Prophase in Mitosis and Meiosis: An Overview

Prophase Definition

In both mitosis and meiosis, the prophase follows the interphase and is generally the longest and first phase of the cell cycle. The chromosomes become visible during the period of DNA unwinding and chromatin condensation.

What Happens in Prophase?

• The DNA that was created during the interphase phase is divided during the prophase to create two daughter cells that are identical to one another.
• During interphase, DNA and proteins combine to form chromatin, which then condenses.
• Condensation causes the chromatin to coil and become dense, generating the observable chromosomes.
• One structured bit of DNA is used to create each chromosome.
• The chromosomes are organized into sister chromatids, which form an X-shape.
• The sister chromatids are connected at a location known as a centromere by pairs of identical DNA copies.
• At the cell’s opposing ends, the formation of mitotic spindles begins.
• They are composed of microtubules, which are long proteins.
• Sister chromatids are divided into two cells by the mitotic spindles.

Prophase in Mitosis

The chromatins wrap and transform into In animal cells, prophase is the initial step of mitosis, while in plant cells it is the second, compact during condensation.

Condensation of Chromosomes

• Condensation of the DNA created during interphase is a key component of this process.
• While in interphase, DNA replication is enabled and DNA molecules are regularly intertwined for easy removal during mitosis.
• Condensation of chromatin into discrete rod-shaped structures called chromatins is referred to as chromatosome condensation.
• Chromatid resolution, a technique that separates identical sister chromatids, is made possible by condensation.
• Condensation creates strong, elastic chromosomes to guard against breakage and damage brought on by the cytoplasmic drags and pulling forces encountered during mitosis.
• Condensin complexes, which contain condensins and topoisomerase, are used to help condense chromosomes.
• Complex proteins called condensins are essential for the cell cycle, namely for chromosomal separation, condensation, chromosome structural integrity maintenance, and resolution of DNA topography during interweaving.
• Topoisomerase, and more especially topoisomerase 2, aids in the cutting and release of the DNA duplex as well as the relaxing and unlinking of supercoils generated during interphase.
• Two sister chromatids that are X-shaped and linked at a location known as a centromere are created by the process of chromosome condensation.

Movement of Centrosomes

• Microtubules at the centromere carry out their job of attracting tubulins.
• Microtubules are added to the nucleus by tubulins in order to function.
• The centromeres from the interphase that are duplicated migrate in the direction of the cell’s opposing poles.
• Motor proteins connected to the centromere are what move things.
• The spindle development and centromere movement during chromatin separation are monitored by the motor proteins associated with the centromeres.
• For movement along the microtubules, the motor proteins transform the chemical energy in the form of ATP into mechanical energy.
• The centromeres are able to shift to opposing poles as a result.

Formation of Mitotic Spindles – Mitotic Spindles are Fibrous

• Centrosomes that have been duplicated are held during separation by the interphase microtubules.
• The radial microtubules called Kinetochore, which is present on every centrosome, help the centrosomes travel to the opposing poles of the cell as they separate.
• Additionally, the interpolar microtubules on both centrosomes work together with the other sets of microtubules to create the fundamental framework of the mitotic spindles.
• The chromosomes nucleate the assembly of microtubules into the mitotic apparatus in cells without centrioles.
• The formation of mitotic spindles in plants is a bit different.
• At the foci, the microtubules begin to create the spindle apparatus by congregating at the cell’s opposing poles.
• As the cell cycle progresses, the sister chromatids are eventually separated by the mitotic spindles.
• Mitotic spindles often include hundreds to thousands of fibrous microtubules and microtubule-associated proteins (MTP) that are engaged in the nucleation, mobility, dynamics, and cross-linking of microtubule networks.

Beginning of Nucleoli Breakdown

• A portion of the nucleus where ribosome synthesis occurs vanishes during the late prophase (prometaphase).
• This is a sign that the nucleus is beginning to disintegrate.
• The cell then devotes its energy from metabolism to the processes involved in cell division.
• The ultimate chromosomal condensation occurs at this point, making them very compact.
• The chromosomes are released when the nuclear membrane disintegrates as well.
• Additionally, the mitotic spindles continue to expand, and some interpolar microtubules begin to seize the chromosomes.

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Prophase in Meiosis

• Because it involves the separation of chromosomes over the course of two cycles, meiosis is a longer process than mitosis.
• The prophase, which occurs in two phases, prophase I and prophase II, is what makes the process lengthier.
• With the pairing of homologous chromosomes and the exchange of genetic material, prophase I is highly complicated. It explains how mitosis and meiosis vary from one another.
• The mitotic prophase and prophase II are quite comparable.

Prophase I

This stage involves DNA exchange to create recombinant chromosomes and homologous chromosomal pairing. Prophase I is composed of five stages.

These include:

Leptotene: The shortest phase of prophase I is leptotene, which is the prophase’s initial stage.

• It is a replicated chromosome’s condensation phase.
• The chromosomes compress, become visible, and condense, allowing for the separation of sister chromatids.
• The beads on the chromosomes’ strings, which resemble strings of beads, are called chromomeres.
• The nuclear envelopes are connected to each of the sister chromatids.

Zygotene: Zygonema is another name for zygotene.

• At this stage, a process known as synapsis occurs, in which homologous chromosomes form close associations to produce pairs of chromosomes. Four chromatids make up each pair of chromosomes (tetrads).
• The synaptonemal complex, which has a zipper-like structure made by the chromatids, is generated when the synaptic connection occurs up and down the chromosome, providing multiple sites of contact.
• The synaptonemal complex holds the aligned chromosomes together to promote synapsis.
• The four chromatids that result from the synapsis of the homologous pairs are known as tetrads or bivalents (two pairs).

Pachytene: This stage occurs when genetic material is exchanged between pairs of homologous chromosomes or non-sister chromatids.

• This results in chiasmata.
• By synapsis, the connection made by the chromatid in a homologous chromosome, a chromosome can cross across.
• The interchange between the genetic components of the mother and father is completed by the synapses, resulting in a variance in the genetic materials.
• The homologous chromosomes stay linked despite the separation of sister chromatids, leading to the formation of the synaptonemal complex, a dense complex.
• The synaptonemal complex facilitates the development of chiasma, which works by enabling allele crossover within discrete chromosomal areas.

Diplotene: At this stage, the synaptonemal complexes vanish, but the homologous pairs are still firmly linked to one another at the chiasmata.

• The two chromosomal arms repel one another as a result of the synaptonemal complex’s breakdown between them.
• As a result, the chromosomes drift apart from one another while being kept together by the chiasmata.
• The chromosomes then begin to gently uncoil, and under a microscope, the chiasmata can be observed advancing toward the ends of the chromatids. The entire process is termed “terminalization”.

Diakinesis: Diakinesis is prophase I’s fifth and last phase. The cell is prepared for metaphase.

• It happens after the sister chromatids have formed a bivalent tetrad under a microscope and the chromatids have condensed.
• The terminalization of the chromosomes is completed when the chiasmata reach the end of the chromatid arms.
• More chromosomes are condensed, but they are still joined by chiasmata and are unable to migrate closer to the poles.
• The nucleolus and nuclear envelope disintegrate at this point, allowing the surviving mitotic spindles and centrioles—the microtubules that form the centrosome and make up the mitotic spindle—to move freely.

Prophase II

• Chromosome condensation and nuclear envelope breakdown occur during this phase.
• Additionally, spindles develop in between the centrosomes when they separate.
• Chromosomes are first captured by these spindles, which are essentially microtubules.
• The difference between this prophase and that of mitosis is that prophase II occurs when the number of chromosomes is haploid, whereas the mitotic prophase occurs when the number of chromosomes is diploid.
• Telophase I is when the chromosomes start to decondense.

References

• Phases of mitosis
• Mitosis vs. Meiosis: Key Differences, Chart and Venn Diagram