Transcription
and Translation
Objective
• At the end of this lecture, student
will be able to
– Describe the steps involved in RNA
synthesis (transcription) and protein synthesis (translation)
Gene to protein
• A gene is used to build a protein in
a two-step process
RNA Synthesis
(Transcription)
• Process
of synthesis of RNA that takes place on a DNA template
• First
step of gene expression, in which a particular segment of DNA is copied into
RNA (mRNA) by the enzyme RNA polymerase
• Genetic information stored in DNA is
expressed through RNA
• For this purpose one of the two strands of DNA serves as a template(
non-coding strand or
sense strand) and produces working copies of RNA molecules
• Other DNA strand which does not
participate in transcription is referred to as coding strand or antisense strand or non-template
strand
• In the newly made RNA, all of the T
nucleotides are replaced with U nucleotides
• RNA polymerase is the main transcription enzyme
• Transcription involves 3 different
stages
• Initiation
• Elongation
• Termination
• Before transcription can take place,
the DNA double helix must unwind near the gene that is getting transcribed
• Region of opened-up DNA is called a transcription
bubble
• Transcription begins when RNA
polymerase binds to a promoter sequence near the beginning of a gene
• RNA polymerase uses one of the DNA
strands as a template to make a new, complementary RNA molecule
• Transcription ends in a process
called termination
Transcription initiation
• To begin transcribing a gene, RNA
polymerase binds to the DNA of the gene at a region called the promoter
• Transcription starts at the promoter
site of DNA
• Each gene has its own promoter
• A promoter contains DNA sequences
that let RNA polymerase attach to the DNA
• RNA polymerase consists of core
enzyme and sigma factor
• Sigma factor recognizes the promoter
sequences
• Once the transcription bubble has
formed, the polymerase can start transcribing
Elongation
• RNA polymerase binds with the
promoter region, the sigma factor is released and transcription proceeds
• RNA is synthesized from 5′ end to 3′ end (5’͢ 3′) antiparallel to the DNA template
• Sequence of nucleotide bases in the
mRNA is complementary to the template DNA strand
• lt is identical to that of coding
strand except that RNA contains U in place of T in DNA
• During elongation, RNA polymerase
moves along template strand, in the 3′ to 5′ direction
• For each nucleotide in the template,
RNA polymerase adds a matching (complementary) RNA nucleotide to the 3′ end of
the RNA strand
Termination
• Process of transcription stops by termination
signals
• Termination is signalled by a
sequence in the template strand of DNA molecule, a signal that is recognized by
a termination protein, Rho (ρ)factor (in prokaryotes)
• At specific termination sites, new
RNA chain is released
• After termination, core enzyme
separates from DNA template and core enzyme binds with sigma factor and takes
part in formation of new RNA molecule
• After end of termination, Rho (ρ)factor dissociates RNA and this factor is again recycled
Protein Synthesis
(Translation)
• Biosynthesis of a protein or a
polypeptide in a living cell is referred to as translation
• mRNA is translated into proteins
with the help of ribosomes and tRNA
• In mRNA, the instructions for
building a polypeptide are RNA nucleotides (As, Us, Cs, and Gs) read in groups
of three called codons
• Three nucleotide (triplet) base
sequences in mRNA that act as code words for amino acids in protein constitute
the genetic code or simply codons
• Each of 20 amino acids is coded by
one or more of these triplets
• Three codons UAA, UAG and UCA do not
code for amino acids
• They act as stop signals in protein
synthesis
• These three codons are collectively
known as termination codons or non-sense codons
• Codon AUG that specifies the amino
acid methionine acts as chain initiating codon
• Genetic code is universal – same
codons are used to code for the same amino acids in all the living organisms
• Genetic code
• Ribosomes are structures where polypeptides
(proteins) are built
• Each ribosome has two subunits, a
large one and a small one, which come together around an mRNA
• Transfer RNAs, or tRNAs, are molecular
“bridges” that connect mRNA codons to the amino acids they encode
• One end of each tRNA has a sequence
of three nucleotides called an anticodon, which can bind to specific
mRNA codons
• Other end of the tRNA carries the
amino acid specified by the codons
Step 1- Initiation
•
In initiation, the ribosome assembles around the
mRNA to be read and the first tRNA
•
mRNA
transcript with start codon AUG attaches to the small ribosomal subunit
•
Small
subunit attaches to large ribosomal subunit
Step 2 –
Elongation
•
As ribosome moves, two tRNA with their amino acids
move into site A and P of the ribosome
•
tRNA binds to matching codon
•
Peptide bonds join the amino acids
•
mRNA is shifted one codon over in the ribosome,
exposing a new codon for reading
Termination
•
After several cycles of elongation and formation of
specific protein/ polypeptide molecule, one of the stop or termination signals
(UAA, UAG and UCA) terminates the growing polypeptide
•
It is the stage in which the finished polypeptide
chain is released
End Product –The Protein!
•
The end
products of protein synthesis is a primary structure of a protein
•
A
sequence of amino acid bonded together by peptide bonds
Protein synthesis inhibitors
Summary
•
Transcription
involves RNA synthesis
•
Protein synthesis(translation)
is of three steps
•
The
ribosome binds to the mRNA at the start codon (AUG) that is
recognized only by the initiator tRNA
•
The
ribosome moves from codon to codon along the mRNA
•
A
release factor binds to the stop codon, terminating translation and releasing
the complete polypeptide from the ribosome
•
Some
antibiotics are protein synthesis inhibitors