4.2 meiosis

Cell division process in which the number of chromosomes is cut in half…

Results in the formation of gametes …such as- eggs and sperm

Gametes have ½ chromosomes of adult

Fusion of an egg and sperm results in a zygote

Zygote now has the same number of chromosomes as adult

4.2.1 State that meiosis is a reduction division…

4.2.2 Define Homologous Chromosome

Homologous chromosomes are chromosome pairs of the same length,

centromere position, and staining pattern with genes for the same

characteristics. One homologous chromosome is inherited from the organism's

mother, the other from the organism's father

4.2.3 Outline the process of Meiosis…

Meiosis includes two rounds of division: Meiosis I, Meiosis II.

Meiosis I Prophase I, Metaphase I, Anaphase I and Telophase I.

Homologous chromosomes are paired

While paired, they cross over and exchange genetic information (DNA)

Homologous pairs are then separated, and two daughter cells are produced

Meiosis II Prophase II, Metaphase II, Anaphase II and Telophase II.

the same as mitosis

sister chromatids of each chromosome separate

result is four haploid daughter cells

Meiosis I:

Prophase I: Nuclear envelope dissolves Spindle of microtubules starts to form Homologues become closely associated in synapsis Crossing over may occur between non-sister

chromatids


Meiosis I

Metaphase I: Microtubules from opposite poles attach to each

homologue, not each sister chromatid. Spindle microtubules move homologous pairs to

equator of the cell. The orientation of each pair of homologues

(maternal and paternal) on either side of equator is random and independent of other homologous pairs.

Spindle fibres


Meiosis I

Anaphase I: Microtubules of the spindle shorten Homologues are separated from each other Sister chromatids remain attached to each other

at their centromeres.


Meiosis

Telophase I Nuclear envelopes form around each set of

chromosomes Each new nucleus is now haploid Sister chromatids are no longer identical

because of crossing over Cytokinesis occurs


Meiosis IIProphase II

Chromosomes, which still consist of two chromatids, condense and become visible.

The new spindle microtubules develop at right angles to the old spindle.

Nuclear envelop breaks down.


Meiosis IIMetaphase II

Chromosomes line up along equator Each chromosome attaches to a spindle fibre by

means of its centromere.


MeiosisAnaphase II

Centromeres separate and chromatids are moved to the opposite poles.

Microtubule fibres


Meiosis IITelophase II

Chromatids reach opposite poles Nuclear envelops forms Cytokinesis occurs.


Comparison of Mitosis and Meiosis

Non-disjunction occurs when chromosomes don’t separate properly during meiosis.

Incorrect number of chromosomes.

Problems with meiotic spindle cause errors in daughter cells

Homologous chromosomes do not separate properly during Meiosis I

Sister chromatids fail to separate during Meiosis II

Too many or too few chromosomes

4.2.4 Explain non-disjunction

4.2.4 Explain non-disjunction

Trisomy 21: 3 copies of chromosome 21 Chromosome 21 is the smallest human chromosomeFrequency of Down syndrome correlates with the age of the mother

4.2.4 Explain non-disjunction: Down syndrome

The number and appearance of the chromosomes in an organism is called

karyotype. A karyotype is an organized image of metaphase chromosomes.

Technicians stain the chromosomes, which results in a banding pattern. The

technician can then organize the chromosomes by their length, the position of

their centromere and by the banding pattern.

4.2.5 Karyotype

Down syndrome karyotipe

4.2.6 Karyotyping for pre-natal diagnosis

4.2.7 Analyse a human karyotype

4.2 meiosis

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