Question

Discuss in details, and in your own words, the steps of protein synthesis. This is a 20 point assignment so answers like "step 1 is transcription and step 2 is translation" are NOT acceptable. Make sure you go into details of each step and use your own words.

Answer 1

The protein synthesis

Proteins are one of building blocks of life. They form enzymes, tissues and muscles. Proteins are made of up long chain of amino acids, synthesise by every cell in our body. The process of protein synthesis is most critical and hence tightly regulated by several mechanisms. The proteins are coded by the information stored on the genetic material I.e DNA. The codes on the DNA are transcribed into sequence of codons on mRNA. The mRNA is then translated into proteins. This is the central dogma of life.

Protein synthesis begins with the transcription of mRNA from respective DNA sequence inside the nucleus. The specialised enzymes called RNA polymerase binds to promoter region of specific DNA sequence and catalysts the transcription. The mRNA is made using one of the strand of DNA called coding strand. The bases are added in anti parallel way and complementary bases are added. The A becomes U, T becomes A, G becomes C and C becomes G. The mRNA once synthesised, undergoes series of modifications like poly A tail at 3’ end, methyl guanine cap at 5’ end...etc. after the modifications are done the mature mRNA is transported out of nucleus into the cytoplasms. In the cytoplasm the mRNA is converted into protein by another process called translation.

The protein synthesis occurs in the specialised structures called Ribosome. The ribosome is a complex of RNA (rRNA) and proteins (ribosomal proteins). It is made of two subunits, 50s and 30s making 80s complex in prokaryotes; 60s and 40s making 80s in eukaryotes. The mRNA has ribosome entry sites before the coding region, this region attaches to the ribosomes and protein synthesis initiates. The ribosome is divided into three compartments. a site, p site and e site. The entry of mRNA occurs in A site. Upon binding, the respective amino acids are brought to the site by tRNA. tRNA bind to specific amino acids. They detect the bases on mRNA as triplets . The codons in combination three nucleotides are detected by tRNA and respective amino acids are attached to the growing chain of protein inside the p site of ribosome. Once all the termination sequence UGA, UGG, UAG are read by the tRNA, amino acid addition stops and protein chain is released from the E site of ribosome. Thus the protein chain is formed from the translation process. The codons are always read in triplets. There can be more than one codon coding for same amino acids, this is called Codon degeneracy. The ribosomes which are attached to endoplasmic reticulum are actively involved in protein synthesis.

The proteins once synthesised undergo modifications inside the endoplasmic reticulum and transported to respective site by Golgi bodies.