A single fertilised egg cell (zygote) develops into a human embryo containing thousands of cells. Explain how mitosis allows this increase in cell number, and why all the cells produced contain the same genetic information as the zygote.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A human zygote contains 46 chromosomes. After repeated rounds of cell division, an embryo consisting of thousands of cells is formed.
Model answer (5 marks)
A fertilised egg cell (zygote) undergoes mitosis, which is a type of cell division that increases cell number.
Before mitosis, the DNA in each chromosome is replicated during the S phase of interphase, producing two identical sister chromatids.
During mitosis, the replicated chromosomes are evenly segregated so that each daughter cell receives a complete set of 46 chromosomes.
Mitosis is repeated many times, allowing a single zygote to give rise to thousands of cells.
Because each division produces identical copies of the DNA, all daughter cells are genetically identical to the zygote and to each other.
Before mitosis, the DNA in each chromosome is replicated during the S phase of interphase, producing two identical sister chromatids.
During mitosis, the replicated chromosomes are evenly segregated so that each daughter cell receives a complete set of 46 chromosomes.
Mitosis is repeated many times, allowing a single zygote to give rise to thousands of cells.
Because each division produces identical copies of the DNA, all daughter cells are genetically identical to the zygote and to each other.
Examiner tips
- Use the term "mitosis" and mention DNA replication; show the sequence of events; emphasise identical chromosome sets; note repeated rounds of division; keep answer concise and structured.
Common mistakes
- Confusing meiosis with mitosis; not mentioning DNA replication; saying cells have different DNA; omitting that all cells are genetically identical.
Mark scheme (5 marks)
- The zygote divides by mitosis (to produce new cells / to increase cell number)
- Before division, the DNA / chromosomes are copied / replicated
- Each daughter cell receives a full / identical copy of the DNA / 46 chromosomes
- This process repeats many times / mitosis is repeated (to produce thousands of cells)
- All cells are genetically identical to the zygote / to each other because each copy of the DNA is identical
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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