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Question
State the number of chromosomes in (a) a human skin cell, (b) a human sperm cell.
Solution
Skin cell = body cell = diploid = 46.
Sperm = gamete = haploid = 23.
Answer
(a) 46 (23 pairs). (b) 23.
Question
Put these stages of the cell cycle in the correct order: cytokinesis; growth + DNA replication; mitosis.
Solution
Cell grows and DNA copies first. Then nucleus divides (mitosis). Then cytoplasm splits.
Answer
Growth + DNA replication → mitosis → cytokinesis.
Question
A diagram shows a cell with two sets of chromosomes being pulled towards opposite ends by long fibres. Which stage of the cell cycle is this?
Solution
Chromosomes pulled to opposite poles = mitosis.
Answer
Mitosis (nuclear division). The fibres are spindle fibres.
Question
What is a stem cell? (2 marks)
Solution
Two properties needed.
Answer
An unspecialised cell (1) that can divide (by mitosis) to produce more cells of the same kind AND can differentiate into one or more specialised cell types (1).
Question
Evaluate the use of embryonic stem cells to treat human diseases. (4 marks)
Solution
Benefit.
Risk.
Ethical issue.
Balanced conclusion.
Answer
Benefit: embryonic stem cells can become any cell type, so they could replace damaged cells in conditions like diabetes or paralysis (1). Risk: cultured stem cells could be infected with viruses, or could divide uncontrollably and form tumours (1). Ethical issue: research destroys the embryo, which some people consider a potential human life (1). Overall: the medical benefits are large, but the ethical concerns and safety risks mean strict regulation is needed (1).
A long molecule of coiled DNA that carries genes, found in the nucleus of a cell.
A short section of DNA on a chromosome that codes for a particular protein.
Two chromosomes that carry the same genes in the same order — one from each parent.
A cell with two copies of each chromosome (in homologous pairs). Human body cells are diploid with 46 chromosomes.
A cell with only one copy of each chromosome. Human gametes are haploid with 23 chromosomes.
The series of stages a cell goes through from being formed to dividing into two daughter cells.
The stage of the cell cycle in which the nucleus divides; produces two genetically identical daughter nuclei with the same number of chromosomes as the parent.
Division of the cytoplasm and cell membrane after mitosis to produce two separate daughter cells.
Process during the cell cycle in which each DNA molecule is copied to produce two identical strands (so the chromosome looks like an X).
An unspecialised cell that can divide by mitosis and differentiate into one or more specialised cell types.
Stem cell from an early embryo, capable of differentiating into any type of body cell.
Stem cell found in adult tissues (e.g. bone marrow); can differentiate only into a limited range of related cell types.
Region of unspecialised, dividing cells at the tip of plant roots or shoots; can differentiate into any plant cell throughout the plant's life.
Producing an embryo with the same genes as a patient, so that stem cells from it can be grown into tissue that will not be rejected by the patient's immune system.
Mistake
Saying 'humans have 23 chromosomes'.
Why it happens
Students remember the number 23 but forget it refers to pairs.
How to avoid it
Always say '23 pairs' or '46 chromosomes' for body cells. Use '23 single chromosomes' for gametes only.
Mistake
Using 'gene' and 'chromosome' interchangeably.
Why it happens
Both are 'DNA things'.
How to avoid it
Hierarchy: gene = small section. Chromosome = whole molecule. One chromosome holds thousands of genes.
Mistake
Treating 'cell cycle' and 'mitosis' as the same thing.
Why it happens
Mitosis is the most memorable part.
How to avoid it
Cell cycle = whole process (growth → mitosis → cytokinesis). Mitosis is the nuclear-division STAGE.
Mistake
Saying mitosis produces 'different' daughter cells.
Why it happens
Confusion with meiosis.
How to avoid it
Mitosis daughters are IDENTICAL. Meiosis daughters are genetically DIFFERENT.
Mistake
Saying mitosis halves chromosome number.
Why it happens
Mixing up with meiosis (which DOES halve).
How to avoid it
Mitosis preserves chromosome number (46 → 46). Meiosis halves it (46 → 23).
Mistake
Saying adult stem cells can become any cell type.
Why it happens
Confusing them with embryonic stem cells.
How to avoid it
Adult stem cells = LIMITED. Embryonic stem cells = unrestricted.
Mistake
Giving only benefits OR only risks in an evaluate question.
Why it happens
Students have a strong opinion.
How to avoid it
Evaluate = balanced. Always include at least one benefit, one risk, one ethical point.
Mistake
Saying plant stem cells are everywhere in the plant.
Why it happens
Plant cells often retain dividing ability.
How to avoid it
Meristem stem cells are found specifically at ROOT TIPS and SHOOT TIPS.