Replication and Division of Nuclei and Cells

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Replication and Division of Nuclei and Cells

Introduction

Cells are the basic unit of life. Recall that some organisms are constituted of only one cell while multicellular organisms have millions or even more cells. The division and replication of cell is necessary for the survival and continuity of life.

In unicellular organisms, cell division is the process by which the organism replicates. It is responsible for the making of new generation by the process of asexual reproduction. In multicellular organisms, cell division is responsible for the normal turnover of the cells. This is necessary for several other functions in the body.

Before the cells divide, the genetic material within the cells must also be replicated. This is done by the process of DNA replication and division of nuclei.

In this article, we will discuss different types of cell division, their stages, and the role they play in the life of an organism.

Types of Cell Division

Cell division is the process by which a parent cell divides to create two or more daughter cells. When we say cell division, it means the division of entire cell i.e., nucleus as well as cytoplasm.

Cell division and replication both imply to the same process. There are two types of cell division:

  • Mitosis
  • Meiosis

Majority of the cells divide by the process of mitosis. Only sex cells undergo division by the process of meiosis.

In between the cycles of cell division, a cell goes through a phase of no division called interphase. During interphase, cells prepare themselves for replication. It should be kept in mind that it occurs between two consecutive mitotic cell divisions. The daughter cells made as a result of meiosis do not undergo further division and thus, there is no interphase.

We will first talk about interphase and then the two types of cell division.

Interphase

As mentioned earlier, it is a non-dividing phase between two consecutive cell divisions. The cell prepares itself for the replication process during the stage of interphase. The processes like replication of DNA and chromosomes that are crucial for cell division are carried out during interphase.

Interphase comprises of the following stages:

  • G1 phase
  • S phase
  • G2 phase

A brief detail of these stages is mentioned below.

G1 Phase

This is the first stage in interphase as well as life cycle of a cell. As soon as a daughter cell is produced by the process of mitosis, it enters the G1 phase. It is also called first growth phase.

During this phase, a cell grows in size, mRNA molecules are made, and proteins are produced for the division of cell. All these processes are essential for the subsequent stages of interphase or cell cycle.

A daughter cell is produced from its parent cells by the division of nucleus and cytoplasm. As a result, the cell is much smaller in size than the parent cell. As soon as the cell is produced, it starts making new organelles as well as cytoplasmic components. This results in considerable increase in size of the cell.

Some important enzymes and other proteins are necessary for the survival, growth and later on, division of the daughter cell. The information for the synthesis of these proteins is encoded within the genes present in the DNA. This information is copied in the form of mRNA for the synthesis of proteins. The mRNA copies thus made are then sent to the cytoplasm via the nuclear pores.

The information encoded in mRNA molecules is translated by ribosomes to make new proteins including enzymes. One mRNA molecule is specific for one single protein. The sequence of nucleotides in the mRNA molecule determines the sequence of amino acids in the protein.

S Phase

It is the second stage of interphase also called the synthetic phase. During this phase, the nuclear material within the cell is replicated to make two copies of DNA. This is essential for the replication of nucleus as well as cell. It ensures that each daughter cell will receive the same amount of DNA as present in the parent cell.

The nuclear material is duplicated by the process of DNA replication. Multiple enzymes are involved in this process such as DNA polymerase, primase, helicase, etc. The replication of DNA takes place via the semi-conservative approach. It makes sure that each daughter DNA molecule has one parent (old) strand and one newly synthesized daughter strand.

This phase also involves the replication of structures associated with DNA. New histone proteins are also made. Thus, the entire genetic material is replicated. At the end of S phase, a cell has double the number of chromosomes as were present in the beginning of interphase.

G2 Phase

It is the last stage in the interphase of cell cycle called the second growth phase. It is very much similar to the G1 stage we have discussed earlier. The cell further increases in size, new proteins are made, etc. One important step that occurs during the G2 phase is DNA repair.

The proteins made in G1 phase were mainly the ones involved in the replication of DNA. However, in the G2 phase, proteins, and organelles necessary for the cell division are synthesized.

Recall that DNA has already been replicated in the S phase. If there has been any error in the process of DNA replication, the defect is removed by the DNA repair enzymes. It specially involves the double strand breaks that are repaired by recombination techniques.

At the end of G2 phase, a cell is fully grown and prepared to undergo cell division and make new daughter cells. It may undergo nuclear and cell division by mitosis or meiosis.

G0 Phase

Cells that are not destined to undergo any type of cell division enter the G0 phase after the first growth phase. It is a silent phase during which no process takes place to prepare the cell for replication process.

Cells continue to grow and undergo normal metabolic processes during the G0 phase. They are fully active and perform all the functions assigned to them.

Some cells permanently remain in the G0 phase and never undergo replication while others can re-enter the normal replicative cell cycle upon the availability of proper signals.

Mitosis

Mitosis is the most common type of cell division we usually talk about. It is the process by which a parent cell divided to form two daughter cells. The nuclear material in both the parent and the daughter cell remains the same. The diploid (2n) parent cells give rise to diploid (2n) daughter cells. It means that both the parent cell and the daughter cells have two copies of each chromosomes.

Mitosis takes place in all the somatic cells. All the cells in the body of an organism are somatic cells except the gametes or sex cells.

Mitosis is mainly a type of division of nucleus. The division of cytoplasm completes the process of cell division that is common to both meiosis and mitosis.

The process of mitosis is divided into the following four phases as mentioned below.

Prophase

It is the first phase in the process of mitosis. It is like a preparatory phase during which the mitotic apparatus is arranged for the division of nucleus. The important processes that occur during this phase are as follows.

Recall that during the various stages of interphase, cell was actively producing proteins. The genetic material in the cell was not tightly packed so that the gene transcription can be promoted. The nuclear material in the cell was in the form of loosely packed chromatin fibres. During prophase, rearrangement of chromatin material takes place. The chromatin material is tightly packed into chromosomes which stops the process of gene transcription. As a result, chromosomes start appearing within the cell. They can be clearly seen under the microscope as fine threads. Chromosomes are only visible in the cell during the process of mitosis. The behavior of chromosome during mitosis can be easily seen under the microscope. They are not visible during the interphase.

With the appearance of chromosomes, the nucleolus disappears. The nuclear envelope also starts to dismantle. As the chromosomes become more and more apparent, nuclear completely disappears at the end of prophase. No nucleus can be distinguished in such a cell.

In plant cells, nucleus is pushed to one side of the cell due to the presence of a large central vacuole. The nucleus is brought to the center of the cell before the beginning of prophase. This is sometimes referred to as pre-prophase.

Centrosomes

These are the organelles that form mitotic spindle fibres during the process of mitosis. Each mature cell contains a single centrosome that is present close to the nucleus. However, centrosomes are duplicated during the interphase so that each cell has two centrosomes at the beginning of mitosis.

During prophase, centrosomes start making mitotic apparatus within the cell. They do so by polymerizing the tubulin protein monomers that were synthesized during the S phase.

As the mitotic spindles are made between the two centrosomes, the centrosomes get pushed to the opposite poles. Thus, the mitotic apparatus spans the cell between these two centrosomes.

At the end of prophase, instead of having a nucleus in the center, a cell contains chromosomes as well as mitotic apparatus in between the centrosomes.

Pro-metaphase

It is an overlapping step between prophase and metaphase during mitosis. During this phase, the nuclear membrane disintegrates completely. Centrosomes have reached the opposite poles of the cell. The spindle fibres get attached to the chromosomes at their centromeres. These centromeres contain special kinetochore proteins that favour the attachment of spindle fibres. Thus, at the end of this phase, no chromosome is free but is attached to the mitotic spindle fibres.

Metaphase

Recall that the spindle fibres have already become attached to the chromosomes. The spindle fibres contract and pull the chromosomes towards the opposite poles. As a result of this tension, chromosomes get arranged in a line along the metaphase plate or equitorial plate of the cell. It is an imaginary line that passes through the center of the cell, dividning it into two halves.

Metaphase check point makes sure that the chromosomes get arranged in the center of the cell because it is necessary for the further steps of mitosis.

Anaphase

The tension in the mitotic spindles further increases. It becomes much greater than the cohesive forces hodling the two chromatids in the chromosome. As a result, the sister chromatids get seperated.

The sister chromatids get pulled towards the oppostide poles of the cell. At the end of anaphase, the divison of chromomes have completed. Equal numbers of chromatids are present at the opposite poles of the cell.

Another important event that occurs during anaphase is the lenghtening of the cell. It occurs due to the elongation of mitotic spindle fibres.

Telophase

It is the last phase in mitosis and is opposite of prophase. During telophase, new nuclear envelope forms around the sister chromatids present at the opposite poles of the cell. The cell also elongates further due to elongation of the spindle fibres.

At the end of telophase, one daughter nucleus can be seen at each pole of the cell. One centrosome is also present in the neighbourhood of each daughter nucleus.

Finally, the mitotic apparatus starts to disintegrate as the process of mitosis has been completed.

The next step is the division of cytoplasm that occurs by cytokinesis.

Cytokinesis

The division of cytoplasm occurs once the daughter nuclei has been formed and are present at the opposite poles of the cell.

Cytokinesis in animal cells occurs by the formation of a cleavage furrow at the site of equatorial plate. The furrow pinches inwards and divided the parent cell into two.

Cytokinesis in plant cells takes place by the formation of a cell plate in the centre of the cell. A new cell wall is later made along this cell plate, dividing the parent cell into two daughter cells.

Meiosis

Meiosis is a type of cell division seen only in organisms that undergo sexual reproduction. In such organisms, gametes are formed by the process of meiosis. For example, in humans, sperm in males and egg in females are made by the process of meiosis. Cells in the body other than the gametes do not undergo meiosis.

Meiosis consists of two rounds of division that results in formation of four haploid daughter cells from single diploid parent cell. The two rounds of division involve:

  • Meiosis I
  • Meiosis II

Both these phases further consist of four steps. A brief detail of this type of cell division is mentioned below.

Meiosis I

It is the first round of division. Before the beginning of meiosis I, chromosomes have been duplicated in the cell and are present in the form of homologous pairs. It involves 4 stages.

Prophase I

It is the first and the longest phase in the process of meiosis. It involves disintegration of nuclear envelope so that the chromosomes become apparent. The genetic exchange between the chromosomes also takes place during this stage. It is further divided into five sub-phases.

  • Leptotene: Chromosomes start appearing within the cell as thin threads.
  • Zygotene: Homologous chromosomes come close to each other to form pairs in a process called synapsis.
  • Pachytene: The pairing process started in zygotene complete during this phase. As a result, they are apparent as a tetrad chromosome having four chromatids. Recombination of chromosomes also takes place. The non-sister chromatids in each homologous pair exchange some fragments in a process called crossing over. This process is responsible for genetic variations.
  • Diplotene: The homologous chromosomes undergo uncoiling during this phase. The tetrad is not broken during the process of uncoiling.
  • Diakinesis: The chromosomes become further condensed, nuclear envelope disentangles, and the nucleoli disappear during this stage. The mitotic apparatus also starts forming.

Metaphase I

The mitotic spindles get attached to the tetrad chromosome at the kinetochores. A tension develops in the fibres that causes the arrangement of tetrad structure along the metaphase plate.

Anaphase I

It is the phase during which the tetrad structure formed during the prophase I breaks. The kinetochore spindle fibres start shortening, pulling the individual chromosomes towards the opposite poles.

As a result, the chromosomes that contain sister chromatids with crossed over fragments get pulled towards the opposite poles. The sister chromatids do not get separated but the pair of homologous chromosomes breaks.

Another important event during this phase is the elongation of the cell. This again occurs due to elongation of spindle fibres.

Telophase I

It is the final stage of first division. The mitotic apparatus disappears, and a new nuclear envelope is formed around the chromosomes present at the opposite poles of the cell. Thus, two daughter nuclei are present in the cell by the end of telophase I.

The daughter nuclei thus formed are haploid as they contain only one copy of each chromosome. Each nucleus has 23 chromosomes, each having two sister chromatids.

Once the nuclear division is completed, cytoplasm divides by the process of cytokinesis that has already been described. Thus, two daughter cells are formed. Each of the daughter cell thus formed undergoes meiosis II without any intervening interphase.

Meiosis II

It is the second division that occurs during meiosis. Daughter cells formed as a result of meiosis I undergo meiosis II. At the end of meiosis II, four daughter cells are present.

Meiosis II is much similar to the process of mitosis. The only difference is that no interphase occurs prior to meiosis II.

Following are the different phases of meiosis II.

Prophase II

Just like the prophase of mitosis, cells during meiosis II undergo degeneration of nuclear envelope, and the chromosomes become apparent. Centrosomes also duplicate and move to the opposite poles during this phase.

Metaphase II

The chromosome become aligned along the metaphase plate in the centre of the cell.

Anaphase II

It involves the separation of the sister chromatids and elongation of the cell. By the end of anaphase II, sister chromatids are present at the opposite poles of the cell.

Telophase II

It is simply the reverse of prophase II. The nuclear envelope is formed around the sister chromatids so that two daughter nuclei are formed within each cell towards the end of telophase II.

Cytokinesis

The daughter cells are formed after the division of cytoplasm by the process of cytokinesis.

In humans, the formation of sperms in males and eggs in females is an example of meiosis.

Summary

  • Replication and division of the cells is essential for the survival and continuity of life. It is not only necessary for the synthesis of new generation but also for making new cells in multicellular organisms to perform multiple functions.
  • Replication of DNA takes place during the interphase that prepares the cell for division. DNA replication occurs during the S phase. Other stages of interphase are necessary for the synthesis of enzymes, proteins, and growth of the cell.
  • Mitosis is a type of cell division seen in somatic cells. It involves for stages.
  • During prophase, the nucleus is replaced by chromosomes and mitotic apparatus.
  • During metaphase, the chromosomes are aligned along the metaphase plate.
  • Anaphase involves the separation of sister chromatids.
  • Telophase involves the formation of nuclear envelope around sister chromatids so that two daughter nuclei are formed.
  • The division or replication is completed after the division of cytoplasm by cytokinesis.
  • Meiosis takes place only in the gametes or sex cells. It involves two sets of divisions. Meiosis II is similar to mitosis in many terms. Meiosis I is however different.
  • Prophase I is the longest phase of meiosis. It involves formation of tetrad structure as well as crossing over of the non-sister chromatids. The nuclear envelope also disappears.
  • The rest of the phases are similar to mitosis. The only difference is that instead of separation of the sister chromatids, chromosomes get separated during meiosis I.

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