Use of Polymer Containing a Single Type of Nucleotide (Homopolymers)

Use of Polymer Containing a Single Type of Nucleotide (Homopolymers)

	Use of Polymer Containing a Single Type of Nucleotide (Homopolymers) - The landmark experiment which led to the deciphering of the genetic code was performed by Nirenberg and Matthaei (1961). Cells of the bacterium E.coli were broken open by grinding with finely powdered alumina to yield cell sap. The DNA of the cells was broken down by adding the enzyme deoxyribonuclease. The template for synthesizing new mRNA was thus destroyed. The cell sap was centrifuged to remove the heavier fragments of the cell wall and cell membranes.
The cell sap was centrifuged to remove the heavier fragments of the cell wall and cell membranes. The slower sedimenting cell free extract contained ribosomes, enzymes, DNA, mRNA and tRNA. To the cell free system were added energy sources (A TP, GTP) and a mixture of all 20 amino acids at least one of which was labelled with radioactive material (14C). Protein synthesis took place in the cell free system and as the mRNA is unstable, came to an end within a few minutes. Cn addition of a crude fraction of mRNA and incubation of the mixture at 37°C for about an hour protein synthesis was resumed. Addition of trichloroacetic acid stopped the reaction and precipitated the proteins.
The free amino acids remained in solution. The protein precipitate was washed and its radioactivity measured by placing it in a radiation rounding instrument. The amount of radioactivity indicated the amount of labelled amino acids incorporated into the protein. It was found that the ribosomes synthesized protein in response to addition of mRNA. Another crucial component of this experiment was the use of the synthetic polynucleotide poly(U) in the cell free system. Such polymers containing one type of monomer are called homopolymers. It was found that the ribosomes read the code in poly(U) and synthesized polyphenylalanine, a polypeptide containing only phenylalanine repeated over and over again
Since the synthetic messenger RNA poly(U) coded for polyphenylalanine, the codon UUU was identified for phenylalanine. Thus the first code word to be deciphered was UUU. This discovery was extended in the laboratories of Nirenberg and Ochoa. The experiment was repeated using synthetic poly (A) and poly '(C) chains, which gave polylysine and polyproline chains, respectively. Thus AAA was indentified as the code for lysine and CCC as the code for proline. Poly (G) was found to be inactive, because it formed a triple stranded helical structure.

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Third Base Deciphering the Code Use of Polymer Containing a Single Type of Nucleotide (Homopolymers) Use of Mixed Polymers with Random Sequences (Heteropolymers) Use of Mixed Polymers with Defined Sequences (Heteropolymers) Use of Trinucleotides in Filter Binding (Minimessengers) Genetic Code is Universal Colinearity of Gene Structure and its Polypeptide Product

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