Cell Division and Stem Cells
Body cells divide by mitosis, which produces two genetically identical daughter cells with the same number of chromosomes as the parent cell, and this is how organisms grow, replace worn-out cells and repair damage. Stem cells are undifferentiated cells that can divide to produce more cells of one or more types, ranging from embryonic stem cells, which can become any cell type, to adult stem cells in bone marrow, which form a limited range including blood cells.
Method
- Learn the cell cycle in order and know what happens in each stage: growth and DNA replication (the DNA is copied so each chromosome becomes two identical strands, and the number of sub-cellular structures such as ribosomes and mitochondria increases), mitosis (the chromosomes line up in the centre, and one copy of each is pulled to each end and a nucleus forms around each set), then division of the cytoplasm and membrane to give two identical cells.
- Remember the three functions of mitosis: growth, repair and replacement of cells, and asexual reproduction. Always say the daughter cells are genetically identical to the parent cell and to each other.
- Define a stem cell precisely: an undifferentiated cell that can divide to produce more cells of the same type, and can differentiate into other specialised cell types.
- Learn the three sources as a set of contrasts: embryonic (can form any cell type, cloned to be a genetic match in therapeutic cloning), adult, for example from bone marrow (forms a limited range only), and plant meristem (can form any plant cell, at any point in the plant's life).
- For the plant applications, learn that meristem cloning produces identical plants quickly and cheaply, which is used to preserve rare species and to mass-produce crop plants with desirable characteristics.
- For an evaluate question on stem cells, plan a balanced answer: medical benefits (treating paralysis, diabetes, replacing damaged tissue), risks (viral infection during transfer, possible rejection unless therapeutic cloning is used), and ethical objections (an embryo is destroyed, and views differ on the status of the embryo). Finish with a judgement rather than a list.
Worked example
A cell cycle lasts 24 hours. Mitosis takes 1 hour of that cycle. In a sample of 600 cells from the same tissue, estimate how many cells would be seen undergoing mitosis at any one moment.
- Find the proportion of the cycle spent in mitosis: 1 hour out of 24 hours = 1/24.
- Assume the cells are dividing at random points in the cycle, so the fraction of the sample in mitosis matches the fraction of time spent in mitosis.
- Multiply the sample size by that fraction: 600 x 1/24.
- Calculate: 600 / 24 = 25.
- Final answer: about 25 cells would be seen in mitosis, and the estimate assumes all cells in the sample are actively dividing.
Practice questions
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Q1How many pairs of chromosomes are there in a human body cell?Show answer
Answer: 23 pairs (46 chromosomes in total).
Q2State three functions of mitosis.Show answer
Answer: Growth, repair or replacement of damaged and worn-out cells, and asexual reproduction.
Q3What happens to the DNA before mitosis begins?Show answer
Answer: It is replicated, so each chromosome becomes two identical copies joined together.
Q4Define an undifferentiated cell.Show answer
Answer: A cell that has not yet developed into a specialised type and can still become one of a range of cell types.
Q5Where in a plant are stem cells found?Show answer
Answer: In the meristem tissue, at the tips of roots and shoots.
Q6Give one advantage of therapeutic cloning over using unrelated embryonic stem cells.Show answer
Answer: The stem cells produced have the same genes as the patient, so they are less likely to be rejected by the patient's immune system.
Q7Give one risk of stem cell treatment.Show answer
Answer: Viruses could be transferred with the cells if the culture is contaminated, causing infection in the patient.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Describe what happens during the cell cycle, up to and including the formation of two new cells.
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Explain why stem cells from a patient's own bone marrow cannot be used to treat every condition.
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Some people object to the use of embryonic stem cells in medicine. Evaluate the use of embryonic stem cells to treat human diseases.
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