DNA as the Genetic Code

A gene is a length of DNA with a base sequence that codes for the synthesis of a particular polypeptide. The genetic code is a triplet, degenerate, non-overlapping code.

Within the nucleus, the DNA is transcribed onto many mRNA moleucles that move through pores in the nuclear envelope into the cytoplasm to be translated into amino acid sequences

Gene mutations are spontaneous changes in a single base on the DNA molecule:

• substitution replaces one base with another, and causes an amino acid substitution in the polypeptide formed (or not since the code is degenerate)
• deletion removes a base causing a frameshift (altering all triplet codes after the point of mutation).

One gene one polypeptide theory - is the most accurate description of the link between the DNA  and its products.

Transcription

• The strands of DNA are separated ('unzipped') along the length of the gene by the enzyme helicase
• One strand acts as a template for the assembly of complementary mRNA, catalysed by enzyme RNA polymerase
• Introns ('junk' DNA) may be removed and the exons, containing coding regions, are spliced together to form functional mRNA

Translation

• The first two codons of the mRNA enter a ribosome
• tRNA molecules with the complementary anticodons bind with the RNA codons
• The specific amino acids carried onto the ribosome by tRNA are joined by peptide bonding
• The ribosome moves along the mRNA one codon length, the codon is translated and the process is repeated until the polypeptide is complete