chp#08

• Chromosome


  Thread-like structure made up of highly condensed chromatin material, appear during cell-division in specific numbers according to species, and carry genes
• Chromosomes


  • Consist of two identical threads called sister chromatids
  • Have a primary constriction called centromere
  • May have a secondary constriction called nuclear organizer
• Types of chromosomes based on centromere position

  • Metacentric
  • Submetacentric
  • Subtelocentric or Acrocentric
  • Telocentric
• Nucleosome


  200 nucleotide containing DNA duplex coiled around an octamer of histone protein
• Chromosome organization


  1. Nucleosome string formation

  2. Chromatin fiber

  3. Supercoil formation to form chromatids
• Gene


  Basic unit of heredity, a small part of DNA which has information to synthesize specific polypeptide chain
• Gene locus


  Fixed position of a gene on a chromosome
• Homologous chromosomes


  Morphologically similar chromosomes with the same gene loci
• Alleles


  Alternative forms of a gene located at the same gene locus on homologous chromosomes
• Mendel observed two alleles for each trait in Pisum plants
• Many traits have more than two alternative alleles
• Chromosomes are made up of DNA and protein
• DNA is the hereditary material
• Allele


  Alternative forms of a gene located at the same gene locus on homologous chromosomes
• Many traits have a number of alternative forms, not only two like Mendelian traits
• Hereditary units are located on chromosomes and transformed through chromosomes from one generation to the other
• Chromosomes are made up of only DNA and protein
• Streptococcus pneumonia has two strands: capsulated smooth form (S-type) and non-capsulate (R-type)
• 1. type is virulent while R-type is non-virulent, both are genetically variable
• Griffith's Experiment


  1. Injected R-type in laboratory mice (no ill effect)

  2. Injected S-type in laboratory mice (proved fatal)

  3. Heated both strains (both killed at high temperature)

  4. Injected heat killed S-type and live R-type in mice (high mortality observed)
• Transformation


  The phenomenon in which heat killed S-type could have hereditary effect on R-type
• Avery, MacLeod, and McCarty discovered and identified the transforming material as DNA
• Hershey and Chase Experiment

  1. Radio labeled DNA of bacteriophage with P32 and its protein coat with S35

  2. Permitted labeled viruses to attack bacteria

  3. New phages contained only P32, not S35

  4. Showed that only P32 containing DNA entered the host and directed the production of new phages
• The Watson and Crick Model of DNA suggests that DNA is a double helix where specific pairing of nitrogenous bases occurs
• The exact copying of DNA is called duplication of DNA, which occurs before cell-division in S-Phase of interphase during the cell-cycle
• Models of DNA Replication

  • Conservative
  • Semi-conservative
  • Dispersive
• Conservative Model


  The parental double helix remains intact and the new molecule is formed entirely from scratch
• Semi-conservative Model


  The two strands of parental DNA molecule separate, and each functions as a template for the synthesis of new complementary strands
• Dispersive Model


  All four strands of DNA after replication have a mixture of old and new DNA
• Meselson and Stahl Experiment

  1. Grew bacteria for several generations in a medium containing heavy isotopes of Nitrogen (N15)

  2. Transferred the growing cells to a new medium containing lighter isotope of Nitrogen (N14) and harvested the DNA at various intervals

  3. After one round of DNA replication, the density of the bacterial DNA decreased to a value intermediate between all light isotope and all heavy isotope DNA

  4. After another round of replication, two density classes were observed, one intermediate and one light isotope, corresponding to DNA that include none of the heavy isotope
• Meselson and Stahl's experiment confirmed the semi-conservative model of DNA replication
• Process of DNA Replication

  1. Origin of Replication: Helicase recognizes origin sites and opens the DNA duplex, forming a replication bubble and replication fork

  2. Elongation of New Strands: DNA polymerases synthesize a continuous leading strand and discontinuous Okazaki fragments for the lagging strand

  3. Priming DNA Synthesis: Primase adds a short RNA primer for DNA polymerase to begin synthesis

  4. Types of DNA Polymerase: Polymerase I, II, and III perform different functions during replication

  5. Termination: Polymerase I removes RNA primers and replaces them with DNA, DNA ligase joins Okazaki fragments
• Proofreading during DNA replication

  Removes wrong nucleotide if it is added mistakenly
• Termination phase of DNA replication

  1. Replacement of primer by DNA nucleotide

  2. Joining of Okazaki fragment in lagging strand
• Removal of primer nucleotides

  DNA nucleotides are replaced by DNA polymerase-I
• DNA polymerase-I


  Performs dual function of exonuclease and polymerase
• Joining of Okazaki fragments


  Occurs by DNA ligase enzyme
• Steps and enzymes involved in DNA replication

  • DNA Helicase & single strand binding protein
  • Primase
  • DNA polymerase-III
  • DNA polymerase-I
  • DNA ligase
• DNA replication is not perfect, with about one mistake for every 10,000 base pairs
• In mammalian cells, the completed DNA strand contains only about one mistake for every billion base pairs