Transcription and Translation

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

 

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