How do human cells divide by mitosis and become specialised, and why are stem cells so valuable?
Cell division by mitosis, the control of the cell cycle, cellular differentiation, and the nature and therapeutic and research value of stem cells (embryonic and tissue).
An SQA Higher Human Biology answer on division and differentiation, covering the phases of mitosis, control of the cell cycle, how cells differentiate by selective gene expression, and the properties and uses of embryonic and tissue (adult) stem cells.
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What this dot point is asking
The SQA wants you to describe how human cells divide by mitosis, explain how the cell cycle is controlled, describe how cells become specialised through differentiation, and explain what stem cells are and why their pluripotency and multipotency make them valuable in research and medicine.
Mitosis and the cell cycle
A cell spends most of its life in interphase, during which it grows, carries out its normal functions and copies its DNA so that each chromosome is made of two identical sister chromatids joined at the centromere. The mitotic phase then follows:
- The chromosomes condense and become visible, and the nuclear membrane breaks down.
- The chromosomes line up on the equator of the cell, attached to spindle fibres at their centromeres.
- The spindle fibres pull the sister chromatids apart to opposite poles, so each pole receives one complete set.
- Two new nuclear membranes form and the cytoplasm divides (cytokinesis), giving two identical daughter cells.
Because each daughter receives an identical copy of every chromosome, mitosis maintains the diploid chromosome number (46 in humans) in body cells and underlies growth, tissue repair and the replacement of dead cells.
Control of the cell cycle
This control matters because uncontrolled cell division produces a tumour, a mass of abnormal cells. If the abnormal cells fail to attach to one another properly they can spread through the body and form secondary tumours, the basis of cancer. The genes that drive and restrain the cycle are therefore central to understanding how cancer develops, which links this key area to mutations.
Cellular differentiation
Every body cell carries the same full genome, but a differentiated cell switches on only the genes characteristic of its cell type and switches the rest off. A muscle cell, for example, expresses genes for the contractile proteins actin and myosin, while a pancreatic beta cell expresses the gene for insulin. The selective control of gene expression is what gives a multicellular organism its many specialised cell types from one genetic blueprint.
Stem cells
Two types appear in the Higher course:
- Embryonic stem cells come from the early embryo and are pluripotent: they can differentiate into all the cell types of the body. They self-renew almost indefinitely in culture.
- Tissue (adult) stem cells are found in specific tissues such as bone marrow and are multipotent: they replenish only the limited range of cells found in that tissue (bone marrow stem cells, for example, form the different blood cells).
Stem cells are valuable for two reasons. Therapeutically, they repair or replace damaged tissue: bone marrow transplants treat leukaemia, cultured skin grafts treat severe burns, and corneal stem cells repair the eye surface. In research, they model how diseases develop and provide cells for testing new drugs before clinical trials. Because embryonic stem cells are taken from embryos, their use raises ethical issues about the status of the embryo, which is why their use is tightly regulated.
Examples in context
Example 1. Bone marrow transplants. A patient with leukaemia has their cancerous bone marrow destroyed and replaced with healthy multipotent bone marrow stem cells from a matched donor. Those tissue stem cells then divide and differentiate to rebuild a healthy population of red cells, white cells and platelets, restoring the blood system. This shows multipotency in clinical use.
Example 2. Modelling disease with stem cells. Researchers grow pluripotent stem cells into nerve cells carrying a disease-causing mutation, then watch how the cells behave and test candidate drugs on them. This avoids early experiments on patients and speeds up understanding of conditions such as motor neurone disease.
Try this
Q1. State the type of cell division that produces genetically identical daughter cells. [1 mark]
- Cue. Mitosis.
Q2. Explain why a differentiated muscle cell makes actin and myosin but not insulin. [1 mark]
- Cue. It expresses only the genes needed for its specialised function, while the insulin gene is switched off.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20193 marksDescribe the events that take place during mitosis and explain why mitosis must be tightly controlled.Show worked answer →
A 3-mark describe-and-explain answer needs the sequence of mitosis and a reason for control.
During mitosis the chromosomes condense and become visible, the nuclear membrane breaks down, and the chromosomes line up on the equator of the cell. The spindle fibres attach to the centromeres and pull the sister chromatids to opposite poles, so each new nucleus receives a full, identical set of chromosomes.
Mitosis must be controlled so that cells divide only when needed. Markers reward the idea that uncontrolled mitosis produces a mass of abnormal cells (a tumour), and that checkpoints in the cell cycle stop division if the DNA is damaged or not fully copied.
Award (1) chromosome condensation and alignment, (2) separation of chromatids to give identical sets, and (3) a valid reason for control.
SQA Higher 20212 marksState one difference between embryonic stem cells and tissue (adult) stem cells, and give one therapeutic use of stem cells.Show worked answer →
This is a 2-mark recall question with one mark for the difference and one for a use.
Embryonic stem cells are pluripotent, meaning they can differentiate into any cell type in the body. Tissue (adult) stem cells are multipotent, so they differentiate only into the limited range of cell types found in their own tissue (for example, bone marrow stem cells give rise to the different blood cells).
A valid therapeutic use is the repair or replacement of damaged tissue, such as using bone marrow transplants to treat leukaemia, or skin grafts grown from stem cells to treat burns. Markers also accept corneal repair and research uses such as drug testing and modelling disease.
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Sources & how we know this
- SQA Higher Human Biology Course Specification (X840 76) — SQA (2019)
- Higher Human Biology course overview — Planit (SQA partner) (2024)