Revision Notes Of Principles Of Inheritance And Variations For NEET

PRINCIPLES OF INHERITANCE AND VARIATIONS

GENE, ITS EXPRESSION AND REGULATION

        Genetic material is the substance that not only control the formation and expression of traits in organisms, but can also replicate and pass on information from a cell to its daughter cells.

        It is now proved that genetic material is the nucleic acids of the cell.

        Nucleic acids are long polymers of nucleotides. Two types of nucleic acids are found in living system, i.e. RNA and DNA.

        DNA acts as a genetic material in most organisms. It is a long polymer of deoxyribonucleotides, etc. which codes for all the metabolic processes of life.

        RNA is the first genetic material to evolve and DNA was derived from RNA. But still RNA acts a genetic material in some plant viruses.

        A nucleotide is composed of a nitrogenous base, a pentose sugar and a phosphate group while, a nucleoside is formed when a nitrogenous base is linked to a pentose sugar through a N-glycocidic linkage.

        Nitrogenous bases are two types, i.e. purines (adenine, guanine) and pyrimidines (cytosine, uracil, thymine). Uracil is present in RNA only in place of thymine.

        In 1953, JD Watson and FHC Crick proposed the 3-d model of physiological DNA on the basis of X-ray diffraction data of DNA obtained by Franklin and Wilkins.

        In a DNA, adenine pairs with thymine through two H-bonds while, guanine pairs with cytosine through three H-bonds. This makes one strand complementary to the other.

        Replication of DNA is the process by which DNA molecule makes its identical copies.

        Watson and Crick in 1953 proposed a semiconservative mechanism of replication on the basis of their model of DNA molecule. According to them, two strands of double helix would separate and act as a template for the synthesis of new complementary strands.

        Evidence in support of semiconservative model of DNA replication was given by Meselson and Stahl (1958), Cairn (1963) and Taylor (1969) independently in their experiments.

        DNA replication begins at a specific and fixed position in a DNA molecule known as origin of replication (ori).

        On 3’ 5’ strand replication is continuous and on 5’ 3’ strand it is discontinuous.

        The process of replication is guided by complimentary H-bonding. The process is catalysed by various sets of catalysis (enzyme).

        DNA dependent DNA polymerase which uses DNA template to catalyse the polymerisation of deoxynucleotides.

        DNA helicase, which unwinds DNA strand for the formation of a replication fork.

        DNA ligase, which facilitates the joining of DNA strands together by catalysing the formation of phosphodiester bond.

        These are three types of RNAs found in all living cells, i.e. mRNA, tRNA and rRNA. Out of which mRNA provides the template for transcription, tRNA brings amino acids and reads the genetic code and rRNA plays structural and catalytic role during translation.

        tRNA is an adapter molecule that read the code of mRNA and on the other end would kind to the specific amino acid. tRNA has five loops, i.e. D-loop, anticodon loop, amino acid loop, T-loop, and variable loop. It is a clover leaf shape molecule.

        Central dogma is the unidirectional flow of information from DNA to RNA and from RNA to polypeptide.

        DNA contains hereditary information in the form of sequence of nitrogenous bases. Three bases together constitute a codon and the whole array of these codon is termed genetic code.

        Genetic code has following features

        It is unambiguous and specific, i.e. one codon codes for only one amino acid.

        Codon is triplet and degenerate.

        The genetic code is nearly universal, i.e. one codon codes for same amino acid is all organisms except in protozoa.

        AUG codon has dual function, i.e. it codes for the amino acid methionine (met) and also act as an initiator codon.

        Transcription is the process through which genetic information of DNA is transferred to mRNA by an enzyme called RNA polymerase in the presence of several other factors.

        Capping is the addition of methyl guanosine triphosphate at 5’ and mRNA. Tailing is the addition of adenylate residues at 3’ end.

        In eukaryotes the genes are split. The coding sequences, i.e. exons are incrupted by non-coding sequences, i.e. introns. These introns are removed from the mRNA by splicing.

        The process of polymerisation of amino acids from a polypeptide is known as translation. The proteins are synthesized from mRNA with the help pf ribosomes.

        Different phases of translation are

        Activation of amino acids

        Initiation of polypeptide synthesis at the start codon (AUG)

        Elongation of polypeptide chain

        Termination of polypeptides synthesis on reaching stop codons (UAG, UAA or UGA).

        Regulation of gene expression occurs at various levels. It results in the formation of a polypeptide.

        In prokaryotes, it is regulated by the rate of initiation of transcription and in eukaryotes, regulation is achieved at the following four levels.

                  Transcriptional level (formation of primary transcript)

                   Processing level (regulation of splicing)

                   Transport of mRNA from nucleus to the cytoplasm.

                   Translational level

        Lac-operon was proposed by Jacob and Monad. It is a transcriptionally regulated system, where a polycistronic structural gene is regulated by a common promotor and regulatory gene.

        Inducible operon system normally remains switched off, but becomes functional in the presence of an inducer, e.g. lac operon in E. coli.

        In repressible operon system regulator produces aporepressor. These aporepressor binds with the corepressor to form active repressor. It binds to the operator gene and blocks the functions of RNA polymerase.


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