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

1. DNA helicase unwinds the DNA double helix
2. Free RNA nucleotides bind to the exposed bases on one of the DNA strands (the ‘template’ strand)
3. RNA polymerase joins the sugar-phosphate backbone of these RNA nucleotides
4. 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

1. the mRNA attaches itself to a ribosome and the tRNA brings amino acids
2. A tRNA molecule that has a complementary anticodon for the first codon on the amino acid binds to it
3. Another tRNA that is complementary for the the second codon binds
4. The amino acids they carry are joined together by the ribosome
5. 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