Cell Division and Growth

It is one of the fundamental properties of all living organisms to divide and reproduce. A single cell is converted to multicellular organisms by the process of division. This process of division is really important in the life of all living organisms because only their cells will and also new cells will be formed because all living organisms or cells have a defined life span. The cells divide by the process of cell division and cell growth which is accompanied by the cell cycle. It is defined as the event when the genome of the organism starts to duplicate and new daughter cells are formed from this process. When the cell enters the cell cycle, it undergoes various changes. 

Changes in the activity of cells, synthesis of enzymes and chemicals responsible for the division of the cell. The DNA and also the cell organelles start to duplicate. This process helps in ensuring the smooth run of the process of cell division. This divided daughter cell matures and undergoes changes and when it will be capable, this will also undergo cell division and reproduction. This gives us a rough understanding of what is cell division. 

Phases of Cell Cycle

Interphase and the mitotic phase are the two phases of the cell cycle. As the name suggests, the interphase is a phase between two mitotic divisions. Here, the cell undergoes changes, not in terms of division but the content of cell organelles and the formation of chemicals. This phase is present in almost 95% of the division of cells. The DNA or the genetic material starts to replicate in this phase so due to this, it is known as the biosynthetic phase of the cell cycle. The mitotic or the M phase is the phase of the actual division of cells. The duration of cell division can vary from organism to organism. The materials which were formed in the interphase are used up in the mitotic phase for the division of cells. We will study the interphase and mitotic phase and about the process of cell division and more about what is cell division in detail below. 

Interphase

As we read above, the interphase is the longest phase of the cell cycle and comprises almost 95% of the cell cycle. This phase is also known as the resting phase because no activity of the division of cells takes place in this phase. But, that does not mean this phase is not important. This phase is of equal importance as that of the mitotic phase because the cell prepares itself for the process of cell division in this phase. The biomolecules and the chemicals that will be required in the process of cell division are synthesized in the interphase process. The interphase is divided into three phases that are G1 phase, the S phase and the G2 phase. 

• G1 Phase: This phase is present between the DNA replication phase and the mitosis phase. No synthesis or replication of DNA takes place in this phase but the cell is very much metabolically active. ATP, Proteins, amino acids are synthesized in this phase and the cell grows in size. As we know that some cells such as the nerve cells in our body do not divide and these cells do not enter the G1 phase but instead they enter a G0 phase which is a non-dividing phase. The cells remain in this phase and can undergo division if it is required at any point in life. 

• S Phase: As the name suggests this phase is known as the synthesis phase. All the materials that are needed for the process of cell division are synthesized in this phase. The amount of nucleic acids, DNA starts to double itself but no doubling of the chromosomes takes place in this phase. The division of chromosomes takes place in the mitotic phase. 

• G2 Phase: This is the second gap phase and this phase is present between the S phase and the M phase. In this phase, only some selected cell organelles divide. These are Mitochondria, chloroplasts and Golgi bodies. It is one of the smallest phases in the interphase. 

M Phase

As we read above, this phase is known as the M phase and this is the actual phase of cell division. The chromosome number starts to double in this phase. Mitosis and meiosis are the two types of division that takes place in a cell. But before that, we will understand cytokinesis and karyokinesis. Cytokinesis is the process by which the division of the cytoplasm takes place. This phase is not divided into sub-phases. In karyokinesis, division of the nucleus takes place and it is divided into 4 sub-phases that are the prophase, metaphase, anaphase and telophase. Now we will study mitosis that is important for cell division and growth. 

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Mitosis

It is a type of cell division and growth and it is known as equational division. In this division, the daughter cells are formed from the parent cell. The chromosomes number is not reduced and they are present as it is in the daughter cells. Strasburger noticed and reported it for the first time in a plant cell. This division takes place in the somatic cells. The body cells are known as somatic cells. The nucleus is divided by the process of karyokinesis and as we read above karyokinesis is further divided into 4 stages that are the prophase, metaphase, anaphase and telophase. We will study all these 4 stages one by one. 

Prophase

This is the first stage in the cell division process. The stage is marked by the condensation of the chromatin material. The nuclear membrane also starts to disintegrate and the chromosomes start to untangle themselves. The centromere helps to join the two sister chromatids together. This is also called a spireme stage because here the chromosomes start to appear like a ball of wool. The centromere that is present in the cell starts to move towards opposite poles. Also, we can see the mitotic spindle fibres. The Golgi complex, the endoplasmic reticulum and the nuclear membrane start to disappear at the start of prophase. 

Metaphase

This is the second phase in the division of the cell cycle after the prophase. When this phase starts, the nuclear membrane starts to completely disintegrate from the cell. The chromosomes are spread throughout the cell in this phase. There are two chromatids present in the chromosomes and they are seen attached to the centromere. The main attraction of this phase is that the chromatids start to align at the equator of the cell. This phenomenon when the chromatids are brought at the centre of the cell or the equator is known as congression. 

As we read above the centromere is the point where the two chromatids are joined together. Then from the centrioles that are present at the opposite poles, spindle fibres start to attach to the chromatids that are present at the equator or metaphase plate. These spindle fibres are made up of microtubules and they serve the purpose of attachment. 

Anaphase

Ana means up and this phase marks the upward movement of the chromatids. As we read above that spindle fibres help in their attachment to the centrioles and these centrioles then in the anaphase pull these chromatids upwards to their respective poles. When the chromatids are undergoing the movement to their opposite poles then they form some interesting shapes and these shapes can be studied in the study of chromatids and chromosomes. The spindle fibres that are attached to the chromatids then start to shorten up and this results in the movement of the chromatids to their opposite poles. 

Telophase

Telo means end and this is the end phase of the cell division. When the chromatids have reached their respective poles, now they start to become chromosomes. The nuclear membrane starts to reappear and the cell organelles also. Around the chromatin cluster, the nuclear envelope starts to assemble. The chromosomes then start to become thin, slender and start uncoiling. 

Meiosis

Meiosis is known as the reductional division. This means that the number of chromosomes is reduced to half. This division takes place in the sex cells or sex gametes. DNA is replicated only once but the process of karyokinesis occurs twice which means that the division of the nucleus happens twice. The prophase-1 is the longest phase and it is divided into 5 substges that are leptotene, zygotene, pachytene, diplotene and diakinesis. With the help of the synaptonemal complex,  the homologous chromosomes are paired together. Initially, the chromosomes appear like a single thread. Two parallel plates that are metaphase-1 plate and metaphase-2 plate are formed. Chromosomes numbers are reduced to half after the end of telophase-1. This meiosis helps in maintaining a constant number of chromosomes from one generation to the other generation. 

Significance of Meiosis

The process of meiosis holds much importance in the process of cell division. Some of the important characteristics are:

• Formation of Gametes: For the sexual reproduction process, meiosis is helpful as it helps in producing the gametes. These gametes that are produced with the help of meiosis hold much importance in sexual reproduction. The number of chromosomes is reduced to half and then at the time of fusion of gametes, the number is restored. 

• Introduction of Variants: As in the process of meiosis, recombination takes place and this helps in the exchange of genetic material. This exchange of genetic material helps in the formation of new variants every time. 

• Maintenance of Chromosome Number: As the chromosome number is reduced to half at the time of formation of gametes with the help of meiosis and then they are reduced to their actual amount when the sexual reproduction takes place. 

Regulation of Cell Cycle

For the smooth functioning of the cell cycle, there are various regulations that take place in the cell cycle. The decision that cells take to whether to go in the process of cell division takes place in the G1 phase. If the cell wants to divide then it will enter the G1 phase and if the cell does not want to divide then it enters the G0 phase. This is known as the quiescent stage of the cell cycle. In this, the cell does not divide and remain as it is. They can divide when they want to and do so by exiting the G0 phase. 

G1 to S phase is the first checkpoint and S to G2 is the second checkpoint in the cell cycle. Cyclin-dependent proteins are present in it that are known as the kinases and they are present for the regulation of the cell cycle. These kinases are activated by the cyclin proteins. They help in the regulation of the eukaryotic cell cycle. G1 cyclins are responsible for carrying out the G1 to S transition in the cell cycle. G2 to M transition is triggered by the help of MPF proteins that are known as maturation promoting factors. They are formed with the help of mitotic cyclins and kinases.