Meiosis Stages: Meiosis 1 and Meiosis 2

Meiosis Stages: Meiosis 1 and Meiosis 2

Meiosis is  reductional division which means if the cell was diploid first ,after division it will become haploid.Now you must first know the meaning of haploid(n) and diploid.(2n)

• Suppose the number of chromosomes in an organism A is 2 (1 red and 1 blue) ,it is the characteristic of that organism .

• Now organism A can have 2 sets of each of these 2 chromosomes(diploid ,2n).

• or just one set of these 2 chromosomes(haploid ,n).

• At the end of meiosis 1 (second last step) only 1 set of the 2 chromosomes is present :one blue and one red(ignoring crossing over) in both the daughter cells. So it is haploid.

• meiosis 2 is exactly like mitosis so haploid cells give rise to haploid cells.

• We will now ,try to understand each of the 2 meiosis stages ,step by step in detail.

Meiosis 1

Step 1:Prophase1

• Leptotene:

• Chromosomes differentiate from chromatin fibers.

• Each chromosome is attached by both ends to the nuclear matrix .

• The centrioles ,already duplicated ,move apart in pairs .

Zygotene

• The homologous come to lie side by side in pairs.

• The pairing of the  homologous chromosome is called synapsis.

• The two chromosomes are called bivalent .

• The chromosomes are still not visible .

• A ladder-like , organelle ,called synaptonemal complex ,develops between the synapsed homologous chromosome .

• It is thought to stabilize the paired chromosome till crossing over takes place.

Pachytene

• The chromatid of each chromosome become visible.

• The two visible chromatid of a chromosome is called a dyad.

• The group of 4 chromatids of the two chromosomes is called a tetrad.

• Crossing over(recombination)occurs during pachytene.It takes place by breakage and reunion of chromatid segments.

Diplotene

• The homologous chromosomes start separating.

• The homologous chromosomes separate except at the site of crossing over.(chiasmata)

Diakinesis

• The chiasmata disappear by sliding toward the tips of the chromosome.This process is called terminalization

   

Step 2: Metaphase 2

• The bivalents align on the equatorial plate

• The  microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.

Step 3: Anaphase 2

• The homologous chromosomes separate , while the sister chromatin remains associated to their centromere.

Step 4: Telophase 2

• The nuclear membrane and nucleolus reappear

• Cytokinesis follows and this is called as dyad of cells .

• The stage between meiosis 1 and meiosis 2 is called interkinesis.

Meiosis 2 

• meiosis 2  is exactly like mitosis.