Transcript video slide
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
5C
3C
Nucleoside
Nitrogenous
base
Cytosine (C)
Thymine (T, in DNA) Uracil (U, in RNA)
Purines
Phosphate
group
5C
Sugar
(pentose)
Adenine (A)
Guanine (G)
(b) Nucleotide
3C
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.
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
Note Card-Explain these concepts
• Semi conservative replication
• Bi-directional replication
• Discontinuous replication