Next Lesson - Epigenetics, Genomic Imprinting and Mitochondrial Inheritance
Core
Gene expression is the process which involves transcription of DNA to messenger RNA (mRNA), and then the translation of mRNA to a protein.
A gene is a stretch of DNA that acts as the ‘recipe’ for a protein (or, for some genes, for a functional RNA such as transfer RNA (tRNA), which is not translated into a protein). Each gene has a chromosomal locus, which is its physical address on a chromosome. For example, the CFTR gene (the cystic fibrosis transmembrane conductance regulator gene) has the locus 7q31.2. This means it sits on the q arm (the long arm) of chromosome 7, at location 31.2. At first, it may seem useless to give each gene a locus. However, humans have about 20,000 protein-coding genes, and most of them are present in two copies in each diploid cell (a cell with two sets of chromosomes), so we need a system for finding them.
Transcription is the process of synthesising mRNA from a DNA template. mRNA is built through polymerisation, whereby RNA polymerase (the enzyme that builds RNA) adds one nucleotide to the end of the mRNA chain being built.
To initiate transcription, a transcription factor (a protein that helps switch a gene on) must bind to a region at the start of the gene, known as the promoter region. Many promoters contain a short DNA sequence that starts TATA, known as the TATA box. The TATA box is present in only some genes (about a quarter of human genes have a TATA-like sequence), and it is one of several sequences that transcription factors can recognise. Genes without one use other promoter sequences. The promoter orientation determines the direction of transcription, and RNA polymerase synthesises RNA in a 5’ to 3’ direction (5’ and 3’ name the two ends of a strand of nucleic acid).
In genes with a TATA box, RNA polymerase starts making mRNA about 30 bases downstream of it (further along the gene, in the direction of transcription). It then adds nucleotides one at a time, a stage called elongation. One strand of DNA is used as the template strand, and the mRNA is built complementary to it, so the regular base pairing rules apply. The only exception is that mRNA does not use thymine (T) as a base: it uses uracil (U) instead, so A pairs with U and C pairs with G. This process continues until the entire gene is transcribed.
After transcription, mRNA is described as pre-mRNA, meaning it is not yet mature. If pre-mRNA were to be released into the cytosol (the watery fluid inside the cell), it would be quickly broken down. To prevent this, mRNA is processed in 3 steps:
- Capping: a cap is placed on the 5’ end of the pre-mRNA strand.
- Tailing (aka polyadenylation): at the 3’ end, a tail of about 250 adenine nucleotides is added to the pre-mRNA in mammalian cells.
- Splicing: non-coding regions of pre-mRNA called introns are removed, and the exons (the parts that stay in the mature mRNA) are joined together. Many exons code for amino acids, but some are non-coding. Alternative splicing joins the exons of one gene in different combinations, so one gene can code for several related proteins.
At this stage, the mRNA is mature and can enter the cytosol.
Translation is the process of using mature mRNA as a template for the building of a polypeptide chain. This is achieved through the use of transfer RNA (tRNA) and ribosomes and occurs in the cytoplasm.
In their cytoplasm, humans (and all other eukaryotes) have 80S ribosomes (with a 60S and a 40S subunit), which are made of 4 ribosomal RNA (rRNA) molecules and about 80 proteins. Prokaryotes in contrast have 70S ribosomes (with a 50S and a 30S subunit). The S stands for the Svedberg unit, which measures how fast a particle sinks in a centrifuge, so the subunit values do not add up to the total (60 plus 40 is not 80). Mitochondria also have their own smaller ribosomes (55S in mammals).
In order to translate mRNA to a protein, we need to convert the 4 base ‘DNA language’ to the ‘protein language’ of 20 standard amino acids (human cells also use a rare 21st, selenocysteine) - this conversion is done through the use of tRNA.
mRNA is read at a ribosome 3 bases at a time; each set of 3 bases is called a codon. The specific set of 3 bases in the codon codes for a single specific amino acid e.g. UAC codes for tyrosine (see the diagram below).
Figure: By moving from the centre of the wheel to the edge it can be determined which amino acid is coded for by a sequence of bases in a codon.
Creative commons source by Mouagip (talk) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
As you can see from the above image, the triplet AUG on mRNA codes for methionine and is the usual start codon, so a new polypeptide chain begins with methionine. This methionine is often removed later, so it is not always the first amino acid of the finished protein. The following triplets on mRNA are then read, and the corresponding amino acids are added to the chain until a stop codon (a codon that signals the end of the chain) is reached. A stop codon is one of UAA, UAG or UGA. It does not code for an amino acid. Instead, proteins called release factors bind to it and the ribosome releases the finished polypeptide chain.
Figure: Shows the clover model of a tRNA molecule with the anticodon labelled.
Creative commons source by Yersinia~commonswiki, edited by Dr. Joshua Sturgeon [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Above is the clover model of tRNA (so called because the folded molecule looks like a cloverleaf). The anticodon is a section of three bases on the tRNA molecule. It is called an anticodon as it is complementary to a specific codon on the mRNA strand being translated, which allows translation from the 'DNA language' to the 'protein language'. tRNA moves towards the mRNA-ribosome complex and brings with it a specific amino acid. The codon on mRNA is ‘recognised’ by the tRNA anticodon, and the attached amino acid is held in place. A peptide bond then forms between this newly delivered amino acid and the previous one in the protein chain. This process continues until a stop codon is reached.
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
Edited by: Dr. Ben Appleby
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