Prophase 1

Prophase 1 is the first phase (Substage) of Meiosis 1. This substage has five sequential phases- Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis. The most important event of the meiotic cell division is crossing over, which occurs here in Prophase 1. The substage Prophase 1 is explained step by step below. The mnemonic of the sub-stages of the Prophase 1 is provided here. Prophase 1 of meiosis 1 is different from the prophase of mitosis type of cell division.

Duplicated Homologs pair during Meiotic Prophase 1

1. The gradual juxtaposition of homologs occurs during a prolonged period called meiotic prophase (or prophase 1), which can take hours in yeasts, days in mice, and weeks in higher plants.
2. Like their mitotic counterparts, duplicated meiotic prophase chromosomes first appear as long threadlike structures in which the sister chromatids are so tightly attached together that they appear as one.
3. It is during early prophase 1 that the homologs begin to associate along their length in a process called pairing, which, in some organisms at least, begins interactions between complementary DNA sequences (called pairing sites) in the two homologs.
4. The homologs become more closely juxtaposed as prophase progresses, generating a four-chromatid structure called bivalent.
5. In most species, homolog pairs are then locked together by homologous recombination: DNA double-strand breaks are formed at several locations in each sister chromatid, leading to large numbers in DNA recombination events between the homologs.
6. Some of these events lead to reciprocal DNA exchanges called crossovers, where the DNA of a chromatid crosses over to become continuous with the DNA of a homologous chromatid.

Homolog pairing leads up to the formation of a Synaptonemal Complex

1. The paired homologs are brought into close juxtaposition, with their structural axes (axial cores) about 400 nm apart, by a mechanism that depends in most species on the double-strand DNA breaks that occur in sister chromatids.
2. One possibility is that the large protein machine called a recombination complex. The recombination complex assembles on a double-strand break in a chromatid, binds the matching DNA sequences in the nearby homolog and helps reel in this partner.
3. This so-called presynaptic alignment of the homologs is followed by synapsis, in which the axial core of a homolog becomes tightly linked to the axial core of its partner by a closely packed array of transverse filaments to create a synaptonemal complex, which bridges the gap, now only 100 nm, between the homologs.
4. The morphological changes that occur during homolog pairing are the basis for dividing meiotic prophase into five sequential stages– Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
5. Prophase starts with leptotene, which homologs condense and pair and genetic recombination begins.
6. At Zygotene, the synaptonemal complex begins to assemble at sites where the homologs are closely associated and recombination events are occurring.
7. The assembly process is complete at Pachytene, and the homologs are synapsed along their entire lengths.
8. The Pachytene stage can persist for days or longer, until desynapsis begins at Diplotene with the disassembly of the synaptonemal complexes and the concomitant condensation and shortening of the chromosomes.
9. It is only at this stage, after the complexes have disassembled, that the individual crossover events between non-sister chromatids can be observed as inter-homolog connections called chiasmata (singular chiasma), which now play a crucial part in holding the compact homologs together, The homologs are now ready to initiate the process of segregation.

Homolog pairing and crossing-over:

1. The structure formed by two closely aligned duplicated homologs is called a bivalent.
2. As in mitosis, the sister chromatids in each homolog are tightly connected along their entire lengths, as well as their centromeres.
3. At this stage, the homologs are usually joined by a protein complex called as synaptonemal complex.
4. A later-stage bivalent in which a single crossover has occurred between non-sister chromatids.
5. It is only when the synaptonemal complex disassembles and the paired homologs separate a little at the end of prophase 1.

Schematic diagram of a synaptonemal complex:

Homolog Synapsis and desynapsis during the different stages of prophase 1:

Other related notes:

1. Cell cycle checkpoints: https://thebiologyislove.com/cell-cycle-checkpoints/
2. Useful numbers in cell culture: https://thebiologyislove.com/useful-numbers-for-cell-culture/

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