The characters in an organism are controlled by genes. The genes lie in chromosomes. Each chromosome contains one molecule of hereditary material called D.N.A. So, the Gene contains a part of D.N.A. Every biochemical reaction needs an enzyme. The enzymes are the proteins and are synthesized on ribosomes. All the enzymes information lies in D.N.A.
The process of transmission of information from gene or D.N.A. and the synthesis of protein according to the information together called Gene Expression.
Many genes are silent and are expressed when their products are needed. But some genes are expressed always as their products are needed at every moment. These genes are called house-keeping genes.
Garrod first observed that some diseases in the human beings are due to the absence of some enzymes. In the healthy normal human beings, all the required enzymes are synthesized. Due to mutation insome genes, some required enzymes are not synthesized. Hence diseases will occur. Garrod observed alkaptoneuria and phenylketoneuria in some humans. These diseases are caused due to gene mutations. Hence Garrod stated as one mutant gene-one metabolic block.
Later Beadle and Tatum did many experiments on fungus, Neurospora. They said that one enzyme synthesis is under the control of one gene. So, they replaced the one mutant gene - one metabolic block as one gene - one enzyme concept.
Ingram observed the synthesis of haemoglobin. It contains '4' polypeptide chains. Each is controled by one gene. Ingram proposed as one gene - one polypeptide concept.
A.VIRAL GENE EXPRESSION
Viruses are intracellular obligatory parasites. They cannot multiply on their own. The viruses need host machinery (enzymes, ribosomes) for their essential products. The genetic material in the viruses is D.N.A. or R.N.A.The viruses that infect bacteria are called bacteriophages.
i. Gene expression in bacteriophages:
The bacteriophages gene expression is of two types. They are lytic cycle and lysogenic cycle.
T4 is the large bacteriophage. It contains D.N.A. T4 infects bacterium, E.coli.The D.N.A. of T4 is injected into the bacterium through the hole created in the membrane of bacterium. The D.N.A. of T4 uses the machinery of bacterium for its transcription and multiplication. After many phage particles are formed, the
D. N.A. of phage synthesizes the enzyme, lysozyme. This enzyme dissolves the membrane of bacterium. Hence many phages are released. This is known as the lytic cycle.
Phage Y (Lamda) is another virus. It contains D.N.A. It infects E.coli. The D.N.A. of Phage X is attached to the D.N.A. of host chromosome. Hence the D.N.A of Phage is also transcribed along with host D.N.A. The phenomenon by which the phage D.N.A. exists as a part of the host D.N.A. is known as lysogeny.
ii.Gene expression in Retroviruses:
The retroviruses contain genetic material R.N.A. These are
hepatitis - B virus, HIV (Human Immuno Defeciency virus or AIDS Virus) etc. Normally the gene expression is unidirectional. It is
D.N.A. -------> R.N.A ------- > Protein.
The R.N.A. of retroviruses enters into the host cell. Then the R.N.A. is changed into D.N.A by reverse transcriptase. This newly formed D.N.A. is incorporated in the host D.N.A. Now the viral D.N.A is also transcribed along with the host D.N.A. After the discovery of reverse transcriptase, we can say that the gene expression is bidirectional. It is
D.N.A. ----> m R.N.A. ------> Protein.
B. GENE EXPRESSION IN EUKARYOTES
Genes control protein synthesis. D.N.A contains essential information for the protein synthesis. The flow of information during protein synthesis is represented as
The sequence of Amino Acids in a protein is determined by the sequence of nitrogenous bases in the D.N.A. Protein synthesis occurs on ribosomes. It consists of two steps. They are transcription and translation.
Formation of 'm' R.N.A. from a strand of D.N.A. is called transcription.
The transfer of information from D.N.A. to 'm' R.N.A. is also called transcription.
It occurs in the nucleus. The two strands of D.N.A. at a specific gene unwind due to temporary breakage of hydrogen bonds. One of the strands acts as a template or code complement or master strand. The nucleoplasm contains the four ribonucleotides.These are A.T.P (Adenine triphosphate) and GTP (Guanine triphosphate), UTP (Uracil triphosphate) and CTP (Cytosine triphosphate). These are assembled on template DNA as a complementary strand. The assembling of ribonucleotides is by an enzyme, RNA Polymerase.
RNA formed on template DNA is called HnRNA (heterogenous nuclear RNA). This RNA contains wanted and unwanted parts. The wanted parts (codons) are called exons and unwanted parts are called introns. The introns are removed by splicing enzymes. This process is called splicing. The exons are joined serially. This process is called processing or Sizing. Now the RNA is called messenger RNA. It passes through the nuclear pore into the cytoplasm. The 'm' RNA is now attached to the ribosomes. The'm' RNA transfers the information from DNA to the ribosomes.
The three continuous nitrogenous bases present on'm' RNA forms a codon for aminoacid. There are sixty four codons for 20 aminoacids. Each codon is complementary to the three continuous nitrogenous bases of D.N.A. Among 64 codons, 3 are non-sense codons (UAA, UAG, UGA). These are termination codons also. The remaining 61 codons are for 20 amino acids. So, some amino acids have more than one codon.
The synthesis of proteins according to the information present on'm' RNA is called translation.
Proteins are made up of long chains of amino acids and the sequence of amino acids in the protein molecule should be related to the sequence of bases in the Messenger Rna Molecule. This requires a third kind of RNA called transfer RNA (tRNA). There are possibly 40 - 60 distinct kinds of transfer RNA molecules in the cell. Among these there is at least one specific transfer RNA molecule for each of the 20 amino acids used in protein synthesis. With the help of a specific enzyme and ATP, each amino acid is activated and attached to a particular transfer RNA molecule. Each kind of 't'R.N.A. carrys a specific aminoacid to the ribosome.This brings the amino acids in sequence and they join to form a polypeptide. The sequence of amino acids in the polypeptide chain is controlled by the sequence of bases in the messenger RNA molecule and thus controlled by the sequence of bases in the DNA molecules in the nucleus. The synthesis of proteins in the ribosomes in accordance with the base sequence of mRNA is called translation.
The union of transfer RNA and messenger RNA requires the presence of ribosomes. Electron micrographs and biochemical analysis suggest that the ribosomes attach to the end of a strand of messenger RNA and then move along its length 'reading' its sequence of bases. As they move they pick up the appropriate transfer RNA molecules each with its amino acid, one at a time, these amino acids become linked together to form a polypeptide. When ribosomes reach the end of the messenger RNA strand, the polypeptide chain is complete. The ribosomes and the polypeptide chain are released from the mRNA. Thus the message in the DNA molecule its sequence of bases, which has been transcribed into a molecule of messanger RNA, becomes translated into a specific sequence of amino acids in the polypeptide chain. Although a single ribosome can manufacture a polypeptide chain from a messenger RNA molecule, several ribosomes are usually found in the process, at a time. One messenger RNA molecule with several ribosomes attached to it at various stages of assembling the polypeptide, is called a polyribosome or polysome.