Cell Cycle

Cell Cycle revision for AS Biology OCR

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  • Created on: 23-12-09 17:46
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The Cell Cycle
Cells are not static structures, but are created and die. The life of a cell is called the cell cycle and has four
phases:
In different cell types the cell cycle can last from hours to years. For example bacterial cells can divide every
30 minutes under suitable conditions, skin cells divide about every 12 hours on average, liver cells every 2
years.
The mitotic phase can be subdivided into four phases (prophase, metaphase, anaphase and telophase).
Mitosis is strictly nuclear division, and is followed by cytoplasmic division, or cytokinesis, to complete cell
division. The growth and synthesis phases are collectively called interphase (i.e. in between cell division).
Mitosis results in two "daughter cells", which are genetically identical to each other, and is used for growth and
asexual reproduction.
Cell Division by Mitosis
Mitosis is a type of cell division that produces genetically identical cells. During mitosis DNA replicates in the
parent cell, which divides into two new cells, each containing an exact copy of the DNA in the parent cell. The
only source of genetic variation in the cells is via mutations.
Interphase
This is when the cell is not dividing, but is carrying

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DNA, histones and centrioles all replicated
Replication of cell organelles e.g. mitochondria,
occurs in the cytoplasm.
chromosomes condense and become visible ­ this
prevents tangling with other chromosomes.…read more

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Telophase
chromatids
nucleoli form
Cytokinesis
In animal cells a ring of actin filaments forms round
the equator of the cell, and then tightens to form a
cleavage furrow, which splits the cell in two.
In plant cells vesicles move to the equator, line up
and fuse to form two membranes called the cell plate.
A new cell wall is laid down between the membranes,
which fuses with the existing cell wall.…read more

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You don't need to know the details of meiosis at this stage (It's covered in module 5).…read more

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Meiosis and Sexual Reproduction
Sexual reproduction is the production of offspring from two parent using gametes. The cells of the offspring have
two sets of chromosomes (one from each parent), so are diploid. Sexual reproduction involves two stages:
Meiosis the special cell division that makes haploid gametes
Fertilisation the fusion of two gametes to form a diploid zygote.…read more

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Chromatid single arm of an Xshaped
chromosome
Homologous Chromosomes
If a dividing cell is stained with a special fluorescent dye and examined under a microscope during cell division,
the individual chromosomes can be distinguished. They can then be photographed and studied. This is a
difficult and skilled procedure, and it often helps if the chromosomes are cut out and arranged in order of size.…read more

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