Transcription and Translation

 

Genetic information is transferred from DNA to RNA to protein. There are two steps in this process, transcription and translation.

Transcription:

• The synthesis of RNA from a DNA template.

Initiation:

• Only one strand of DNA is transcribed. This strand is called the template strand. The other strand is not transcribed and is called the coding strand. The coding strand has the same sequence as the product mRNA except it has thymine instead of uracil .

• RNA polymerase: The main enzyme that catalyzes the synthesis of RNA from a DNA template. Promoter region: The RNA polymerase binds to a promoter region on the DNA which consists of a sequence of nucleotides that show the RNA polymerase where to initiate transcription.

• Two promoter sequences are required to show the RNA polymerase complex the correct strand for binding and the correct orientation.

• After binding to the DNA, the RNA polymerase unwinds and opens a section of the DNA.

Elongation:

• RNA polymerase synthesizes mRNA that is complementary to the template strand of DNA with the T being replace with U. The RNA polymerase add new nucleotide to the 3'-OH group moving in the 5'to 3' direction but since only one strand of template DNA is being synthesized  in one direction, Okazaki fragments are not needed.

• While one RNA polymerase moves along the DNA, another RNA polymerase binds to the promoter region synthesizing another mRNA molecule. This allows hundreds of copies of nRNA molecules to be made from one gene at a time.

• Synthesis of mRNA occurs at a faster rate than synthesis of DNA because RNA polymerase does not have a proofreading function. An error in transcription only results an error in one protein molecule, not in the genetic make-up of organism and therefore is not significant. Synthesizing mRNA at a higher speed is more important than minimizing sequence errors.

                                             

Termination:

• Specific nucleotide sequences in template DNA signal the stop of transcription. When RNA polymerases reaches this signal it detaches from the DNA strand.
• The new mRNA strand is released and the DNA double helix rewinds.
• A G-cap is added on the 5'end and a poly-A tail at the 3'end to make the mRNA stable.

Translation:

• The next step is translation where mRNA template is used to produce proteins with amino acid sequences.

Initiation:

• Proteins called initiation factors assemble small and large ribosomal sub-units, mRNA, and initiator tRNA.

• The small ribosomal subunit binds to mRNA near the start codon (AUG). Then, the initiator tRNA carrying an amino acid binds to the mRNA start codon with its complementary anticodon (UAC) and the corresponding amino acid methionine.

• The large ribosomal subunit joins to form the ribosome and starts the process. There are three binding sites for the tRNAs: the P(peptide) site, A(amino acid) site, and E(exit) site with the initiator tRNA occupying the P site and the A site ready for the next tRNA.

Elongation: (4 Steps)

• With the initiator tRNA occupying the P site, the next tRNA carrying second amino acid enters the A site. Elongation factors enable the tRNA anticodons to bind to mRNA condons.

• A peptide bond forms between the first and amino acid and the amino acid in the A site and the chain transfers from the initiator tRNA to the new tRNA at the A site. The resulting dipeptide is attached to the tRNA at the A site.

• The polypeptide chain is one amino acid longer nad the mRNA moves ahead by one codon and the tRNA goes to the P site.  

• The tRNA no longer carries its amino acid and exits from the E site.The new codon is at the A site ready to receive the new complementary tRNA.

                  

Termination:

• Elongation continues until the mRNA reaches a stop codon (with the codon sequences UAG, UGA, or UAA).

• A protein called a release factor cuts the polypeptide chain from the last tRNA.

• The polypeptide is released and folds into a three-dimensional shape ready to carry out its activities.