CELL DIVISION

MEIOSIS

It is a reduced type of cell division taking place in germ cells that results in the formation of 4 haploid daughter cells or gametes.

• ANIMATED VERSION

STAGES OF MEIOSIS:

• MEIOSIS I : RESULTS IN THE FORMATION OF 2 HAPLOID CELLS
• MEIOSIS II :RESULTS IN THE FORMATION OF 4 DAUGHTER CELLS WITH HAPLOID CHROMOSOMAL NUMBER MAINTAINED.

MEIOSIS I:

Consists of 4 phases:

1. PROPHASE I
2. METAPHASE I 
3. ANAPHASE I
4. TELOPHASE I

Figure 7-18. Early primary spermatocytes of the mouse. Left to right: leptotene,
zygotene, early pachytene, midpachytene. Sudan Black squash (x1200).

PROPHASE 1:

It is the longest phase of the cycle and consists of the following substages.

• Leptotene (lepto-Greek thin , -tene -Greek ribbon)

In this first substage of prophase I, the chromosomes have appeared within the nuclear envelope but are not yet fully condensed.  Each is a thin thread of DNA along which clearly defined beads of local coiling (chromomeres) can be seen. The chromosomes, while they have this threadlike form, are called chromatonemata .The chromosomes appear single because the sister chromatids are still so tightly bound to each other that they cannot be separately seen.

• Zygotene (zygonema) (zygo- is Greek for union, fusing)

During zygotene homologs begin to unite (synapse) by coming into approximate alignment .Synapsis ,the process of fusion that occurs between homologs begins at various points along the chromosome and extends outward, zipper-fashion, until complete. When synapsis is finished, the fused homologs look like single chromosome under the light microscope, but that are actually double.
 The interface where two homologs unite is called a synaptonemal complex, which can be seen under an electron microcope.

In the  early zygotene , some  regions of paternal and maternal homologs have fused while In the  late zygotene, both homolog pairs have fused over their entire lengths .

Tetrads: Once the homolog pairs synapse they are called tetrads (each has four chromatids; tetra is Greek for four) or bivalents.Bivalent is the preferred term.

• Pachytene (pachynema). (pachy- is Greek for thick). 

During pachytene the two sister chromatids of each chromosome separate from each other. This makes the chromosomes look thicker .Homologs are still paired at this point.

Each chromosome splits into two chromatids and thus each pair will have four chromatids two paternal and two maternal. They are now called tetrads.

CROSSING OVER:
The non-sister chromatids of the paternal and maternal chromosomes overlap each other. They appear to be joined at several regions along their length. These points are called Chiasmata. Each chiasma is the site of an exchange of genetic material between the two chromatids.

A kind of localized breakage of the DNA occurs, which is followed by exchange of DNA between them. This process is called crossing over or genetic recombination .Crossing over produces "cross-over chromatids," each composed of distinct blocks of DNA, some blocks derived from the mother, others from the father.

• Diplotene OR Diplonema:

At the beginning of this stage each chromatid of each chromosome is still fused to a chromatid of that chromosome's homolog (recall that sister chromatids are already separate at this point).

As diplotene progresses, these initially fused non-sister chromatids begin to separate from each other. However, they cannot separate completely because they are still connected by two strands of DNA at each of the points where exchanges took place.At each such cross-over site, the two strands form an x-shaped structure called a chiasma (pl. chiasmata).

TERMINALIZATION:
Chiasmata then begin moving toward the ends of the chromatids. This process of sliding toward the ends is known as terminalization. 

DICTYOTENE IN OOCYTES:
In oocytes, a special, extremely prolonged form of diplotene occurs called dictyotene. The primary oocyte undergoes the first three of the substages of prophase I (leptotene, zygotene, and pachytene) during late fetal life. The process is then suspended during diplotene until puberty or thereafter. Therefore, in dictyotene (and consequently prophase I) can last months or even years, depending on the type of organism in question.

• Diakinesis

During this, the last stage of Prophase I, the nucleolus disappears, terminalization reaches completion, and the chromosomes coil tightly, and so become shorter and thicker. The nuclear envelope begins to disappear. The centrosomes reach the poles.

• QUIZ ON MEIOSIS