Microbiology Procedure

Index >> Genes >> Genes as a Unit of Function (Cistron)

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Genes All Cases of Genes are DNA Genes of Some Virus are RNA Genome and Plasmon Genes as a Unit of Recombination (Recon) Gene as a Unit of Mutation (Muton) Gene as a Unit of Function (Cistron) Numer of Genes Number of Nucleotides in Average Gene Duplication of Gene

Genes as a Unit of Function (Cistron)

	Genes as a Unit of Function (Cistron) In 1908 Garrod first proposed the one-gene-one-enzyme hypothesis. This concept was clearly demonstrated by Beadle and Tatum in 1941. The gene was considered to be the unit for coding the synthesis of a single enzyme. It was thus identified by its functional product, the enzyme. The one-gene-one-enzyme hypothesis has been expressed in several different ways. Since the gene forms a messenger RNA molecule it can also be said that one gene forms one messenger RNA. In some cases, however, several genes form a single mRNA strand, which is then said to be polycistronic. The concept has also been expressed as one-gene-one-protein, since the messenger RNA molecule serves for coding of a protein (enzyme)
It would be more correct to use the statement one-gene-one-polypeptide chain, since an enzyme or protein may contain more than one polypeptide chain. The chains formed by different genes then aggregate to form the enzyme (protein molecule). Stahl has defined the gene as a polynucleotide chain consisting of segments, each controlling the expression of a particular trait. The functional gene has been called the cistron by Benzer because it is a chromosomal segment within which the cistrons effect operates. The cistron represents a segment of the DNA molecule and consists of a linear sequence of nucleotides which controls some cellular functions. In most cases the cistron is approximately equivalent to the gene of the geneticist. In E. coli the cistron may contain about 1,500 base pairs.
There is evidence that some cistrons may contain as many as 30,000 nucleotide pairs. Because the genetic code is a triplet one, a protein consisting of n amino acids is coded by a cistron of about 3n nucleotides. The cistron starts with an initiating codon and ends with a terminating codon
Each cistron is responsible for coding one messenger RNA molecule, which in turn serves fat the formation of a polypeptide chain. It has been found that hundreds of units of mutation (mutons) and recombination (recons) exist within each cistron. Cistrons, therefore, occupy a much greater chromosomal length than mutons or recons.
	The T4 virus (bacteriophage) contains a single long DNA molecule lying within a protein head. The DNA molecule has several regions, one of which is called the rII region. Benzer (1955, 1956) has studied the rII region and has found that it consists of two cistrons, A and B. Each of these regions can function independently and contains 2,500 and 1,500 nucleotide pairs, respectively. Each cistron contains several small units of mutation and recombination.

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Gene Regulation Loci and Alleles Split Genes Hexon Gene Ovalbumin Gene β-Globin Gene Genes Within Genes (Overlapping Genes)

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