The Molecular Basis of Heredity Chapter 16

5.5 Nucleic acids store, transmit, and help express hereditary information

Nucleotides

DNA Structure

Anti-parallel Structure  Sugar carbons are numbered 1’-5’  One side of DNA runs in the 3’ direction  The other side runs in the 5’ direction  This is important to replication

RNA Structure  Ribose sugar  Uracil replaces thymine  Single stranded

16.1 DNA is the genetic material

Protein as the genetic material?  T.H. Morgan – fruit flies  Discovered genes as part of chromosomes  Chromosomes made of protein and DNA  Protein?  More known  Diverse structures  Specificity of function  DNA?  Little known  Seemed too uniform to be the genetic code of all life

16.2 Many proteins work together in DNA replication and repair

DNA Replication  Making DNA from existing DNA  Semi-conservative  At the end of DNA replication, each daughter molecule has one old strand (from the parent DNA) and one new strand (synthesized during replication)  Model proposed by Meselson and Stahl

Meselson & Stahl

DNA Replication  Step 1:  Helicases unwind DNA at origin of replication by breaking hydrogen bonds between nitrogen bases  Replication bubble forms as two parental strands separate  Replication fork forms at end of each replication bubble

DNA Replication  Step 2:  Single-strand binding proteins hold the unpaired DNA strands apart while new DNA strands are being synthesized  Topoisomerase protein binds to parental DNA to relieve strain untwisting puts on replication fork

DNA Replication  Step 3:  Primase creates a short RNA primer that binds to the parent DNA to signal DNA polymerase III where to begin adding nucleotides  RNA primer will later be replaced with DNA nucleotides

DNA Replication  Step 4:  DNA Polymerase III adds nucleotides to exposed bases in 5’-3’ direction at the RNA primer  Leading strand  Produced continuously in 5’-3’ direction  Elongation moves towards replication fork  Lagging strand  Produced in pieces  Okazaki fragments  Elongation moves in opposite direction of replication fork (5’-3’)

DNA Replication  Step 5:  Lagging strand is completed as DNA ligase seals Okazaki fragments

Proofreading & Repair  Initial error rate in replication is 1 in 100,000 nucleotides  DNA polymerases proofread and correct errors  Error rate in completed replication is 1 in 10 billion bases

Mismatch Repair  For that 1 in 10 billion errors that escapes DNA polymerase or are due to environmental mutations  Many enzymes involved  Cut out damaged section (nuclease)  Replace with new nucleotides (DNA polymerase I)  Seal in place (DNA ligase)

16.3 A chromosome consists of a DNA molecule packed together with proteins