Gene : Structure and function

Gene:

It is a basic source of information that controls how the life of an organism should be. It is also termed as the unit of heredity which carries information such as traits that are passed on from one generation to the other. It has its unit of function and is involved in replication, recombination and mutation

Unit of function:

• Cistron is called the gene's function. It is the coding region for a particular gene. 
• The other functional units of gene include muton, recon and codon. 
• The gene's smallest unit is the muton. The length of the muton is comparatively smaller than the cistron. 
• The functional unit of gene which can undergo recombination is recon. It is also smaller. The codon is a part of the gene which can code for amino acids. It is not a primary functional unit of the gene.

Replication, recombination and mutation:

Replication is termed as the production of a copy of a parental gene. It is an important process as the genetic information present in the newly formed cells should be the same as the parental cells at the time of cell division. Replication happens just before the cell division.

Recombination is the process in which the breaking of the genetic material occurs and the genes are rejoined to form a different gene expressing a different trait. It is the process which leads to diversity in the organisms. The recombination which occurs in the replication of DNA is the replicative recombination.

Mutation is the process which permanently alters the genome of an individual resulting in genetic variation among a particular species. Here, we have to emphasize that recombination leads to genetic diversity, but mutation leads to genetic variation. The types of mutations include chromosomal mutations, point mutations and mutation by location.

Fine structure of gene:

• The prokaryotic genes are very simple and easy to understand. But, the eukaryotic genes contain exons and introns. 
• The exons code for the amino acids whereas the introns do not code for any amino acids but are involved in alternative splicing. 
• The introns regulate the gene expression when they are removed during splicing and enabling the particular gene to code for multiple amino acids. Some of the internal regions of the genes include bar loci, complex loci and rII locus. The test which is used for finding whether the mutations are in a single gene or in different genes is a complementation analysis test.

Bar locus:

It is a particular part in an organism which controls a specific trait and the mutations in the alleles of this locus could lead to abnormal functioning of the specific organ. For example, the bar locus in Drosophila melanogaster influences the shape of its eye. Any mutations in the alleles of this locus could lead to an abnormal size of the eyes in drosophila.

Complex loci:

It is the genes in a type which controls the specific function in biological systems. The contrast to this is simple loci in which a gene controls a single trait in biological systems.

rII locus:

Seymour Benzer, a famous molecular biologist utilized rII locus in his experiments which defined that genes are divisible fragments of DNA. It is a part of the T4 bacteriophage genome used for mapping gene structure. There is a test called complementation analysis to prove the complementarity of the genome.

Complementation analysis:

• The rII locus has two subtypes namely, rIIA and rIIB. When the rIIA mutants of T4 bacteriophage are allowed to infect the K12 strain of E.coli, the bacteriophage did not grow on E.coli with the rIIB kept wild. 
• The same experiment was repeated but, this time by keeping the rIIB mutants and rIIA wild. Again, the bacteriophages did not grow on E.coli. Then, the experiment was repeated by keeping the rIIA mutant in one type of bacteriophage and rIIB mutant in the other type of bacteriophage. 
• Now, the bacteriophages could grow on E.coli, surprisingly proving positive on the complementation test.

Gene function:

The function of genes is to provide instructions to the cells to produce a protein for the biological function of the cells. It was clearly explained by the one gene-one enzyme hypothesis. There are also certain pathways followed by the genes to perform their functions. 

One gene / One enzyme hypothesis:

• It was defined by Edward Tatum and George Beadle. They used the fungal organism Neurospora crassa in which mutation is introduced. 
• When there is no mutation, the fungal organisms were able to synthesize nutrients. But, in the presence of the mutation, the fungal organisms were not able to synthesize nutrients, as there is no enzyme synthesised by a particular gene. 
• This hypothesis is also known as "One gene-One polypeptide hypothesis.

Pathways of gene action:

• The central dogma is the basic pathway followed by a gene to perform its function. 
• In addition to that, there are also the types of functional pathways such as biosynthetic pathways, gene-to-behavior pathways, signal transduction pathways and regulatory pathways. 
• The biosynthetic pathway involves the two genes A and B responsible for producing an intermediate and final product, respectively. 
• Introduction of mutation in either of the gene will not lead to the production of the final product. The gene-to-behavior pathways define how the gene behaves to achieve a final product. 
• The signal transduction pathways function within the cell with its receptors present on the cell surface. The regulatory pathway involves the regulation of one gene by the other in a biochemical reaction.