1. REVISION:
MEIOSIS
Topic 4.2
IB Biology
Miss Werba

2. HAPLOID & DIPLOID Haploid: One set of chromosomes In humans, n = 23
4.2.1
HAPLOID & DIPLOID
Haploid:
One set of chromosomes
In humans, 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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3. HAPLOID & DIPLOID Diploid:
4.2.2
HAPLOID & DIPLOID
Diploid:
each pair of chromosomes have the same genes
they are arranged in the same sequence (loci)
they don’t have the same alleles of all of the genes
They are therefore considered to be homologous, not identical.
The chromosomes are said to exist in homologous pairs.
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4. 4.2.3
PROCESS OF MEIOSIS
Meiosis =cell division to halve the chromosome number of the cell
A reductive division
Must occur in organisms that reproduce sexually to prevent doubling of the DNA during fertilisation
Occurs in germ cells in the gonads - diploid
Production of gametes (sex cells)
Produces four gametes from every germ cell
Gametes are haploid and not identical to original cell
Involves 2 divisions - Meiosis I and Meiosis II
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5. MEIOSIS I Homologous chromosomes pair up. They are called a bivalent.
3.5.3
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.
Two new haploid cells form.
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6. 3.5.3
MEIOSIS I
Ref: Advanced Biology, Kent
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7. MEIOSIS II New spindle apparatus forms.
3.5.3
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 cells 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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8. 3.5.3
MEIOSIS II
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Ref: Advanced Biology, Kent 8

9. CROSSING OVER & RECOMBINATION
4.2.3
CROSSING OVER & RECOMBINATION
Occurs in Prophase I
Occurs when the chromosomes (bivalent) line up in pairs
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. chromosomes that are unlike either parent chromosome)
Increases variation
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10. CHROMOSOMAL NON-DISJUNCTION
4.2.4
CHROMOSOMAL NON-DISJUNCTION
Occurs in Anaphase I
Occurs when homologous chromosomes fail to separate properly.
They are a form of chromosome 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 involves trisomy 21
The chance of non-disjunction increases with maternal age.
Ref: Biology Key Ideas
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11. 4.2.5
KARYOTYPING
Karyotyping is 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
Karyotyping can be used to detect chromosome disorders in foetuses.
DNA is obtained using Chorionic villus sampling or amniocentesis
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12. KARYOTYPING MALE KARYOTYPE FEMALE KARYOTYPE 4.2.5 J WERBA – IB BIOLOGY12

13. 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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14. 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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