Cell Division
Overview
You began as one cell. That cell held a full instruction set, and it did something remarkable: it copied every instruction, split in half, and handed a complete set to each of the two cells it became. Those two did it again, and again, and the repetition never really stopped. The skin you shed this morning is being replaced right now by the same process, running quietly in a layer of cells you will never see.
This lesson follows that process in two very different versions. One makes copies so faithful that the new cells are indistinguishable from the old, and it builds bodies and keeps them stocked. The other deliberately halves the instruction set, and without it sexual reproduction would double the chromosome number in every generation until nothing worked. You will learn what a chromosome actually carries, why body cells hold them in pairs, where each kind of division happens, what comes out of it, how fertilisation puts the pairs back together, and why a single fertilised egg can end up producing cells as unalike as a nerve cell and a red blood cell.
Objectives
- The nucleus of a cell contains chromosomes. Chromosomes carry genes that control the characteristics of the body. Each chromosome carries a large number of genes.
- Many genes have different forms called alleles, which may produce different characteristics.
- In body cells the chromosomes are normally found in pairs.
- Body cells divide by mitosis to produce additional cells during growth or to produce replacement cells.
- When a body cell divides by mitosis: copies of the genetic material are made; the cell then divides once to form two genetically identical body cells.
- Cells in reproductive organs divide to form gametes.
- A cell divides to form gametes by meiosis.
- When a cell divides to form gametes: copies of the genetic information are made; the cell then divides twice to form four gametes, each with a single set of chromosomes.
- Gametes join at fertilisation to form a single body cell with new pairs of chromosomes. This cell repeatedly divides by mitosis to form many cells. As an organism develops, these cells differentiate to form different kinds of cells.
Mind map
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Lesson Note
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Lesson Evaluation
Congratulations on completing the lesson on Cell Division. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- How many chromosomes are present in a human gamete? A. 23 B. 46 C. 92 D. 12 Answer: A
- Which statement describes what happens when a body cell divides by mitosis? A. The genetic material is copied and the cell divides twice B. The genetic material is copied and the cell divides once C. The cell divides once and the genetic material is then copied D. The genetic material is halved and the cell divides once Answer: B
- How many cells are produced when one cell divides by meiosis? A. One B. Two C. Four D. Eight Answer: C
- What is an allele? A. A length of DNA that carries many genes B. An alternative form of a gene C. A cell that has developed structures suited to one job D. A pair of matching chromosomes Answer: B
- A plant body cell contains 24 chromosomes. How many chromosomes are present in one of its gametes? A. 6 B. 12 C. 24 D. 48 Answer: B
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