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

Making DNA from existing DNA Semi-conservative 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