2. 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…
3. 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. 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
5. 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
4.2.3 Outline the process of Meiosis…
6. 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
4.2.3 Outline the process of Meiosis…
7. 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.
4.2.3 Outline the process of Meiosis…
8. 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
4.2.3 Outline the process of Meiosis…
9. Meiosis II
Prophase 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.
4.2.3 Outline the process of Meiosis…
10. Meiosis II
Metaphase II
Chromosomes line up along equator
Each chromosome attaches to a spindle fibre by
means of its centromere.
4.2.3 Outline the process of Meiosis…
11. Meiosis
Anaphase II
Centromeres separate and chromatids are moved
to the opposite poles.
Microtubule fibres
4.2.3 Outline the process of Meiosis…
12. Meiosis II
Telophase II
Chromatids reach opposite poles
Nuclear envelops forms
Cytokinesis occurs.
4.2.3 Outline the process of Meiosis…
14. 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
16. Trisomy 21: 3 copies of chromosome 21
Chromosome 21 is the smallest human chromosome
Frequency of Down syndrome correlates with the age of the mother
4.2.4 Explain non-disjunction:
Down syndrome
17. 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
Meiosis is a type of cell division that is vital for sexual reproduction. Meiosis takes place in the reproductive organs. It results in the formation of gametes with half the normal chromosome numbers. So Haploid sperms are made in the testes and haploid eggs are made in the ovaries.
In flowering plants, haploid gametes are made in the anthers and ovules.
In contrast to Mitosis, Meiosis produces cells that are not genetically identical. So meiosis has a key role in producing new genetic types, that is, it results in genetic variation.
Nuclear envelop breaks down
Spindle of microtubules starts to form
Chromosomes each consist of 2 sister chromatids
Homologous chromosomes pair up (synapsis) and crossing over may occur.
As the chromosomes pair up, they shorten and twist around each other. This cause a tension, and sections of chromatid may break off and exchange with corresponding section of a different chromatid. The point where this exchange of chromatid material occurs is called Chiasmata. This swapping of chromatid material is called crossing over. Crossing over is the interchange of genetic material between homologous chromosomes
At the start of the metaphase 1, the spindle will be formed.
As in mitosis, the chromosomes assemble on the equator of the spindle.
The key difference in meiosis metaphase1 is that chromosomes are joined in homologous pairs (bivalents).
The orientation of paternal and maternal chromosomes on either side of equator is random and independent of other homologous pairs.
The contraction of the spindle fibres pulls the homologous chromosomes apart. Then, homologous chromosomes are separated. One chromosome of each pair moves to each pole.
The separated homologous form a cluster at each pole of the cell, and the nuclear envelop re-forms around each daughter cell nucleus. Cytokinesis occurs. The resulting two cells have half the number of chromosomes as the original cell (versus four in the original cell). Each chromosome is still in the duplicated state and consist of two sisters chromatids, but sister chromatids are not identical because crossing over has occurred.
A completed spindle apparatus is in place in each cell. Chromosomes consisting of (non-identical) sister chromatids joined at the centromere align along the metaphase plate in each cell.
The nuclear membranes re-forms around four different cluster of chromosomes. After cytokinesis, four haploid cells result. No two cells are alike due to the random alignment of homologous pair at metaphase 1 and crossing over during prophase 1.
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 according to their size and structure.
Amniocentesis involves the removal of amniotic fluid which surrounds the fetus.(after 14 weeks pregnancy)
The fluid is produced by the amnion membrane and contains cells from the fetus.
These cells are removed by inserting a needle through the abdominal wall, myometrium and into the amniotic fluid.
The fluid is then centrifuged, cells are incubated and then subject to karyotyping (two weeks to develop).
The fluid (supernatant) can be used to test for neural tube disorders such as spina bifida.
Chorionic villus sampling involves the sampling of the chorion which is one of the extra-embryonic membranes (after 8 weeks pregnancy).
A catheter (tube) is inserted via the vagina and a sample is take of the chorion.
The sample is extracted and cultured to produce cells for karyotyping(few days).
