Prophase

Cell division is a vital process that occurs in all living organisms. DNA replication and growth of cells take place during the process of cell division. The complete process of cell division occurs cyclically and in a very coordinated way to ensure that every cell division occurs correctly. 

The cell cycle process involves a cell division wherein the genome of the cell duplicates, various other constituents of the cells are formed, and, eventually, divide into two daughter cells. During each stage of the cell cycle there occur different kinds of processes such as DNA synthesis, chromosome replication, etc. Similarly, there are different stages of events in the cell division process. 

Cell cycle and cell division

There are two basic phases of a cell cycle, namely, the interphase and the M phase (mitosis phase). The actual cell division or mitosis process occurs during the M phase and the phase between the two successive M phases is termed as the interphase (resting phase). In the M phase, two processes occur: karyokinesis and cytokinesis. 

Karyokinesis is the process in which nuclear division and separation of daughter chromosomes occur. Cytokinesis is where the division of the cytoplasm occurs. The interphase involves three phases:

• G1 (Gap 1) phase: Initiation of DNA replication
• S (Synthesis) phase: DNA synthesis and replication occurs
• G2 (Gap 2) phase: Protein synthesis and cell growth occurs

M phase is the most important phase in the cell division process during which almost all the components of a cell are reorganised. The M phase is divided into four stages: prophase, metaphase, anaphase, telophase.

What is prophase?

The first stage in the mitotic as well as in the meiosis cell division process is prophase. It is a reduction division process. Some of the most remarkable processes which occur during prophase are the condensation of chromosomes, chromosomes become visible, and nuclear envelope disappears. All these features are visible in the prophase diagram.

Prophase is marked by the following events of cell division:

1. Chromosome condensation
2. Disappearance of nucleolus

During prophase, the chromosomes start to become condensed as can be viewed in the prophase diagram below. It is known that the chromosomes consist of two sister chromatids which are attached together at the centre called the centromere. When chromosomes condense during the prophase, the nucleolus starts to disappear. 

During this process, the centromere also divides into two parts and each half contains a pair of centrioles composed of γ-tubulin rings (MTOC, that is, the microtubule organising centre of the cell). Then the centrosomes move away from each other towards the opposite poles of the cells and where each forms a new MTOC and gives rise to mitotic spindles.

Mitotic prophase

During prophase, the initiation of the condensation of chromosomal material occurs. New DNA molecules are synthesised in tangled form and the nucleolus disappears. The said DNA molecules get untangled during the process of chromatin condensation. During the synthesis phase of the interphase, the centrioles are duplicated and these centrioles move towards the opposite poles of the cell during prophase. Prophase occurs after the gap 2 phase of the interphase; that is the time when DNA synthesis and DNA replication have already taken place.

Characteristics of prophase                                             

From the prophase diagram, it can be seen that chromosome condensation occurs during prophase. In the chromosomes, two chromatids attached at the centromere can be seen in the prophase diagram.

Mitotic spindles initiate to get assembled and thus mitotic spindles are formed. During prophase, movement of the centrosomes occurs. The process of the movement of the centrosomes is powered by the effect of centrosome-associated motor proteins and various other proteinaceous components of the cell cytoplasm and microtubules.

Meiotic prophase

Meiotic prophase occurs in two stages, namely, prophase I (the most complex phase as during this phase, the pairing of homologous chromosomes occurs along with the exchange of genetic material) and prophase II (this is similar to the mitotic prophase). The prophase diagram shows the different stages involved in prophase I. The different stages involved in prophase-I are leptotene, zygotene, pachytene, diplotene, and diakinesis.

• Leptotene: Chromosome condensation begins and it marks the start of the prophase (visible in the prophase diagram).
• Zygotene: Pairing of homologous chromosome
• Pachytene: DNA coiling and DNA condensation, that is, the synapsis process is completed; DNA recombination or crossing-over (exchange of chromatid material)
• Diplotene: Homologous chromosomes begin to pull apart in chiasmata
• Diakinesis: Condensed chromosomes attain a maximum condensed state

Conclusion

Cell division is a vital process in all living organisms for cell growth. Prophase is marked as the most important first stage of both mitotic and meiotic cell division. Chromosome condensation and nucleolus disappearance are the two important features of the prophase which can be viewed in the prophase diagram. 

The chromosomes which are shortened and thickened during the interphase start to condense during prophase. Each chromosome consists of two sister chromatids (product of DNA replication). During prophase, these chromatids, which are joined by centriole, start to separate and move towards the opposite poles of the cell. 

During this process, the mitotic spindles are formed and they become visible to form microtubule asters which form spindle poles or mitotic centres. The nucleolus starts to disappear and the nuclear envelope starts to disintegrate to release the chromosomes. The disintegration of the chromosomes marks the end of the prophase stage.