Download Handout on the Central Dogma

THE CENTRAL DOGMA OF LIFE
This is the Central Dogma of life. It is mind-bogglingly complex, and every single living cell does
it. At least 200 genes and 200,000 rungs of the DNA are needed just to set up the machinery.
Incidentally, the machinery must first get itself going -- a sort of ultimate bootstrap: how do things
get kicked off?
• The genetic information is recorded in long ladder-like molecules of DNA. Each rung of
the ladder (a base pair) is a pair of 4 short molecules called nucleotides A,T,C and G: A
(adenine) pairs with T (thymine), and C (cytosine) pairs with G (guanine). The sides of
the ladder are sugar molecules strung end-on-end. The size of the DNA is expressed in
the number of base pairs.
A Codon is a triplet of base pairs. Each codon corresponds to one of twenty Amino acids
-- it’s the amino acids that are the building-blocks of proteins, which do the work of the
cell.
A gene is a sequence of codons. Each gene corresponds to a particular protein that is
used by the cell to do its work1.
To every codon there is a special molecule called tRNA that has that codon on one end
and has its associated amino acid attached to the opposite end. A given codon (virtually)
always associates with the same amino acid -- across all species. Generally a cell must
have at least 30 of these tRNA molecules to make its proteins; each one must have the
genes to build itself as part of the cell's DNA.
• Hanging around the cell's DNA are a number of molecules -- RNA polymerases,
repressors and other regulatory molecules that determine if, when and how often a given
gene will be copied to make proteins. These molecules are part of a complex regulatory
system that is an essential for reading the DNA. Without this regulatory apparatus -which may be as complex as the DNA itself -- the DNA is useless.
• When a given gene is to be copied, the rungs of the DNA ladder split and the left-half is
copied into a messenger molecule called mRNA. After error checking and correcting, this
mRNA moves away from the DNA to the main part of the cell to begin making proteins.
• Complex motor molecules called ribosomes surround the mRNA chain and begin to
transcribe the codons and build an amino acid chain that will be the future protein 2. This
is done by matching a tRNA molecule to a codon, and then detaching the amino acid and
adding it to the building chain. Every living species from the simplest to the most complex
requires ribosome motors to carry out gene processing.
• Once the protein chain is complete, it then detaches from the ribosome and folds into
the shape that it must have to carry out that protein's specific function. Generally this
folding uses other helping molecules that guide it to make the correct shape.
1
Some genes code for RNA molecules -- such as tRNA -- rather than proteins. There are many RNA molecules used in
regulation.
2
It takes over 50 genes just to form the ribosomes. See the wikipedia article on ribosome biogenesis.