Page 352 - Applied Probability

Appendix A: Molecular Genetics in Brief
342
hopelessly tangled, and to control what genes are expressed when, the dou-
ble helix is wrapped around protein structures with a central core of eight
histone proteins. A complex of wrapped DNA and histone core is known as
a nucleosome. Nucleosomes are further organized into chromatid fibers
by supercoiling. Thus, the helical motif is repeated at several levels in the
construction of a chromosome. At the highest level of chromosome organi-
zation are the centromere and two telomeres. The centromere is critical
to proper division of duplicated chromosomes at mitosis and meiosis. The
telomeres protect the ends of a chromosome from degradation and control
the maximal number of divisions a cell line can undergo. In gene mapping,
the centromere and telomeres serve as cytologically visible landmarks. The
adjectives proximal and distal indicate centromeric and telomeric direc-
tions, respectively.
The 22 autosomes and the X chromosome of humans contain a total
9
of 3 × 10 base pairs. Embedded within the human genome are between
30,000 and 100,000 genes, most of which range from 10,000 to 100,000 bases
in length. Much of the genome consists of noncoding DNA whose function
is poorly understood. However, one should be careful in dismissing the
noncoding regions as “junk” DNA. All organisms must control the timing
and level of transcription of their genes. Geneticists have identified regula-
tory regions such as promoters, enhancers, and silencers upstream and
downstream from many genes. Other regions provide recognition sites for
recombination enzymes and attachment sites for the machinery of chromo-
some segregation during meiosis and mitosis. Even the patently junk DNA
of pseudogenes provides a fossil record of how genes duplicate, evolve,
and are eventually discarded. The full significance of the human genome
will become clear only after it is completely sequenced. Exploring and in-
terpreting this treasure trove should occupy geneticists for many years to
come.

TABLE A.1. Amino Acids
Amino Acid Abbreviation Amino Acid Abbreviation
Alanine Ala or A Leucine Leu or L
Arginine Arg or R Lysine Lys or K
Aspartic acid Asp or D Methionine Met or M
Asparginine Asn or N Phenylalanine Phe or F
Cysteine Cys or C Proline Pro or P
Glutamic acid Glu or E Serine Ser or S
Glutamine Gln or Q Threonine Thr or T
Glycine Gly or G Tryptophan TrporW
Histidine His or H Tyrosine Tyr or Y
Isoleucine Ile or I Valine Val or V