Gene Expression In Eukaryotes

Gene Expression in Eukaryotes

The genome of higher eukaryotes is very complex. The eukaryotic genome contains DNA many times as compared to the prokaryotic genome. For example -Drosophilahas 5,000-10,000 genes. Human haploid genome seems to have at least 23,000-100,000 genes. In eukaryotes, most of the DNA is non-functional or inactive and are known as excess DNA or repetitive DNA. The diploid organism has two sets of chromosomes. The genome in eukaryotes controls various functions such as growth and division of cells, differentiation, and specialisation of tissues such as muscles, liver, or heart in animals and parenchyma, chlorenchyma, xylem or phloem in plants. As the eukaryotic genome is very large, the gene expression and its regulation become very complex. Therefore, in eukaryotes-

1) Different structural genes present in eukaryotes do not lie adjacent to each other. They are generally found well spaced on the same or different chromosomes.

2) Each structural gene has its own promoter gene.

3) Eukaryotes possess sensor gene which picks up information of any changes in the intracellular environment and presence or absence of hormones, vitamins, chemicals.

4) Eukaryotes have integrator genes for coordinated functioning of structural genes.

5) Eukaryotes have specific genes: enhancer genes and silencer genes which keep or slow down the expression of certain genes respectively.

6) Eukaryotic structural gene has two regions exons and introns. Exons are essential or coding parts while introns are non-essential parts. The non-coding parts or introns are removed by means of nuclease. The exonic regions are joined together. This is also called as splicing.

7) The freshly formed mRNA undergoes several changes in the 5'-3' end. It receives a cap at the 5' end and poly A tail at 3'. Then mRNA is transported through the pore of the nuclear membrane for transportation with the help of ribosomes and tRNA.

Significance of gene regulation:

The significances of mechanism of gene regulation is as follows :

a) Gene regulation allows the metabolism of specific chemicals by a particular cell.

b) Gene regulation helps in growth and differentiation causing morphogenesis.

c) It permits the cells to adjust to environment changes.

d) Regulation of gene expression allows for the expression of only those genes which are of immediate need of the cell.

e) Gene regulation helps the production of specific chemicals by specific cells.

Differentiation and development

Differentiation is defined as the full sequence of changes involved in the progressive diversification of cells, tissue, organs, system e.t.c so that a cell becomes specialised to carry out specific function more efficiently.

The process of development involves the division of fertilised eggs into many cells. During early cleavage, the blastomeres are totipotent. This means each embryonic cell is able to develop into and embryo and give rise to all kinds of tissues of an adult organism. As cell division progresses, the blastomeres gradually lose their totipotency. These cells collectively form tissues, organs, system and organ system such as leaf and stem in plant and brain, liver, e.t.c in animals. The whole process by which totipotent unspecialized embryonic cells become specialised and give rise to specific tissues is called differentiation.

Types of differentiation

As differentiation is a cellular event, it occurs within groups of similar cells. It can be of two types:

1) Intracellular differentiation-It is the change within the cells (eg- the maturation of sperm).

2) Intercellular differentiation- It is the change among cells. It involves the progressive divergence of two or more cells .

• Molecular basis of differentiation

It is not known what primary events trigger differentiation along a particular pathway. Basically, chemical changes are brought about by the action of an enzyme. Any change in enzyme pattern naturally leads to differentiation. Therefore, the process of differentiation involves gene activity. Different genes act at different times to meet the requirement of the organisms.

Ageing

Ageing can be defined as the progressive deterioration in structure and function of cells, tissues, organs, and organ systems of the organism with the advanced age. The field of developmental biology that deals with the study of ageing is known as gerontology.

The effects of ageing vary widely in different groups. Bacteria, viruses and majority of protozoans are free from ageing. However, none of the multicellular organism lives forever. Even under most favourable conditions (with no accident or disease) every metazoan dies its natural death, though the life span differs widely. While some live only for short periods, other may live for several decades or even the centuries. Some sea anemones are known to have lived for about 78 years, turtles survive up to 150 years.

Genetic basis of ageing

Many theories have been proposed to explain the phenomenon of ageing. According to the genetic theory of ageing, all individuals possess ageing genes in their genome, which determine the rate of ageing and the maximum life span. There is a good deal of evidence to support this view-

1) Annual plants age, despite most suitable environmental conditions

2) Different life spans even in different species of mammals

3) Longevity varies even in different family lines of a single species.

Cancer

Cancer is a problem associated with differentiation and development. Growth and differentiation are regulated by various growth controlling mechanisms. But in the case of cancer (malignant disease), the cell deviates from the usual growth control mechanism. In other words, the cell loses growth controlling and regulating mechanism. This cell behaves differently from the normal cell. As a result, the cell starts inducing and damaging normal tissue leading to cancer. The cells are called malignant or cancer cells. These cells continue to divide unchecked, resulting in an everlasting number of cancer cells and thus a tumour is developed. Therefore, cancer can be defined as unorganised growth of cells in which the controlling and regulating mechanisms have been disappeared or have been ineffective,

A) Cancer may be caused due to alteration in chromosomes or gene mutations in nuclei of somatic cells.

B) Some changes in the cytoplasm, that results in loss of control of nuclear and cytoplasmic division may cause cancer. This says that the genetic change may not be an essential factor for cancer.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.