1. MEIOSIS AHL Topic 10.1 IB Biology Miss Werba
Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.
AHL Topic 10.1
IB Biology
Miss Werba

2. TOPIC 10 – GENETICS and EVOLUTION
10.1
MEIOSIS
10.2
INHERITANCE
10.3
GENE POOLS and SPECIATION
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3. THINGS TO COVER U.1 U.2 U.3 U.4 U.5 U.6 U.7 S.1 Statement Guidance
Chromosomes replicate in interphase before meiosis.
U.2
Crossing over is the exchange of DNA material between non-sister homologous chromatids.
U.3
Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
U.4
Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
U.5
Homologous chromosomes separate in meiosis I.
U.6
Sister chromatids separate in meiosis II.
U.7
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
S.1
Drawing diagrams to show chiasmata formed by crossing over.
Should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma was formed.
NOS 1.8
Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies.
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4. MEIOSIS
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5. https://youtu.be/eaf4j19_3Zg?t=15s

6. HOMOLOGOUS CHROMOSOMES
Homologous chromosomes have the same sequence of genes in the same loci, but slightly different alleles
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7. HOMOLOGOUS CHROMOSOMES
Chromosomes replicate during the synthesis phase (S-phase) of interphase before meiosis.
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8. HOMOLOGOUS CHROMOSOMES
Replication yields two pairs of sister chromatids, each joined at a centromere
chromatid
centromere
sister chromatids
non-sister chromatids
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9. HOMOLOGOUS CHROMOSOMES
The homologous pair of chromosomes line up side by side during prophase I.
This is called synapsis and the pair formed is called a bivalent.
bivalent
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10. CROSSING OVER Might occur in Prophase I
Sections of non-sister chromatids may touch (ie. cross over).
This point is called a chiasma (pl. chiasmata).
Sections of the chromosomes may be swapped between the non-sister chromatids.
This produces recombinant chromosomes (ie. with allele combinations unlike either parent chromosome)
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11. CROSSING OVER
U.3
Increases genetic variation through the recombination of linked alleles.
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12. CROSSING OVER Synapsis Crossing over Recombination
U.3
Synapsis
Homologous chromsomes pair up, forming a bivalent
Crossing over
Non-sister chromatids may overlap, forming chiasmata
Recombination
Alleles are swapped between non-sister chromatids, forming recombinant chromosomes.
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13. CROSSING OVER Y Y y y Z Z z z y y Y Y z Z z Z recombinants U.3
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14. CROSSING OVER
U.3
Skill: you need to be able to draw diagrams to show chiasmata formed by crossing over
Try to differentiate between the 2 sister chromatids in some way (remember no colours or shading).
Remember that the homologous chromosomes are in synapsis and therefore need to be drawn close together.
Make sure the sister chromatids, centromeres and chiasmata are all labeled.
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15. INDEPENDENT ASSORTMENT
U.7
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I
Occurs in Metaphase I
Bivalents move to the equator.
The chromosomes orientate themselves randomly.
increases variation in gametes as there are 223 possible combinations!
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16. INDEPENDENT ASSORTMENT
U.7
Only holds true for unlinked genes (ie. genes on different chromosomes)
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17. SEPARATION OF CHROMOSOMES
U.5
Non-sister chromatids
Homologous pairs are separated in Anaphase I as the spindle fibres contract.
One chromosome from each pair goes to each pole.
This is a reductive division (ie. 2n  n).
Non-disjunction could occur here (ie. failure to separate) and affect the formation of gametes.
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18. SEPARATION OF CHROMOSOMES
U.6
Sister chromatids
The duplicated chromosomes (sister chromatids) separate during Metaphase II.
Again, random orientation contributes to variation in the gametes.
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19. Making careful observations
MENDEL vs MORGAN
NOS 1.8
Making careful observations
Careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for.
Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies.
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20. MENDEL vs MORGAN Gregor Mendel deduced that:
NOS 1.8
Gregor Mendel deduced that:
“The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.”
Independent assortment occurs as a result of the random orientation of chromosomes during Metaphase I.
Mendel made his deduction based on his work with pea plants. It was just lucky that he investigated two traits that were on separate chromosomes (ie. unlinked genes).
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21. MENDEL vs MORGAN
NOS 1.8
Thomas Hunt Morgan investigated traits in fruit flies.
He produced results that could not be explained by Mendel’s work.
The “anomalous” data was repeated and kept appearing in predictable patterns.
Morgan and his colleagues revised Mendel’s ideas:
Discrete pairs of factors (ie. genes) are located on chromosomes
Some traits are sex-linked
Other traits are also sometimes linked
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23. MEIOSIS
Q1. The diagram below shows: A. Four separate chromosomes B. A bivalent C. One pair of sister chromatids D. Non-disjunction
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24. MEIOSIS
Q2. The diagram below shows: A. Two separate chromosomes B. A bivalent C. One pair of sister chromatids D. Crossing over
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25. MEIOSIS
Q3. The diagram below shows: A. Two separate chromosomes B. A bivalent C. One pair of sister chromatids D. Homologous chromosomes
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26. MEIOSIS
Q4. The diagram below shows chromosomes during meiosis. How many chromosomes and chiasmata are visible?
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27. MEIOSIS Q5. Which processes result in recombination?
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28. MEIOSIS
Q6. Outline how meiotic division results in almost an infinite genetic variation in the gametes produced. [2] Q7. State how chromosome number can increase in human beings. [1]
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29. MEIOSIS A1. B A2. C A3. A A4. C A5. A A6. crossing over / chiasmata;
shuffles alleles;
random orientation of chromosomes;
at metaphase I:
at metaphase II; 2 max
A7. non disjunction
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