Cell Cycle and Division | Zoology Optional Notes for UPSC PDF Download

Significance of Cell Reproduction

• All cells are formed by division of pre-existing cells
• Each new individual begins its life as a new single cell commonly the fertilized egg or zygote
• A large number of cells are being torn or killed every moment in the body of a multicellular organism through skin peelings, lining of digestive tract, old red blood corpuscles, etc.
• An injury is healed through formation of new cells by healthy cells around the area of injury
• It is a mode of multiplication in unicellular organisms. In multicellular organisms, cell reproduction is required to form propagules and gametes
• The mechanism of cell reproduction or cell division is fundamentally similar in all the organisms showing kinship and unity of life

Factors Controlling Cell Reproduction

• Minimum growth: A newly formed cell does not divide immediately. Some amount of minimum growth in cell and its component is required before a cell attains the ability to undergo division.
• Surface –Volume ratio: Increase in cell size results in decrease of surface-volume ratio. This disturbs efficiency of surface exchange required for maintaining optimum metabolism. As it reaches a critical stage the cell undergoes division.
• Nucleocytoplasmic or kernplasma ratio: Cell functions are controlled by nucleus. The size of nucleus does not change while that of cytoplasm increases during cell growth. As nucleocytoplasmic ratio decreases, the cell is stimulated to divide.
• Mitogens: they are substances or factors which bring about cell division. Cytokinin is a plant hormone which functions as mitogen. There are several mitogenic substances known in human beings. E.g. EGF (Epidermal growth Factor), PDGF (platelet-derived growth factor), and lymphokines.
• There are some agents which inhibit cell division. They are called mitotic poisons. Example, azides, cyanides, chalones, colchicines. Colchicines is obtained from atucrocus (Colchicum autumnale). It arrests cell division at metaphase due to non-formation of spindle.

Cell Cycle

• Cell cycle (Howard and Pelc, 1953) is a genetically controlled series of changes that occur in a newly formed cell by which it supplicates its contents, undergoes growth and division to form two daughter cells. It consists of two states or periods, a long non-dividing growth I-phase and a short dividing M-phase. Both have substages. I-phase represents interphase.
• The regular sequence of G1, S, G2 (interphase) and M phase (mitotic phase) is called the cell cycle.
• Interphase is called ‘resting stage,” but it is, in fact, a period of great activity. Three important processes, which are preparatory to cell division, take place during interphase. Thus it is also known as the preparatory phase.

Interphase

• G1 phase: G1 phase is also known as the first growth phase or post-mitotic gap phase. It is the longest phase of cell division. In this phase, different types of RNA (mRNA, tRNA, rRNA) and proteins are synthesized.
• S-phase: S-phase is known as the synthetic phase. In this stage, replication of DNA takes place by the synthesis of histones. As a result, each chromosome undergoes replication, producing two chromatids.
• G2-phase: G2 is also called the second growth phase or pre-mitotic gap phase. In this phase, synthesis of DNA stops and synthesis of RNAs and proteins continues. All cell organelles multiply and spindle formation takes place.
• G0-phase: The phase in which cells fail to divide further (do not undergo S-phase after G1-phase) and undergo differentiation is known as G0 phase or quiescent stage. It occurs due to the non-availability of mitogen and energy-rich compounds.

Mitosis

• Mitosis is a type of cell division in which chromosomes of parent cells are duplicated and equally distributed into two daughter nuclei. It is also known as equational division due to the equal distribution of chromosomes in daughter nuclei. It is often known as somatic cell division due to its occurrence in somatic cells.
• The process of cell division is found to be essentially the same in all living organisms, and the events are chiefly centered in the nucleus. Three types of cell divisions have been distinguished: Amitosis or direct cell division, Mitosis or indirect cell division, Meiosis or reduction division.
• Mitosis and meiosis are the two major types of cell division. The basic stage in both types of divisions is almost identical.

Amitosis

• Amitosis is a direct division characterized by the splitting of the nucleus followed by that of the cytoplasm. Nuclear division is followed by cytokinesis (division of cytoplasm).
• Amitosis was first described by Robert Remak (1855) in red blood corpuscles of chick embryos. The term was coined by Flemming (1882).
• Amitosis occurs through cleavage or constriction, for example, cartilage cell degenerate cells meganucleus of Paramecium, cells of the fetal membranes of vertebrates.

Mitosis

• Mitosis is a type of cell division in which chromosomes of parent cells are duplicated and equally distributed into two daughter nuclei. Mitosis was first observed by Strassburger in plant cells (1870) and Boveri and Flemming in animal cells (1879). The term was coined by Flemming in 1882.
• Mitosis is completed in two steps: karyokinesis and cytokinesis. Karyokinesis is the process of nuclear division, while cytokinesis is the process of cytoplasmic division.
• Karyokinesis starts with the division of the nucleus, known as prophase. It is followed by metaphase, anaphase, and telophase.

Meiosis

• Meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. Meiosis takes place in the reproductive organs and results in the formation of gametes or spores.
• There are three types of meiosis: gametic, zygotic, and sporic meiosis. Gametic meiosis occurs during the formation of gametes, while zygotic meiosis occurs in the zygote. Sporic meiosis occurs during sporogenesis, the formation of spores.
• Meiosis involves two divisions of the cell, meiosis I and meiosis II. Each division includes prophase, metaphase, anaphase, and telophase.
• Meiosis plays a significant role in the formation of gametes, introducing genetic variation through crossing over and independent assortment. It ensures stable sexual reproduction and the maintenance of genetic stability in populations.

Abnormal Cell Growth

• Uncontrolled cell division may lead to the formation of undifferentiated aggregates of cells termed tumors or neoplasms.
• Uncontrolled cell division leads to hyperplasia, hypertrophy, metaplasia, neoplasia, and He La cells.
• Hyperplasia is the increased production and growth of normal cells in a tissue or organ.
• Hypertrophy is the increase in the size of a tissue or organ brought about by the enlargement of its cells.
• Metaplasia is the conversion of normal tissue cells into an abnormal form in response to stress, injury, or infection.
• Neoplasia is the new and abnormal development of cells that may be benign or malignant. Benign growth is restricted to a particular site of the body and does not spread to different parts. Malignant growth spreads to different parts of the body and is cancerous.
• He La cells are human cancerous cells that have been maintained in culture since 1952. They are used for studying cellular processes.