Chapter 16: Molecular Basis of Inheritance

• Chargaff's Rules


  Adenine=Thymine

  Guanine=Cytosine

  DNA composition varies between species
• Components of a nucleotide

  A nitrogenous base, a pentose sugar (deoxyribose or ribose) and a phosphate group
• Purine


  2 rings

  Adenine and guanine
• Pyrimidines


  1 ring

  Cytosine, thymine, and uracil
• 5' is Phosphate group

  3' is OH group
• Conservative model


  Parental strands direct synthesis of an entirely new double-stranded molecule. Parental strands are fully conserved.

  2 blue, two gray
• Semi-Conservative model


  2 parental strands each make a copy of itself. After a round of replication, the two daughter molecules each have one parent and one new strand

  Gray and blue, then blue and gray
• Dispersive Model


  The material in two parental strands is dispersed randomly between daughter cells. After duplication, daughter molecules have a random mix of parental and new DNA.

  A mix of blue and gray in each strand
• Helicase


  Unwinds DNA strands at the replication fork
• Single strand binding proteins (SSBPs)


  Bind to DNA to keep it open
• Topoisomerase


  Helps to prevent strain ahead of the replication fork by relaxing supercoiling.
• RNA Primase


  Initiates replication by adding short segments of RNA, called primers to parental DNA strand. The enzymes that synthesize DNA can only attach new nucleotides to existing nucleotides. Primers act as the foundation for DNA synthesis.
• DNA Polymerase III 


  Attaches to primers on the parental strand and moves in 3-5 directions. As it moves, adds nucleotides to the NEW strand in 5-3 direction.

  DNAP III that follows helicase is leading strand. Only needs 1

  DNAP that moves away from helicase is lagging strand. Needs many
• DNA Polymerase I


  Replaces RNA nucleotides with DNA nucleotides
• DNA ligase


  Joins okazaki fragments forming a continuous DNA strand
• Telomers


  Repeating units of short nucleotide sequences that do not code for genes. They form a cap at the end of DNA to help postpone erosion. Telomerase adds telomers to DNA

  TTAGGG in humans, and reproductive cells are high in telomerase
• Proofreading and Repair


  If errors occur, mismatch repair takes place.

  Enzymes remove and replace incorrectly paired nucleotides.

  Nuclease can remove segments of nucleotides and DNA poly/ligase can replace the segments
• Heterochromatin vs Euchromatin


  H= higher degrees of organization/densely arranged nucleosomes.

  E= loosely arranged, open to proteins that carry out transcription and genes to be expressed