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DNA and protein synthesis
- The genome of a cell is the complete set of genes that it contains
- The proteome of a cell is the full range of proteins that the cell is able to produce
- The proteome will be different for different types of cell
- Protein synthesis has two main parts: transcription and translation
Transcription
- Transcription is the production of mRNA from DNA
- mRNA is a single polynucleotide strand that contains groups of three bases called codons
- DNA helicase unwinds the DNA double helix
- Free RNA nucleotides bind to the exposed bases on one of the DNA strands (the ‘template’ strand)
- RNA polymerase joins the sugar-phosphate backbone of these RNA nucleotides
- In prokaryotes, the RNA strand formed is now mRNA. In eukaryotes, it is called pre-mRNA and it needs to be spliced (edited) to remove the introns and leave just the exons before it can be called mRNA
Translation
- Translation is the production of polypeptides from the sequence of codons carried by mRNA
- It involves tRNA (transfer RNA) which is a single strand of RNA folded into a ‘clover-leaf’ shape
- On one end is a specific sequence of three bases called an ‘anticodon’, and on the other end is an amino acid binding site
- the mRNA attaches itself to a ribosome and the tRNA brings amino acids
- A tRNA molecule that has a complementary anticodon for the first codon on the amino acid binds to it
- Another tRNA that is complementary for the the second codon binds
- The amino acids they carry are joined together by the ribosome
- This continues until the ribosome meets a codon called a ‘stop’ codon and this signals for translation to stop and the polypeptide to detach
- All the active processes in transcription and translation occur with the energy released from the breakdown of ATP into ADP + Pi
