Cell Division
- Created by: Emily Cartwright
- Created on: 30-03-14 15:03
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- Cell Division
- 'A process by which a cell splits to form daughter cells'
- Two types of cell division
- Mitosis
- In mitosis the cell divides once to produce two diploid daughter cells that are genetically identical to the parent cell
- In mitosis dividing cells undergo a regular pattern of events called the cell cycle
- Uncontrolled mitosis can lead to the formation of groups of cells called tumours
- Benign tumours are slow growing and harmless
- Malignant tumours are cancerous and can spread to other tissues
- Meiosis
- In sexual reproduction meiosis is used to form gametes containing half the number of chromosomes
- In meiosis, the cell divides twice to produce 4 daughter cells which are genetically different to the parent with half the number of chromosomes
- It is important that the daughter cells only have half the number of the parents chromosomes to prevent doubling of the chromosomes when the gametes fuse in sexual reproduction
- Mitosis
- Chromosomes
- Made up of DNA, protein and a small amount of RNA
- DNA occurs as a single strand in the form of a double helix running the length of the chromosome
- Each DNA molecule is made up of many sections called genes
- DNA occurs as a single strand in the form of a double helix running the length of the chromosome
- It is only at the onset of cell division that chromosomes become visible
- Just before cell division starts each DNA molecule makes a copy of itself
- The single thread of DNA becomes two identical threads. These are called chromatids. They are joined at the centromere
- Just before cell division starts each DNA molecule makes a copy of itself
- Replicated Chromosome
- Replicated chromosomes are two strands of identical chromosomal material called chromatids (sister chromatids)
- Humans always have 46 chromosomes
- Chromosomes are found in matching pairs called homologous pair. Humans have 23 homologous pairs of chromosomes
- The total number of chromosomes is called the diploid number
- Gametes have half the diploid number, called the haploid number. Human gametes have 23 chromosomes
- Chromosomes are found in matching pairs called homologous pair. Humans have 23 homologous pairs of chromosomes
- Made up of DNA, protein and a small amount of RNA
- Cell Cycle
- 1. Interphase
- Makes up about 90% of the cell cycle
- The phase between cell divisions and although called the resting phase the cell is metabolically active during this phase
- Nucleus and Nucleolus are visible
- The DNA in the nucleus replicates and doubles in quantity
- New organelles are made e.g. Mitochondria
- All the ATP needed for mitosis is made
- No chromosomes are visible because chromatin is dispersed throughout nucleus in diffuse form
- 2. Prophase
- Longest phase, so in a sample of cells this will be the most commonly seen
- The nucleus disappears and the nuclear membrane disintegrates
- The chromatin material coils up and becomes shorter and thicker forming chromosomes
- Centrioles divide and migrate to opposite poles of the nucleus and begin to form the spindle
- By the end of this phase, spindle fibres have developed from the centrioles and extend from pole to pole
- 3. Metaphase
- A relatively short phase
- Chromatids move to the equator and attach to the spindle fibres by their centromeres
- (Plant cells do not contain centrioles)
- 4. Anaphase
- Also a short phase (the fastest phase)
- The centromeres divide to separate the chromatid pairs
- The spindle fibres contract pulling the chromatids to opposite poles (centromere first)
- 5. Telophase
- The chromatids have now reached the poles and can be regarded as distinct chromosomes
- A nuclear membrane forms around each set of chromosomes and the nucleus reappears
- By the end of this phase the chromosomes uncoil to form diffuse chromatin which are no longer visible
- 6. Cytokinesis
- At the end of telophase, the cell divides by cytokinesis to form two daughter cells
- In animal cells this is done by 'pinching in' of the cell membrane
- In animals the cell cycle lasts for 8-24 hours (20 minutes in bacteria) but the actual period of cell division accounts for less than 10% of the total cycle
- 1. Interphase
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