Diagram showing structure of nitrogenous bases and how they bond together
DNA carries the information in a code which describes the organism and allows this information to be passed on to the next generation of cells so each cell can carry an exact replica of the information code.
DNA can be made from many million nucleotides, in the case of mammalian cells up to 2.5 billion units. This means the sequence of bases is almost infinite, providing an immense store of genetic information.
Diagrams showing a) the polymerisation of nucleotides by condensation and b) the formation of a double stranded DNA molecule
The code is carried in the information strand of the DNA molecule, the other strand is made from the complimentary bases to the information strand bonded to an identical sugar phosphate backbone and contains no code. The information has to code for 20 amino acids so cannot be made from single bases as it would only code for 4 amino acids, nor can it be from paired bases as it would code for 16 amino acids. Instead the code is made from triplets of bases which have enough combinations to code for 64 amino acids, as the body only needs to manufacture 20 amino acids several codes can form one amino acid so the code is said to be degenerate. Each triplet of the code is known as a "codon"
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To form an exact copy of itself DNA is unwound by an enzyme, DNA Gyrase, into two strands with unpaired bases. Each chain acts as a template to form a new complimentary strand along side it. As the bases are specific about with base can hydrogen bond with each other, T in the original strand will only lie along side A in the new complimentary strand and so on. As the appropriate nucleotide is brought into place it is joined to the growing molecule by DNA Polymerase which also proof reads the growing strand to make sure it is accurate. DNA is replicated always in the same direction, from 5' to 3', this means that the anti-parallel strand of DNA cannot be replicated from 3' to 5' and so replication occurs in short strips which are then polymerised together to form the new complimentary stand by another enzyme known as ligase.the correct sequence of codons to manufacture proteins.
Diagram showing transcription of mRNA from DNA
In the ribosome the mRNA is translated into the correct protein, a specific sequence of amino acids are formed to compliment the codons on the strand of mRNA. The amino acids are combined with a transfer RNA molecule which bind with a specific amino acid. The ribosome acts as a framework which holds the mRNA and tRNA together until the two amino acids form a peptide bond between each other. Once combined the ribosome will move along to the next codon written on the mRNA and the next amino acid is bonded to the growing peptide chain. Eventually the mRNA will contain a code that does not have an appropriate amino acid, this is one of the "stop" codes which terminates the protein synthesis and the polypeptide chain is released into the cytoplasm where it is assembled into a protein. The mRNA is easily broken down as it is unstable due to it's single stranded structure.
