Prophase I of meiosis I : It is more complicated and prolonged as compared to the similar stag of mitosis. It may be divided into the five sub- phases
(i) Leptotene or leptonema:
(a) It is the first stage of meiotic division following the interphase.
(b) The chromatin fibres of interphase nucleus gets shorten and elongated, chromosomes become clear.
(ii) Zygotene or zygonema:
(a) The homologous chromosomes come to lie side by side and get attached laterally due to the development of nucleoprotein between them. This pairing of homologous chromosomes is called synapsis or syndesis.
(b) On account of synapsis, chromosomes form pairs or bivalent. The number of bivalents is half number of the chromosomes.
(iii) Pachytene or pachynema:
(a) The paired chromosomes or bivalent become short and thick. The chromatids of each paired chromosome slightly separate and become visible. Thus, each bivalent or chromosome pair is made up of actually four chromatids, two of each chromosome.
(b) Each group of four chromatids, is referred as tetrad. During pachytene, exchange of the corresponding segments of non-sister chromatids of homologous chromosomes, occurs. This process is called crossing over.
(c) After crossing over, the two chromatids of a chromosome become dissimilar.
(iv) Diplotene or Diplonema :
(a) The nucleoprotein fusion complex of the synapsed chromosomes dissolves.
(b) The homologous chromosomes tart separating except in the region of crossing over.
(c) The points of attachment between the homologous chromosomes are called chiasmata.
(v) Diakinesis:
(a) This is marked by terminalisation of chiasmata.
(b) During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.
(c) The nucleolus/ nucleoli disappear and Nuclear envelope breaks down into vesicles.
Fig. Prophase I of meiosis I
