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DNA Replication

Notes on DNA Replication

Susan Innes

on 15 December 2009

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Transcript of DNA Replication

DNA Replication
2 Main processes:
Step 1:
DNA helicase unwinds double helix
by breaking Hydrogen bonds
The place where the DNA is opened is called the replication fork.
Step 2:
To prevent base pairs from rebonding (annealing),
single-stranded binding proteins (SSB's)
bind to the base pairs to keep the 2 strands apart
Step 3:
DNA gyrase relieves tension in the molecule that
occurs during unwinding (this happens before DNA
helicase kicks in)
Now that the DNA is unwound, time to build!
Step 1:

RNA primase creates a 'start signal' by binding a 3 nucleotide primer to the 5' end leading strand
Step 2:

DNA polymerase III starts adding complimentary nucleotides from the 5' to 3' direction - towards the replication fork
(the free nucleotides come from the cytoplasm)
Step 3:
DNA polymerase I removes
the primer and replaces it
with correct nucleotides
Step 4: Quality Control

DNA polymerase I and III
'proof-read' the new strands

If error found an exonuclease is
called in to remove and repair
Leading Strand
Where to start - anyone?
Lagging Strand
Step 1:

Replication moves away
from replication fork,
RNA primase places
primer to 'start'
Step 2: in fragments
DNA polymerase III encodes fragment
called: Okazaki fragments - only 100-200
nucleotides at a time
Step 3:
DNA 1 removes
primers and encodes
Step 4:
DNA ligase joins
Okazaki fragments
Step 5: Quality Control

DNA polymerase I and III
proof-read new strand for errors

If errors found, an
exonuclease removes errors
and replaces with correcct ones.
Protein Synthesis
RNA polymerase finds a "promoter" region to bind to. This region is strong in adenine (A) and thymine (T). A & T only require less energy to break as they only have 2 bonds, where G & C have 3.

RNA binds to promoter region which is always upstream of gene to be encoded.
mRNA single strand built in 5'-3' direction
doesn't need primer
replication starts right after promoter region
only 1 DNA strand used = template strand
DNA not used = coding strand
mRNA should be identical to ________?
transcription to end of gene until terminator sequence reached
mRNA released
RNA polymerase moves on to next promoter region
Post-transcriptional Modifications:
Capping and Tailing:
exon 1
exon 2
exon 3
5' cap
1) 5' cap protects RNA from digentsion by
harsh enzymes as it enters the cytoplasms
2) 200 - A added to 3' tail by poly-A polymerase
DNA has exons (coding) and introns (non-coding) regions
these are transcribed to RNA, introns need to be removed for translation to occur and
protein to fold and function properly
spliceosomes remove introns
exon 2
exon 3
5' cap
mRNA !
codon: group of 3 nucleotides
2 subunit ribosomes clamp
together and bind at 5' mRNA
ribosomal RNA (rRNA) is part of the ribosome; structural site of building proteins
sequence of codons makes up the reading frame
For example: mRNA must be positioned correctly
can be read as:
but if the ribosomes starts from the 2nd nucleotide:
transfer RNA (tRNA):
starts from the AUG (methionine)
"start" codon
tRNA carries amino acid and
pairs with complimentary codon on mRNA
next tRNA binds with next complementary mRNA codon in ribosome site A
AUG tRNA (start codon) moves left to site P
bonds are formed between 2 amino acids
next tRNA binds at site A and shift second amino acid to site P
process continues as chain is formed
Elongation continues until 'stop'
codon is reached: UAG,UGA or UAA.
ribosome stalls; disassembles and
releases mRNA and newly formed polypeptide
protein is folded and sent to area of cell where needed
Interactive Websites:
Mutations in DNA
Substitution Mutation
Missense Mutation
where a codon is changed due to a base DNA
sequence change
results in a different amino acid being placed
in the polypeptide chain
e.g. sickle - cell anemia

Nonsense Mutation
change in the DNA sequence causes a "stop"
codon to replace a regular amino acid
non-developed protein fragment is then
digested by cytoplasm enzymes

Point Mutations and Frameshift Mutations
occurs when one additional nucleotide is added into the DNA sequence
occurs at a specific point
this changes the reading frame

opposite of insertion
one nucleotide is omitted from the DNA sequence
occurs at a specific point
changes the reading frame

mutation that occurs on an intron
the intron is excised during transcription
mutation is irrelevantt = has no effect

redundancy of the genetic code = 4 ways to
code for 1 amino acid
if there is a third base change, there are three
other ways to code for the same amino acid

Translocation Mutation:
larger mutations
sections of 2 different chromosomes
change location on each other

if transcribed and translatedthe result
is a fused protein and disease: e.g. leukemia

1) spontaneous: DNA I misses an error
2) induced: mutagenic agents induce or cause
a mutation, e.g. UV radiation, X rays or chemical

Mutations can result in a postive, negative or have no effect on the organism
OK! Let's draw these:
Mrs. Innes will draw the translocation mutation.
I need 5 groups to draw one of the others, draw from a hat.

Video: Secret of Life "Accidents of Creation"

very cool
Full transcript