Structure and Organization of Genome

Structure and Organization of Genome :

The information that drives the process of life is encoded in the genome of the organism. Structurally defined, a genome is an organization of genetic information consisting of nucleic acids (deoxyribonucleic acid or ribonucleic acid). Arrangement of nitrogenous bases in the nucleic acid codes the information for
execution of cellular processes. Four types of bases (Adenine, Guanine, Cytosine and Thymine) are present in deoxyribonucleic acid (DNA) while in RNA Uracil replaces thymine group of any three bases on transcribed RNA represent a codon. With four types of bases, sixty four types of codons can be formed. Genetic information encrypted in these codons is released in the form of arrangement of polypeptides consisting proteins via mRNA intermediate. This universal process of flow of genetic information
from DNA to RNA to protein is known as the central dogma of life. The whole set of information coded in 64 codons, out of which 61 codes for one or more amino acids (building block of polypeptides/proteins) and three indicating where to stop is called the book of life or genetic code. Depending on the versatility
of living organism, variations of central dogma exist, where initial information can flow from RNA through DNA or RNA intermediate.
     To be considered as the driving force of life and information storehouse, a genetic material must have fulfilled some basic criteria. First of all, it should have a mechanism of storing vast informationl  in a limited place, yet in a manner simple enough to encrypt the information rapidly and easily by the organism as and when required. As explained above, arrangement of codons in DNA encrypt the information, which fulfill these criteria by being linear, continuous and simple to understand. A complete set of codon that drives synthesis of a functional polypeptide with defined start and end points is known as a gene. Genome, therefore, is a collection of genes that direct the function of life. Secondly, the language of the code should be .,niirsa1 so that decoding of information in organism X should not be a different process than in organism Y. I-low then, organisms vary in their genetic constitution and phenotypic expression which generates the
biodiversity of the world? This is achieved by altering the information contained in the genome, although the method of reading remains the same. A gene may become nonfunctional by single or multiple changes in its base composition, parts or whole of gene may be deleted, duplicated or rearrangements can take
place in genome to alter the acne structure and position. Basic of all these changes is mutation, a change in base composition of gene and is the driving force of evolution. A single change in the composition of codon can make a protein nonfunctional, as evident from the variant of lmemoglobin protein that cause sickle
cell anemia which arises through a single amino acid replacement in the sixth position of the 3-polypeptide. The other one, recombnat ion, is a process of genetic material exchange in sexually mating species and contributes to genetic variability in a population. Mutation followed by natural selection is the basis of evolution and specification, i.e., generation of species diversity from bacteria to human and higher plants.

        Third, this information should be transformed into working units to execute cellular process. Formation of structural and enzymatic proteins from messenger RNA (translation) which themselves are coded from genes by the process of transcription is the central feature of life releasing the information stored in the genome. Depending on the organismic complexity, the scheme varies and is regulated at various phases of execution by several proteins and short RNA molecules. In prokaryotes, where no separate compartment exists; for storing genome, transcription and translation is a coupled process and many a times common

regulatory meclianisnis are adopted by cell to coordinate the processes. In eukaryotic cellular environment genome is protected in separate envelope called nucleus where transcription takes place to generate mRNA. After import into cytoplasmic environment and several modifications, the processed mRNA is used as template for translation of polypeptides.

         Fourth and perhaps the most intriguing question relates to copying the information contained in the genome. Every organism, from single cell to complex multicellular animal or plant multiplies for propagation. When a cell multiplies through mitosis or produces gametes for sexual reproduction through meiotic

divisions, the genome is copied and distributed evenly in the progeny cells. Considering the linear nature of gene, there are about 30,000 genes in human genome which occupies only about 5% of the total genome. Each of the genes being several kilobases in size, the linear length of total genome would be more than one

meter. Being supercoiled as 24 (22 + X + Y) chromosomes in a nucleus of diameter of few micrometer only. the task of replicating the genome by untangling each of the chromosome and repacking them again in the form of chromosomes seems to be an Herculean task. The cell accomplishes this by exploiting the unique secondarij double helical structure of DNA, where two linear strands of polynucleotide (combination of pentose sugar, nitrogenous base and phosphate group) twist around each other and form hydrogen

bonding by complementary base pairing. Using the complementary nature, new DNA strands are synthesized and packed with one old strand so each new chromosome contains one old and one new strand in the double helix.