Question

1. Try to compose a definition of a gene that includes all of its known properties. Is it useful to distinguish between different types of genes? If so, what would be the basis for making the distinction between gene types?

2. Is the “one gene, one polypeptide” axiom helpful in understanding the properties of a gene or are the exceptions so frequent that this principle is misleading?

Answer 1

1. A gene is a sequence of DNA that codes for a molecule that has a function. DNA contains a chain made from four types of nucleotide subunits, each of which is composed of: a five-carbon sugar (2-deoxyribose), a phosphate group, and one of the four bases adenine, cytosine, guanine, and thymine. Genes can acquire mutations in their sequence, which leads to different variants, known as alleles, in the population. These alleles may encode little different versions of a protein, which may cause different phenotypical traits. Gene has 3 basic parts. 1) Protein coding sequence or Open reading frame (ORF) 2) Promoter and 3) Regulatory sequence. Eukaryotic open reading frame consists of untranslated introns which are removed before the exons are translated. The sequences at the ends of the introns, determine the splice sites to generate the final mature mRNA which encodes the protein. Many prokaryotic genes are organized into operons where multiple protein-coding sequences are transcribed as a unit. The promoter gets recognized and bound by transcription factors and RNA polymerase to start transcription. The recognition occurs as a consensus sequence like the TATA box. Flanking the open reading frame, genes consist of a regulatory sequence that is required for their expression. Regulatory regions locate upstream or downstream of the open reading frame that alter expression. There are 2 types of regulatory sequences, enhancers and silencers. Enhancers increase transcription by binding an activator protein which then helps to recruit the RNA polymerase to the promoter and silencers bind repressor proteins to make the DNA less available for RNA polymerase.

It is useful to distinguish between different types of genes, based on their function. There are 6 types of genes.

1. Complementary genes: These genes are present on different genetic loci but interact with each other to express a single character in combination. Thus they both together produce a particular phenotypic trait in an individual.
2. Duplicate genes: Duplicate genes are essential sources of genetic material that evolution can act on, but new duplicates are under constant risk of being inactivated by mutations and subsequently lost.
3. Polymeric genes: Polymeric gene interaction is the combination of two dominant alleles that intensifies the phenotype or creates a median variation.
4. Modifier genes: Instead of masking the effects of another gene, these genes can modify the expression of a second gene.
5. Lethal genes: They can cause death of the organism, which carries them.
6. Movable genes: They can become incorporated and function at a number of locations on the genome.

2. " One gene, one polypeptide" is not helpful in understanding the properties of a gene. There are many exceptions. Because the theory states that each gene is responsible for the synthesis of a single polypeptide. But as a single gene contains number of axons, and they are spliced in a different way to generate different function polypeptides.