How does one cell divide into two identical cells, and why does the body need this?
The cell cycle (growth and DNA replication, mitosis, cytokinesis), mitosis producing two genetically identical diploid daughter cells, and the role of mitosis in growth, repair and asexual reproduction.
A focused answer to the OCR Gateway GCSE Biology A topic B2 on the cell cycle and mitosis, covering the stages of the cell cycle, how mitosis produces two genetically identical cells, and its role in growth, repair and asexual reproduction.
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What this dot point is asking
OCR wants you to describe the stages of the cell cycle, explain how mitosis produces two genetically identical cells, and state the roles of mitosis in growth, repair and asexual reproduction.
The cell cycle
The stages in order are:
- Growth and DNA replication. The cell grows larger, makes more sub-cellular structures (such as ribosomes and mitochondria), and copies its DNA so that each chromosome becomes two identical copies joined together.
- Mitosis. The copied chromosomes line up in the centre of the cell, and one copy of each is pulled to each end. The nucleus then divides into two, each with a full set of chromosomes.
- Cytokinesis. The cytoplasm and cell membrane divide, producing two new cells, each with a nucleus.
Mitosis produces identical cells
The result of mitosis is two genetically identical daughter cells. They are identical because the DNA was copied exactly before division, and mitosis shares one complete copy of every chromosome to each new cell. Both daughter cells therefore have:
- The same number of chromosomes as the parent cell (they are diploid).
- The same genes as each other and as the parent.
The roles of mitosis
Mitosis is used wherever the body needs more cells that are exact copies:
- Growth. As an organism develops, mitosis makes the many new cells needed to get bigger.
- Repair. When tissues are damaged, mitosis replaces the dead or damaged cells, for example healing a cut.
- Replacing cells. Some cells wear out and are replaced constantly, such as skin cells and the lining of the gut.
- Asexual reproduction. Some organisms (such as bacteria, and plants grown from cuttings) reproduce by mitosis, producing offspring that are genetically identical clones of the parent.
When the cell cycle goes wrong
Normally the cell cycle is tightly controlled, so cells divide only when more are needed. If mutations cause this control to be lost, cells divide uncontrollably by mitosis, forming a lump of cells called a tumour. This is the basis of cancer, a link you meet again when studying non-communicable diseases and risk factors.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksDescribe what happens during the cell cycle, including mitosis, and explain why the two cells produced are genetically identical.Show worked answer →
A B2 structured question worth 4 marks.
Cell cycle (Describe): first the cell grows and the number of sub-cellular structures (such as mitochondria and ribosomes) increases; then the DNA is replicated so there are two copies of each chromosome; then mitosis occurs, in which the chromosomes line up and are pulled to opposite ends, and the nucleus divides; finally the cytoplasm and cell membrane divide (cytokinesis) to form two cells.
Genetically identical (Explain): because the DNA was copied exactly before division, and mitosis shares one complete copy to each new cell, both daughter cells have the same chromosomes and the same genes as each other and as the parent cell. Markers reward the link from accurate DNA replication to identical genetic information in both cells.
OCR 20213 marksState two processes in the human body that rely on mitosis, and explain why a cancer is described as uncontrolled cell division.Show worked answer →
A 3-mark recall and application question.
Two processes relying on mitosis: growth (making more cells as the body gets bigger) and repair of damaged tissues (replacing dead or damaged cells). Replacing worn-out cells (such as skin or blood cells) is also accepted.
Cancer: normally the cell cycle is controlled so cells only divide when needed. In cancer, changes (mutations) cause cells to divide uncontrollably by mitosis, forming a mass of cells called a tumour. Reward the idea that the normal control of the cell cycle is lost so cells divide far too often.
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