Question

Write an essay in which you explain how the sequence of proteins is encoded in the mRNA and how it is translated at the ribosome. Also using a drawing give a detailed description of how initiation, elongation and termination occur in bacteria.

explain how the sequence of proteins is encoded in the mRNA and how it is translated at the ribosome.

using a drawing give a detailed description of how initiation, elongation and termination occur in bacteria.

Answer 1

In the process of transcription, the genetic information present on the double-stranded DNA is transcribed to a single-stranded mRNA in the form of genetic code or codons that encode specific proteins with the help of RNA Polymerases. The Thymine in the DNA is replaced as Uracil in the RNA.

Genetic code is a triplet of nucleotides- A, T, G, C, U in random order that gives specific instructions to form a protein.

Three RNA's are involved in the process of RNA expression into proteins.:

• mRNA- messenger RNA- they carry transcripts of the coding sequences for protein expression.
• rRNA- ribosomal RNA- and important RNA in the ribosome structure.
• tRNA- transfer RNA- during protein expression, they carry the amino acids to the ribosomes.

A wide array of mRNA is present, forming the transcriptome. The transcriptome varies in different cell types. In the pancreas, it will have transcripts for insulin.

Ribosomes are the sites of protein synthesis. They are made of proteins and rRNAs. The rRNA's present inside the ribosomes, play a role in the catalytic step of joining of amino acids into a protein molecule.

mRNA's define the order in which new amino acids should be added to the growing protein chain.

tRNA finds the right codon for each amino acid. The tRNA has two ends, on one side amino acid is bound and to the other end, respective complementary codons bind. They will all assemble in the rRNA. The end chains will have thousands of amino acids, and these are synthesized into protein molecules.

Stages in translation for bacteria:

• Initiation
• Elongation
• Termination

Initiation:

In E.coli, the initiator complex involves:

• 30S ribosome,
• mRNA template
• 3 initiation factors for ribosome assembly
• Guanosine triphosphate (GTP) for energy
• N-formyl-methionine carrying initiator tRNA

The initiator tRNA identifies AUG start codons called the Shine Dalgarno sequence, which notifies the beginning of a coding sequence. The tRNA carrying anticodons will interact with the complementary codons of the rRNAs that form intact ribosomes after binding of the 50S ribosomal subunit.

Elongation:

The 50S subunit of ribosomes forms three sires:

• The aminoacyl (A) site- binds to the incoming aminoacyl tRNAs
• The peptidyl (P) site- binds to the tRNAs that have already formed peptide bonds in the polypeptide chains but haven't yet dissociated from the tRNA.
• The exit (E) site- helps in the exiting of dissociated tRNAs.

There is the formation of peptide bonds between the amino group of amino acids of A site and the carboxyl group of amino acids of the P site, by peptidyl transferases.

The amino acid now bound to the P site will go on to attach to the growing polypeptide chains. The old amino acid in the P site that was replaced will proceed to the E site and finally expelled.

This is a GTP dependant process.

Termination:

The termination step comes when the moving tRNA's are stopped by a non-sense codon like UAA, UGA, UAG. Release factors recognize these codons for the dissociation of formed polypeptides. After this, the large and small subunit of the ribosomes gets dissociated and they get involved in another translation process.