase I.
Even more variation!
Random fertilisation during sexual reproduction ensures even greater variation within the population.
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51. 3 applications!
Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.
Application: Studies showing age of parents influences chances of non-disjunction. An understanding of karyotypes has allowed diagnoses to be made for the purposes of genetic counselling. This raises ethical issues over selective abortion of female fetuses in some countries.
Application: Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

57. Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.