Replication and Division of Nuclei and Cells

This section explores the processes of DNA replication and cell division, focusing on how disruptions to these processes can lead to uncontrolled cell growth and the formation of tumours.

DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. It is a highly accurate process essential for ensuring that each daughter cell receives a complete and identical copy of the genetic information.

The Process of DNA Replication

1. Unwinding and Separation: The double helix of DNA is unwound and separated by the enzyme helicase, creating a replication fork.
2. Priming: Short RNA sequences called primers are added to the DNA strands by the enzyme primase.
3. Elongation: DNA polymerase enzymes then add complementary DNA nucleotides to each template strand, following the base-pairing rules (A with T, and G with C).
4. Leading and Lagging Strands: One strand is synthesized continuously (leading strand), while the other is synthesized in short fragments (Okazaki fragments) that are later joined together (lagging strand).
5. Proofreading: DNA polymerase also has proofreading capabilities to correct errors during replication.

Cell Division

Cell division is a fundamental process by which cells multiply. There are two main types of cell division: mitosis and meiosis.

Mitosis

Mitosis is the process of nuclear division that results in two identical daughter nuclei. It is used for growth, repair, and asexual reproduction.

Mitosis consists of several phases:

• Prophase: Chromosomes condense and become visible.
• Metaphase: Chromosomes line up in the middle of the cell.
• Anaphase: Sister chromatids separate and move to opposite poles of the cell.
• Telophase: Chromosomes arrive at the poles, and the nuclear membrane reforms.

Meiosis

Meiosis is a process of nuclear division that results in four daughter nuclei with half the number of chromosomes as the parent cell. It is used for sexual reproduction.

Tumour Formation: Uncontrolled Cell Division

A tumour is an abnormal mass of tissue that results from uncontrolled cell division. This uncontrolled growth can occur in any part of the body.

Causes of Uncontrolled Cell Division

Uncontrolled cell division can be caused by:

• Mutations in genes that regulate cell growth and division: These mutations can lead to the production of faulty proteins that disrupt the normal cell cycle.
• Exposure to carcinogens: Certain chemicals, radiation, and viruses can damage DNA and increase the risk of mutations.
• Hormonal imbalances: In some cases, hormonal imbalances can stimulate cell growth and division.

The Cell Cycle and Checkpoints

The cell cycle is a series of events that lead to cell growth and division. It is tightly regulated by checkpoints that ensure that each stage of the cycle is completed correctly. These checkpoints monitor DNA integrity and other critical factors.

Checkpoint	What it monitors	Outcome of failure
G1 Checkpoint	DNA damage, cell size, nutrient availability	Cell cycle arrest, DNA repair, or apoptosis
G2 Checkpoint	DNA damage, DNA replication completion	Cell cycle arrest or apoptosis
M Checkpoint	Chromosome attachment to spindle fibres	Cell cycle arrest or apoptosis

How Mutations Lead to Tumours

Mutations in genes that control cell growth and division can disrupt the normal cell cycle checkpoints. This can lead to cells dividing even when they should not, resulting in the accumulation of cells and the formation of a tumour. These mutations can be:

• Oncogenes: Genes that promote cell growth and division. Mutations can turn proto-oncogenes into oncogenes, which are constantly stimulating cell growth.
• Tumour suppressor genes: Genes that inhibit cell growth and division. Mutations can inactivate tumour suppressor genes, removing brakes on cell growth.

Types of Tumours

Tumours can be classified as benign or malignant:

• Benign tumours: Non-cancerous tumours that do not spread to other parts of the body.
• Malignant tumours: Cancerous tumours that can invade surrounding tissues and spread to other parts of the body (metastasis).

Conclusion

Uncontrolled cell division, often resulting from mutations that disrupt the regulation of the cell cycle, is the fundamental cause of tumour formation. Understanding the processes of DNA replication and cell division, as well as the mechanisms that regulate these processes, is crucial for understanding the development and treatment of cancer.