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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…

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out its normal cellular functions.




chromatin not visible


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.


Due to DNA replication during interphase, each
chromosome consists of two identical sister
chromatids…

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energy for movement


spindle fibres disperse


nuclear membranes from around each set of
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…

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identical.



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





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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…

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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…

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