CCEA A Level Biology
CCEA A Level Biology 1.6 Continuity of Cells Exam Questions
Free CCEA A Level Biology revision for 1.6 Continuity of Cells: what you need to know, common mistakes and exam-style questions with answers. Covers the cell cycle, checkpoints and cancer, mitosis, anticancer drugs, haploid and diploid cells, meiosis, crossing over and independent assortment.
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About CCEA A Level Biology 1.6 Continuity of Cells
Section 1.6 of Unit AS 1 covers how cells divide. You need the stages of the cell cycle (G1, S, G2, mitosis and cytokinesis), the role of checkpoints and the G0 state, and how loss of control leads to cancer. CCEA names two anticancer drugs: vincristine, a mitotic poison that inhibits microtubules, and 5-fluorouracil, an antimetabolite acting in S phase. You must describe each stage of mitosis and meiosis, explain how crossing over and independent assortment create variation, and identify stages in root tip squashes and photomicrographs. This underpins inheritance and population genetics in A2 2.
What you need to know
- Cell cycle Interphase is G1 (growth, organelle synthesis), S (DNA replication) and G2 (preparation for division), followed by mitosis and cytokinesis. Checkpoints in the gap phases stop damaged cells progressing; cells can leave the cycle into G0. Loss of checkpoint control can lead to uncontrolled division and cancer.
- Chromosomes Eukaryotic chromosomes are DNA wound around histone proteins. After S phase each chromosome consists of two sister chromatids joined at the centromere.
- Mitosis Prophase: chromosomes condense and nuclear envelope breaks down. Metaphase: chromosomes line up at the equator attached to spindle fibres. Anaphase: chromatids are pulled to opposite poles. Telophase: nuclear envelopes reform. Daughter cells are genetically identical.
- Anticancer drugs Vincristine stops microtubules forming so the spindle cannot form and mitosis halts. 5-fluorouracil is an antimetabolite that interferes with DNA synthesis in S phase.
- Meiosis Two divisions produce four haploid cells. In prophase I homologous chromosomes pair and crossing over occurs at chiasmata. In metaphase I homologous pairs line up; in anaphase I they separate. Meiosis II separates chromatids.
- Sources of variation Crossing over swaps sections between non-sister chromatids, producing new allele combinations. Independent assortment of homologous pairs at metaphase I gives 2^n combinations of chromosomes.
- Practical Prepare a root tip squash: fix, hydrolyse in acid, stain and squash; then identify stages and calculate a mitotic index.
Common mistakes in 1.6 Continuity of Cells
- Saying DNA replicates during prophase. It replicates in S phase of interphase.
- Mixing up chromosomes and chromatids in anaphase. In mitosis and meiosis II sister chromatids separate; in anaphase I homologous chromosomes separate.
- Saying crossing over occurs between sister chromatids. It occurs between non-sister chromatids of homologous chromosomes.
- Stating that cancer is caused by fast mitosis. Cancer is uncontrolled division due to loss of cell cycle control.
- Describing vincristine as stopping DNA replication. It stops spindle formation; 5-fluorouracil acts on DNA synthesis.
Try three 1.6 Continuity of Cells exam-style questions
1. Explain how vincristine slows the growth of a tumour. (3 marks)
Show the mark scheme answer
Inhibits formation of microtubules; spindle cannot form; chromosomes or chromatids cannot be separated so mitosis stops at metaphase; fewer new tumour cells produced.
2. A species has a diploid number of 8. Calculate the number of different chromosome combinations in gametes from independent assortment alone. (2 marks)
Show the mark scheme answer
Haploid number n = 4; 2^4 = 16.
3. Describe two ways meiosis produces genetic variation. (4 marks)
Show the mark scheme answer
Crossing over in prophase I; exchange of sections between non-sister chromatids of homologous chromosomes forms new combinations of alleles; independent assortment in metaphase I; random orientation of homologous pairs gives different combinations of maternal and paternal chromosomes.
1.6 Continuity of Cells FAQs
What does CCEA A Level Biology 1.6 Continuity of Cells cover?
It covers the cell cycle and cancer, mitosis, anticancer drugs, haploid and diploid cells, meiosis and how meiosis produces variation.
Which unit is 1.6 Continuity of Cells examined in?
It is examined in Unit AS 1: Molecules and Cells, a 1 hour 30 minute written paper of structured questions and an essay. Related practical skills can also be tested in Unit AS 3.
How do I learn the stages of mitosis and meiosis?
Draw each stage from photomicrographs, focusing on what chromosomes are doing, and practise identifying stages from root tip squash images.
