The Molecular Basis of Inheritance Chapter 16 Biology – Campbell • Reece

History of DNA Scientists knew… Genes are located on chromosomes Chromosomes are made of DNA and Protein Proteins are more complex than DNA So the question is: Is it protein or DNA that is the genetic material?

History of DNA Frederick Griffith (1928) – studied the bacteria, Streptococcus pneumoniae, to show transformation Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944) – isolated various chemicals to try to identify which one caused transformation Concluded that it was DNA

Transformation

History of DNA Alfred Hershey and Martha Chase (1952) – used bacteriophages to prove that DNA is the transforming factor Erwin Chargaff (1947) – found that the number of adenines = thymines and cytosines = guanines Rosalind Franklin – used X-ray crystallography to show that DNA is a helix

Protein or DNA?

Chargaff’s Data Source of DNA A T G C Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 29.4 19.9 19.8

X-ray photo of DNA

History of DNA James Watson and Francis Crick (1953) Proposed the double-helix model of DNA The phosphate-sugar chains were the backbone The base pairing followed Chargaff’s Rules (A-T and C-G) Always paired a purine with a pyrimidine

Double-Helix DNA Model

Purines Paired With Pyrimidines

5’ to 3’

DNA Replication Watson and Crick then proposed that each strand of DNA would act as a template to make new copies of DNA

The Process of Replication… Origins of Replication – special sites where DNA replication begins Proteins recognize this sequence and attach to the DNA, separating the two strands, forming a replication “bubble” Replication then proceeds in both directions, until the entire molecule is copied

Origins of Replication

Primers Primer - A short stretch of RNA that initiates synthesis (about 10 bases long) Primase – the enzyme that makes the primer The RNA is then later replaced with DNA by DNA polymerase

The Process cont. DNA polymerases – enzymes that add the nucleotides to the new DNA strand The two DNA strands are antiparallel meaning their sugar-phosphate backbones run in opposite directions Nucleotides are only added to the 3’ end Leading strands and lagging strands The segments of the lagging strand are called Okazaki fragments

Leading and Lagging Strands

Other Enzymes… DNA ligase – joins segments of DNA together to make one strand of DNA Helicase – untwists the double-helix at the replication fork, separating the strands Single-strand binding protein – holds the DNA strands apart while they are replicated

Proofreading DNA polymerase proofreads each nucleotide as soon as it is added If the base is incorrect, the DNA polymerase removes it and continues If DNA polymerase doesn’t catch it, nuclease, will cut out the incorrect base and it will be replaced by DNA polymerase and ligase

Nucleotide Excision Repair

Telomeres Since DNA Polymerase can only add nucleotides to the 3’ end, there is no way to complete the 5’ ends of the daughter strands Each replication results in a shortening of the DNA strand Regions of repetitive sequences, called telomeres, prevent the loss of genes

Shortening of DNA

DNA Packing