Before studying the most important part of the cell cycle, it is important to understand the basics of a cell cycle.
Cell division is a vital process for all living organisms. During this process, cells grow, and the DNA replicates itself. The combination of cell division, replication, growth, and DNA replication occurs in a coordinated way to ensure cells’ correct formation and division to keep genomes intact.
This sequence of cell division, replication and division, from a parent cell to a daughter cell, is known as the cell cycle. The cell cycle occurs in different phases.
Different Phases of the Cell Cycle
The duration of a cell cycle depends on different organisms. In humans, cells divide almost every 24 hours, while this process can be done in only 90 minutes in yeast.
Cell division takes place in two phases:
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Interphase
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Mitosis phase or M phase
Interphase
The resting phase or interphase is when a cell is preparing for division. This is an essential phase in the cell cycle. This phase leads to an orderly growth of cells and the application of DNA.
Interphase is divided into three phases.
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G1 phase (Gap 1): This phase is the interval between the initiation of DNA replication and mitosis. Here, the cell is metabolically active and grows continuously. However, the DNA does not replicate itself.
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S phase (Synthesis): In this phase, the number of DNA per cell doubles, but the number of chromosomes remains the same, i.e. 2n.
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G2 phase (Gap 2): In this phase, the body prepares proteins for mitosis while cell growth continues.
Mitosis phase or M phase
Mitosis or mitotic division is meant for the multiplication of cells. The development of complete organisms from zygotes is using mitotic cell division. So mitosis is necessary for maintenance, growth, repair and continuity of life. Mitosis generally takes place in vegetative or somatic cells.
In this process, one parent cell divides into two daughter cells, where chromosome number remains the same as in the parent cell, i.e., daughter cells exactly resemble parent cells both quantitatively as well as qualitatively. So mitosis leads to an increase in the number of cells without any change in genetic make-up.
W. Flemming (1882) gave the term mitosis (Gr.: Mitos-thread + osis-state). However, mitosis was first observed in plants by E. Strasburger (1875) and in animals by W. Flemming (1879).
The actual mitotic phase (M-phase) is completed in two steps:
(A) Karyokinesis: i.e., division of the nucleus into two.
(B) Cytokinesis: Karyokinesis is followed by cytokinesis, i.e., division of cytoplasm into two cells or wall formation.
(A) Karyokinesis
Karyokinesis (division of nucleus) consists of the following four stages:
(i) Prophase (ii) Metaphase (iii) Anaphase (iv) Telophase
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Prophase: After interphase, prophase comes, which is the first and longest stage of mitosis. Prophase is the most active of all the stages of the M-phase. The chromosomes appear as thin, filamentous, uncoiled structures, which later on become coiled, shortened and thickened. These two sister chromatids are attached only at a point called centromere or kinetochore or primary constriction. The nuclear membrane breaks down to form membranous vesicles. The nucleolus disappears or gets dispersed. Centrosomes (particularly in animal cells) divide into two centrioles, which move to opposite poles. The spindle is also formed in the late prophase (or early metaphase).
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Metaphase: At this stage, the chromosomes become maximally distinct due to further contraction and thus the size of the chromosomes is measured at mitotic metaphase. Due to the attachment of spindle fibres at the centromeres of chromosomes, the chromosomes are arranged in the centre or at the equator or equatorial plate or metaphase plate due to their active movements. In animal cells, the spindle apparatus is formed due to the division of centrosome or centriole, but in plants, the spindle is still formed even in absence of centrosome in plant cells. The mechanism of the formation of the spindle is, however, not clear in plant cells.
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Anaphase: This is the shortest stage of mitosis. Chromosomes divide at the point of centromere or kinetochore and thus two sister chromatids are formed, which are now called chromosomes. These sister chromatids or future chromosomes or daughter chromosomes now move towards the opposite poles of the spindle and this movement is due to repulsion between centromeres, and contraction of spindle fibres. Different shapes of chromosomes are observed at anaphase (V or J or L or I-shaped), depending upon the position of the kinetochore or centromere.
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Telophase: In the final stage of the M phase, the parent cell divides into identical daughter cells. This stage starts when cell replication ends, and the paired chromosomes move towards the opposite pole of the cell.
(B)Cytokinesis-
Karyokinesis results in the formation of two nuclei inside a cell and now it is followed by the division of cytoplasm (Cytokinesis), thus forming two cells (daughter cells). Cytokinesis is by 2 methods:
(i) Cell furrow method: This is characteristic of animal cells. Due to the absence of rigid cell walls here, the more flexible plasma membrane forms the outer layer of the cell. A circular constriction or invagination appears at the centre or equator, which deepens gradually and finally, two daughter cells are separated.
(ii) Cell plate method: This is characteristic of plant cells. Here, vesicles provided by the Golgi apparatus unite to form phragmoplasts, which join to form cell plates. The cell plate is first laid down in the centre and then proceeds towards the periphery (i.e., centrifugal plate-formation). Cell wall materials are now laid down on both sides of the cell plate and thus forming two daughter cells.
Thus by mitotic division or mitosis, two daughter cells (exactly similar to the parent cell) are formed from a single parent cell.
Importance of the Cell Cycle
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It helps repair damaged cells in the body.
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It helps produce new cells from older ones.
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It helps maintain the number of chromosomes.
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It helps in growth and development by providing more cells.
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It helps in the proper growth and development of a person.
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It helps living organisms survive.
Conclusion
A series of events during cell growth and division is called the cell cycle. The cell cycle or division is important in all living organisms. Interphase is an essential phase of the cell cycle. It helps a cell prepare for division. This phase leads to an orderly growth of cells and the application of DNA.
The cell cycle is important because it helps living organisms survive, grow and develop.