Transcript Protein Synthesis

Protein Synthesis
Also Known As…
Decoding the Central Dogma
Why the Central Dogma?
• The process of protein synthesis is summarized by the
central dogma of modern biology. (DNA  RNA  Protein)
• Why this strategy?
– DNA must stay within the nucleus. Why?
– The environment of the cytoplasm is very harsh in
comparison to the nucleus. You can’t risk the DNA being
damaged.
– If DNA could exit nucleus to meet with ribosomes, it
would need an exiting and reentry method for passing
through the nuclear membrane.
– How could the DNA make preparations for a second
type of protein if it were already preoccupied with a
protein already?
• To solve all of these problems, the process uses an
intermediary – RNA – which is capable of passing from the
nucleus to the cytoplasm with losing its form or function.
Transcription (DNA  RNA)
INITIATION
• RNA Polymerase will enter the area of
the gene and bind to the DNA
upstream of the gene. This area
upstream of the gene is called the
promoter. This is an area that has
many A’s and T’s so there are fewer Hbonds to split apart in order to access
the gene – energy conservation.
Transcription (DNA  RNA)
ELONGATION
• Once the DNA is opened up the RNA
polymerase begins to build the mRNA
molecule in a 5' to 3' direction. There is
no primer required to get the RNA
polymerase to go (unlike DNA
replication).
• The polymerase will only make an mRNA
copy of one of the strands – this is the
template strand. The other strand is
known as the coding strand.
• The promoter is not transcribed in this
process – only the area within the gene.
Transcription (DNA  RNA)
TERMINATION
• RNA polymerase moves down the
template strand until the end of the
gene is reached. At the end of the gene
is a terminator sequence which tells the
RNA polymerase to stop and get off the
DNA. The mRNA is now free to go while
the RNA polymerase may move on to
another gene’s promoter and make
more mRNA.
Transcription (DNA  RNA)
Post-Transcriptional Mods
• Once the mRNA is released from the DNA it is
modified in order to perform its role in the cytoplasm
successfully.
• A 5' Cap is added to the start of the primary
transcript (made 5' to 3'). This “cap” is an inverted,
tri-phosphate guanine nucleotide. The 5' cap help
protect the mRNA strand from enzymes that would
digest it. The 5' cap also aids in the initiation of
translation as it can signal the start point for the
ribosome.
• A 3' Poly-A tail is added to the 3' end of the mRNA.
The 3' poly-A tail helps the mRNA against
degradation by other molecules.
• Lastly, the non-coding regions of the mRNA – known
as the introns – are spliced out of the strand much
like an editing room floor of a movie studio. This
cutting and removing is carried out by spliceosomes.
The remaining, useful parts of the mRNA (called
exons) are joined together to make the finished
product – a complete and functional mRNA
transcript.
Translation (RNA Pn)
INITIATION
• Translation is performed by the ribosome – the
protein builder of the cell.
• The ribosome consists of two smaller parts – the 60S
and the 40S subunits. (The number refers to the size and
the S is for the “sedimentation rate” of the molecule when placed
in a centrifuge.)
• The ribosome recognizes the 5' cap of the mRNA
transcript and begins the process of translation at
this end of the mRNA. The ribosome moves along the
mRNA transcript in a 5' to 3' direction.
• The ribosome reads the mRNA in three nucleotide
segments at a time – these segments are called
codons on the mRNA. The role of the initiator
sequence becomes very important so that the
codons are read correctly in order to make the
protein according to specificity standards.
Translation (RNA Pn)
The Role of Transfer RNA (tRNA)
• The tRNA molecules deliver the correct
amino acid to the ribosome to be
bonded together.
• tRNA is a single stranded molecule of
RNA that has an anticodon loop that
consists of three nucleotides that are
complementary to the mRNA codons.
• Each tRNA has its own specific
anticodon that matches up with a
specific amino acid. There are three
mRNA codons that do not have a
matched tRNA anticodon and so these
are called terminator sequences.
Translation (RNA Pn)
ELONGATION
• The ribosome has two compartments – an A-site (acceptor) and
a P-site (peptide). The first tRNA with its amino acid (MET) is
brought into the P-Site and the anticodon and codon verify the
match/sequence.
• The next tRNA and its amino acid enter the A-site according to
sequence.
• A peptide bond is formed between the two amino acids by the
ribosome.
• The ribosome then slides one codon length (three nucleotides)
down the mRNA strand. The tRNA that was in the P-site is now
ejected from the ribosome and the tRNA that was in the A-site is
now in the P-site. The amino acids are connected with the
peptide bond.
• The ejected tRNA will let go of its amino acid and can be used
again to pick up an amino acid and bring it back to the ribosome
if the sequence is required.
• This continues until the ribosome hits the terminator codon on the
mRNA – there is no matching tRNA (or amino acid) for the
terminator sequences.
Translation (RNA Pn)
TERMINATION
• The ribosome hits the stop codon on the
mRNA (no matching tRNA or amino
acid) and just stops. A protein called a
release factor sees the stalled ribosome
and helps separate the ribosome and
the polypeptide chain.
• The two subunits of the ribosome will
let go. They can be used again.
• The polypeptide chain will begin to
assume its 3-D conformation/shape.
Translation (RNA  Pn)
Polysomes
• The cell often needs many copies of the
protein being made so to maximize
product, with minimal energy cost, the
mRNA strand may be translated by
many ribosomes at the same time in
order to get many proteins from one
mRNA molecule.
• The mRNA molecule can’t last forever
because of digestive enzymes that will
eventually degrade it and break it up so
you have to make the most of its brief
existence.
Polysomes
The Big Picture
DNA  RNA  Pn
That’s All I Got Genetics Fans!!!