TRANSCRIPTION AND TRANSLATION
Introduction
The majority of genes are expressed as the proteins they encode. The process occurs in two step:
· Transcription = DNA ^ RNA
· Translation = RNA ^ protein
Gene Transcription: DNA ^RNA
DNA serves as the template for the synthesis of RNA much as it does for its own replication.
The Steps:
• An enzyme, an RNA polymerase, binds to the complex of transcription factors.
• Working together, they open the DNA double helix
• The RNA polymerase proceeds down one strand moving in the 3'⁵’direction.
• In eukaryotes, this requires — at least for protein-encoding genes — that the nucleosomes in front of the advancing RNA polymerase (RNA PII) be removed. A complex of proteins is responsible for this. The same complex replaces the nucleosomes after the DNA has been transcribed and RNAPII has moved on.
• As the RNA polymerase travels along the DNA strand, it assembles rib nucleotides (supplied as triphosphates, e.g., ATP) in to a strand of RNA.
• Each ribonucleotide is inserted into the growing RNA strand following the rules of base pairing. Thus for each C encountered on the DNA strand, a G is inserted in the RNA; for each G, a C; and for each T, an A. However, each A on the DNA guides the insertion of the pyrimidine uracil
(U, from uridine triphosphate, UTP). There is no T in RNA.
• Synthesis of the RNA proceeds in the 5' ^ 3' direction.
• As each nucleoside triphosphate is brought in to add to the 3' end of the growing strand, the two terminal phosphates are removed.
• When transcription is complete, the transcript is released from the polymerase and, shortly thereafter, the polymerase is released from the DNA.
Note that at any place in a DNA molecule, either strand may be serving as the template; that is, some genes “run” one way, some the other (and in a few remarkable cases, the same segment of double helix contains genetic information on both strands!). In all cases, however, RNA polymerase proceeds along a strand in its 3' ^ 5' direction.
Transcription
Transcription is the synthesis of RNA from a DNA Template. A single gene (DNA) is transcribed using only one of the two DNA strands, the coding strand. Its complement, the silent strand, is not used.
The two DNA strands of the gene move apart to provide access by RNA polymerase. This enzyme attaches to the initiation site at the 3’end of the coding strand of the gene (DNA). The enzyme moves along the coding strand, inserting the appropriate RNA nucleotides in place as dictated by the nucleotide sequence of the gene.
Transcription consists of three steps-
1. Initiation: Occurs at an initiation site at the 3’ end of the gene. The initiation site is a part of larger promoter region has its own for motor the promoter consists of a Tata box of about hundred employed mostly t and a a dash and an initiation sequence the Tata box is upstream of the initiation sequence protein protein Asia transcription factors attached to the promoters and help the help polymerases find the the and attach to the initiation site RNA polymerase attached to the initiation sequence.
2. Elongation: RNA polymerase moves along, unwinding one turn of the double helix at a time thus exposing about 10 bases. New RNA nucleotides are added to the 3’end of the growing mRNA molecule at a rate of about 60sec-. The double helix reforms behind the enzyme. Many RNA polymerase molecules can transcribe simultaneously (remember, the gene is hundreds of thousands of nucleotides long). Only one DNA strand, the coding strand, is transcribed. The other strand is not used (silent). But, which strand is coding and which silent varies from gene to gene. The reading direction is 3’ to 5’ along the coding strand.
3. Termination: When the RNA polymerase reaches the 5’end of the coding strand of the gene it encounters a termination site, usually AATAAA. Here the enzyme falls off the coding strand and releases the pre mRNA strand (which must now be modified). This has produced a strand of pre mRNA which contains many areas of non –sense known as introns interspersed between useful areas known as exons. This pre RNA must be modified to remove the introns. These non-sense areas are faithful transcriptions of similar non sense areas of the gene (DNA). Most genes have introns, but why is not known.
Translation
Translation is the process of converting the RNA codon sequences into an amino acid sequence. Genes determine phenotype through the synthesis of proteins. The next step is translation in which the nucleotide sequence of the m-RNA strand is translated into an amino acid sequence. This is accomplished by t-RNA and ribosomes. The amino acid sequence is encoded in the nucleotide sequence. This code is a (nearly) universal one that is now known in its entirety.
The initiator codon (AUG) codes for the amino acid N-formylmethionine (f-Met). No transcription occurs without the AUG codon. F-Met is always the first amino acid in a polypeptide chain, although frequently it is removed after translation. The initiator t-RNA/ m-RNA/ small ribosomal unit is called the initiation complex. The larger subunit attaches to the initiation complex. After the initiation phase the message gets longer during the elongation phase.
References
G.Peterson, R. (2019). Molecular Biology Applied Genetics (Vol. 1). (H.-C. Yang, & J. R. McCarthy, Eds.) Noida: wiley.
