1. MEIOSIS
Topic 4.2
IB Biology
Miss Werba

2. TOPIC 4 - GENETICS 4.1 CHROMOSOMES, ALLELES, GENES & MUTATIONS 4.4 4.2
MEIOSIS
4.3
THEORETICAL GENETICS
4.4
GENETIC ENGINEERING & BIOTECHNOLOGY
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3. THINGS TO COVER Haploid v diploid Define homologous chromosomes
Process of meiosis
Chromosomal non-disjunction
Karyotyping & non-disjunction
Pre-natal diagnosis of chromosome abnormalities
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4. MEIOSIS Command term = STATE
4.2.1
MEIOSIS Command term = STATE
Meiosis is a reduction division of a diploid nucleus to form haploid nuclei
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5. HAPLOID vs DIPLOID Haploid: Diploid: One set of chromosomes
4.2.1
HAPLOID vs DIPLOID
Haploid:
One set of chromosomes
In humans, number (n) = 23
In humans, gametes (sperm & ova) are haploid
Diploid:
Two sets of chromosomes
In humans, 2n = 46
In humans, all body cells (other than gametes) are diploid
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6. HOMOLOGOUS CHROMOSOMES Command term = DEFINE
4.2.1
HOMOLOGOUS CHROMOSOMES Command term = DEFINE
Homologous chromosomes are chromosomes that share the same structural features and the same genes, but may have a different combination of alleles
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7. HOMOLOGOUS CHROMOSOMES
4.2.1
HOMOLOGOUS CHROMOSOMES
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8. 4.2.3
PROCESS OF MEIOSIS
Meiosis must occur in organisms that reproduce sexually to prevent doubling of the DNA during fertilisation
Involves 2 divisions - Meiosis I and Meiosis II
Occurs in the diploid germ cells in the gonads (testes & ovaies)
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9. PROCESS OF MEIOSIS Produces four gametes (sex cells – sperm or ova)
4.2.3
PROCESS OF MEIOSIS
Produces four gametes (sex cells – sperm or ova)
Gametes are haploid and not identical to original cell
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10. PROCESS OF MEIOSIS MEIOSIS I
4.2.3
PROCESS OF MEIOSIS MEIOSIS I
Homologous chromosomes pair up. They are called a bivalent.
Non-sister chromatids may cross over at points called chiasmata. They may exchange genetic material – crossing over.
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11. two non-sister chromatids
4.2.3
two sister chromatids
two non-sister chromatids
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12. CROSSING OVER & RECOMBINATION
4.2.3
CROSSING OVER & RECOMBINATION
Occurs in Prophase I
Sections of non-sister chromatids may touch (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. unlike either parent chromosome)
Increases variation
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13. PROCESS OF MEIOSIS MEIOSIS I
4.2.3
PROCESS OF MEIOSIS MEIOSIS I
Homologous chromosomes pair up. They are called a bivalent.
Non-sister chromatids may cross over at points called chiasmata. They may exchange genetic material = crossing over.
Homologous pairs line up at equator. Maternal and paternal chromosomes of each pair line up independently of other pairs = independent assortment.
Homologous chromosomes separate and move towards opposite poles. If they fail to separate = non-disjunction
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14. CHROMOSOMAL NON-DISJUNCTION
4.2.4
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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15. PROCESS OF MEIOSIS MEIOSIS I
4.2.3
PROCESS OF MEIOSIS MEIOSIS I
Homologous chromosomes pair up. They are called a bivalent.
Non-sister chromatids may cross over at points called chiasmata. They may exchange genetic material = crossing over.
Homologous pairs line up at equator. Maternal and paternal chromosomes of each pair line up independently of other pairs = independent assortment.
Homologous chromosomes separate and move towards opposite poles. If they fail to separate = non-disjunction
Two new haploid cells form.
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16. PROCESS OF MEIOSIS MEIOSIS I
4.2.3
PROCESS OF MEIOSIS MEIOSIS I
Ref: Advanced Biology, Kent
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17. PROCESS OF MEIOSIS MEIOSIS II
4.2.3
PROCESS OF MEIOSIS MEIOSIS II
New spindle apparatus forms.
Chromosomes line up at the equator in a single line.
Centromeres divide and sister chromatids move towards opposite poles.
Each cell divides, resulting in a total of four haploid cells. Each cell formed is genetically unique due to crossing over and independent assortment.
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18. PROCESS OF MEIOSIS MEIOSIS II
4.2.3
PROCESS OF MEIOSIS MEIOSIS II
Ref: Advanced Biology, Kent
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19. 4.2.3
PROCESS OF MEIOSIS

20. 4.2.3
PROCESS OF MEIOSIS

21. MEIOSIS vs MITOSIS

22. PCR

23. 4.2.5
KARYOTYPING
Karyotyping is the process of arranging the chromosomes in pairs according to their structure.
Chromosomes are easily identifiable and are arranged depending upon:
their length
the position of their centromere
their banding pattern
Can be used to detect chromosome disorders in foetuses.
Utilises foetal DNA obtained using chorionic villus sampling or amniocentesis
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24. KARYOTYPING MALE KARYOTYPE FEMALE KARYOTYPE 4.2.5 J WERBA – IB BIOLOGY24

25. CHORIONIC VILLUS SAMPLING
4.2.6
CHORIONIC VILLUS SAMPLING
11-12 weeks of pregnancy
Sample of the chorionic villi (formed from the outer cells of the zygote) is taken
A karyotype is made & analysed
Procedural risk is ~1%
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26. AMNIOCENTESIS 16 weeks of pregnancy
4.2.6
AMNIOCENTESIS
16 weeks of pregnancy
Sample of amniotic fluid (contains foetal cells) is taken
A karyotype is made & analysed
Procedural risk is ~0.5%
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27. Sample questions
Q1. In the following diagram, which pair represents homologous chromosomes?
A. 1 and 2
B. 3 and 4
C. 2 and 5
D. 4 and 6
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28. Sample questions
Q2. If the haploid number of a species is 14, how many chromatids will there be in metaphase I in a dividing diploid cell?
A. 7
B. 14
C. 28
D. 56
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29. Sample questions
A1. D
A2. D
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30. Sample questions
Q3. The karyotype below shows the chromosomes from a person with Down syndrome.
[Source: U.S. Department of Energy Human Genome Program (genomics.energy.gov, genomicscience.energy.gov)]
(a) State the evidence provided by the karyotype that shows this person has Down syndrome. (1)
(b) Outline how Down syndrome occurs due to meiosis. (2)
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31. Sample questions
A3.
(a) three copies/extra copy/trisomy of chromosome/pair 21 Do not accept chromatid.
(b) occurs due to non-disjunction; (homologous) chromosome/pairs/sister chromatids fail to separate; some gametes have an extra chromosome;
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