Genetics—Transcription and Translation
Before reading the rest of this hub you should probably familiarise yourself with the following definitions:
- Gene - a length of DNA forming part of a chromosome, the order of which determines the order of monomers in a polypeptide.
- Genome - the entire DNA sequence of an organism. There are about 3 million nucleotide base pairs in the entire human genome.
- Polypeptide - a linear organic polymer consisting of a number of amino-acid residues bonded together, by peptide bonds, in a chain.
- Protein - one or more large polypeptide chains that are usually 100 or more amino acids long.
- Transcription - The first stage of protein synthesis whereby a single stranded mRNA copy is made from a DNA coding strand.
- Translation - the synthesis of proteins at ribosomes.
- Codon - a triplet of nucleotide bases.
The bullet points below outline the key parts of the process of transcription:
- The hydrogen bonds between the bases in the DNA double helix breaks and the molecule 'unzips'.
- The RNA nucleotides (ATP, GTP, CTP and UTP) have 2 extra phosphate groups attached
- Activated RNA nucleotides from the nucleolus bond (with hydrogen bonds) to the complementary base pairs on the DNA template strand. This is catalysed by the enzyme RNA polymerase.
- When the activated nucleotides bond a phosphate group is released which releases energy that helps the adjacent nucleotides to bond.
- A sugar-phosphate backbone is formed when the sugar-phosphate groups of neighbouring nucleotides bond together.
- The single-stranded mRNA molecule that is produced is released from the nucleus and passes through a pore in the nuclear envelope to a ribosome.
Transfer RNA (tRNA) is another type of RNA molecule that is essential for the translation process. Essentially the tRNA molecule is made of lengths of RNA that fold into 'hairpin' like structures. At one end of the structure is 3 exposed bases where an amino acid can temporarily bind. At the other end there is something called an 'anti-codon' which is 3 unpaired nucleotide bases which are able to bind (again, temporarily) with complementary codons on the mRNA strand.
Translation takes place on ribosomes which are either free in the cytoplasm or bonded to rough endoplasmic reticulum (rough ER). The ribosome's structure consists of two subunits which have a small groove in the middle which allows the strand of mRNA to pass through.
- The process of translation starts when a strand of mRNA binds to a ribosome.
- At any time only 2 codons are bound to the small subunit (and exposed to the large subunit) of the ribosome.
- The first exposed codon (triplet of base pairs) is always AUG.
- A tRNA molecule that bound to the amino acid methionine on one side and the anti-codon UAC on the other will bond to the exposed AUG codon on the mRNA strand forming hydrogen bonds.
- A second tRNA molecule with a complimentary anti-codon (and a different amino acid) will bond to the next codon on the mRNA strand and a peptide bond will form between the neighbouring amino acids.
- Specific enzymes catalyses the reactions between the anti-codon and the codon and between the amino acids.
- The ribosome will then move along the mRNA strand the process of the tRNA molecule binding to the codon and the amino acid forming peptide bonds with adjacent amino acids will continue until a 'stop codon' is reached.
- This stop codon (either UAA, UAG or UGA) consists of 3 nucleotides that have no corresponding tRNA molecules and thus the process is complete.
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