Cell Cycle and Cell Division

All organisms start life from a single cell, and growth and reproduction depend on cell division. The sequence of events by which a cell duplicates its genome, synthesises other constituents and divides into two daughter cells is the cell cycle. A typical human cell in culture divides about once every 24 hours, while yeast takes about 90 minutes. The cycle has interphase, made of G1, S and G2 phases and taking more than 95% of the cycle, and the short M phase. In S phase DNA content doubles from 2C to 4C while the chromosome number stays the same; non-dividing cells enter the quiescent G0 stage. You will study mitosis, an equational division in four stages (prophase, metaphase, anaphase and telophase) followed by cytokinesis, which occurs by furrowing in animal cells and cell plate formation in plants. The chapter then explains meiosis, the reductional division that halves the chromosome number to form haploid gametes. It covers the substages of prophase I (leptotene, zygotene, pachytene, diplotene and diakinesis), synapsis, crossing over and chiasmata, and the significance of mitosis and meiosis.

Detailed Worksheet: Cell Cycle and Cell Division Section A - Definitions (10 marks) 1. What is the cell cycle? Name its two basic phases. (2 marks) 2. What is the G0 phase? Give an example of cells in this phase. (2 marks) 3. What is a bivalent or tetrad? In which stage is it formed? (2 marks) 4. Define crossing over. In which stage does it occur? (2 marks) 5. What is cytokinesis? How does it differ in plant and animal cells? (2 marks) Section B - Calculations and Applications (15 marks) 6. A cell has a DNA content of 2C and 20 chromosomes in G1 phase. State the DNA content and chromosome number at G2, after mitosis in each daughter cell, after meiosis I, and after meiosis II. (3 marks) 7. A human cell cycle lasts 24 hours, and the M phase lasts about 1 hour. Calculate the percentage of time spent in M phase and in interphase. If a cell divides continuously by mitosis, how many cells will be formed from one cell in 5 days? (3 marks) 8. How many mitotic divisions are needed to produce 256 cells from a single cell? How many meiotic divisions are needed to produce 100 pollen grains? Explain. (3 marks) 9. An onion root tip squash shows 400 cells, of which 20 are in prophase, 8 in metaphase, 6 in anaphase, 6 in telophase and the rest in interphase. Calculate the mitotic index (percentage of cells dividing). If the cell cycle lasts 20 hours, estimate the duration of mitosis. (3 marks) 10. A diploid plant cell has 16 chromosomes. Calculate the number of chromatids at metaphase of mitosis, the number of bivalents at pachytene, and the number of chromosomes in each cell after meiosis II. (3 marks) Section C - Diagrams (10 marks) 11. Draw labelled diagrams of the stages of mitosis in an animal cell. (4 marks) 12. Draw a labelled diagram of the cell cycle showing G1, S, G2 and M phases. (3 marks) 13. Draw labelled diagrams of the stages of meiosis I. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Compare mitosis and meiosis on the basis of the number of divisions, chromosome number, pairing of homologues, crossing over and significance. (5 marks) 15. Explain the events of prophase I of meiosis in detail. Why is it the longest phase? (5 marks) 16. 'Meiosis maintains the chromosome number across generations and creates genetic variation.' Explain this statement with examples. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw diagrams neatly with labels. Show the steps of calculations.