Page 351 - Applied Probability

Appendix A: Molecular
Genetics in Brief

A.1 Genes and Chromosomes

All of life is ultimately based on biochemistry and all of genetics on the
biochemistry of DNA and RNA. The famous DNA double helix discovered
by Watson and Crick carries the information necessary for the develop-
ment, maintenance, and reproduction of all organisms, from bacteria to
humans. The genetic code consists of an alphabet of four letters (or bases)
organized into words (or codons) of three letters each. Codons are further
grouped into genes or parts of genes known as exons. Genes are trans-
lated as needed by a cell into proteins; these in turn catalyze the many
reactions taking place in the cell and serve as structural components of
cellular organelles and membranes.
The four bases of DNA are adenine, guanine, cytosine, and thymine,
abbreviated A, G, C, and T, respectively. The first two of these bases are
purines; the latter two are pyrimidines. In RNA, a sister compound to
DNA, the pyrimidine uracil (U) is substituted for thymine. Both RNA and
DNA are polymers constructed by linking identical sugar units—ribose
in the case of RNA and deoxyribose in the case of DNA—by identical
phosphate groups. These sugar/phosphate linkages along the backbone of
the polymer (or strand) occur at carbon sites on the sugars designated by

the abbreviations 3 and 5 . One base is attached to each sugar; a single
repeat unit consisting of a sugar, phosphate group, and base is known as a
nucleotide. Codons are read in the 5 to 3 direction.

DNA is distinguished from RNA by its stronger tendency to form double
helices of complementary strands. The two strands of DNA are held to-
gether by hydrogen bonds between the bases projecting into the center of
the double helix from the backbones. The geometry of base pairing dictates
that adenine is paired to thymine and cytosine to guanine. These hydrogen
bonds are strong enough to stabilize the double helix but weak enough to
permit unzipping of the DNA for transcription of a gene on one strand
or replication of both strands when a cell undergoes division. Note that
in transcribing a gene, the antisense DNA strand serves as the template
rather than the sense strand so that the copied messenger RNA will
carry sense rather than antisense codons.
A chromosome is more than just a naked double helix. To protect DNA
from the occasionally harsh environment of the cell, to keep it from getting