General Genetic

• Bio 221 Lab 2

MeiosisDefinition : The process in cell division in sexually

reproducing organisms that reduces the number of chromosomes from diploid to haploid (half the original number). Meiosis involves two divisions .

The two haploid cells (23 chromosomes, N * each of the chromosomes consisting of two sister chromatids) produced in meiosis I to the end result is production of four haploid cells (23 chromosomes, N in humans) in meisis II .

(leading to gametes animals and spore in plant ) .

Place : 1-In most sexually reproducing organisms 2-it occurs during animal gameto-genesis or sporo-genesis in plants

الحازمي : إيمان إعداد

2-gametogenesis (in the animal)

1-sporogenesis (in the Plant )

1N 1N – Secondary spermatocytes

2N

2N

    

       • Interphase I:genetic materials are duplicated

due to active DNA replication.

• A. Meiosis I

• Prophase I:Prophase-I is a very long phase, it is divided into five-sub-phases.

leptonema, zygonema, pachynema, diplonema, and diakinesis.

Stages of meiosis

Prophase I

• Leptonema:- (leptos = thin; = band / stripe)

The chromosomes are very thin and so can be hardly seen.

Chromosomes duplicated and the two chromatids of each chromosome are twined around each other and appear to have

small nodules known as "Chromomeres."

Prophase I• Zygonema:- (zygon = touching another) In this phase the

homologous chromosomes come to each other to form pairs

Synapses. The paired chromosomes are known as "bi-valents".

Prophase I• Pachynema:- (pachus = thick) The chromosomes become

thick and dense on the completion of pairing in this phase. As they are paired, there appear to be four chromatids in all"tetra-valent".

• Crossing over is now exhibited by the inner chromatid of each homologous pair of chromosome.

Prophase I• Diplonema:- (diplous = in twofold )It is a longer sub-

phase. The chromosomes in each pair now stay moving away from each other. The inner chromatids are in a state of crossing over and remain attached. These local are referred to as chiasmata.

الحازمي : إيمان إعداد

Prophase I• Diakinesis:- (dia = apart; kinein = to move) there is an

increase in the opposite pulling between the homologous chromosomes. Because of this the chromatids break at the chiasmata and exchange their parts. The opposite pulling of the homologous chromosomes here does not take them away from each other but they remain nearby.

الحازمي : إيمان إعداد

Metaphase-I• At this moment the nuclear membrane starts

disappearing. The spindle fibers developing from the centrosome and get attached to the two chromosomes of each homologous pair opposite ends. As a result the chromosomes appear to be arranged in two lines on equatorial plane of the nuclear spindle.

Anaphase-I•   Anaphase-I :the chromosomes of each

homologous pair are pulled towards the opposite ends because of the contraction and shortening of spindle fibers.

Telophase-I• The nuclear spindle disappears on completion of the

contraction of the spindle fibers. A new nuclear membrane is formed and two daughter nuclei come into existence. Each daughter nuclei has half the number of Chromosomes.

 

B-MeiosisII• The same like mitosis except in the end we have

4 tetrads not identical

الحازمي : إيمان إعداد

Meiosis II

Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis. The four main steps of Meiosis II are: Prophase II, Metaphase II, Anaphase II, and Telophase II .In prophase II we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division .In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.

This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids are now called sister chromosomes as they move toward opposing poles , while all chromatids are Chromsome stand-alone The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells.

• http://www.youtube.com/watch?v=D1_-mQS_FZ0

• http://www.youtube.com/watch?v=35ncSrJOwME

1- add the anthers in to the tube .

2- Fill the tube about 2-3ml with 1N HCL and cover the tube by gauze then tie it with rubber

3- Place the tube in 60 C water bath and incubate for 10min.

4- Remove the tube from water bath

5- remove the HCL from the test tube and rinse the anther in the water about 3 times

6- cover the root with the aceto carmine stain and incubate the root in the stain for 12 min.

7- Transfer anther to the slide and remove outer cortex .

8-Cover the root tip by a cover slip .

Pollen grains squash method