Monday, 9 September 2013

Autobiography - the structure and function of DNA (part 2)

Structure of DNA

"....It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."
Watson and Crick, 1953

Watson and Crick proposed that DNA forms a double helix: two separate DNA strands, each consisting of a row of subunits (A, T, G, C) attached to a ribbon-like backbone, are wound around each other. Within the double helix the subunits on one strand face and bind subunits on the other strand; only A binds to T and G binds to C.


Therefore, DNA is replicated by unzipping the double helix and each separated strand forms a template to allow individual subunits to bind their partner; thus generating two DNA molecules from the original molecule.

ATCG           ATCG           ATCG
TAGC                                TAGC
                    TAGC           TAGC

DNA encodes proteins

Proteins provide the structure of every cell in your body and carry out each function needed within the cell to keep you alive. DNA initiates and controls life by controlling which proteins are manufactured in the cell. Each protein is made from a gene, which is a specific sequence of DNA subunits.

DNA is said to express proteins: three subunits of DNA code for one amino acid (amino acids are the subunits of proteins) and it is the sequence of amino acids that determines the function of proteins.

The sequence of DNA is converted into proteins via RNA: DNA is unzipped but only one strand is a template to bind new subunits (A, C, G and U (instead of T)). A short sequence of these new subunits (called RNA) corresponding to a gene is then released from the strand of DNA.

CAGGAT                              CAGGAU        CAGGAU

RNA then encounters ribosomes, which mediate the interaction between RNA and amino acids.

                            Q    D          QD...

Amino acids are sequentially added according to the sequence of RNA and therefore DNA. The resulting protein is released into the cell.

See Nature 421 395-453

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