Role of Genes

Genes play а crucial role in cells. Their main contributions are listed below—

1. Genes code for proteins, structural аs well as functional, to control the phenotype and metabolism.

2. Genes соdе for RNAs essential for protein synthesis.

3. Genes regulate transcription to generate proteins as and when required in the cells.

4. Cenes constitute the, genetic material, the basis for inheritance.

5. Genes contribute to reproduction through their replication.

6. Genes undergo mutation and recombination to help in evolution.

7. Genes repair themselves, if damaged, to maintain specificity.

8. Genes cause differentiation of cells during development. Only certain genes remain functional in particular cells. This enables the cells having similar genes to assume different structure and function.

9. Genes are considered to play а role in ageing process also.

Gene Expression in Prokaryotes

Bacterial Chromosome. The bacterial chromosome is described in chapter 2 of this unit. An average bасterium contains one thousandth the DNA content of а eukaryotic cell. The genome of the bacterium Е. соli соmprises about 2,000 to 3,000 genes. The bacterium can live on glucose diet, indicating that its genes have information for the synthesis of practically ail the organic compounds it needs. The pleuropneumonia - like оrganisms (PPLOs) or mycoplasmas have а genome of а few hundred genes, which code for only а few proteins. Therefore, the PPLOs usually live as parasites on other organisms and get most of their requirements from the host.

Genetic Transfer between Bacterial Cells. The bacteria multiply by an asexual process of division. The doubling time, also called generation tip just 20 minutes in some cases. Replicanterial DNA proceeds in both direction single origin of replication for quick cell. Though the bacteria reproduce asexually of genetic material between two differ does occur in them. It takes place in the transformation - the uptake of naked transduction, i.e., by viral infection.

Gene Expression in Eukaryotes

The genome of higher eukaryotes is very соmplex. This is evident from the number of genes present in the eukaryotic cells. For instance, Drosophila has 5,000 to 10,000 genes. Human haploid genome seems to have at least 23,000 to 100,000 genes. The eukaryotic genome has not only to control growth and division оf the cells but also their differentiation into specialised tissues such as muscles, liver or heart in animals and parenchyma, chlorenchyma, xylem or phloem in plants. This makes the gene expression and its regulation in eukaryotic cells very complex рrосеssеs. The structure оf thе eukaryote gene is also complex. Whereas the concept of colinearity of gene and protein is fully true for the prokaryotes, it partly holds good for the eukaryotes This is so because а eukaryotic gene has stretches of bases which do not соdе for amino acids inserted between stretches of bases which code for аminо acids. The coding segments of the gene are called exons and noncoding inserts are termed introns. Thus, the information for assembling а polypeptide is not continuous in а eukaryotic gene. Instead, it is split into pieces.

Тhе mRNA transcribed from а eukaryotic ,gene has unwanted RNA regions. It is called heterogeneous nuclear RNA (Hn RNA). Its unwanted regions are removed by nucleases, and the ' regions coding for amino acids are joined together.

This processing of mRNA known as splicing. Thus, the processed mRNA and а polypeptide it codes for are colinear even ш the eukaryotes, although the genes are split. The processed mRNA passes out of the nucleus and joins the ribosomes. Here it is ехрrеssеd through the synthesis of а polypeptide.