Cell Cycle and Cell Division

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Zygotene (Zygonema): Chromosomes become more condensed. Similar chromosomes

start pairing together (synapsis) with the help of a complex structure called synaptonemal

complex. The paired chromosomes are called homologous chromosomes. Each pair of

homologous chromosomes is called a bivalent. (NEET 2013)

Pachytene (Pachynema): Comparatively longer phase than the first two stages of

prophase-I. Bivalent chromosomes split into similar chromatids. This stage is called

tetrads. During this, recombination nodules appear at which crossing over occurs

between non-sister chromatids of a homologous chromosome pair. It leads to

genetic recombination on homologous chromosomes.

Crossing over: The exchange of genetic material between non-sister chromatids

of two homologous chromosomes in presence of an enzyme, recombinase.

(Odisha NEET 2019)

Recombination is completed by the end of pachytene leaving the chromosomes

linked at the sites of crossing over. (NEET 2014)

Diplotene (Diplonema): Dissolution of the synaptonemal complex occurs. The

recombined homologous chromosomes of the bivalent. (NEET 2020)

Separate from each other except at the sites of crossovers. These X-shaped

structures are called chiasmata. In oocytes of some vertebrates, diplotene lasts for

months or years.

Diakinesis: Terminalisation of chiasmata. Chromosomes are fully condensed. The

meiotic spindle fibres originate from the poles assembled to prepare the homologous

chromosomes for separation. Nucleolus & nuclear envelope disappear.

It represents transition to metaphase.

Metaphase I:

Spindle formation

is completed.

The bivalent

chromosomes

align on the

equatorial plate.

The microtubules

from the spindle

attach to the pair

of homologous

chromosomes.

Telophase I:

The nuclear membrane

and nucleolus reappear

and 2 haploid daughter

nuclei are formed. This

is called dyad of cells

After this, cytokinesis may

or may not occur. After

a short interphase, it is

followed by meiosis II. This

short stage between the

two meiotic divisions is

called interkinesis. DNA

replication does not occur

in this phase

Anaphase I:

The homologous

chromosomes

separate, while

sister chromatids

remain

associated

at their

centromeres.