Biology Reference
In-Depth Information
Research has shown that a number of other mammalian cells, tissues,
and organs have their own biological clocks that can operate in the
absence of the SCN. Such clocks have been demonstrated to function in
organs, such as the lung, liver, heart, skeletal muscle, and other parts
of the brain, as well as in cultured fibroblasts (connective tissue cells).
Yamazaki et al. (2000) used transgenic rats containing a reporter gene
(one that makes a readily detectable protein) under the direction of
circadian gene control regions to look for circadian rhythms in rats (see
Section F later in this chapter). The particular gene arrangement was
engineered so that the reporter protein would be produced in a rhythmic
fashion in all tissues generating circadian rhythms. Rhythmicity was
observed in the SCN, liver, lung, and skeletal muscle. Further research
has demonstrated rhythmicity in other organs and tissues (Abe et al.
[2002]). In mammals, the SCN has been found to act as the controller or
pacemaker, keeping the other clocks coordinated through neural and
hormonal mechanisms. Thus, a relationship between the body's many
biological clocks exists, forming an internal temporal order (Richter et al.
[2004]). For additional details, we refer the reader to the review articles
by Richter et al. (2004) and Bell-Pedersen et al. (2005).
C. The Molecular Bases of Biological Clocks
The biological clocks responsible for circadian rhythms spring from
multiple feedback mechanisms involving both positive and negative
controls. Control of gene expression, protein-protein interactions,
post-translational protein modification, nuclear transport, and protein
degradation are all involved. In order to understand the control
mechanism, we must first briefly review the flow of genetic information
in the cell.
The cell's repository of genetic information is the deoxyribonucleic acid
(DNA). Information stored in the DNA is copied into ribonucleic acid
(RNA) and then used to direct the production of a protein. This idea,
which is called the central dogma of molecular biology, is shown in
Figure 11-1. DNA and RNA are polymers made up of subunits called
nucleotides. A nucleotide consists of a sugar (ribose for RNA or
deoxyribose for DNA), a phosphate group, and a nitrogenous base. Only
four different nitrogenous bases are used for each nucleic acid. They
are adenine (A), guanine (G), cytosine (C), and thymine (T) for DNA and
adenine (A), guanine (G), cytosine (C), and uracil (U) for RNA. The
structures of the four nucleotides used to make DNA are shown in
Figure 11-2. The only parts that differ among the four structures are the
nitrogenous bases. The genetic information in the nucleic acids is stored
in the sequence of the nitrogenous bases along the chain.
Proteins are the functional elements in the cell. They are the enzymes
that catalyze the cell's chemical reactions and are responsible for turning
