transcription & translation
TRANSCRIPT
Transcription &
Translation are the two major steps in protein
synthesis
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
See the “missing” oxygen atom?
What’s the
difference here?
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.
DNA
Many mRNAs being
formed
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