1. MEIOSIS Topic 3.3 IB Biology Miss Werba
Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
Topic 3.3
IB Biology
Miss Werba

2. GENETIC MODIFICATION and BIOTECHNOLOGY
TOPIC 3 – GENETICS
3.1
GENES
3.2
CHROMOSOMES
3.3
MEIOSIS
3.4
INHERITANCE
3.5
GENETIC MODIFICATION and BIOTECHNOLOGY
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3. THINGS TO COVER U.1 U.2 U.3 U.4 U.5 U.6 U.7 U.8 Statement Guidance
One diploid nucleus divides by meiosis to produce four haploid nuclei.
U.2
The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
U.3
DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
U.4
The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
The process of chiasmata formation need not be explained.
U.5
Orientation of pairs of homologous chromosomes prior to separation is random.
U.6
Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
U.7
Crossing over and random orientation promotes genetic variation.
U.8
Fusion of gametes from different parents promotes genetic variation.
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4. THINGS TO COVER A.1 A.2 A.3 S.1 Statement Guidance NOS 1.8
Non-disjunction can cause Down syndrome and other chromosome abnormalities.
A.2
Studies showing age of parents influences chances of non-disjunction.
A.3
Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.
S.1
Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
Drawings of the stages of meiosis do not need to include chiasmata.
NOS 1.8
Making careful observations.
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5. MEIOSIS
U.1
Meiosis is a reduction division of a diploid nucleus to form four haploid nuclei.
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6. Making careful observations
DISCOVERY OF MEIOSIS
NOS 1.8
Making careful observations
Meiosis was discovered by microscope examination of dividing germ-line cells.
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7. DISCOVERY OF MEIOSIS
NOS 1.8
First observed in 1876 by Oscar Hertwig, as he studied sea urchin eggs under a microscope.
Eduord van Beneden observed the behaviour of chromosomes during meiosis in the eggs of parasitic worms.
In 1890, August Weismann observed the two phases needed to complete the division of eggs.
The term meiosis was first used in 1905.
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8. MEIOSIS
U.2
The halving of the chromosome number prevents doubling of the DNA with fusion of gametes during fertilisation.
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9. MEIOSIS
U.2
Occurs in the diploid germ cells in the gonads (testes & ovaries)
Meiosis involves 2 divisions - Meiosis I and Meiosis II
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10. MEIOSIS Produces four gametes (sex cells – sperm or ova)
Gametes are haploid and not identical to original cell
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11. non-sister chromatids
MEIOSIS
U.3
DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
sister chromatids
non-sister chromatids
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12. MEIOSIS I
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13. MEIOSIS I Interphase DNA replication and preparation of the cell U.4
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14. MEIOSIS I
U.4
U.5
U.6
Prophase I
Homologous chromosomes pair up (now called a bivalent).
Crossing over may occur: Non-sister chromatids may cross over at points (called chiasmata) and exchange genetic material
Spindle microtubule forms
The nuclear envelope breaks down.
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15. MEIOSIS I Metaphase I Homologous pairs line up randomly at equator.
Maternal and paternal chromosomes of each pair line up independently of other pairs = independent assortment.
This greatly increases genetic variety in the gametes.
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16. MEIOSIS I Anaphase I The microtubules contract.
Homologous chromosomes separate and move towards opposite poles.
The chromosome number is halved from diploid to haploid.
Chromosomal non- disjunction occurs if they fail to separate
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17. MEIOSIS I Telophase I + Cytokinesis Spindles break down.
U.4
U.5
U.6
Telophase I + Cytokinesis
Spindles break down.
New nuclear envelope forms around each new nucleus.
Two new haploid cells form.
Each chromosome still has 2 sister chromatids at this point.
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18. CROSSING OVER & RECOMBINATION
U.4
U.7
Occurs in Prophase I
Sections of non-sister chromatids may touch
This crossing-over point is called a chiasma (pl. chiasmata).
Sections of the chromosomes may be swapped between the non-sister chromatids.
This produces recombinant chromosomes (ie. unlike either parent chromosome)
Increases variation
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19. CHROMOSOMAL NON-DISJUNCTION
Occurs in Anaphase I
Homologous chromosomes fail to separate properly.
This is a form of chromosomal mutation.
Extra or fewer chromsomes are drawn to each pole and packaged into the new cells.
Results in trisomy (3 copies) or monosomy (1 copy)
eg. Down Syndrome = trisomy 21
The chance of non-disjunction increases with maternal age.
Ref: Biology Key Ideas
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20. MEIOSIS II
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21. MEIOSIS II
U.4
U.5
U.6
Prophase II
New spindle microtubules form in each of the two cells.
The spindles form at right angles to the original division.
The nuclear envelope breaks down.
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22. MEIOSIS II Metaphase II Individual chromosomes line up at equator.
Spindle fibres attach to each chromatid at the centromere.
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23. MEIOSIS II Anaphase II The microtubules contract.
Sister chromatids separate and move towards opposite poles.
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24. MEIOSIS II Telophase II + Cytokinesis Spindles break down.
U.4
U.5
U.6
A.1
Telophase II + Cytokinesis
Spindles break down.
New nuclear envelopes form around each new nucleus.
The chromosomes uncoil.
Four haploid cells form.
Each cell formed is genetically unique.
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25. https://youtu.be/eaf4j19_3Zg?t=15s

26. MEIOSIS PROMOTES GENETIC VARIATION
U.5
U.6
U.7
U.8
Each chromosome of the homologous pair carries different alleles.
Homologous pairs line up and separate independently.
Crossing over may occur during prophase I leading to entirely new combinations of alleles.
There is fusion of gametes from different parents during fertilisation.
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27. DOWN SYNDROME
A.1
A.2
A.3
Down syndrome is a condition resulting from a non- disjunction in the chromosome 21 homologous pair.
It is also called Trisomy 21.
Karyotyping can be used to detect Down syndrome in foetuses.
The process utilises foetal DNA obtained using chorionic villus sampling (CVS) or amniocentesis.
New techniques use a maternal blood test to access foetal DNA.
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28. DOWN SYNDROME Why is prenatal testing needed?
All women are at risk of having a baby with Down syndrome, although the risk increases with the age of the woman.
If the mother is over 35, prenatal testing is strongly encouraged.
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29. DOWN SYNDROME MALE KARYOGRAM FEMALE KARYOGRAM A.1 A.2 A.3
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30. PRE-NATAL TESTING CHORIONIC VILLUS SAMPLING 11-12 weeks of pregnancy
Sample of the chorionic villi (from the placenta) is taken
A karyogram is made & analysed
Procedural risk is ~1%
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31. PRE-NATAL TESTING AMNIOCENTESIS 16 weeks of pregnancy
Sample of amniotic fluid (contains foetal cells) is taken
A karyogram is made & analysed
Procedural risk is ~0.5%
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32. PRE-NATAL TESTING NON-INVASIVE PRE-NATAL TESTING (NPT)
Any stage of pregnancy (usually before end of 2nd trimester)
Examines DNA from the unborn baby circulating in the mother’s bloodstream
A karyogram is made & analysed
Procedural risk is 0%
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33. MEIOSIS
Q1. If the haploid number of a cell is represented by the letter X, what would be the number of chromosomes in the cells at the end of the first division of meiosis be? 4X X 2X
0.5X
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34. MEIOSIS
Q2. If the haploid number of a species is 14, how many chromatids will there be in metaphase I in a dividing diploid cell? 56 7 28 14
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35. MEIOSIS Q3. What is chorionic villus sampling?
Sampling cells from the foetal digestive system
Sampling cells from the placenta
Sampling cells from the amniotic fluid
Sampling cells from the umbilical cord
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36. MEIOSIS Q4. Outline how Down syndrome occurs due to meiosis. (2 marks)
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37. MEIOSIS A1. B A2. A A3. B A4. occurs due to non-disjunction;
(homologous) chromosome/pairs/sister chromatids fail to separate;
some gametes have an extra chromosome;
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