Introduction
Genetic information in the form of the sequence of nucleotide
triplets in the DNA determines the structure and functions of cells by
specifying the nature of the proteins present in the cell. The process is done by
two steps which are now reasonably well understood. In the first step, the
nucleotide sequence from one of the two DNA stranded is transcribed on to mRNA,
hence this process is known as transcription. The newly transcribed mRNA is
complementary to one of the DNA that served as its template. The process is
known as transcription. In the second step, the mRNA sequence is converted into
specific sequences of amino acids in proteins where triplet bases of the mRNA
coded for each individual amino acid. This latter process is known as
translation.
Gene expression determines the state of differentiation of the cell
and has role in the regulation of cell activity in various physiological
states. Gene activation and repression caused by differentiation is stable and
inheritable. When gene expression is controlled by physiological signal, it is
transient and easily reversed. Despite these differences, many of the events
surrounding these two different kinds of regulation by gene expression are
common.
Cells at different periods during the life cycle may differ
significantly in structure and function. In fact, the multicellular organisms
possess different type of cells. Each type specialized to perform a specific duty.
These cells are said to be differentiated. The different types of cells differ
from each other in that each type expresses only a small portion of the genetic
information of the genome. Through the differentiation, part of the genome has
been lost, though it is not generally.
Gene Expression : Genotype
and Phenotype
The DNA of an organism is very long double-stranded helical molecule
made up of linear series of molecular units called nucleotides, of which there are
four types: Adenine, Guanine, Cytosine, and Thymine. Genes comprise specific
segments of this DNA, and the sequences of nucleotides within a gene constitutes
a code which specifies the order of amino acids in the proteins that are
building blocks of tissues and cells, as well as the enzyme and hormones that
allow metabolism to proceed. Each gene specifies the construction of different
protein. Enzyme plays an important role of physiological processes in a
biochemical reaction of an organism. Due to most of the metabolism activity in
an organism are catalyzed by enzyme. The product of metabolism is being used to
grow and develop the cell. However, a problem arises if the nature or
arrangement of the nucleotides in gene is altered. An aberrant amino acid
sequence may be specified in the particular protein that arises from such a modified
or mutated gene and in turn, this variant protein may have serious consequences
to the organism.
Nucleotide sequences in DNA, however, do not encode proteins. Many
sequences lying outside the coding segments of genes form part of the machinery
that can adjust the activity of specific genes. Not only do nucleotide
sequences in the DNA of an organism determine precisely the type and structure
of proteins that an organism will make, but they also comprise part of the
signaling machinery that controls the level to which these proteins are
produced by the organism in response to environmental conditions.
Another important feature of DNA molecule is their ability to serve
as templates for the manufacture of copies them selves. When cells divide and
when egg and sperm cells are produced, each progeny cell has a complete set of
genes which are virtually identical to the set in the original cells. Although
not completely error free, this DNA replication process forms the basis whereby
nucleotide sequence variations within genes and related DNA segments are
inherited.
While an organism‘s complement of genes (genotype) sets its
potential, the properties actually realized (phenotype) depends on the
interactions of all of these genes with the environment. An organism’s
phenotype unfolds during development and maturation when genes and the products
derived from them, interact with one another and with environmental factors,
made a morphological characteristic of an organism. A trait cannot be
attributed solely to genes or to the environment, since an organism requires an
inter play of both for successful survival. Genes provide the initial
guidelines for the development of an organism, and a range of possible
phenotypes. Within that predetermined range, a specific phenotype is molded by
environmental influences. When one is dealing with very complex characteristics
such as behavior, the interactions between heredity and environment are far too
complicated to be understood at this time. Nevertheless, there are inherited to
have a behavioral component as part of their phenotype.
Even if gene is present in an individual organism or cell, it may
not be active, or switched-up. Moreover, the number of copies of proteins
produced per unit time by different genes varies to satisfy cellular
requirements. Many gene products are needed only under certain conditions, and
regulatory mechanisms that function like an on-off switch allow such products to
be made only when required. Other more refined mechanisms can make minor
adjustment in the intracellular concentration of a particular protein in
response to needs imposed by the environment. On the other hand, when a gene is
active, its protein product has the potential to interact with other elements
within the environment of the cells as well as with external environmental
factor such as dietary components temperature.
The interaction between the organism and environment are complex,
but crucial to these are the precise molecular effect of environmental factor
in relation to the DNA of genes. Such interactions are of reaching significance
for the mechanisms of adaptation, acclimation, and evolution. It is also clear
that they are very relevant to the development of important organism
degenerative diseases and to processes that contribute the ageing.
A specific class of genes can be activated to yield special protein
that is used to protect vital cellular structure from environment adversity. In
relation to gene activity they are may layers of feedback regulation from both
the physiology of the organism itself and the external environment. Certain
types of stressful environmental conditions can activate stress gene to produce
stress protein that enable organisms to tolerate such stresses.
References
Anonim. 2006. E-book : Chapter 03, Gene Expression in Differentiated Cells.pdf.
http://albany/edu/03 part1.html.
Burdon, R. H. 1999. Genes and
The Environment. University
of Stratclyde. Scotland
Campbell, NA., and J.B. Reece. 2005. Biology. Pearson Education, Inc. San Francisco.
Tri Wibowo, Y. 2005. Biologi Molekular. Erlangga. Jakarta.
