Download Summary Notes on Meiosis File

Meiosis Notes for G12LB by Nadine Hage
Terms that you should know:
Chromosomes:






Carriers of genetic material
Located in the nucleus of a cell
Consists of 2 identical chromatids (double chromosome) or 1 chromatid (single chromosome)
depending on the phase of the cell division.
Each chromatid contains one DNA double helix molecule.
In humans, there are 46 chromosomes or 23 pairs of homologous chromosomes (where each
pair includes a maternal and a paternal chromosome). These pairs include 22 pairs of
homologous autosomes (pair 1 through 22) and the last 23rd pair of gonosomes or sex
chromosomes (XX for a female and XY for a male) (note: the Y chromosome also look X in
shape however it is smaller in size as compared to the X chromosome).
Homologous pairs or homologues have similar size, shape, centromere position, genes and
banding patterns when stained because they contain genes for the same traits in the same
order. (Note that similar alleles or different alleles exist on those homologues).
Karyotype:




A picture of organism’s chromosomes that are arranged according to size from biggest to smallest.
In order to get this picture, during Metaphase the chromosomes are isolated, stained, and examined
under the microscope. Most often, this is done using the chromosomes in the white blood cells.
It allows the detection of chromosomal abnormalities like:
o Trisomy (having three copies of a chromosome instead of two)
o Monosomy (having one copy of a chromosome instead of two)
It also allows the determination of the chromosomal formula. In humans the chromosomal formula is:
o For females: 2n= 44 + XX
o For males: 2n= 44 + XY
Meiosis




Cell division responsible for the formation of the gametes: ovum in females and sperm in males.
Consists of two successive divisions:
o Meiosis I which is reductional as it reduces the number of chromosomes from 2n into n.
o Meiosis II which is equational as Mitosis as it maintains the number of chromosomes as n.
Results in the formation of 4 haploid daughter cells ( that are neither identical to one another nor to the
mother cell) starting always from a diploid germ mother cell which are present in the sex organs or
gonads (ovaries of the female and testes of the male).
Meiotic cells have an interphase stage before the start of meiosis I which is similar to the Interphase
preceeding mitosis. It includes G1, S and G2 phases. (See notes on mitosis) After meiosis I there is
another brief interphase stage which is followed by meiosis II.
Meiosis I: (reductional division)
Prophase I:





Chromatin condenses into chromosomes (each chromosome made up of two chromatids joined by a
centromere).
Nuclear membrane starts to break down.
Nucleolus disappears.
Centrosomes duplicate into asters that move to opposite poles.
Spindle fibers are formed.

Homologous chromosomes come together and line up side by side (due to reasons that we don’t know)
forming a tetrad. This process is called Synapsis. At the chiasmata, crossing over or exchange of
genetic material between non-sister adjacent chromatids occurs.
Metaphase I:


Paired up homologous chromosomes or tetrads line up at the equator held at their centromeres by
the spindle fibers forming the equatorial plate.
Note that as the tetrads align, the maternal or paternal homologue may be oriented towards either pole.
Anaphase I:


Homologous chromosomes separate independently in a random manner by the process of
Independent Assortment and are pulled to opposite poles. This halves the chromosome number
however each chromosome is still composed of two sister chromatids.
Each chromosome from the homologous pair is found at an opposite pole.
Telophase I:



Daughter cells have one chromosome from each pair.
Nuclear membrane reforms around each daughter nucleus.
Cell membrane then divides through cytokinesis.
Cell has divided into two haploid daughter cells; however, the process does not end here as these two cells
immediately start to divide again.
Meiosis II: (equational division)
Prophase II:



The spindle fibers are reformed.
Asters move to opposite pole again.
Nuclear membrane breaks down.
Metaphase II:

Chromosomes in each cell line up at the middle of the cell held by the spindle fibers attached to the
centromere of each chromosome forming the equatorial plate.
Anaphase II:



Chromosomes divide.
Each sister chromatid move to opposite poles in both cells pulled by the spindle fibers.
Each sister chromatid then becomes a chromosome.
Telophase II:





The nuclear membrane reforms around the four sets of daughter chromosomes.
Nucleolus reforms.
Spindle fibers disappear.
Chromosomes uncoil into chromatin.
Cytokinesis then follows to divide the cytoplasm of the two cells and so the result is four daughter cells
each with a haploid set of chromosomes.
Note: Meiosis & Genetic Diversity: no daughter cells formed during meiosis are genetically identical to either
mother or father. Meiosis results in such a great genetic variation due to Independent Assortment Shuffling of
maternal and paternal chromosomes and crossing-over.
The random arrangement of chromosomes at Metaphase I lead to variation within offspring. The number of
possible chromosome combinations for each gamete is 2n.