Chromosomes, DNA and Mitosis
Chromosomes, DNA and mitosis, at IGCSE, require the structural relationship between DNA, genes and chromosomes in more depth than GCSE, plus a precise, staged account of the cell cycle and mitosis. Students must describe DNA as a double helix of two strands held together by complementary base pairing (A with T, C with G), define a gene as a section of DNA that codes for a specific protein (via a specific sequence of amino acids), and give mitosis as an ordered sequence of stages (interphase, prophase, metaphase, anaphase, telophase) that produces two genetically identical diploid daughter cells, each with the same chromosome number as the parent cell.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Build the structural hierarchy in order: a DNA molecule is a double helix made of two strands of nucleotides, coiled around each other and held together by complementary base pairing; a chromosome is a single, very long, coiled molecule of DNA (found in the nucleus); a gene is a specific section of a DNA molecule (chromosome) that codes, through its base sequence, for a specific sequence of amino acids, which forms a specific protein.
- Learn the base-pairing rule precisely: adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G), joined by hydrogen bonds between the two strands; this complementary pairing is why, if you are told the percentage or sequence of one base, you can calculate or state the others.
- State the human chromosome number and how it is organised: a normal human body cell nucleus contains 46 chromosomes, arranged as 23 pairs (one chromosome of each pair inherited from each parent), making the cell diploid.
- Give the stages of the cell cycle and mitosis in order, one clear action per stage: interphase (the cell grows, and DNA replicates so each chromosome now consists of two identical sister chromatids joined at a centromere); prophase (chromosomes condense/become visible, and the nuclear membrane breaks down); metaphase (chromosomes line up individually along the middle/equator of the cell); anaphase (the sister chromatids of each chromosome are pulled apart to opposite poles of the cell); telophase (a new nuclear membrane forms around each set of chromosomes at each pole); finally the cytoplasm divides, producing two new, genetically identical diploid daughter cells.
- Explain why mitosis produces genetically identical cells: because DNA replication in interphase produces an exact copy of each chromosome before division, and the two identical sister chromatids of each chromosome are simply separated (not mixed or recombined) into the two new cells, so each daughter cell receives one complete, identical set of the parent cell's genetic information.
- Link the purpose of mitosis to named biological contexts: growth (increasing the number of cells in an organism), repair (replacing damaged or dead cells), and asexual reproduction (producing genetically identical offspring, as in Topic B2).
- For questions using a diagram or micrograph of a cell mid-division, identify the stage from the visible chromosome arrangement (chromosomes lined up in the middle = metaphase; chromosomes being pulled to opposite ends = anaphase) rather than the overall shape of the cell.
Worked example
A DNA sample is found to contain 30% adenine bases. Calculate the percentage of each of the other three bases (thymine, cytosine and guanine) in the sample, and explain your reasoning.
- Recall the base-pairing rule: adenine (A) always pairs with thymine (T) in equal amounts, and cytosine (C) always pairs with guanine (G) in equal amounts.
- Apply the rule to adenine and thymine: since A is 30%, and A pairs equally with T, thymine (T) must also be 30%.
- Calculate the combined percentage of A and T: 30% + 30% = 60%.
- Find the remaining percentage for C and G together: 100% - 60% = 40%.
- Split this equally between C and G, since they pair in equal amounts: 40% / 2 = 20% each.
- Final answer: thymine = 30%, cytosine = 20%, guanine = 20%.
Practice questions
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Q1Define a gene.Show answer
Answer: A section of a DNA molecule (chromosome) that codes for a specific sequence of amino acids, forming a specific protein.
Q2State the base that pairs with cytosine in a DNA molecule.Show answer
Answer: Guanine.
Q3State the number of chromosomes found in a normal human body cell.Show answer
Answer: 46 (23 pairs).
Q4Name the stage of mitosis during which chromosomes line up along the middle of the cell.Show answer
Answer: Metaphase.
Q5A DNA molecule contains 24% guanine. Calculate the percentage of adenine in the molecule.Show answer
Answer: 26%. Guanine (24%) pairs equally with cytosine, so cytosine is also 24%, giving 48% for C and G combined; the remaining 52% is split equally between A and T, so adenine = 26%.
Q6Explain why the two daughter cells produced by mitosis are genetically identical to each other and to the parent cell.Show answer
Answer: Before division, DNA replication produces an exact copy of each chromosome (as two identical sister chromatids); during mitosis these identical chromatids are simply separated, one to each new cell, so both daughter cells receive a complete and identical set of the parent cell's chromosomes.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
State the stage of mitosis during which the nuclear membrane breaks down and chromosomes become visible, and the stage during which sister chromatids are pulled to opposite poles of the cell.
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A researcher analyses a section of DNA and finds it contains 840 thymine bases out of a total of 2400 bases. Calculate the number of cytosine bases in this section of DNA, showing your working.
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Describe the sequence of stages in mitosis, from interphase to the production of two daughter cells, and explain why this process results in two cells that are genetically identical to the original parent cell.
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Free printable worksheet
Want more practice on paper? Download the chromosomes, dna and mitosis worksheet pack - 6 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 7 of IGCSE Science Workbook, the whole course as one free printable PDF.
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