Sunday, September 27, 2009

Regulation of Gene Expression

The hereditary apparatus of the body works in much the same way as that of most primitive bacteria; all organisms use the same mechanism. RNA copy of each active gene is made and at a ribosome RNA copy directs the sequential assembly of a chain of amino acids. There are many minor differences in the details of gene expression between bacteria and eukaryote and a single major difference. The basic apparatus used in gene expressions appear to be the same in all organisms. It apparently has persisted virtually unchanged since early in the history of life. The process of gene expression occurs in two phases which are called transcription and translation.

(1) TRANSCRIPTION: First stage of gene expression is the production of RNA copy of the gene called messenger RNA or mRNA. Like all classes or RNA that occur in cells mRNA is formed on DNA template. The production of RNA is called transcription; messenger RNA molecule (as well as other t RNA and r RNA) is said to have been transcribed from DNA.

Transcription is initiated when a special enzyme called RNA Polymerase binds to a particular sequence of nucleotides on one of DNA strands. This sequence is located at the edge of a gene.


Starting at that end of the gene, RNA polymerase proceed to assemble a single strand of RNA with a nucleotide sequence Complementary to that of the DNA strand it has bound. Complementarity refers to the way in which two single strands of DNA that form a double helix relate to one another, with A (ademine) pairing with T (thymine) and G (guanine) pairing with C (cytosine). RNA strand complementary to thymine is uracilc.

As the RNA polymerase moves along the strand into gene, encountering each DNA nucleotide in turn. It adds the Corresponding Complementary RNA nucleotide to the growing RNA strand. When the enzyme arrives at a special stop signal at the for edge of the gene, it disengages from the DNA and releases the newly assembled RNA chain. This chain is complementary to DNA strand from which the polymerase assembled it; thus it is RNA transcript (copy) called primary RNA transcript, of the DNA nucleotide sequence of gene.


The second stage of gene expression is the synthesis of a Polypeptide by ribosomes, which use the information contained in and mRNA molecule to direct the choice of amino acids. This process of mRNA directed Polypeptide synthesis by ribosomes is called ‘Translation’ because nucleotide sequence information is translated into amino acid sequence information.

Translation begins when r RNA molecule within the ribosome binds to one end of mRNA transcript. Once it has bound to mRNA molecule, a ribosome proceeds to move along mRNA molecules in increments of three nucleotides. At each step it adds amino acid to a growing polypeptide chain. It continues to do this until it eucounters a ‘stop’ signal that indicates the end of Polypeptide. It then disengages from mRNA end releases newly assembled Polypeptide.

Genetic Code: There is sued the word code several times to refer to the information stored in DNA and ultimately translated into amino acid sequence of Proteins. This genetic code is conceptually similar to Morse Code. One set of symbols (bases in nucleic acids, dots and dashes in Morse Code) can be translated into another set of symbols (amino acids in Proteins, letters of the alphabet). The question is what combinations of bases stand for which amino acids?

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