A student is studying onion root tip cells undergoing mitosis. Explain why mitosis is important for the growth of the onion root tip, and describe what happens to the chromosomes during mitosis so that each new cell receives the correct genetic information.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Onion root tips are frequently used in biology to study cell division. The root tip contains a region called the meristem where cells actively divide by mitosis.
Model answer (5 marks)
Mitosis is essential for the growth of the onion root tip because it produces new cells, increasing the number of cells in the meristem and allowing the root to elongate.
During mitosis the chromosomes are first copied: DNA replication in the S phase creates two identical sister chromatids for each chromosome.
In anaphase the sister chromatids are pulled apart to opposite poles of the cell.
Cytokinesis then separates the cytoplasm, producing two daughter cells.
Each daughter cell receives an identical set of chromosomes, the same number as the parent cell, so the new cells are genetically identical to the parent and to each other (clones).
During mitosis the chromosomes are first copied: DNA replication in the S phase creates two identical sister chromatids for each chromosome.
In anaphase the sister chromatids are pulled apart to opposite poles of the cell.
Cytokinesis then separates the cytoplasm, producing two daughter cells.
Each daughter cell receives an identical set of chromosomes, the same number as the parent cell, so the new cells are genetically identical to the parent and to each other (clones).
Examiner tips
- Use the word "mitosis" to show you know the process; mention DNA replication, chromosome separation, cytokinesis.
- Show the flow: growth → new cells → DNA copied → chromatids pulled apart → identical daughter cells.
- Include the key terms: meristem, sister chromatids, poles, cytokinesis, genetically identical.
Common mistakes
- Confusing meiosis with mitosis; mentioning gametes or recombination.
- Omitting the step that chromosomes are copied before division.
- Failing to state that each daughter cell gets the same number of chromosomes as the parent.
Mark scheme (5 marks)
- Mitosis produces new cells for growth (of the root tip) / increases the number of cells
- The chromosomes are copied / DNA is replicated before the cell divides
- The copies of each chromosome are separated / pulled to opposite ends / poles of the cell
- Each new (daughter) cell receives an identical set of chromosomes / the same number of chromosomes as the parent cell
- The new cells are genetically identical to the parent cell / to each other (clones)
Key terms in this question
mitosis · chromosomes · genetic information
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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