Biology Reference
In-Depth Information
1.10 Genes
The concept of a “gene” has evolved and has become increasingly difficult to
define (
Muller 1947, Maienschein 1992, Nelkin 2001, Pearson 2006, Pesole 2008,
Brosius 2009
). Genes can be a specific location on a chromosome (the bead on
a string analogy), a particular type of biochemical material, and a physiological
unit that directs development. Genes can consist of DNA sequences that can be
spliced in alternative ways so that they are essentially coding for more than one
protein (
Pearson 2006
). We also know that genes transcribed into RNA can regu-
late other genes, and many genes are never translated into proteins.
Genes are segments of a DNA molecule that may vary in size from as few as
75 nt to
>
200
kilobases
(kb) of DNA. A kilobase is 1000 nt. Genes contain biologi-
cal information by coding for the synthesis of an RNA molecule. The RNA may
subsequently direct the synthesis of a protein molecule or the RNA may be the
end product (e.g., transfer RNAs [tRNAs], ribosomal RNAs [rRNAs], regulatory
RNAs). Proteins may regulate other genes, form part of the structure of cells, or
function as enzymes. Expression of the information contained in protein-coding
genes involves a two-step process of
transcription
and
translation
(
Figure 1.1
).
Genetic information is determined by one of the two strands of the dou-
ble-helix DNA molecule. The DNA containing the genetic information is called
the
coding strand
, and the other strand is the noncoding complement to it.
Sometimes, the coding strand is known as the
sense
strand and the noncoding
strand is known as the
antisense
strand. A few examples are known in which
both strands are the “coding strand” for part of the length of the DNA mol-
ecule, but the genes occur in different specific regions. Thus, one strand of the
double helix may be the sense strand over part of its length but be the antisense
strand over other segments (
Figure 1.8
). A protein-coding gene typically includes
a variety of regulatory structures and signals, as is described in Chapter 2.
Nonprotein-coding genes include genes that code for RNAs that are them-
selves the end products: the synthesized RNAs may be used directly as tRNAs,
rRNAs, small nucleolar RNAs (snoRNAs), small nuclear RNAs (snRNAs), and other
regulatory elements (
Eddy 2001, Sharp 2009, Tuck and Tollervey 2011
).
1.11 The Genetic Code for Protein-Coding Genes is a Triplet
and is Degenerate
The genetic code for a protein-coding gene is based on the sequence of three
nucleotides in the DNA molecule. The triplet sequence (or
codon
) determines
which amino acids are assembled in a particular sequence into proteins. It is
