Transcript Transcription & translation

®
AP
Review:
Protein Synthesis
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Transcription
&
Translation
are the two major
steps in protein
synthesis
Transcription Animated
Translation Animated
From DNA an RNA template must
be made
The RNA template must then be
“read” so that a protein can be
synthesized
DNA  mRNA  tRNA protein
• DNA must first unzip so
that ONE side may be
read
• Which side? The one
that allows mRNA to be
synthesized in the 5’  3’
direction
• mRNA must leave the
nucleus and travel to the
ribosome so protein
synthesis can take place.
Leaving the nucleus
• The nucleus has
pores formed by
proteins
• DNA is double
stranded, too fat to fit
through the pore!
• mRNA is a single
strand and once it is
properly modified
[more later…] it can
exit through the pore
so translation can
occur at a ribosome
Complementary base pairs are added.
Uracil replaces thymine in RNA
Ribonucleic Acid
vs.
Deoxyribonucleic Acid
What’s
the
difference
here?
See the
“missing
” oxygen
atom?
Before mRNA can leave the
nucleus, it must be modified
• Not all of the DNA is
expressed at once.
• Usually only one gene
or a few genes at a
time
• Exons are the part of
the mRNA transcript
that are EXPRESSED
• Introns are the INERT
part
An enzyme cuts out the intron and
splices the exons together
This saves valuable ENERGY for
the cell so it is not making proteins
it doesn’t need.
Opening and closing the nuclear
pore
• Next, the ends of the
mRNA must be modified
to signal to the pore to
“open” and “close”
• A cap is added 5’ end
• A poly A tail is also added
to the 3’ end
• Remember synthesis is
5’3’ so the 3’ end is
always the tail!
Summary of mRNA processing
What makes transcription begin?
• There is a regulatory
region ON THE DNA
that regulates [go
figure] the
transcription of DNA
into mRNA
Both sides of the DNA can be utilized
at the same time, but synthesis occurs
in the 5’3’ direction
More about the regulatory region
later…for now, know that a
promoter “promotes” transcription!
We say that the regulatory region is
“upstream” from the coding region
of the gene. The promoter bases
are NOT transcribed.
Once transcription begins, many RNA
polymerases may read the DNA at
once to get hundreds of mRNA
templates made simultaneously—very
efficient!
What stops transcription? A sequence of bases
on the DNA that cause the RNA polymerase and
mRNA strand to be released—remember mRNA
has to be modified before it can leave the
nucleus!
This is an electron micrograph of many
RNA polymerases acting at once.
Many
mRNAs
being
formed
DNA
Transcription Animation
There are other types of RNA
tRNA is modified by adding a
trinucleotide CCA to the 3’ end to which
the amino acid can be attached
Remember there are 20 amino acids
necessary for life. 23 actually exist.
Three different ribosomal RNAs are
made from a long primary transcript by
removing sequences separating the
rRNAs.
RNA structure: not always a single
strand
Translation
Overview
• First the mRNA leaves
the nucleus
• The 2 subunits of the
ribosome attach to mRNA
• tRNA carrying an amino
acid matches its
anticodon to the codon on
mRNA
• Peptide bonds form
between the amino acids
• tRNA is released from the
ribosome
What does rRNA do?
• It is thought to have a
catalytic role
The start codon on mRNA always
codes for methionine
Easy to
remember…
…we always
start school in
AUGust!
There are three stop codons—they
do NOT code for any amino acid.
3 base pairs coding for an amino acid
allows for minor mistakes in the DNA
not becoming major mistakes in the
protein!
These are the most forgiving—get
the first 2 bases in the third doesn’t
matter!
Notice tryptophan is not at all
forgiving—it is only coded for by
UGG
Here’s the whole picture: just
memorize AUG as the start codon and
remember all polypeptide chains begin
with methionine. No need to memorize more.
What begins translation?
The ribosomal subunits consist of
rRNA and proteins—don’t
memorize!
Oversimplified view of tRNA
Now you know why we simplify!
One region contains the anitcodon.
The amino acid is attached to the 3’
end of the convoluted strand.
AUG on the mRNA is matched with
UAC on the tRNA and methionine
is now in place.
The next codon is “read” and the next aa is
placed. The peptide bond is formed
between the amine group of one aa and
the carboxylic acid group of the next aa.
A peptide bond is a dehydration
synthesis!
Once the peptide bond forms, tRNA releases its
aa and is recycled. The ribosome shifts down
the mRNA strand in a, you guessed it, 5’3’
direction.
Now we simple repeat until a stop
codon is reached….elongation
continues.
Translation stops when the stop
codon is reached at the 3’ end of
mRNA
Once a stop codon is reached, the
polypeptide is released.
Then the mRNA is released.
Translation Animation