DNA contains the instructions for making all the proteins in the body (roughly 25,000 in humans). Each gene carries instructions for one protein. To produce a protein, the cell goes through 2 steps :
Transcription : the code on the DNA is copied to mRNA. The 2006 Nobel prize for Chemistry was given to Roger Kornberg who worked out the details of transcription.
Translation : the mRNA code is read by ribosomes in the cytoplasm, which join amino acids together.
The code on DNA and mRNA is a triplet code : 3 bases code for 1 amino acid eg GGA codes for Glycine.
One code on mRNA is the "start" code : AUG. Several codes on mRNA signal "stop" eg UGA.
Transfer RNAs ( tRNA) bring each amino acid to the ribosome. Each tRNA has an anticodon which matches the 3 base codon on mRNA. The ribosome moves along the mRNA strand, adding amino acids one at a time.
Small pieces of "interfering RNA" could be used medically to switch off particular genes, for example to cure cancer or genetic diseases. Gene-blocking therapy. The 2006 Nobel Prize for Medicine was awarded to the two scientists who discovered RNA interference.
In eukaryotic cells, genes have introns ( that do not code for proteins "junk DNA" ) and exons ( that code for proteins ). Some of the introns probably have some function (so they are not really "junk"): "Junk DNA"
Mutations (changes in the DNA code ) can cause a different protein to be made. Fig 13-20. Most mutations are harmful eg cancer, but very occasionally they produce a better protein than the original, which is important in evolution.
Last edited April 2015 by David Byres, David.Byres@fscj.edu