Genetic Transcription

& Translation

‘To continue the linguistic, information-theory metaphor within which genetic theory was now to be formulated, the directed synthesis of RNA on DNA was termed transcription, and the synthesis of protein on the RNA was translation.’  Steven Rose, Lifelines: Biology, Freedom, Determinism, 1997

‘TRANSCRIPTION - The cell makes a copy of the gene whose message is to be interpreted. The copy is made of RNA, ribonucleic acid, a chemical similar to DNA. RNA has a different sugar in its sugar-phosphate backbone and is usually a single-stranded molecule.  And wherever there is a T in DNA, there will be a U in RNA, also pairing with A. The enzyme that carries out this process of copying is called RNA polymerase. It can recognise the 'start here' and 'stop here' signals that appear in the DNA code, and catalyse the formation of an RNA molecule using bases, sugars and phosphate molecules from the nucleus. The RNA molecule created is called messenger RNA (mRNA) because it is responsible for carrying the message from the nucleus to the cytoplasm, the outer part of the cell, where the code in the mRNA will be translated into protein. The mRNA passes through the pores in the nuclear membrane, and makes its way to the part of the cell where proteins are made, called the rough endoplasmic reticulum (ER). Called 'rough' ER because, under the microscope,  it has a bumpy, blobby appearance. The 'blobby' structures are ribosomes - the factories of the cell. Inside each ribosome are three different types of RNA molecule, called ribosomal RNA (rRNA).’ Yourgenome.org

‘TRANSLATION - Ribosomal RNA is responsible for translation, in which the mRNA code is used to create a protein molecule. The mRNA message is 'read' in groups of three bases at a time. Each group of three bases is called a triplet, or 'codon'. There are 4 x 4 x 4 = 64 possible codons, or combinations of three bases. Most of the codons correspond to a specific building block of protein - an amino acid. Many of the amino acids have more than one triplet coding for them. Because there are more codons (64) than there are amino acids (20), the code is described as 'degenerate'. Three of the possible codons don't actually code for an amino acid; instead they indicate 'stop' signals. One codon (ATG - for methionine) is the 'start' signal for proteins. It is down to another RNA molecule, called transfer RNA (tRNA). Unfolded tRNA is roughly the same shape as a clover leaf. At one end of the 'leaf', are three crucial bases. These bases are called an 'anticodon' and are complementary to one of the codons on the mRNA molecule. When two bases will bind to each other, they are said to be complementary – the base A always binds to T, or  (U in RNA) and C always binds to G. For the triplet GUC, the complementary codon would be CAG. These two codons would bind firmly together, with hydrogen bonds forming between each of the complementary bases. Each tRNA molecule becomes attached specifically to one of the 20 amino acids. As the protein is being formed, each codon on the mRNA molecule is read, one at a time. For each codon, the tRNA molecule with the complementary anticodon temporarily binds to the mRNA. The amino acid that is joined to the end of the tRNA molecule is brought in line with the growing polypeptide chain, and the amino acid links to the end of that chain. The tRNA disengages from the mRNA molecule, and the next codon on the mRNA molecule is available to be 'read'. The appropriate tRNA molecule is again joined to the mRNA molecule, and its amino acid joined to the polypeptide chain. The process of making a protein is called translation and is very similar to translating from one language to another - in this case from the four-letter language of DNA (interpreting all the full stops and starts of 'sentences') into the 20-letter language of proteins. ‘ Yourgenome.org?

‘Developmental biologists can observe genomic activiation taking place, more or less directly. For example, RNA contains the base known as uracil, whereas DNA contains thymine instead. If you add radioactive-labelled uracil to young embryos in culture you can see at that stage they start to incorporate it. They incorporate the uracil only when they start to make RNA – which, of course, signals the start of transcription.’ Ian Wilmut and Keith Campbell, Mammals Cloned, The Second Creation, Headline, 2001