Transcription and Translation

Transcription and Translation


       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


       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




       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


       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


       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


       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


       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


       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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