Life Cycle of Somatic Cells

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The Zygote:

Human life begins as a fertilized ovum called zygote. A zygote is a diploid cell from all cells of the human body arise by a series of mitoses. While mitosis is crucial for the growth and differentiation of cells, it only takes up a small portion of the somatic cell's entire life cycle. So what goes on in the interphase - the period between two successive mitoses?

Somatic cell cycle

The G1 phase:

According to the diagram above, mitosis is the shortest of the four stages of the cell cycle. Immediately after mitosis, the cell enters a phase called G1 phase in which there is no DNA synthesis. Some cells can spend days or even years in the G1 and some can only spend a few hours. The timing of each step in mitosis is controlled at certain cellular checkpoints. Furthermore, these checkpoints also monitor and control the accuracy of DNA syntheses as well as the assembly and attachment of microtubules which facilitate chromosome movement. If the cellular genome is found to be damaged in anyway, these mitotic checkpoints halt cell-cycle progression until the damage is repaired. Needless to say, if the extent of cellular damage is extensive, the cell is instructed to commit suicide by programmed cell death - a process called apoptosis.

The S Phase:

The S phase - the stage of DNA synthesis - follows the G1 phase. During the S phase, each chromosome which was a single DNA molecule in the G1 replicates and becomes a bipartite chromosome consisting of two sister chromatids, and each chromatid is identical to the original chromosome from the G1 phase. Specialized DNA sexuences called telomers mark the ends of each chromatid. These telomers ensure the integrity of each chromosome during replication. The two sister chromatids are held together physically at the centromere, a DNA region part of the kinetochore. This entire structure helps the chromosome attach to microtubules of the mitotic spindle and ensure smooth chromosome movement during mitosis. DNA synthesis during the S-phase is asynchronous, i.e., each chromosome has multiple origins of DNA replication, and each individual chromosome segments have their own characteristic time of replication during the 6- to 8- hour S-phase.

The G2 phase:

By the end of the S phase, the DNA content of the somatic cell is doubled and the cell enters the G2 stage. The cell doubles its mass before the next mitosis because of continuous production of ribonucleic acids and proteins. Mitosis ends when G2 begins, i.e., when chromosomes condense and become visible under the microscope as thin elongated threads.

Telophase:

The telophase consists of the G1, S, and G2 phases. These three phases last for a total of 16-24 hours, while mitosis lasts for 1-2 hours. The overall length of the cell cycle is variable; rapidly dividing cells of the skin and gastrointestinal tract replicate rapidly, while neurons and RBCs don't replicate at all and are forever stuck in a phase known as the G0 phase. Needless to say, some cells like the liver cell may enter the G0 but may progress to the G1 and continue on with the cell cycle if there is organ damage.

Additional Readings:

Basic Genetics

1. Single-Gene Disorders
2. Population Genetics
3. Genetic Disease Transmission Probability
4. What is Cytogenetics?
5. Notes on Bloom's Syndrome
6. Introduction to Human Chromosomes
7. Life Cycle of Somatic Cells

Related Topics

1. Genetic and Developmental Disorders
2. Genetic Code, Mutations, and Translation

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