Translation

Following the process of Transcription and RNA processing covered in our previous topic, we will now delve into the process of translation.

Translation

Translation describes the synthesis of a polypeptide using information from the mRNA. This process occurs at the ribosome, with a nucleotide sequence becoming an amino acid sequence. 

tRNA is plays a significant role in translating mRNA to an amino acid sequence.

Transfer RNA

tRNA has an anticodon region that is complementary and antiparallel to mRNA. tRNA carries the amino acid that mRNA codon codes for. The enzyme aminoacyl-tRNA synthetase attaches amino acids to tRNA. When tRNA carries an amino acid it is charged. 


Ribosomes

Translation occurs at the ribosome as mentioned earlier. Ribosomes are classified into two subunits, small and large. Prokaryotic and eukaryotic ribosomal subunits differ in size.

Prokaryotes small units = 30s, large subunits =40s
Eukaryotes small units = 40s, large subunits = 60s

The large subunit has three sites: A, P, E

A site is the Amino acid site, that holds the next tRNA carrying an amino acid

P site is the Polypeptide site, that holds the tRNA carrying the growing polypeptide chain

E site is the Exit site

Translation

Translation occurs within three stages identical to transcription

Step 1: Initiation

In initiation, begins when a small ribosomal subunit binds to the mRNA and a charged tRNA binds to the start codon, AUG on the mRNA. The tRNA carries methionine, subsequently the large subunit binds. Make note that the first tRNA carrying methionine will go to the P site, every other tRNA will go the A site first.

Step 2: Elongation 

Elongation begins when the next tRNA comes into the A site. mRNA is then moved through the ribosome and codons are read. Each mRNA codon codes for a specific amino acids. Codon charts are used to determine the amino acid similarly to transcription. Due to all organism sharing the same genetic code, this process supports the idea of common ancestry.

Step 3: Termination

Termination occurs in the event of a stop codon in the mRNA that reaches the A site of the ribosome. Stop codons do not code for amino acids, contrastly they stop codon signals fora  release factor.

This hydrolyzes the bond that holds the polypeptide to the P site, releasing polypeptides, and all translational units disassemble.

Protein Folding

As translation continues, the growing polypeptide chain begins to coil and fold (THINK BACK TO AMINO ACID STRUCTURES!)

Genes determine the primary structure, which determines the final shape.

Certain polypeptides require chaperone proteins to fold correctly and others require modification before it can function within the cell.

Retroviruses

As promised in previous topics we will cover how certain viruses can be expressed within genetic information.

Retroviruses namely HIV, are exempted from the standard flow of genetic information.

Information flows from RNA to DNA, via an enzyme known as reverse transcriptase.