Question

Explain the transcription and translation mechanism of protein and the role of GTP in translation process

Answer 1

Transcription

• Transcription is the process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule.

• RNA polymerase is the main transcription enzyme.

• Transcription begins when RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins).

• RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule.

• Transcription ends in a process called termination. Termination depends on sequences in the RNA, which signal that the transcript is finished.

Transcription can be broken into five stages: pre-initiation, initiation, promoter clearance, elongation, and termination:

1.The first step of transcription is called pre-initiation. RNA polymerase and cofactors (general transcription factors) bind to DNA and unwind it, creating an initiation bubble. This space grants RNA polymerase access to a single strand of the DNA molecule. Approximately 14 base pairs are exposed at a time.

2.The initiation of transcription in bacteria begins with the binding of RNA polymerase to the promoter in DNA. Transcription initiation is more complex in eukaryotes, where a group of proteins called transcription factors mediates the binding of RNA polymerase and the initiation of transcription.

3.The next step of transcription is called promoter clearance or promoter escape. RNA polymerase must clear the promoter once the first bond has been synthesized. Approximately 23 nucleotides must be synthesized before RNA polymerase loses its tendency to slip away and prematurely release the RNA transcript.

4.One strand of DNA serves as the template for RNA synthesis, but multiple rounds of transcription may occur so that many copies of a gene can be produced.

5.Termination is the final step of transcription. Termination results in the release of the newly synthesized mRNA from the elongation complex. In eukaryotes, the termination of transcription involves cleavage of the transcript, followed by a process called polyadenylation. In polyadenylation, a series of adenine residues or poly(A) tail is added to the new 3' end of the messenger RNA strand.

Translation

Initiation

Translation begins with the binding of the small ribosomal subunit to a specific sequence on the mRNA chain. The small subunit binds via complementary base pairing between one of its internal subunits and the ribosome binding site, a sequence of about ten nucleotides on the mRNA located anywhere from 5 and 11 nucleotides from the initiating codon, AUG.Once the small subunit has bound, a special tRNA molecule, called N-formyl methionine, or fMet, recognizes and binds to the initiator codon. Next, the large subunit binds, forming what is known as the initiation complex. With the formation of the initiation complex, the fMet-tRNA occupies the P site of the ribosome and the A site is left empty. This entire initiation process is facilitated by extra proteins, called initiation factors that help with the binding of ribosomal subunits and tRNA to the mRNA chain.

Elongation

With the formation of the complex containing fMet-tRNA in the peptidyl site, an aminoacyl tRNA with the complementary anticodon sequence can bind to the mRNA passing through the acceptor site. This binding is aided by elongation factors that are dependent upon the energy from the hydrolysis of GTP. Elongation factors go through a cycle to regenerate GTP after its hydrolysis. Now, with tRNA bearing a chain of amino acids in the p site and tRNA containing a single amino acid in the A site, the addition of a link to the chain can be made. This addition occurs through the formation of a peptide bond, the nitrogen-carbon bond that forms between amino acid subunits to form a polypeptide chain. This bond is catalyzed by the enzyme peptidyl transferase. The peptide bond occurs between the carboxyl group on the lowest link in the peptide chain located at the p site and the amine group on the amino acid in the A group. As a result, the peptide chain shifts over to the A site, with the original amino acid on the A site as the lowest link in the chain. The tRNA in the A site becomes peptidyl RNA, and shifts over to the P site. Meanwhile, the ribosome engages in a process called translocation: spurred by elongation factors, the ribosome moves three nucleotides in the 3' prime direction along the mRNA. In other words, the ribosome moves so that a new mRNA codon is accessible in the A site.

Termination

Translation ends when one of three stop codons, UAA, UAG, or UGA, enters the A site of the ribosome. There are no aminoacyl tRNA molecules that recognize these sequences. Instead, release factors bind to the P site, catalyzing the release of the completed polypeptide chain and separating the ribosome into its original small and large subunits.

During the elongation stage of translation, GTP is used as an energy source for the binding of a new amino-bound tRNA to the A site of the ribosome. GTP is also used as an energy source for the translocation of the ribosome towards the 3' end of the mRNA.