DNA is the molecule that carries instructions for our development. The genetic code is the sequence of nucleotides and amino acids in a polypeptide chain.
DNA is a polynucleotide made up of nucleotides containing deoxyribose (a sugar), a phosphate and a base. DNA is a double-helix structure, with hydrogen bonds between the bases, joined by complementary base pairing.
- The genetic code is non-overlapping – codons do not overlap so a single point mutation would only affect one amino acid rather than 3 if it were overlapping
- The code is degenerate/redundant – contains more information than it needs as it is the first 2 nucleotides which are responsible for determining the amino acid – if the final base in a triplet is changed by a mutation, this could still produce the same amino acid without affecting the polypeptide made. So a degenerate code helps protect organisms from point mutation.
- Transcription and transferring the genetic code into a complementary mRNA strand
- Translation and transferring the mRNA into a sequence of amino acids in the polypeptide chain
DNA à pre-mRNA à mRNA à polypeptide à protein
Occurs in the nucleus
- Hydrogen bonds between the bases are broken, separating two DNA strands, this is catalysed by DNA helicase
- RNA polymerase attaches to one of the DNA strands at a start codon
- This strand is called the template strand and is transcribed to give a complementary single strand of mRNA. This is brought about by the DNA polymerase
- Nucleotides join by complementary base pairing
- When RNA polymerase reaches a stop codon, it stops making mRNA and detaches from DNA
- This newly synthesised mRNA strand has the same sequence as the non-template
- The DNA strand with the same sequence as the mRNA is the sense strand. The strand which acts as the template is the antisense strand
- The mRNA then leaves the nucleus through membrane pores into the cytoplasm for translation
Post-transcriptional changes in mRNA:
mRNA is modified before translation
- Extrons – Contain coding information
- Introns – Non-coding information (nonsense sections) – pre-mRNA contains both introns and extrons
Changes take place in pre-mRNA before it leaves the nucleus and attaches to a ribosome. RNA splicing occurs – introns are removed and extrons are rejoined to form a single continuous coding strand of mRNA. This is carried out large enzymes called spliceosomes.
Sometimes extrons are removed too so the code on the final mRNA is different from the code on the DNA which was transcribed.
So strands of mRNA from the same bit of DNA may not be the same so will code for polypeptides containing a slightly different sequence of amino acids.
Occurs on ribosomes in the cytoplasm
- The mRNA enters the cytoplasm and attaches to the ribosomes
- The ribosome starts reading the mRNA at the start codon
- As each codon is read, the tRNA with the specific anticodon picks up the specific amino acid from the cytoplasm and carries it to the ribosome
- The tRNA lines up its anticodon alongside a complementary codon in the mRNA. There will always be two tRNA molecules held on the ribosome
- Hydrogen bonds between the two bind the tRNA to the ribosome while amino acids are joined by peptide bonds, catalysed by an enzyme
- Once the peptide bond is formed, the tRNA leaves the ribosome
- The ribosome moves along the mRNA molecule until it reaches a stop codon, leaving a completed polypeptide chain