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DNA REPLICATION
REVIEW
1. When does DNA divide?
2. Why does it happen at this time?
• What is the first step in DNA replication?
• Next, what is the job of the enzyme Helicase?
What is the replication fork?
1. What happens next?
2. What enzyme is responsible for this
action?
3. What is the significance of the three
phosphates on the free nucleotides?
LEADING STRAND VS. LAGGING STRAND
Why does the DNA always have to build from
the original DNA’s 3’?
Why does the DNA always have to build from
the original DNA’s 3’?
The Flow of Genetic Information.
How you get from a gene to a protein.
DNA
RNA
protein
DNA gets
all the glory,
but proteins do
all the work!
replication
We’ve talked about replication, now we
will focus on TRANSCRIPTION…
Transcription
The process of copying
a segment of DNA
into RNA.
This is the first step in protein synthesis.
DNA holds the instructions to make proteins, but can
NOT leave the nucleus.
Proteins are made outside of the nucleus on the tiny
organelles called ribosomes.
What’s the problem?
The DNA needs a messenger to take a
copy of the instructions to the
ribosomes so the proteins can be
made.
• Other than DNA, what is another type of
nucleic acid?
• There are 3 types of RNA, but today we will
only mostly focus on one- mRNA.
Messenger RNA (mRNA)
delivers a copy of DNA’s instructions from the
nucleus to the ribosomes.
• RNA’s structure differs from DNA’s structure
in three ways.
#1
RNA is single stranded—it looks like one-half of a
zipper—whereas DNA is double stranded.
#2
• The sugar in
RNA is ribose;
• REMEMBER:
DNA’s sugar is
deoxyribose.
#3
• Finally, both DNA and RNA can contain four
nitrogenous bases, BUT RNA does not have
Thymine.
• Thymine is replaced by a similar base called
uracil (U).
Uracil forms a base
pair with adenine in
RNA.
Steps of Transcription
1. DNA is uncoiled by enzymes and unzips
using Helicase, just as it does in DNA
replication.
Steps of Transcription
2. An enzyme called RNA polymerase
recognizes and binds to a DNA at a site
called a “promoter”.
A promoter is a base sequence in DNA that
signals the start of a gene.
Steps of Transcription
3. The RNA Polymerase then moves along the
DNA strand’s 3’ 5’prime direction, joining
one free RNA nucleotide after another until
it reaches a termination sequence.
Steps of Transcription
4. Once the termination sequence is reached,
the entire ‘coding gene’ has been
transcribed into an mRNA strand, and the
mRNA is cut away from the DNA.
Transcription differs from DNA replication in
three key respects…
1. Instead of copy the whole DNA strand, only
a selected gene within the DNA is copied.
2. Instead of DNA polymerase attaching free
nucleotides, it is RNA polymerase.
3. At the end of transcription there is a single,
free strand of RNA nucleotides, not a double
helix.
Transcription is complete at this point.
The mRNA at this point is considered “pre-mRNA”.
HOWEVER, the mRNA is not yet ready to leave the nucleus
and deliver DNA’s message.
It first needs some “finishing touches”……
called Post-Transcriptional mRNA processing.
Post-transcriptional processing
• First a cap and tail are added to the mRNA strand.
– Why?
• The cap helps the mRNA bind to the ribosome
5 GTP cap (modified Guanine nucleotide)
• The tail keeps enzymes in the cytoplasm from
“attacking” the mRNA strand.
poly-A tail (string of Adenine nucleotides)
3'
mRNA
5'
P
G P
P
A
Remember - Eukaryotic genes have
“junk”!
• Eukaryotic genes are not continuous
– exons = the real gene
• expressed / coding DNA
– introns = the junk, stuff you don’t need in
order to build a protein.
• inbetween sequence
intron = non-coding (in-between) sequence
eukaryotic DNA
exon = protein coding (expressed) sequence
mRNA splicing – Taking out the “Trash”
• Edit out the introns
• A “mature” mRNA strand has now been formed
– It is much shorter than the original
– Exons are Exported, Introns stay IN the nucleus
intron = noncoding (inbetween) sequence
~10,000 bases
eukaryotic DNA
exon = coding (expressed) sequence
pre-mRNA
primary mRNA
transcript
mature mRNA
transcript
~1,000 bases
spliced mRNA
Splicing must be accurate
• No room for mistakes!
– a single base added or lost affects the entire
protein
AUGCGGCTATGGGUCCGAUAAGGGCCAU
AUGCGGUCCGAUAAGGGCCAU
AUG|CGG|UCC|GAU|AAG|GGC|CAU
Met|Arg|Ser|Asp|Lys|Gly|His
AUGCGGCTATGGGUCCGAUAAGGGCCAU
AUGCGGGUCCGAUAAGGGCCAU
AUG|CGG|GUC|CGA|UAA|GGG|CCA|U
Met|Arg|Val|Arg|STOP|
RNA splicing enzymes
• Spliceosome
snRNPs
snRNA
– Made of small nuclear exon
ribosomes
5'
intron
3'
– recognize splice site
sequence
• cut & paste gene
exon
spliceosome
5'
3'
lariat
5'
mature mRNA
exon
5'
3'
exon
3'
excised
intron
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From gene to protein
nucleus
cytoplasm
transcription
DNA
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translation
mRNA
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protein
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ribosome
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