DNA translation

Translation is the final step on the way from DNA to protein. It is the synthesis of proteins directed by a mRNA template. The information contained in the nucleotide sequence of the mRNA is read as three letter words (triplets), called codons. Each word stands for one amino acid.

During translation amino acids are linked together to form a polypeptide chain which will later be folded into a protein. The translation is dependent on many components, of which two are extra important. First of all; the ribosome which is the cellular factory responsible for the protein synthesis. It consists of two different subunits, one small and one large and is built up from rRNA and proteins. Inside the ribosome the amino acids are linked together into a chain through multiple biochemical reactions.

The second component is the tRNA, a specialised RNA molecule that carries an amino acid at one end and has a triplet of nucleotides, an anticodon, at the other end. The anticodon of a tRNA molecule can basepair, i.e form chemical bonds, with the mRNA's three letter codon.Thus the tRNA acts as the translator between mRNA and protein by bringing the specific amino acid coded for by the mRNA codon.

Translation process The translation process is   divided into three steps:
	Initiation: When a small subunit of a ribosome charged with a tRNA+the amino acid methionine encounters an mRNA, it attaches and starts to scan for a start signal. When it finds the start sequence AUG, the codon (triplet) for the amino acid methionine, the large subunit joins the small one to form a complete ribosome and the protein synthesis is initiated.
	Elongation: A new tRNA+amino acid enters the ribosome, at the next codon downstream of the AUG codon. If its anticodon matches the mRNA codon it basepairs and the ribosome can link the two aminoacids together.(If a tRNA with the wrong anticodon and therefore the wrong amino acid enters the ribosome, it can not basepair with the mRNA and is rejected.) The ribosome then moves one triplet forward and a new tRNA+amino acid can enter the ribosome and the procedure is repeated.
	Termination: When the ribosome reaches one of three stop codons, for example UGA, there are no corresponding tRNAs to that sequence. Instead termination proteins bind to the ribosome and stimulate the release of the polypeptide chain (the protein), and the ribosome dissociates from the mRNA. When the ribosome is released from the mRNA, its large and small subunit dissociate. The small subunit can now be loaded with a new tRNA+methionine and start translation once again. Some cells need large quantities of a particular protein. To meet this requirement they make many mRNA copies of the corresponding gene and have many ribosomes working on each mRNA. After translation the protein will usually undergo some further modifications before it becomes fully active.

Types of RNA:

·         mRNA - messenger RNA is a copy of a gene. It acts as a photocopy of a gene by having a sequence complementary to one strand of the DNA and identical to the other strand. The mRNA acts as a busboy to carry the information stored in the DNA in the nucleus to the cytoplasm where the ribosomes can make it into protein.

·         tRNA - transfer RNA is a small RNA that has a very specific secondary and tertiary structure such that it can bind an amino acid at one end, and mRNA at the other end. It acts as an adaptor to carry the amino acid elements of a protein to the appropriate place as coded for by the mRNA.

·         rRNA - ribosomal RNA is one of the structural components of the ribosome. Its sequence is the compliment of regions in the mRNA so that the ribosome can match with and bind to an mRNA it will make a protein from.

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