Transcript DNA Replication Notes

DNA Replication
Advanced Biology
Replication
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Replication- the process of copying DNA in a cell
(specifically in the nucleus)
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End result is two identical copies of DNA
Use for cell division (one copy will end up in each cell)
DNA replication is called semi-conservative
replication because one of the original strands is
present or conserved in each new molecule of DNA
Can also be used to repair DNA that has been
damaged from heat, radiation, or chemicals
Replication Vocabulary
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Template strand
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Original DNA strand
Non-template strand
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Newly formed DNA strand
Process of Replication
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DNA strands will unwind
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Allows for more room to work on the DNA strands
The entire strand will not unwind all at once; only
a few parts will unwind
Process of Replication
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DNA strands “un-zip”
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Enzyme called helicase “un-zips” the strands at the
complementary bases
Two chains of nucleotides separate (template
strands)
Hydrogen bonds between two nitrogen bases are
broken
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Adenine and thymine have a double bond
Cytosine and guanine have a triple bond
Process of Replication
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“Extra” nucleotides attach to each DNA strand
by DNA polymerase (forms non-template
strand)
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RNA polymerase lays down a “primer” of RNA so
that the DNA has something to attach to, since
DNA polymerase can’t start the synthesis of DNA
Binding proteins support the original strand
(just after it separates from the other strand)
Strands will be built from the 5’ 3’ end
Replication
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Since replication occurs in one direction, one
strand will be working towards the replication
fork (leading strand) and the other will be
working outwards from the replication fork
(lagging strand)
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As helicase will be moving along the origianl
DNA, one side will be discontinous
Will have fragments of DNA  Okazaki
fragments
Replication
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RNA primers are taken off of the non-template
strand and DNA ligase will connect the
Okazaki fragments
Covalent bonds attach the backbone together
Hydrogen bonds attach the strands back
together
Replication
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Does not start at one end of each DNA strand,
it will simultaneously start at various points on
the strand
Enzymes (DNA polymerase) “proofread” the
strand before completion to minimize
mutations
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Mutation: a change in the nucleotide sequence
If a mutation occurs early in development of a
fetus, that mutation can be transferred as the DNA
is replicated during its growth and development
Replication
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Cancer is usually associated with rapidly
dividing cells
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Sometimes cancerous cells are treated with
chemotherapeutic drugs
Act like the four different nucleotides found in
DNA
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If the cancerous cells mistake the drugs for their DNA,
replication stops and the cells die
Semi-Conservative Replication
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In 1958, Matthew Meselson and Franklin Stahl
confirmed the theory of semi-conservative replication
Grew E. coli in a medium that had 15N (heavy isotope
of nitrogen)
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Nitrogen isotope was used by the DNA during protein
synthesis and replication
Bacteria grown in “normal” (14N) media have a light
density
DNA grown in 15N rich medium had very heavy DNA
(very dense)
Semi-Conservative Replication
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Bacteria (with nitrogen heavy DNA) were
transferred to a “normal” growth medium and
were allowed to reproduce once
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DNA after these replications had a density that was
between the “normal” DNA and the “heavy” DNA
Tells us that half of the DNA has the 15N and the
other half (non-template strand) has 14N or has an
“intermediate” density
Semi-Conservative Replication
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When the bacterial DNA was allowed to
replicate again (for a third time), there were
two versions (densities) of DNA
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Half the DNA had the “normal” density (both
strands used the 14N)
Half the DNA had the “intermediate” density (one
strand—non-template—using the 14N and the other
strand—template—used the 15N)