the cell cycle

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  • Created on: 05-05-12 19:37
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The cell cycle
Before a nucleus divides, its DNA must be replicated to ensure that the
daughter cells have the genetic information to produce enzymes and
other proteins. How is DNA replicated? Answer semiconservative
replication. In Semi conservative replications the enzyme DNA helicase
breaks the hydrogen bonds linking the base pairs of DNA. As a result
the double helix separates into its strands and unwinds. Both strands act
as templates. Complementary nucleotides are attracted to these
exposed polynucleotide strands. Energy is needed to activate these
nucleotides. These activated nucleotides are joined together by the
enzyme DNA polymerase to form the `missing ` polynucleotide strand
on each other two original polynucleotide strands of DNA. Each of the
new DNA molecules contains one of the original strands this is why the
process is called semi conservative.
In the experiment, used to provide evidence for the semiconservative
replication, bacteria were grown in a medium containing 15N. The DNA of
the bacteria will be made of DNA with 15N. The bacteria were transferred
into a medium containing 14N, the semi conservative model suggest that
every bacteria will have DNA that is made up of 50% 14N and 50% 15N.
Experimental proves this is what happens.
Mitosis produces daughter cells that are identical to the parent cell. The
only way mitosis can lead to variation is via mutation. Mitosis always
occurs after Interphase (where chromosomes are not visible). Mitosis is
summarised into four stages.
1. Prophase, chromosome become visible and the nuclear envelope
2. Metaphase, chromosome arrange themselves at the equator of the
3. Anaphase, the chromatids of each chromosome migrate to opposite
4. Telophase, nuclear envelope reforms

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The importance of mitosis & producing identical daughter cells
1. Growth
2. Differentiation ­ specialised cells can be replicated exactly to produce tissues
that function efficiently
3. Repair ­ mitosis allows repaired tissues to function as well as they did prior to
The cell cycle has three stages:
1. Interphase, occupies most of the cell cycle. In this stage no division
takes place. Interphase can be split up into three further stages:
a.…read more


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