Answer to Question #166967 in Molecular Biology for Aliya

Question

1.     Describe the process of protein synthesis. You answer must include the two major steps, all major enzymes and molecules involved, where each step occurs in the cell and a summary of what happens in each step. (4 marks)

 

 There are two processes that occur during protein synthesis, transcription and translation. In transcription, DNA is used as a template to make the messenger RNA (mRNA). Transcription occurs in the nucleus of eukaryotic cells. However, translation occurs when the mRNA leaves the nucleus and goes to a ribosome in the cytoplasm of eukaryotic cells. During translation, the genetic code is read by tRNA and the protein is synthesized according to the message present on mRNA. There are three steps in translation: initiation, elongation and termination. Initiation: always starts with the AUG codon therefore, it is always the first amino acid to be added.  Elongation: are amino acids that are added on one at time to the growing protein chain. The tRNA associates the anticodon to the codon of the mRNA and transports each time the correspondent amino acid, the peptidyl transferase forms the peptide bonds between amino acids.

Termination: is when there is a stop codon (UGA, UAG and UAA) the tRNA recognizes it, and it released and the translation ends.

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Answer 1

Protein synthesis involve two major steps - Transcription and Translation.

1. Transcription - It is the process in which mRNA is formed from one of the strands of DNA (called the template/antisense/non-coding strand). It takes place in the nucleus of a cell. The main enzyme involved here is RNA Polymerase (DNA dependent RNA Polymerase). Steps involved in this process are -

• DNA helicase unwinds the DNA helix.
• RNA polymerase binds to the promoter.
• The RNA chain elongates due to polymerisation as nucleotides are added to the chain.

As the polymerase reach the terminator region, transcription terminates and the nascent mRNA is released. This mRNA undergoes splicing, capping and tailing. Now the fully processed mRNA moves out of the nucleus into the cytoplasm. 

2.Translation - It is the process during which protein is synthesised from mRNA. The steps involved in this process are -

• Activation of amino acids - ATP and aminoacyl tRNA synthetase activate the amino acid.
• Charging of tRNA - Activated amino acids bind to its specific tRNA. This is is called charging/aminoacylation of tRNA.
• Initiation of polypeptide synthesis - As the mRNA comes into cytoplasm, ribosome binds to it. The first codon of mRNA is always AUG , so the first amino acid added by tRNA is Methionine. tRNA binds to the mRNA via its anticodon which is complimentary to the codon on mRNA.
• Elongation of the polypeptide chain - Ribosome moves over the mRNA chain from 5' to 3' end, and tRNA accordingly adds amino acids one at a time, thus elongating the chain. The amino acids are joined by peptide bond formed by the enzyme peptidyl transferase.
• Termination of the polypeptide chain - As the ribosome reaches the terminator region of mRNA (having codons UAG, UGA or UAA), protein synthesis is stopped and the newly synthesised polypeptide is released.