Rafia khan
Assignment 1. Sir Abdul Rehman.

Meiosis: –
Definition: –
Cell division of a diploid cell into four haploid cells, which develop to produce gametes.
OR: –
A process of cell division which takes place in germ cells during gametogenesis. Division that involves first and second meiotic divisions.

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An Overview of Meiosis: –
As meiosis involve first and second meiotic divisions, each divisible into prophase, metaphase, anaphase and telophases as I and II. Prophase is long and subdivided into leptotene, zygotene, pachytene, diplotene and diakinesis. During meiosis, four chromatids belong to replicated homologous pair, and each is distributed in four daughter nuclei because of two nuclear divisions with a short intervening resting stage without DNA replication. Each daughter cell has only one member of each pair of homologous chromosomes paired during prophase I.

First Meiotic Division: –
As mitosis, meiosis is also a continuous process. Three important events take place in the initial stages of prophase I.
First: – The chromatin fibers of interphase become condensed into discrete chromosomes.
Two: – Synapsis between each homologous pair of chromosomes.

Three: – Crossing over among synapsed homologous pair.
That’s why prophase is divided into five sub stages. The DNA of chromosomes has already been duplicated during interphase.

Prophase I: –
This stage is characterized by most profound and genetically most significant modifications.

Leptotene: –
The first part of the prophase of meiosis, characterized by thread like structure of chromosomes.

Zygotene: –
The intimate pairing of homologous chromosomes during the synaptic stage (The association of homologous maternal and paternal chromosomes during meiosis is called synaptic stage)
Pachytene: –
In this stage of prophase chromosomes shorten and divide into four chromatids.

Diplotene: –
In this stage of prophase homologous pairs begin to separate and chiasmata become visible.

Diakinesis: –
Last stage of prophase in which nucleolus and nuclear envelope disappear, spindle fibers form, and the chromosomes shorten.

Crossing over is important phenomenon that occur in these stages of prophase I.

Metaphase I: –
Stage that comes after prophase and before anaphase. In this stage condensed chromosomes become aligned before being separated. The bivalents attach to spindle fibers by centromere and move towards the equatorial plate. The two centromeres of each pair of homologues appear to each other opposite on metaphase Plante. The pairing of homologous chromosomes makes metaphase I of meiosis distinct from the mitotic metaphase where no such pairing exists.

Anaphase I: –
The stage in which chromosomes separate, the chromatid moving to opposite poles of the cell. Migration of chromosomes toward the poles take place in this stage. The centromere of each homologous chromosomes moves towards opposite pole. This movement of chromosomes is called non-disjunction. The chromosomes moving towards opposite poles are the chromosomes of either maternal or paternal in origin. Some of them may have exchanged their non-sister chromatids.

Telophase I: –
In this stage the daughter chromosomes move towards opposite ends of the nuclear spindle. A nuclear membrane is formed around each set of homologous chromosomes.

Cytokinesis: –
Cytokinesis is the process in which the cytoplasm of cell divides following the division of the nucleus. The cell divides into two daughter cells. In this way two haploid cells are formed.

Second Meiotic Division: –
Meiosis II resembles mitosis. At the onset of meiosis II, the separated maternal and paternal chromosomes are already duplicated and the sister chromatids are attached to a common centromere. The two cells have haploid chromosomes. The stages of meiosis II are prophase, metaphase, anaphase and telophase II.

Prophase II: –
In prophase II the centrosomes divide into two centrioles, each of them migrates to the opposite poles and microtubules are arranged in the form of spindle at right angle to the spindle of first meiotic division. The nuclear membrane and nucleolus disappear. The chromosomes having two chromatids, become shorter and thicker.

Metaphase II: –
At metaphase II of the meiosis, the chromosomes attach to spindle fibers by centromere and align on the equatorial plate.

Anaphase II: –
At the beginning of anaphase II, only the centromere of each chromosome divides for the first-time during meiosis and the two sister chromatids become daughter chromosomes that move to opposite poles.

Telophase II: –
As the migration of chromosomes to the opposite pole’s advances, the telophase II sets in. In this nuclear envelope is reformed around each of the complex. Spindle fibers disappear and nucleolus makes its appearance.

Cytokinesis: –
The cytokinesis creates the partition in each complex and results in the formation of four haploid cells.

Note: –
Meiosis I start with diploid chromosomes per cell and ends with two cells or chromosome complexes, having haploid chromosomes.
Meiosis II ends with four cells, each containing haploid chromosomes.

Crossing Over: –
Mechanism of crossing over in Meiosis: –
The behavior of chromosomes during meiosis itself explains the mechanism of crossing over. The events start with the synapsis in the prophase I of meiosis. The maternal and paternal chromosomes of each homologous pair come close to each other and align in juxtaposition. The process is aided by heterochromatin sequences to join the homologues along their entire length. The sister chromatid of each homologue also shows intimate pairing due to the cohesion complex aided by the process called sister chromatid cohesion.

Beside the synaptonemal complex, a unique structure called recombination nodule appears later, it is closely connected to recombination process. The nodule complex contains enzymatic machinery vital to break and rejoin the chromatids of homologous chromosomes. As a result of this event, DNA segments of non-sister chromatids are interchanged, during which the crossing over between sister chromatids is actively suppressed. Each chromosome arm usually shows one or more crossovers per meiosis, irrespective of the size of chromosome. With the completion of crossing over, the synaptonemal complexes break down. However, the homologous chromosomes remain attached at some places.

The structure four chromatids do not completely separate because of the two reasons. First is because the paired homologues are held at points involving crossing over around the site of exchange and the Second is that, the two sister chromatids by DNA replication of each homologue are held together by their common respective centromere. Much later the chiasmata move to the end of chromosome.

Note: –
The cytological observation of chiasmata is called crossing over and its genetic detection is called recombination.

Basic Points about Meiosis: –
Meiosis takes place in germ cells during gametogenesis.

Chromosomes number become haploid after meiosis.

Two successive divisions of a cell occur one after the other; four cells are formed in the end.

Genetic variation is important in meiosis. It is long having 4 stages of prophase. In this stage crossing and genetic recombination take place.

Prophase, metaphase, anaphase and telophase are represented in meiotic I and meiotic II. In the first division, homologous chromosomes separate and in the second meiotic division, chromatids separate resulting four cells.

Reference: –
Book: Genetics and Genomics
Waseem Ahmad (Faridi)


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