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Cell Division
Cell Division Revision
Cell Division
Cells must replicate and divide to produce new cells and therefore enable organisms to grow and survive. Eukaryotic organisms divide by mitosis and meiosis. Prokaryotic organisms divide by binary fission.
Chromosomes
The nucleus of eukaryotic cells contains genetic material called DNA which is coiled up into structures called chromosomes.
Each chromosome contains a large number of genes which are sections of DNA that control the development of specific characteristics in an organism.
Different species have different numbers of chromosomes. Human body cells usually have 23 pairs of chromosomes, one received from their mother and one from their father. This means they have 46 chromosomes altogether.
Before cells divide they must replicate their DNA so the new cells have their own copies.
The Cell Cycle and Mitosis
Eukaryotic cells divide in a process called mitosis which is a vital stage of the cell cycle. Mitosis produces 2 daughter cells that are identical to the original cell with the same number of chromosomes.
The cell cycle has 2 main stages:
The first portion of the cell cycle is the growth phase.
- The cell grows, then prepares for mitosis by replicating all of its subcellular structures like mitochondria and ribosomes.
- The cell also duplicates all of its DNA so each new cell will have a copy of the DNA. When not dividing, the DNA is spread out in long strings but when copied ready for mitosis, it forms X-shaped chromosomes with both sides of the cross containing the same DNA.
The other part of the cell cycle is mitosis and is where the cell divides into 2 identical cells.
- It begins with the nuclear membrane breaking down and the chromosomes lining up at the equator (centre) of the cell.
- Fibres then pull the arms of the chromosome to opposite ends of the cell.
- Membranes form around the chromosomes which creates the nuclei for the new cells.
- Finally, the cytoplasm and cell membrane divide in a process called cytokinesis, producing 2 identical daughter cells that contain the same DNA as each other and the parent cell.
Note:
In an exam you may be given a photograph or drawing of a cell and be asked to identify which stage of the cell cycle it is in.
The best way to do this is to work out what the DNA is doing; if the DNA is in long, non-distinct strands then it will be in the growth phase but if the DNA is more visible and in chromosomes then it will be in mitosis.
The Importance of Mitosis
Mitosis is an essential process in organisms.
Multicellular organisms need to carry out mitosis in order to grow and replace damaged cells.
Newly fertilised egg cells (zygotes) must undergo several rounds of mitosis to increase the number of cells and develop into embryos. Embryos then go through many more rounds of mitosis and differentiation to create fully formed foetuses.
Organisms that reproduce asexually divide by mitosis.
Cell Division Example Questions
Question 1: What is a gene?
[1 mark]
A gene is a section of DNA that controls the development of specific characteristics in an organism.
Question 2: Describe what happens in mitosis.
[5 marks]
- The nuclear membrane breaks down.
- The chromosomes line up along the centre of the cell (equator).
- Cell fibres pull the arms of the chromosome to opposite ends of the cell.
- Membranes form around the chromosomes forming the new nuclei.
- The cytoplasm and cell membrane divide to form the 2 identical daughter cells.
Question 3: Name two purposes of mitosis.
[2 marks]
Any 2 from:
Growth
Repair/replace damaged cells
Development
Cell Division Worksheet and Example Questions
Cell Cycle and Mitosis Questions
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