Overview: Life’s Operating Instructions DNA, the substance of heredity, is the most celebrated molecule of our time Hereditary information is encoded in.

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

• Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research

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

Watson and Crick relied on other scientists’ data

Rosalind Franklin produced some of the important X ray crystallographic images

5 end

Nucleoside

Nitrogenousbase

Phosphategroup Sugar

(pentose)

(b) Nucleotide

Polynucleotide, or nucleic acid- a polymer made of nucleotide monomers Nucleotide = base + sugar + phosphate

Nucleoside = base + sugar

3 end

3C

3C

5C

5C

Nitrogenous bases

Pyrimidines

Cytosine (C) Thymine (T, in DNA) Uracil (U, in RNA)

Purines

Adenine (A) Guanine (G)

Sugars

Deoxyribose (in DNA) Ribose (in RNA)

(c) Nucleoside components: sugars

The nucleotides arelinked byphosphodiester bonds

Sugar–phosphate backbone

5 end

Nitrogenous

bases

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

DNA nucleotide

Sugar (deoxyribose)

3 end

Phosphate

(c) Space-filling model

Hydrogen bond 3 end

5 end

3.4 nm

0.34 nm

3 end

5 end

(b) Partial chemical structure(a) Key features of DNA structure

1 nm

Watson and Crick’s key contribution was the base-pair

Cytosine (C)

Adenine (A)Thymine (T)

Guanine (G)

Watson-Crick base pairs

Other types can form-and they do!

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

Fig. 16-9-3

A T

GC

T A

TA

G C

(a) Parent molecule

A T

GC

T A

TA

G C

(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand

(b) Separation of strands

A T

GC

T A

TA

G C

A T

GC

T A

TA

G C

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

Biology Eighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

(a) Origin of replication in an E. coli cell (b) Origins of replication in a eukaryotic cell

Origin ofreplication Parental (template) strand

Double-strandedDNA molecule

Daughter (new)strand

Replicationfork

Replicationbubble

Two daughterDNA molecules

Origin of replicationDouble-strandedDNA molecule

Parental (template)strand

Daughter (new)strand

Bubble Replication fork

Two daughter DNA molecules

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

Biology Eighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Origin of replication

Overview

Leadingstrand

Leadingstrand

Laggingstrand

Lagging strand

Overall directionsof replication

12

DNA polymerase works in only one direction.

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

Note Card-Explain these concepts

• Semi conservative replication

• Bi-directional replication

• Discontinuous replication