Gene expression

Meaning of Gene Expression. Gene expression refers to the molecular mechanism by which а gene shows its potential in the phenotype of an organism.

Mechanism of Gene Expression. А gene contains the blue print or code for а particular polypeptide in the form of а specific sequence of its base pairs. It transfers its code to mRNA transcribed from it. The process is called transcription. Other genes transcribe rRNA and tRNAs. The mRNA binds to ribosomes and with the help of suitable tRNAs selects the required amino acids and links them in а proper sequence received from the gene to form а polypeptide. This process occurs on the ribosomes and is called translation. The polypeptide chain so formed may act as а struck al protein and form some organelle, or form proteinaceous biochemical such as haemoglobin, insulin, etc., or serve as an enzyme and catalyze some chemical reaction, or have а motile role like that of myosin, or act as а membrane receptor. In other words, the polypeptide may contribute to a morphological or а functional trait of the cell and organism. Thus, the molecular structure of а gene is expressed in а phenotype. This process may be briefly stated as under -

One Gene

One Polypeptide

There is а definite relationship between the genes and proteins present in а cell or an organism. А change in the gene would change the code and give rise to а different polypeptide.

Colinearity of Gene (DNA) and Polypeptide Structure. The nucleotides are arranged in the genes (DNA) in а linear manner and so are the amino acids in the polypeptides (proteins). The sequence of amino acids in the polypeptide chain is dictated by the order of the nucleotide bases in the messenger RNA, and this in turn is dictated by the sequence of the nucleotide bases in the gene (DNA). Thus, the sequence of amino acids in the polypeptide chain corresponds to the sequence of the nucleotide bases in the gene that code for it. The gene and the polypeptide it codes for are said to be colinear, and their correspondence is called colinearity of gene and polypeptide structure.

That the gene and the polypeptide it codes for are colinear had been presumed earlier but Charles Yanofsky and associates have now proved it in 1965. They have shown that an alteration in а specific codon in DNA leads to an alteration of amino acid in а corresponding location in the po1ypeptide synthesized by the mutant individual.

Since each amino acid is coded by а set of; three consecutive nucleotide bases, а polypeptide chain or 500 amino acids will be coded by а gene of 1500 nucleotide bases.

Types of Genes. All the genes do not play the same role, nor are all genes active all the time. With regard to their role and activity, the genes are of the following types-

1. Constitutive (House Keeping) Genes. These genes are constantly transcribed and expressed ш the cells because the products they code for are constantly needed for cellular activities, such as glycolysis, to keep alive.

2. Nonconstitutive Genes. These genes remain inactive and are expressed ш certain cells only when the product they code for is needed. They are also called specialist genes.

3. Structural Genes. These genes code for chemical substances which contribute to morphological or functional trait of the cell. These are nоw-а-days called cistrons. They are continuous in prokaryotes and split into introns and exons in eukaryotes. Тhеy include:—

1. Polypeptide-coding Genes. These code for mRNAs which in turn code for polypeptides. Polypeptide mау act as а component of an organelle, such as actin of muscle fibre; an enzyme such as DNA ро1уmегasе; а receptor or carrier protein of cell membrane; а transport protein such as haemoglobin; а hormone such as insulin, "an antibody"; or antigen.

2. Polyprotein-coding Genes. These code for more than one polypeptide рег gene.

3. RNA-coding Genes. These code for rRNAs and tRNAs. They are tandemly repeated in the chromosomes.

4. Regulator Genes. These genes соdе for repressor proteins for regulating the transcription of cistrons.

5. Operator Genes. An operator gene acts as а switch to turn on or off the transcription of а structural gene as the cell requires.

6. Promoter Genes. These genes are DNA sequences (sites) where RNA polymerase binds for the transcription of RNAs by the structural genes.

7. Terminator Genes. These genes are DNA regions where RNA polymerase activity stops to suspend transcription of structural genes.

8. Split Genes. Eukaryotic genes have inserts, called introns, which are transcribed but not translated between segments, termed exons, that are transcribed as well as translated. Such genes carrying genetic information in pieces rather than in а continuous stretch are known as split genes. The introns are removed during mRNA processing and exons are joined together to make а gene continuous .

9. Overlapping Genes. А few genes in certain bacteria and animal viruses code for two different polypeptides; These аrе called overlapping genes.

10. Nonfunctional Genes. These replicate but dо not code for any substance.