Friday, September 25, 2009

DNA and mRNA and their Co-Ordinated Role in Living Cells

They are important constituents of living cells. RNA occurs in cytoplasm and DNA occurs in the chromosomes of the nuclear reticulum. They consist of 5-carbon ribose sugar and phosphate but DNA has deoxyribose with one less oxygen atom in its molecule. DNA is double stranded molecule while RNA is single stranded one. Secrets of life are embodied in DNA or in other words it is the chemical basis of life. DNA is controlling centre of all vital activities of the cell. DNA is sole genetic (hereditary) material migrating from generation to generation through the reproductive units or gametes and is responsible for the development of specific characters of a plant. It also controls the biosynthetic processes of the cell including protein synthesis.

mRNA is chemical messenger and plays a key role in the process of protein synthesis. It is produced in nucleus from the coded instruction in the DNA and then passes into the cytoplasm where it becomes associated with the ribosomes. It carries chemical information from the DNA of the gene to ribosome for protein synthesis. Molecule of mRNA are 753000 nucleotides long and are not folded in any special way.

Spiegelman and his colleagues in 1961 infected E. coli bacteria with 32 p-labelled phages, and isolated labelled RNA. They used this labelled RNA in hybridization to DNA of both phages and bacteria in separate experiments. RNA hybridized only with phage DNA showing that it was complimentary in base sequence to the viral genetic information. The result of the experiment suggests that messenger RNA (mRNA) is made on a DNA template and it directs the synthesis of specific protein in association with ribosomes. The concept was formerly proposed by Francois Jacob and Jacques Monod in 1961.

Each messenger RNA molecule is a long single strand of RNA that passes from nucleus to the cytoplasm. During polypeptide synthesis mRNA molecules bring information from the chromosomes to the ribosomes to direct the assembly of amino acids into a polypeptide. These molecules together with ribosomal Proteins and certain enzymes constitute a system that carries out the task of reading the genetic message and producing the polypeptide that the particular message specifies. They are the principal components or an apparatus that a cell uses to translate its hereditary information. This information is a message written in the code specified by the sequence of nucleotides in DNA. The cell’s polypeptide producing apparatus reads this particular polypeptide. Biologists have also learnt to read this code and in so doing have learnt great deal about what genes are and how they work in dictating what a Protein will be like and when it will be made.

The codes are transmitted to different parts of the cell for their proper functioning.

mRNA acts as chemical messenger or carrier of such codes. The amount of DNA in the cells remain constant, no matter under what metabolic conditions the cell might be placed, once DNA of chromosomes is synthesized, it does not break down or undergo any metabolic change although DNA does participate in controlling various cellular events. The mitosis is characterized by exact replication and exact distribution of DNA and nuclear proteins become condensed to form chromatin threads of which the chromosomes are composed of obviously the initial event during mitosis is doubling DNA in the nucleus.

During transcription the genetic message coded in DNA is transcribed into mRNA which moves out of the nucleus and provides the information to the protein synthesizing organelles the ribosomes.

The sequence of nucleotide on mRNA directs the precise synthesis of amino acid sequence of a protein molecule. M. Nirenberg and H. G. Khorana said that the unit that codes for a given amino acid, consists of a group of three adjacent nucleotides on mRNA, the next three nucleotides on mRNA code fro the net amino acid.

Gene expression is the production of RNA, copy of the gene called messenger of RNA or mRNA. Like all classes of RNA that occur in cells mRNA is formed on DNA template. The production of RNA is called transcription mRNA molecule is said to have been transcribed from DNA. As the RNA polymerase moves along the strand into gene, encountering each DNA nucleotide in turn, it adds the corresponding complimentary RNA nucleotide to the growing RNA strand. This chain is complimentary to DNA strand from which the polymerase assembled it; thus, it is RNA transcript called Primary RNA transcript of DNA nucleotide sequence of the gene.

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