Overview: Life’s Operating Instructions DNA, the substance of heredity, is the most celebrated molecule of our time Hereditary information is encoded in DNA and reproduced in “all” cells of the body This DNA program directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits

Concept 16.1: DNA is the genetic material Early in the 20th century, the identification of the molecules of inheritance loomed as a major challenge to biologists The discovery of the genetic role of DNA began with research by Griffith in 1928 Griffith showed that bacteria contained a substance that could cause a genetic transformation In 1944, Avery, McCarty and MacLeod announced that the transforming substance was DNA

More evidence for DNA as the genetic material came from studies of viruses that infect bacteria Bacteriophages (or phages), are widely used in molecular genetics research Simple systems that help to understand more complex ones: the “atom” of molecular biology

Bacteriophages were widely accepted as a model system Consist of DNA and protein Known to re-program genetics of infected cell

Alfred Hershey-Martha Chase “Blender” Experiment

In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA Figure 16.1 How was the structure of DNA determined?

Watson and Crick relied on other scientists’ data Rosalind Franklin produced some of the important X ray crystallographic images

(c) Nucleoside components: sugars 5 end Nitrogenous bases Pyrimidines 5C 3C Nucleoside Nitrogenous base Cytosine (C) Thymine (T, in DNA) Uracil (U, in RNA) Purines Phosphate group Sugar (pentose) 5C Adenine (A) Guanine (G) 3C (b) Nucleotide Sugars 3 end Polynucleotide, or nucleic acid - a polymer made of nucleotide monomers Nucleotide = base + sugar + phosphate Nucleoside = base + sugar Deoxyribose (in DNA) Ribose (in RNA) (c) Nucleoside components: sugars

Sugar–phosphate backbone 5 end Sugar (deoxyribose) 3 end Nitrogenous bases The nucleotides are linked by phosphodiester bonds Thymine (T) Adenine (A) Cytosine (C) Phosphate DNA nucleotide Sugar (deoxyribose) 3 end Guanine (G)

Watson and Crick’s key contribution was the base-pair 5 end Hydrogen bond 3 end 1 nm 3.4 nm Figure 16.7 The double helix For the Cell Biology Video Stick Model of DNA (Deoxyribonucleic Acid), go to Animation and Video Files. For the Cell Biology Video Surface Model of DNA (Deoxyribonucleic Acid), go to Animation and Video Files. 3 end 0.34 nm 5 end (a) Key features of DNA structure (b) Partial chemical structure (c) Space-filling model Watson and Crick’s key contribution was the base-pair

Watson-Crick base pairs Other types can form-and they do! Adenine (A) Thymine (T) Figure 16.8 Base pairing in DNA Guanine (G) Cytosine (C)

Concept 16.2: Many proteins work together in DNA replication and repair The relationship between structure and function is obvious in the double helix Watson and Crick noted that the specific base pairing suggested a possible copying or replication mechanism for genetic material

The Basic Principle: Base Pairing to a Template Strand Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

(b) Separation of strands Fig. 16-9-3 A T A T A T A T C G C G C G C G T A T A T A T A A T A T A T A T G C G C G C G C (a) Parent molecule (b) Separation of strands (c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand Figure 16.9 A model for DNA replication: the basic concept

Which Model? Conservative (top)? Semi-conservative (middle)? Dispersive (bottom)? Meselson-Stahl experiment

The place on a DNA molecule where replication begins is an origin of replication or ori Special sequence of DNA bases that signals the replication machinery to assemble Many enzymes are involved: DNA helicase, DNA topoisomerase, DNA ligase. The “growth point” is the replication fork

(a) Origin of replication in an E. coli cell (b) Origins of replication in a eukaryotic cell Origin of replication Double-stranded DNA molecule Parental (template) strand Origin of replication Daughter (new) strand Parental (template) strand Daughter (new) strand Replication fork Double- stranded DNA molecule Replication bubble Bubble Replication fork Two daughter DNA molecules Two daughter DNA molecules Figure 16.12 Origins of replication in E. coli and eukaryotes. 0.5 m 0.25 m

DNA polymerase (Dpol) multiple types involved Unidirectional enzyme (5’ to 3’ synthesis) How to replicate both strands of the double helix at each replication fork? The replication machinery has to go back and forth at each fork to copy both strands: leading strand and lagging strand. Dpol enzymes not good at initiating-require help from RNA polymerase: primer

Overall directions of replication DNA polymerase works in only one direction. Overview Leading strand Origin of replication Lagging strand Lagging strand 2 1 Figure 16.16 Synthesis of the lagging strand. Leading strand Overall directions of replication

Unidirectional enzyme has problems at the ends of a linear DNA template Special adaptations required Some systems have modifications to the ends of the linear DNA More common-a special enzyme for synthesis of DNA ends is used: telomerase A polymerase with a portable RNA template

Proofreading and Repair DNA polymerases check their work as they go along and correct mistakes in real time: proofreading Remaining mistakes are removed later by a separate system of enzymes: mismatch repair Two important ways to ensure the integrity of DNA information