Identify and describe Why Cells Need to Divide, The Cell Cycle and Chromosomes: Coiled Packets of DNA in the context of chromosomes.
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GCSE Biology Revision
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GCSE Biology revision
Chromosomes
Cell division
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Explain the relationships between Why Cells Need to Divide, The Cell Cycle and Chromosomes: Coiled Packets of DNA and how they contribute to chromosomes.
Apply and analyse knowledge of Why Cells Need to Divide, The Cell Cycle and Chromosomes: Coiled Packets of DNA to interpret unfamiliar information about chromosomes.
Revision summary
Key knowledge
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Why Cells Need to Divide
- All multicellular eukaryotic organisms require a continuous supply of new cells for growth, development, and repair
- Young organisms need new cells to increase in size, whilst older organisms must replace lost cells, such as skin cells that continuously fall off
The Cell Cycle
- The cell cycle is the life cycle of a cell, from the time it is made to the time it divides into two new cells
- There are three main stages in the cell cycle: growth, DNA replication, and mitosis followed by division (cytokinesis)
- In the growth stage, the cell increases in size and multiplies its subcellular structures, such as mitochondria and ribosomes
- In the DNA replication stage, the cell's DNA is duplicated so that each new daughter cell will receive a full set of genetic information
Chromosomes: Coiled Packets of DNA
- When a cell is not dividing, DNA exists as long, spread-out strings, but as division approaches, it condenses into chromosomes
- Chromosomes are coiled-up packets of DNA, each containing a large number of genes that control the development of characteristics such as eye colour
- Eukaryotic cells, such as those in animals, contain two copies of each chromosome (one from each parent), known as a homologous pair
Human Chromosomes
- Humans have 23 pairs of chromosomes, giving a total of 46 chromosomes in each body cell
- Different species have different numbers of chromosomes, which is one reason why animals of different species cannot reproduce to produce fertile offspring
DNA Replication & Chromosome Structure
- During preparation for division, each of the 46 chromosomes is duplicated, but the duplicate remains attached to the original, forming an X shape
- Each half of the X-shaped chromosome is referred to as an arm, and the DNA on both arms of a single chromosome is identical
Mitosis: Separating the Chromosomes
- All 46 X-shaped chromosomes line up along the centre of the cell during mitosis
- Spindle fibres from either side of the cell attach to each arm of every chromosome and pull them to opposite ends, called poles
- This process separates each X-shaped chromosome into two individual arms, ensuring each side of the cell receives a complete set of genetic material
Cytokinesis: Forming Two Daughter Cells
- In cytokinesis, the cell membrane and cytoplasm divide, forming two separate daughter cells
- Each daughter cell contains the same DNA as the other and as the original parent cell, making them genetically identical
- The resulting daughter cells can contribute to growth, development, or repair, and can undergo the cell cycle all over again
Key Vocabulary Recap
- Mitosis is the stage of the cell cycle in which the duplicated chromosomes are separated to opposite poles of the cell
- Cytokinesis is the physical division of the cytoplasm and cell membrane to produce two daughter cells
- A chromosome is a condensed, coiled structure made of DNA, carrying many genes that determine an organism's characteristics