1. MEIOSIS
AHL Topic 10.1
IB Biology
Miss Werba

2. DIHYBRID CROSSES & GENE LINKAGE POLYGENIC INHERITANCE
AHL TOPIC 10 – GENETICS
10.1 MEIOSIS
10.2
DIHYBRID CROSSES & GENE LINKAGE
10.3
POLYGENIC INHERITANCE
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3. THINGS TO COVER Process of meiosis – in detail
Process of crossing over & recombination
Genetic variety through movement of chromosomes
Mendel’s Law of Independent Assortment
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4. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Reminder: Homologous chromosomes have the same sequence of genes in the same loci, but slightly different alleles
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5. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Homologous chromosomes replicate during the synthesis phase (S-phase) of interphase
Yields two pairs of sister chromatids, each joined at a centromere
sister chromatids
chromatid
centromere
non-sister chromatids
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6. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
The homologous pair of chromosomes line up side by side during prophase I.
This is called synapsis.
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7. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
The homologous pair of chromosomes line up side by side during prophase I.
This is called synapsis.
The pair formed is called a bivalent.
bivalent
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8. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1,
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Crossing over may occur between non-sister chromatids during prophase I.
Crossing over can lead to recombination of alleles.
The crossover points are called chiasmata.
chiasma
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9. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1,
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
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10. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Meiosis reduces the chromosome number in each nucleus.
It forms haploid gametes (n) from diploid somatic cells (2n).
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11. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Interphase
DNA replication occurs
Sister chromatids are formed
Growth and other preparation events also occur
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12. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Prophase I
Chromosomes condense
Nuclear membrane disappears
Nucleolus becomes invisible
Spindle formation
Synapsis:
homologous chromosomes move side by side
form bivalents
crossing over may occur(forming chiasmata)
 results in new allele combinations , thus increasing variation in gametes
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13. CROSSING OVER Command term = OUTLINE
10.1.1
10.1.2
10.1.3
CROSSING OVER Command term = OUTLINE
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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14. CROSSING OVER Command term = OUTLINE
10.1.1
10.1.2
10.1.3
CROSSING OVER Command term = OUTLINE
Crossing over
Increases variation in gametes.
Can break up genes that are usually inherited together due to their proximity to each other. These are called linked genes.
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15. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Metaphase I
Bivalents move to the equator.
The chromosomes orientate themselves randomly.
 also increases variation in gametes as there are possible combinations!
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16. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Anaphase I
The spindle fibres contract
Homologous pairs are separated
One chromosome from each pair goes to each pole
Non-disjunction could occur here (ie. failure to separate) and affect the formation of gametes
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17. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Telophase I
Chromosomes arrive at poles
Spindle disappears
New nuclei form
The cytoplasm divides by cytokinesis.
The nuclei are now officially haploid.
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18. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Prophase II
New spindle forms
Forms at right angles to the previous spindle
Nuclei break down
No crossing over occurs
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19. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Metaphase II
Sister chromatids align at the equator
Spindle fibres attach to the centromeres
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20. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Anaphase II
Spindle fibres contract
Centromeres break
Sister chromatids are pulled to opposite poles
Non-disjunction could also occur here
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21. CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
10.1.1
CHROMOSOMES IN MEIOSIS Command term = DESCRIBE
Telophase II
Chromosomes arrive at poles
Spindle disappears
Nuclear membrane reappears
Nucleolus becomes visible
Cytokinesis (cell division) is not part of meiosis but is often considered to be the last stage of telophase II
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23. GENETIC VARIATION Command term = EXPLAIN
10.1.3
GENETIC VARIATION Command term = EXPLAIN
Genetic variation is increased dramatically as a result of meiosis.
Results from:
Crossing over in prophase I
Random orientation of chromsomes in metaphase I
Random fertilisation during reproduction
Mate selection
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24. INDEPENDENT ASSORTMENT Command term = STATE
10.1.4
INDEPENDENT ASSORTMENT Command term = STATE
Mendel’s Law of Independent Assortment:
The presence of an allele of one gene in a gamete has no influence over which allele of another gene is present.
Only true for unlinked genes
ie. those far away from each other
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25. INDEPENDENT ASSORTMENT Command term = EXPLAIN
10.1.5
INDEPENDENT ASSORTMENT Command term = EXPLAIN
Random orientation of the chromosomes during metaphase I allows the genes to become independently assorted into the gametes.
Mendel came up with his ideas before the specifics of meiosis were known (ie. random orientation).
Chromosomes and DNA had not been discovered yet.
He was looking at the traits themselves and it’s just lucky that he chose two unlinked traits.
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26. Sample questions
Q1. The diagram below shows chromosomes during meiosis. How many chromosomes and chiasmata are visible?
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27. Sample questions Q2. Which processes result in recombination?
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28. Sample questions
Q3. Outline how meiotic division results in almost an infinite genetic variation in the gametes produced. [2] Q4. State how chromosome number can increase in human beings. [1]
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29. Sample questions A1. C A2. A crossing over / chiasmata;
shuffles alleles;
random orientation of chromosomes;
at metaphase I:
at metaphase II; 2 max
A4. non disjunction
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