Cell Division
Aperçu
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 and repairs bodies. 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 where each one happens, what comes out of it, why a fertilised egg can become a nerve cell or a muscle cell, what makes stem cells so valuable to medicine, and how to build the kind of balanced judgement about them that earns marks on Paper 1.
Objectifs
- 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.
- Most types of animal cell differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement.
- Cells from human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types of human cell.
- In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo will not be rejected by the patient’s body so they may be used for medical treatment.
- Treatment with stem cells may be able to help conditions such as paralysis. Knowledge and understanding of stem cell techniques is not required. Students should be able, when provided with appropriate information, to make informed judgements about the social and ethical issues concerning the use of stem cells from adult bone marrow and embryos in medical research and treatments.
Carte mentale
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Note de cours
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Évaluation de la leçon
Félicitations, vous avez terminé la leçon sur Cell Division. Maintenant que vous avez exploré le concepts et idées clés, il est temps de mettre vos connaissances à lépreuve. Cette section propose une variété de pratiques des questions conçues pour renforcer votre compréhension et vous aider à évaluer votre compréhension de la matière.
Vous rencontrerez un mélange de types de questions, y compris des questions à choix multiple, des questions à réponse courte et des questions de rédaction. Chaque question est soigneusement conçue pour évaluer différents aspects de vos connaissances et de vos compétences en pensée critique.
Utilisez cette section d'évaluation comme une occasion de renforcer votre compréhension du sujet et d'identifier les domaines où vous pourriez avoir besoin d'étudier davantage. Ne soyez pas découragé par les défis que vous rencontrez ; considérez-les plutôt comme des opportunités de croissance et d'amélioration.
- 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 can differentiate into many types D. A pair of matching chromosomes Answer: B
- Why are stem cells taken from an embryo made by therapeutic cloning not rejected by the patient who receives them? A. They have no nucleus and so carry no genes B. They carry the same genes as the patient C. They are produced by meiosis rather than mitosis D. They have had their chromosome number halved Answer: B
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