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  1. Bases     These are either purines or pyrimidines structure derivatives.
  2. Purines Adenine, Guanine
  3. Pyramidines   Cytosine, uracil, thymine
  4. Nucleoside   Purine or pyrimidine base + ribose or deoxyribose sugar.
  5. Nucleotide   Nucleoside + phosphoric acid.
  6. Nucleic acid (DNA or RNA) double helical structures of two long nucleotide chains.
  7. Gene   Unit of DNA that carries the information for synthesis of a specific protein.
  8. Codon Trinucleotide unit of gene that codes for single amino acid.
  9. Chromosome   A singular piece of DNA (which contains many genes, regulatory elements, other nucleotide             sequence) plus DNA binding protein, i.e. chromosome =  organized structure of DNA + protein.
  10. Genome   Refers to a full set of chromosomes present in human. Somatic cells contain two sets of 23 chromosomes in their genome while gamete (ovum or sperm) contains one set of 23 chromosome in its genome. That means somatic cells have 46 chromosomes in their genome and somatic cell has 23 chromosomes in its genome.
  11. Allele   Somatic cells contain two sets of 23 chromosome, so for every protein there are two genes (pair of gene), i.e. one on chromosome of one set and second on chromosome of other set. These two genes for one protein are  allele for each other.
  12. Exons   Parts of genes that are translated to protein.
  13. Introns   Parts of genes that are not translated into proteins.
  14. Transcription Transferring the code from DNA to RNA, i.e. formation of messenger RNA from DNA.
  15. Translation   Formation of protein from m RNA

Lyon's hypothesis

  1. Only one of the X-chromosome is genetically active. .
  2. Other X of the paternal or maternal origin undergoes pyknosis and is rendered inactive.
  3. Inactivation of either maternal or paternal X occurs at random among all the cells of blastocyst by about 16th day of embryonic life.       
  4. Inactivation of the same X chromosome persists in all the cells derived from each precursor cell


  1. A mutation is a permanent change in the DNA.
  2. Mutations that affect germ cells (sperm or ovum) are transmitted to progeny and may give rise to inherited disease.
  3. Mutations that affect somatic cells are not transmitted to progeny but are important in the genesis of cancers and congenital malformations.
  4. Mutations may be classified into three categories-
  1. Gene mutations
  • The vast majority of mutations associated with hereditary disease are gene mutations.
  • These may of different types depending whether it involves complete gene or single base -
  1. Point mutation
  • A single nucleotide base is substituted by a different base.
  • When a pyrimidine base is substituted by other pyrimidine base or a purine base is substituted by other purine Transition.
  • When a purine is substituted by a pyrimidine or vice-versa Transversion.
  • This may alter the code in a triplet of bases, i.e. in codon and leads to replacement of one amino acid by another in the gene product.
  • Because these mutations alter the meaning of the genetic code, they are often termed missense mutation.
  • Example is sickle mutation in which CTC codon in ï-chain of hemoglobin that codes for glutamic acid is changed to CAC codon that codes for valine.
  • Another type of point mutation is nonsense mutation in which a point mutation may change an amino acid codon to a stop codon.
  1. Deletion and insertions
  • Deletion or insertion of one or two base lead to alterations in the reading frame of the DNA strand frame shift mutation.
  • If the number of base pairs involved is three or a multiple of three frameshift does not occur (because codon is triplet), instead an abnormal protein missing one or more amino acids is synthesized.
  1. Trinucleotide repeat mutation
  • Normally a codon is triplet i.e. trinucleotide.
  • In this type of mutation a codon, i.e. trinucleotide sequence undergoes amplification and the same codon is repeated continuously so many times in the chain.
  • For example in fragile X-syndrome, CGG codon is repeated 250-4000 times, i.e. there are 250-4000 tandem repeats of CGG

Genetics Polymorphism

  • The occurrence of different DNA sequence in at least 1% of population, i.e. presence of more than one genetically distinct type in single population of a species.
  • It causes diversity between individuals, e.g. genetic polymorphism might give rise to blue eyes versus brown eyes, or straight hair versus curly hair.
  • Two types of genetic polymorphisms are there:
  1. Single nucleotide polymorphisms (SNPs) as the name suggest there is variation in a single nucleotide in every stretch of approximately 1000 base pairs.
  2. Repeat-length Polymorphism In contrast to SNPs there are short repetitive sequences of variation: 
    (a) Microsatellites Repeat size of 2-6 base pairs and less than 1 kilobase.
    (b) Minisatellites 15 to 70 base pairs and 1-3 kilobase.
  • Most common type of polymorphism is single nucleotide polymorphism (SNP).
  • The best example of genetic polymorphism is blood groups


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