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Revision for AS OCR Biology Topic 3 (Cell Division, Cell Diversity and Cellular Organisation)
Cell cycle is the process by which all the cells in a multicellular organism grow and divide.
Interphase is the growth stage and mitosis is the division stage.
Mitosis only occupies a small percent of the cell cycle, the rest is interphase where DNA is copied and
checked for errors. If errors are detected the cell may kill itself or mutation may occur.
Mitosis is used to repair damaged tissue.
DNA is copied and checked
Chromosomes condense and become shorter and fatter.
Centrioles replicate and move to opposite ends of the pole and form a network of protein fibres across
The chromosomes, each with 2 chromatids each, line up along the equator and become attached to
the spindle by their centromere.
The centromere divides, splitting the two sister chromatids. The spindle contracts pulling each pair of
chromatids to opposite poles of the cell, centromere first.
Once the chromatids reach to opposite ends of the pole, they uncoil and become long and thin again,
called chromosomes again.
A nucleur envelope forms around the two sets of chromosomes forming two nuclei.
Cytoplasm divides, creating two new genetically identical daughter cells.
A human has 46 chromosomes, 23 pairs. One chromosome from each pair came from the mum and one from
the dad. The chromosomes that make up each pair are the same size and have the same gene although they
could be different versions of that genes called alleles. These pairs of chromosomes are called Homologous
pair of chromosomes.
Budding is the asexual reproduction in fungi. It produces genetically identical yeast cells.
A bud forms at the surface of the cell
The cell undergoes interphase and replicates its DNA and organelles.
The cell then undergoes mitosis.
After nucluer division is complete, there are two identical nuclei. The bud has the same genetic
information as the parent cell.
The bud then separates off and is a genetically identical yeast cell.
Cell produced by meiosis are not genetically identical.
Stem Cells are unspecialised cells that differentiate to become specialised.
In adult cells, stem cells are found in the bone marrow. These stem cells can only differentiate into neutrophils
and erythrocytes, they are not that flexible.
Embryonic stem cells can differentiate into any type of cell, they are very flexible and can help scientists to
treat medical conditions.
In plants, stem cells are found in the cambium. These stem cells differentiate into xylem and phloem.
Biconcave disc shape gives it a larger surface area for carrying oxygen
No nucleus means it has more space for Hb.
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Flexible shape allows it to engulf unwanted bacteria and pathogens.
Lots of lysosomes in cytoplasm which contains digestive enzymes to break down unwanted bacteria
Epithelial cells line a surface for example in the trachea.
Some epithelial cells like in the lungs have cilia on them to waft mucus to the back of the throat to be
swallowed, preventing lung infections.
Some epithelial cells like in the small intestine, fold into microvilli to give it a large surface area.…read more