GRADE 12: GRADE 12: MEIOSISMEIOSIS

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Why do meiosis?Meiosis is “REDUCTION DIVISION”so Reduces chromosome number, parent cell is 2n while daughter cells are n.

Why do gonosome undergo meiosis?

Haploid gametes (n 23)

Egg cell

Sperm cell

Fertilization

n

n

Meiosis

Ovary Testis

Diploidzygote(2n 46)

2n

MitosisKey

Haploid stage (n)Diploid stage (2n)

Multicellular diploidadults (2n 46)

A life cycle

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QUIZ TIME…

1. In streptomycin fungus n = 11. What is the diploid number for this species?

2. If a horse egg has 98 chromosomes how many does a horse egg cell have?

3. If n = 16 for goldfish how many chromosome in a fin cell?

4. If 2n = 108 for black spruce trees then what is the haploid number? 2n = 14 for this species. Fill in

the chromosome numbers for each cell.

A

BC

What is process A, B, and C?

Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male

Meiosis I (four phases)Meiosis I (four phases)

• Cell division Cell division that reduces the chromosomechromosome number by one-half.one-half.

• four phasesfour phases:a.a. prophase Iprophase Ib.b. metaphase Imetaphase Ic.c. anaphase Ianaphase Id.d. telophase Itelophase I

Interphase IInterphase I

Interphase IInterphase I

nuclear membrane

nucleolus

cell membrane

chromatin

MEIOSIS I : prophase i

MEIOSIS I : prophase i

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Metaphase IMetaphase I• Shortest phaseShortest phase

• TetradsTetrads align on the metaphase platemetaphase plate.

• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n

Example:Example: 2n = 42n = 4then then n = 2 n = 2

thusthus 2 22 2 = 4 combinations= 4 combinations

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Anaphase IAnaphase I

• Homologous chromosomes Homologous chromosomes separate and move towards the poles.

• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

Anaphase IAnaphase I

Meiosis i: telophase i

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Meiosis ii

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Meiosis ii: prophase II

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Prophase IIProphase II

• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II

metaphase platemetaphase plate

Anaphase IIAnaphase II

Telophase IITelophase II

Crossing over

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Crossing over

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= Non-separation of chromosomes resulting in diploid or empty gametes.

Non-disjunction

Downs syndrome = Trisomy 21 - 3 copies of chromosomes 21 equalling a total of 47 chromosomes. = 2n + 1(chance of occurring in oogenesis increases with maternal age)

Turners syndrome = Monosomy X - has only oneone XX chromosomes totalling only 45 chromosomes in her body cells. (monosomy X) = 2n-1

Klinefelter’s syndrome = XXY - male which has an extra Xextra X chromosome = 2n + 1

Conditions caused bynon-disjunction

A simple check for the presence of a bone in the nose could more accurately test unborn babies for Down's syndrome, scientists say. Researchers say combining the nose test with existing screening methods (amniocentesis, CVS) could lead to a five-fold reduction in the number of miscarriages linked to an invasive procedure used to confirm Down's syndrome.

Down’s syndrome – trisomy 21

Normal boy who develops some female secondary sex character-istics at puberty.

-slightly lower IQ, infertile, delayed motor, speech, maturation

-treated with testosterone.

Klinefelter’s syndrome XXY