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Transcription and Translation HL
2014!
Lets do this!!!
7.3. Transcription
• Protein Synthesis occurs in two stages:
transcription and translation
– Transcription
• Occurs in the nucleus
• Purpose is to create a molecule of
messenger RNA
7.3
• carried out in a 5’3’ direction (the 5’ end of
the free RNA nucleotide is added to the 3’ end
of the RNA molecule which is already
synthesized) (7.3.1)
• DNA is read 3’  5’
• No helicase is needed in this process RNA
polymerase breaks the H bonds in DNA and
repairs them
• RNA polymerase binds to parts of the DNA
called promoters in order for separation of DNA
strands to occur
Promoters
• Promoter region is piece of DNA that is noncoding but has a very important function
• TATA box –
– TATAAAA on 3’5’ strand
– ATATTTT on 5’  3’ strand
– This signals the start of transcription
• Transcription proceeds as nucleosidetriphosphates
(type of nucleotide) bind to the DNA template and
are joined by RNA polymerase in the 5' to 3'
direction.
7.3
• Transcription ends when RNA
polymerase reaches a termination site
on the DNA
• When it reaches the terminator, the
RNA polymerase releases the RNA
strand.
• The two DNA strands are labeled as
“sense” and “anti-sense”
•The sense strand is the non-template strand and
has the same base sequence as the mRNA (with
uracil instead of thymine).
•The antisense strand is the template strand
(strand being transcribed) and has the same base
sequence as tRNA
• In eukaryotes, some “editing” must occur
in order to produce a mature strand of
mRNA
• After RNA has been transcribed
enzymes(splicesome) will “cut” out the
introns (useless sequences of repetitive
nucleotides”junk DNA”)
• “glue” the exons (portions that code for
protein ”expressed”) back together
Viruses
– In some viruses (i.e. HIV) RNA is the
primary nucleic acid
– DNA must be transcribed from RNA in
order to enter the host cells genome
– The enzyme that helps to catalyze this
process is called “reverse transcriptase”
– Reverse transcriptase can be used for
biotechnology
• for example, it can make DNA from a
section of mature mRNA that codes for
the gene for human insulin
– this DNA section can then be spliced into
host DNA (eg E. coli) without the introns
– That bacteria will then create human insulin
Prokaryotes
– Transcription in Prokaryotes is slightly different
than transcription in Eukaryotes due to the fact
that they have a circular chromosome
– The best of example of prokaryotic
transcription is the lac operon model:
• operons are found only in prokaryotes and
are sections of the chromosome that code
for a particular gene or set of genes as well
as a promoter and an operator region
(which will be explained momentarily).
•
E. Coli bacteria use three enzymes to
break down lactose (milk sugar).
–Lac Z
–Lac Y
–Lac A
• These three genes are part of one
operon – the lac operon
• A regulator gene, located outside of
the operon, codes for a repressor
protein that binds to the operator
region
• Binding of the repressor to the operator
prevents RNA polymerase from binding to the
promoter region and thus prevents
transcription of the three lac enzyme genes
• However, an isomer of lactose will bind to the
repressor protein, and change its shape so it
doesn't fit with the operator
• This allows for the production of the three
enzymes
• Therefore, if there is no lactose (and therefore
no isomers of lactose), the repressor protein
will prevent the transcription of the enzymes
Translation
– Translation
• Takes place on the ribosomes
• Free ribosomes (in the
cytoplasm) synthesize
proteins for use primarily
within the cell
– ribosomes on the rough
endoplasmic reticulum
synthesize proteins for
secretion or for lysosomes
• Translation occurs in the 5’-3’
direction
– during translation, the ribosome
moves along the mRNA towards the
3’ end. The start codon is nearer to
the 5’ end than the stop codon.
• Translation consists of three stages:
initiation, elongation, and termination
• Ribosomes consist of two
subunits -light and heavy
(sometimes called the 30s and
50s subunits respectively)
–the light piece
»composed of one rRNA and
33 different proteins
–the heavy piece
»composed of three
different types of rRNA and
45 different protein
molecules
– Initiation
• After transcription is complete, mRNA
goes to the cytoplasm and attaches its 5'
end to the small subunit of the ribosome
• AUG is called the start codon because it
initiates the translation process
–remember: codon is a base triplet on
the mRNA strand
• mRNA inserts itself in between the
two ribosomal subunits
• There are three binding sites for tRNA
on the ribosome:
–A site (for tRNA binding)
–P site (for peptide bonding)
–E site (exit site)
• The anticodon
on one end of a
tRNA molecule
is
complimentary
to a specific
codon on the
mRNA
• The codon AUG hydrogen bonds to the
anticodon of a tRNA molecule holding
the amino acid methionine (called the
initiator tRNA).
• The initiator tRNA is now in the ‘A’ site
• As the ribosomal subunits slide over
mRNA the initiator tRNA moves from the
‘A’ site to the ‘P’ site
– Elongation
• Another tRNA (let's call it tRNA "X") carrying a
specific amino acid attaches itself to the next
codon at the A site of the larger subunit
• these two amino acids (methionine and the
amino acid on X) now make a peptide bond
with each other
• The initiator tRNA lets go of methionine, moves
to the E site, then leaves the ribosome
• At the same time, tRNA X moves
from the A site to the P site
• Another tRNA molecule (tRNA "Y")
attaches to the codon in the A site
• The amino acid that is attached to
tRNA Y makes a peptide bond with
the amino acid from tRNA X
• tRNA X lets go of it’s amino acid, moves
to the E site and leaves the ribosome
• tRNA Y moves to the P site to make room
for the next tRNA to enter the A site
• The process repeats itself
– Termination
• The stop codon is one that does not code for an
amino acid and that terminates the translation
process
• the polypeptide is released and the mRNA
fragments return to the nucleus
• These nucleotides are recycled and used for RNA
and DNA synthesis
• tRNA also is returned to its free state and
attaches to its specific amino acid so as to be
ready for the translation process when needed.
Polysomes
• Many ribosomes can translate the same
mRNA at the same time
• This allows for multiple proteins to be created
in a very short time from the same piece of
mRNA.
• tRNA specificity
– Each amino acid has a specific tRNA-activating
enzyme that helps tRNA to combine with its
complimentary mRNA codon.
– the enzyme has a 3-part active site that
recognizes three things:
• a specific amino acid
• ATP
• A specific tRNA
– The enzyme attaches the amino acid to
the 3' end of the tRNA.
– The amino acid attachment site is always
the base triple CCA.
– It is important to note that each tRNA
molecule can attach to one specific
amino acid, but an amino acid can have a
few tRNA molecules with which is can
combine