One "gene" codes for one "protein"
Central Dogma (of cell biology) TRANSPARENCY (Fig. 5.28)
DNA (nucleus, virus) ONE GENE
mRNA (nucleus to cytoplasm in eukaryotic cell, although retrovirus, like
HIV, makes DNA from a template of RNA using an enzyme called reverse transcriptase).
Protein ONE PROTEIN
RNA's: m (messenger), r (ribosomes), t (transfers aa's)
Nucleic acids: nucleotide = sugar, PO4 & base (no essential nucleic
acids) TRANSPARENCY (Fig. 5.29)
Sugar (ribose or deoxyribose), phosphate (PO4), base
4 bases in DNA: Adenine, Guanine, Thymine, Cytosine
4 bases in RNA: The same except Uracil instead of Thymine
purines - A & G, pyrimidines - C, T, U
TRANSPARENCY (Fig. 1.5) so fundamental, it is the 5th figure in the whole
Two in a row nucleotides in a row would give 4 x 4 = 16 possibilities.
This is < 20, the number of amino acids, so 2 would not be enough for
However 4 x 4 x 4 = 64 is more than enough (redundant) and the word for
this is degeneracy, in that there are several codes for certain of the amino
4 x 4 x 4 - there is a 3 letter word (codon) consisting of 4 letters for
each amino acid.
It takes a longer macromolecule of DNA to code for a protein than the protein
it codes for.
Nucleotides are also usded (1) in the molecules ATP (used for energy and
to donate phosphate to reactions), (2) cAMP and cGMP (c=cyclic, intracellular
signalling molecules where diester bonds make the molecule cyclic) and (3)
and (combined with other groups) coenzymes
DNA is a double helix, with A across from T and C across from G TRANSPARENCY
(Fig. 5.30); this pairing is essential for DNA to reproduce itself. In making
RNA, the same pairing applies except that U is across from A.DNA is quite
stable and accurate in its replication. However, sometimes factors such
as chemical mutagens and ionizing radiation cause alterations called mutations.
In the fully evolved organism, mutations are usually deliterious, but they
can sometimes create an advantage. On the evolutionary time scale, mutations
have been the driving force of divergent evolution and adaptive radiation.
TRANSPARENCY (Fig. 5.19) shows that a disorder which turns human red blood
cells (erythrocytes) sickle shaped (sickle cell anemia) is caused by a mutation
substituting Val for Glu at amino acid #6 in the beta chain of hemoglobin.
This disorder is high in Blacks of equatorial African origin. Homozygous,
it is very bad, but heterozygous, it confers resistance to malaria.
This page was last updated 6/14/02
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