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 reprogram
genetics of
infected cell
Alfred Hershey-Martha Chase “Blender”
Experiment
In 1953, James
Watson and
Francis Crick
introduced an
elegant doublehelical model for
the structure of
deoxyribonuclei
c acid, or DNA
Watson and Crick relied on other scientists’ data
Rosalind Franklin produced some of the important X ray
crystallographic images
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
5C
Sugar
(pentose)
Adenine (A)
Guanine (G)
(b) Nucleotide
3C
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
The nucleotides
are
linked by
phosphodiester
bonds
Sugar–phosphate
backbone
5 end
Nitrogenous
bases
Thymine (T)
Adenine (A)
Cytosine (C)
DNA nucleotide
Phosphate
Sugar (deoxyribose)
3 end
Guanine (G)
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
3 end
0.34 nm
(a) Key features of DNA structure (b) Partial chemical structure
5 end
(c) Space-filling model
Watson and Crick’s key contribution was the
base-pair
WatsonCrick base
pairs
Adenine (A)
Thymine (T)
Other types
can formand they do!
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
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
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
Origin of
replication
(b) Origins of replication in a eukaryotic cell
Parental (template) strand
Origin of replication
Daughter (new)
strand
Doublestranded
DNA molecule
Replication
fork
Parental (template)
strand
Replication
bubble
Bubble
Double-stranded
DNA molecule
Daughter (new)
strand
Replication fork
Two daughter
DNA molecules
Two daughter DNA molecules
PowerPoint® Lecture Presentations for
0.5 m
Eighth Edition
Neil Campbell and Jane Reece
0.25 m
Biology
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
DNA polymerase works in only one direction.
Therefore-it must back up from second to second: replication is
DISCONTINUOUS
Overview
Leading
strand
Origin of replication
Lagging
strand
Lagging strand
2
1
PowerPoint® Lecture Presentations for
Biology
Leading
strand
Overall directions
Eighth Edition
Neil Campbell and Jane Reeceof replication
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Define:
Semi-conservative replication
Bi-directional replication
Replication fork
Discontinuous Replication