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Chapter 16

BIOL 135

Professor Ott

For this lecture:

• Will go over:
  • Discovering that genetic material was DNA
  • DNA replication
    • The semiconservative model
    • The process of replication
    • Errors and repair
    • Telomeres
• Please refer to Sec. 16.1-2 of textbook 2

Scientists were trying to discover what were the molecules of

inheritance

• In early 1900s, identification of molecules of inheritance posed a challenge
• Morgan showed that genes are located on chromosomes
• 2 components of chromosomes—DNA and protein—became candidates for genetic material Figure 16.1b 4

Scientists discovered that the genetic material was DNA

• This research began by working with 2 strains of a bacterium: one pathogenic and one harmless
• Heat-killed pathogenic strain was mixed with living cells of harmless strain
• Some living cells became pathogenic
• Called transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA Figure 16.2 5

Scientists discovered that the genetic material was DNA

• More evidence for DNA as genetic material came from studies of viruses that infect bacteria (viruses known as bacteriophages or phages)
• Virus is DNA (sometimes RNA) enclosed by protective coat, usually protein
• Phages have been widely used as tools by researchers in molecular genetics Figure 16.3 7

Scientists discovered that the genetic material was DNA

• Showed that DNA is genetic material of a phage
• Experiment showed that only 1 of 2 components of genetic material (DNA or protein) enters an E. coli cell during infection
• Concluded that injected DNA of phage provides genetic information Figure 16.4 8

Scientists discovered that the genetic material was DNA

• Chargaff’s rules:
  • Base composition of DNA varies between species
  • In any species, number of A and T bases is equal and number of G and C bases is equal
• These rules were not understood until discovery of double helix Figure 16.5 10

Scientists accepted that the genetic material was DNA

• After DNA was accepted as genetic material, challenge was to determine how structure accounts for role in inheritance
• Maurice Wilkins and Rosalind Franklin used technique called X-ray crystallography to study molecular structure
• Franklin produced a picture of DNA molecule Figure 16.6 11

Scientists accepted that the genetic material was DNA

• Watson and Crick built models of a double helix
• Franklin concluded there were 2 outer sugar-phosphate backbones with nitrogenous bases paired in molecule’s interior
• Watson built a model where backbones were antiparallel (their subunits run in opposite directions) Figure 16.7 and 5.24 13

Scientists accepted that the genetic material was DNA

• At first, Watson and Crick thought bases paired like with like (A with A, and so on)
• But did not result in uniform width
• Instead, pairing a purine (A or G) with a pyrimidine (C or T) resulted in uniform width consistent with X-ray data Figure 16.8 14

_______ make up the two outer backbones of DNA, while

_____ are paired in the interior of the helix.

A. Phosphate groups; nitrogenous bases B. Nitrogenous bases; sugar-phosphate groups C. Sugar-phosphate groups; nitrogenous bases D. Nitrogenous bases; ribose-phosphate groups 16

_______ make up the two outer backbones of DNA, while

_____ are paired in the interior of the helix.

A. Phosphate groups; nitrogenous bases B. Nitrogenous bases; sugar-phosphate groups C. Sugar-phosphate groups; nitrogenous bases D. Nitrogenous bases; ribose-phosphate groups 17

DNA replication is the process of copying DNA

• Watson and Crick noted that specific base pairing suggested possible copying mechanism for genetic material
• DNA replication = copying of DNA Figure 16.10 19

DNA replication is the process of copying DNA

• Since 2 strands of DNA are complementary, each strand acts as a template for building new strand in replication
• Yields 2 exact replicas of “parental” molecule Figure 16.10 20