24/05/2022
The biochemical process of protein synthesis in a cell ‼
Transcription
Transcription occurs when an enzyme, RNA polymerase, binds to a section of DNA and begins building a chain of RNA nucleotides to form a complementary strand of RNA. The Figure shows the details of this process. (The number of each step in the description below matches the sequence of numbered steps in the Figure)
1 RNA polymerase binds to a part of the DNA called the promoter and the DNA ‘unzips’ – that is, the DNA unspirals, hydrogen bonds between the two strands break, and the strands separate over a short length. This happens only in that part of the DNA that contains the gene to be used. Only one strand of DNA contains the genetic information to make a protein; rather confusingly, it is called the noncoding strand or sense strand; the other strand is called the coding strand (it has the same code as the mRNA being made) or antisense strand.
2 Transcription of the gene is controlled by the enzyme RNA polymerase. The sense strand of the DNA acts as a template and RNA nucleotides are assembled, forming a complementary single-stranded mRNA molecule (that is, DNA is transcribed into mRNA). The sequence of nucleotide bases on the mRNA molecule is the same as the DNA coding strand, except that it has U instead of T. (In eukaryotes, ‘editing’ or splicing of pre-mRNA may take place at this point).
3 The mRNA moves out of the nucleus and into the cytoplasm, where it encounters some of the millions of ribosomes in the cell. Usually one mRNA molecule is read by a large number of ribosomes, so multiple chains of the same polypeptide product are produced from one mRNA template molecule.
Translation
4 Translation occurs when the ribosomes move along the mRNA molecule and, as they do so, they attach tRNA molecules to mRNA by temporarily pairing the bases of the tRNA anticodons with their complementary triplets of bases (codons) on the mRNA.
5 The amino acids from the tail end of each tRNA are linked to one another by an enzyme to form a polypeptide chain. Each amino acid is then spliced off its tRNA carrier.
6 The tRNAs move away from the mRNA, leaving the growing chain of amino acids, and move back into the cytoplasm where they can pick up another amino acid and be reused.