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NEET Biology Molecular Basis of Inheritance

Molecular Basis of Inheritance PDF Notes, Important Questions and Synopsis

SYNOPSIS

  • Structure of DNA: DNA is a double-stranded structure in which one strand coils around the other and gives it a helical structure.
  • Chargaff’s Rule of Base Pairing:
  1. In any DNA, the purine and pyrimidine base pairs are in equal amounts.
    A + G = T + C
  2. The proportion of A is equal to T and the proportion of G is equal to C, but the amount of A + T is not necessarily equal to G + C.
  • Watson and Crick Model of DNA: Nucleotides in a polynucleotide chain are linked together by phosphodiester bonds. These chains remain held together by hydrogen bonds and run in antiparallel directions with the carbon atom at the 5’ position in the sugar molecule in one direction in one chain and in the opposite direction 3’ in the other chain.

  • Differences between DNA and RNA

     

    DNA

    RNA

    Sugar

    DeoxyriboseRibose

    Ribose

    Nitrogenous bases

    Adenine, guanine, cytosine and thymine

    Adenine, guanine, cytosine and uracil

    Occurrence

    Chromosomes (in the nucleus)

    Cytoplasm

    Base composition

    A/T = G/C = 1

    May vary

    Structure

    Double stranded, helically coiled and one chain is complementary to the other.

    Single stranded and not helically coiled.

    Reactivity

    Chemically less reactive and structurally more stable.

    Chemically more reactive and structurally less stable

  • Central dogma is the expression of genetic information from DNA to m-RNA and then to proteins.
  • Negatively charged DNA wraps around the positively charged histone octamer to form a structure called the nucleosome.
  • Bacterial transformation is the process in which DNA isolated from one type of bacterial cell, when introduced into another type of bacterial cell, is able to bestow some of the properties of the former to the latter. S. F. Griffith demonstrated bacterial transformation by performing an experiment on Diplococcus pneumoniae. 
  • Hershey–Chase experiment proved that DNA and not protein is the genetic material.
  • Steps in DNA Replication:

       
  • Meselson and Stahl demonstrated by their experiment that DNA replication is semi-conservative in nature.
  • Transcription is the formation of m-RNA on a DNA strand in the nucleus.
  • Introns are non-essential regions of the gene which do not form a part of m-RNA.
  • Exons are the essential regions of the gene which become a part of m-RNA during transcription and code for different regions of the amino acid.
  • Splicing is the removal of non-coding regions (introns) and fusion of coding regions (exons) of RNA

  • Capping is the addition of an unusual nucleotide (methyl guanosine triphosphate) to the
    5'-end of hnRNA.

  • Tailing is the addition of adenylate residues (200–300) to the 3'-end in a template-independent manner.

  • Cistron is a segment of DNA coding for a polypeptide.

  • Types of RNA:

    r-RNA

    m-RNA

    t-RNA

    It accounts for about 80% of total RNA in the cell.

    It accounts for about 5% of total RNA in the cell.

    It accounts for about 15% of total RNA in the cell.

    It is small, moderate to large-sized and is most abundant and highly coiled.

    It is moderate to large-sized and is least abundant.

    It is the smallest and coiled like a clover leaf.

    It carries no coding message.

    It carries a coding message for many amino acids.

    It carries a coding message for only one amino acid.

    It is called insoluble RNA and forms ribosomes.

     

    It is called template/ nuclear/ messenger/informational RNA as it carries genetic information provided by DNA.

    It is called soluble or adapter RNA and carries amino acids to mRNA during protein synthesis.


    Out of 64 codons, AUG and GUG act as initiation codons, whereas UAA, UAG and UGA do not represent any amino acid and are called nonsense or termination codons.
  • The genetic code is triplet, universal, degenerate, non-ambiguous, commaless and acts in a non-overlapping fashion.
  • Mutations and Genetic Code:

    Point mutation

    • It is the abrupt change in a single base pair of DNA.

    • In sickle cell anaemia, a change of single base pair in the gene for beta globin chain results in the change of amino acid residue glutamate to valine.

    Frame shift mutation

    • It is the elimination or addition of one or two base pairs or a segment of DNA in a gene

    • In thalassemia, an inherited blood disorder, the β chain of haemoglobin changes due to frame shift mutation.

  • Translation is the synthesis of a polypeptide chain where a particular nucleotide sequence on m-RNA is translated into a particular amino acid.
  • Steps in Translation:

            
  • Gene expression is the mechanism at the molecular level by which a gene is able to express itself in the phenotype of an organism.
  • Types of Gene Expression:

  • Lac operon is an inducible operon which is switched on in response to the presence of the substrate lactose. 

  • A mega project of sequencing and mapping the human genome and storing human genome information in a database is called the Human Genome Project.
  • DNA fingerprinting is a technique to identify individuals on the basis of their respective profiles.