All cells arise from other cells (3.2.2)
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Not all eukaryotic cells retain the ability to divide, but those that can undergo the cell cycle, a series of stages involving growth and division. The outcome is two genetically identical daughter cells.
The cycle consists of interphase, followed by mitosis and cytokinesis (cytoplasmic division).
Interphase is the stage during which the cell grows and prepares for division. It occupies approximately 90 of the cell cycle and consists of three phases:
- G1 (first growth)
- S (DNA synthesis)
- G2 (second growth).
Mitosis is the nuclear division process during which a eukaryotic cell separates its duplicated chromosomes into two identical nuclei. It consists of prophase, metaphase, anaphase and telophase and is followed by cytokinesis.
Cytokinesis is the division of the cytoplasm following nuclear division, resulting in the formation of two daughter cells.
Prophase is the first stage of mitosis. Chromatin condenses into chromosomes, each consisting of two sister chromatids joined at the centromere.
The nuclear envelope breaks down and the nucleolus disappears.
Centrosomes (centrioles) move to opposite poles of the cell, forming spindle fibres that guide chromosome movement.
During metaphase, chromosomes align at the cell equator, also called the metaphase plate.
Spindle fibres from opposite poles attach to each centromere.
During the anaphase, the centromeres divide, allowing sister chromatids to separate.
The spindle fibres shorten, moving the chromatids towards opposite poles of the cell.
During telophase, the chromatids are at the cell’s poles and are now chromosomes. The nuclear envelope reforms around each set of chromosomes, creating two nuclei.
The chromosomes uncoil back into chromatin, and the nucleolus is formed.
The mitotic spindle breaks down as the cell prepares for cytokinesis.
Viruses are non-living, so they cannot undergo cell division.
Viruses attach to a host cell and inject their genetic material (DNA or RNA) into it. The host cell then synthesises the viral components and assembles new viruses.
Binary fission is the method of cell division (asexual reproduction) in prokaryotic cells (e.g., bacteria).
- Replication of circular DNA and plasmids.
- The cell increases in size.
- The circular DNA molecules move to opposite ends of the cell.
- The cytoplasm divides.
- A new cell wall and cell membrane form between the two circular DNA molecules.
Two genetically identical daughter cells are produced, containing one copy of the circular DNA and a variable number of plasmids.
If mitotic control mechanisms fail, cells may divide uncontrollably, forming a tumour (a mass of abnormal cells). Tumours may be benign (harmless) or malignant.
Benign tumours grow slowly, remain contained and are unlikely to be life-threatening.
Malignant tumours are cancerous, grow rapidly and can invade surrounding tissues and spread to other parts of the body, making them more likely to be life-threatening.
Many cancer treatments work by controlling or stopping cell division.
Examples include:
- Chemotherapy – uses drugs to prevent mitosis.
- Radiotherapy – uses radiation to damage DNA and stop cell division.
These treatments also affect normal dividing cells, causing side effects such as hair loss, nausea, and a weakened immune system.
There are multiple types of gene mutations. In substitution (point) mutations, one DNA base is replaced by another during DNA replication. There are three different types of substitution mutation:
- Silent: the base sequence changes, but the same amino acid is coded for, so the protein is unchanged.
- Missense: the base substitution changes the codon, resulting in a different amino acid being incorporated into the protein. This may alter the protein’s structure and function.
- Nonsense: the base substitution creates a stop codon, causing translation to end prematurely and producing a shortened, usually non-functional protein.
In insertion / deletion (indel) mutations, one or more bases are added or removed. Unless the number of bases inserted or deleted is a multiple of three, this causes a frameshift mutation altering the triplet reading frame.
As a result, amino acid sequence from the mutation onwards is changed, often producing a protein with an altered tertiary structure that is unable to function normally.