16
The Chapter Molecular Basis of
Inheritance
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
Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Life s Operating Instructions
In 1953, James Watson and Francis Crick
introduced an elegant double-helical model for
the structure of deoxyribonucleic acid, or DNA
DNA, the substance of inheritance, 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
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-1
1
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
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Search for the Genetic Material: Scientific
Inquiry
When T. H. Morgan s group showed that
genes are located on chromosomes, the two
components of chromosomesDNA and
proteinbecame candidates for the genetic
material
The key factor in determining the genetic
material was choosing appropriate
experimental organisms
The role of DNA in heredity was first
discovered by studying bacteria and the
viruses that infect them
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Evidence That DNA Can Transform Bacteria
The discovery of the genetic role of DNA began
with research by Frederick Griffith in 1928
Griffith worked with two strains of a bacterium,
one pathogenic and one harmless
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
2
When he mixed heat-killed remains of the
pathogenic strain with living cells of the
harmless strain, some living cells became
pathogenic
He called this phenomenon transformation,
now defined as a change in genotype and
phenotype due to assimilation of foreign DNA
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-2
Mixture of
EXPERIMENT
heat-killed
Living S cells Living R cells Heat-killed
S cells and
(control)
(control)
S cells (control) living R cells
RESULTS
Mouse dies Mouse healthy Mouse healthy Mouse dies
Living S cells
In 1944, Oswald Avery, Maclyn McCarty, and
Colin MacLeod announced that the
transforming substance was DNA
Their conclusion was based on experimental
evidence that only DNA worked in transforming
harmless bacteria into pathogenic bacteria
Many biologists remained skeptical, mainly
because little was known about DNA
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
3
Evidence That Viral DNA Can Program Cells
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
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-3
Phage
head
Tail
sheath
Tail fiber
Bacterial
cell
100 nm
DNA
In 1952, Alfred Hershey and Martha Chase
performed experiments showing that DNA is
the genetic material of a phage known as T2
To determine the source of genetic material in
the phage, they designed an experiment
showing that only one of the two components
of T2 (DNA or protein) enters an E. coli cell
during infection
They concluded that the injected DNA of the
phage provides the genetic information
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
4
Fig. 16-4-3
EXPERIMENT
Phage
Empty
protein
Radioactive shell
protein
Radioactivity
(phage
protein)
in liquid
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Phage
DNA
Centrifuge
Pellet (bacterial
cells and contents)
Radioactive
DNA
Batch 2:
radioactive
phosphorus (32P)
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
Additional Evidence That DNA Is the Genetic
Material
It was known that DNA is a polymer of
nucleotides, each consisting of a nitrogenous
base, a sugar, and a phosphate group
In 1950, Erwin Chargaff reported that DNA
composition varies from one species to the
next
This evidence of diversity made DNA a more
credible candidate for the genetic material
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Chargaff s rules state that in any species there
is an equal number of A and T bases, and an
equal number of G and C bases
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
5
Fig. 16-5
Sugarphosphate
backbone
5+ end
Nitrogenous
bases
Thymine (T)
Adenine (A)
Cytosine (C)
DNA nucleotide
Phosphate
Sugar (deoxyribose)
3+ end
Guanine (G)
Building a Structural Model of DNA: Scientific
Inquiry
After most biologists became convinced that
DNA was the genetic material, the challenge
was to determine how its structure accounts for
its role
Maurice Wilkins and Rosalind Franklin were
using a technique called X-ray crystallography
to study molecular structure
Franklin produced a picture of the DNA
molecule using this technique
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-6
(a) Rosalind Franklin
(b) Franklin s X-ray diffraction
photograph of DNA
6
Franklin s X-ray crystallographic images of
DNA enabled Watson to deduce that DNA was
helical
The X-ray images also enabled Watson to
deduce the width of the helix and the spacing
of the nitrogenous bases
The width suggested that the DNA molecule
was made up of two strands, forming a double
helix
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-7
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 built models of a double helix
to conform to the X-rays and chemistry of DNA
Franklin had concluded that there were two
antiparallel sugar-phosphate backbones, with
the nitrogenous bases paired in the molecule s
interior
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
7
At first, Watson and Crick thought the bases
paired like with like (A with A, and so on), but
such pairings did not result in a uniform width
Instead, pairing a purine with a pyrimidine
resulted in a uniform width consistent with the
X-ray
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-UN1
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data
Watson and Crick reasoned that the pairing
was more specific, dictated by the base
structures
They determined that adenine (A) paired only
with thymine (T), and guanine (G) paired only
with cytosine (C)
The Watson-Crick model explains Chargaff s
rules: in any organism the amount of A = T,
and the amount of G = C
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
8
Fig. 16-8
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
Concept 16.2: Many proteins work together in
DNA replication and repair
The relationship between structure and
function is manifest in the double helix
Watson and Crick noted that the specific base
pairing suggested a possible copying
mechanism for genetic material
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
9
Fig. 16-9-3
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
A
T
A
T
A
T
A
T
A
T
A
T
A
T
A
T
G
C
G
C
G
C
G
C
(a) Parent molecule
(c) Daughter DNA molecules,
each consisting of one
parental strand and one
new strand
(b) Separation of
strands
Watson and Crick s semiconservative model
of replication predicts that when a double helix
replicates, each daughter molecule will have
one old strand (derived or conserved from
the parent molecule) and one newly made
strand
Competing models were the conservative
model (the two parent strands rejoin) and the
dispersive model (each strand is a mix of old
and new)
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-10
Parent cell
First
replication
Second
replication
(a) Conservative
model
(b) Semiconservative model
(c) Dispersive
model
10
Experiments by Matthew Meselson and
Franklin Stahl supported the semiconservative
model
They labeled the nucleotides of the old strands
with a heavy isotope of nitrogen, while any new
nucleotides were labeled with a lighter isotope
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The first replication produced a band of hybrid
DNA, eliminating the conservative model
A second replication produced both light and
hybrid DNA, eliminating the dispersive model
and supporting the semiconservative model
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-11
EXPERIMENT
1 Bacteria
cultured in
medium
containing
1 5N
2 Bacteria
transferred to
medium
containing 14N
RESULTS
3 DNA sample
centrifuged
after 20 min
(after first
application)
4 sample
centrifuged
after DNA 40 min
(after second
replication)
Less
dense
More
dense
CONCLUSION
First replication
Second replication
Conservative
model
Semiconservative
model
Dispersive
model
11
DNA Replication: A Closer Look
The copying of DNA is remarkable in its speed
and accuracy
More than a dozen enzymes and other proteins
participate in DNA replication
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Getting Started
Replication begins at special sites called
origins of replication, where the two DNA
strands are separated, opening up a
replication bubble
A eukaryotic chromosome may have hundreds
or even thousands of origins of replication
Replication proceeds in both directions from
each origin, until the entire molecule is copied
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-12a
Origin of
replication
Parental (template) strand
Daughter (new) strand
Doublestranded
DNA molecule
Replication fork
Replication
bubble
0.5 m
Two
daughter
DNA
molecules
(a) Origins of replication in E. coli
12
Fig. 16-12b
Origin of replication Double-stranded DNA molecule
Parental (template) strand
Daughter (new) strand
0.25 m
Bubble
Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
At the end of each replication bubble is a
replication fork, a Y-shaped region where
new DNA strands are elongating
Helicases are enzymes that untwist the double
helix at the replication forks
Single-strand binding protein binds to and
stabilizes single-stranded DNA until it can be
used as a template
Topoisomerase corrects overwinding ahead
of replication forks by breaking, swiveling, and
rejoining DNA strands
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-13
Primase
Single-strand binding
proteins
3+
Topoisomerase
5+
3+
5+
RNA
primer
3+
5+
Helicase
13
DNA polymerases cannot initiate synthesis of a
polynucleotide; they can only add nucleotides to
the 3+ end
The initial nucleotide strand is a short RNA
primer
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
An enzyme called primase can start an RNA
chain from scratch and adds RNA nucleotides
one at a time using the parental DNA as a
template
The primer is short (510 nucleotides long), and
the 3+ end serves as the starting point for the
new DNA strand
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Synthesizing a New DNA Strand
Enzymes called DNA polymerases catalyze
the elongation of new DNA at a replication fork
Most DNA polymerases require a primer and a
DNA template strand
The rate of elongation is about 500 nucleotides
per second in bacteria and 50 per second in
human cells
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
14
Each nucleotide that is added to a growing
DNA strand is a nucleoside triphosphate
dATP supplies adenine to DNA and is similar to
the ATP of energy metabolism
The difference is in their sugars: dATP has
deoxyribose while ATP has ribose
As each monomer of dATP joins the DNA
strand, it loses two phosphate groups as a
molecule of pyrophosphate
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-14
New strand
5+ end
Sugar
Template strand
3+ end
5+ end
3+ end
T
A
T
C
G
C
G
G
C
G
C
T
A
A
Base
Phosphate
DNA polymerase
3+ end
A
T
Pyrophosphate 3+ end
C
Nucleoside
triphosphate
C
5+ end
5+ end
Antiparallel Elongation
The antiparallel structure of the double helix
(two strands oriented in opposite directions)
affects replication
DNA polymerases add nucleotides only to the
free 3+ end of a growing strand; therefore, a
new DNA strand can elongate only in the 5+ to
3+ direction
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
15
Along one template strand of DNA, the DNA
polymerase synthesizes a leading strand
continuously, moving toward the replication fork
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-15a
Overview
Origin of replication
Leading strand
Lagging strand
Primer
Leading strand
Lagging strand
Overall directions
of replication
Fig. 16-15b
Origin of replication
3+
5+
RNA primer
5+
Sliding clamp
3+
5+
Parental DNA
DNA pol III
5+
3+
5+
3+
5+
16
To elongate the other new strand, called the
lagging strand, DNA polymerase must work in
the direction away from the replication fork
The lagging strand is synthesized as a series of
segments called Okazaki fragments, which
are joined together by DNA ligase
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-16b1
3+
Template
strand
Fig. 16-16b2
5+
5+
3+
5+
3+
3+
Template
strand
3+
5+
RNA primer
5+
1
3+
5+
17
Fig. 16-16b3
3+
5+
5+
Template
strand
3+
RNA primer
5+
3+
3+
1
5+
3+
5+
Okazaki
fragment
3+
1
5+
Fig. 16-16b4
3+
5+
5+
Template
strand
3+
RNA primer
5+
3+
3+
5+
1
5+
5+
2
Fig. 16-16b5
3+
5+
1
3+
5+
5+
Template
strand
3+
RNA primer
5+
3+
3+
1
3+
5+
1
5+
5+
2
3+
5+
Okazaki
fragment
3+
3+
5+
Okazaki
fragment
3+
3+
3+
1
1
3+
5+
5+
2
1
3+
5+
18
Fig. 16-16b6
3+
5+
3+
5+
Template
strand
3+
RNA primer
3+
5+
1
5+
5+
2
3+
5+
1
5+
3+
3+
5+
1
2
3+
5+
Okazaki
fragment
3+
3+
3+
1
5+
5+
3+
5+
1
2
Overall direction of replication
Table 16-1
Fig. 16-17
Overview
Origin of replication
Lagging strand
Leading strand
Leading strand
Lagging strand
Overall directions
of replication
Single-strand
binding protein
Helicase
5+
3+
Parental DNA
Leading strand
3+
DNA pol III
Primer
5+
Primase
3+
5+
DNA pol III
4
3+ 5+
Lagging strand
DNA pol I
3
2
DNA ligase
1
3+
5+
19
The DNA Replication Complex
The proteins that participate in DNA replication
form a large complex, a DNA replication
machine
The DNA replication machine is probably
stationary during the replication process
Recent studies support a model in which DNA
polymerase molecules reel in parental DNA
and extrude newly made daughter DNA
molecules
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Proofreading and Repairing DNA
DNA polymerases proofread newly made DNA,
replacing any incorrect nucleotides
In mismatch repair of DNA, repair enzymes
correct errors in base pairing
DNA can be damaged by chemicals, radioactive
emissions, X-rays, UV light, and certain
molecules (in cigarette smoke for example)
In nucleotide excision repair, a nuclease cuts
out and replaces damaged stretches of DNA
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-18
Nuclease
DNA
polymerase
DNA
ligase
20
Replicating the Ends of DNA Molecules
Limitations of DNA polymerase create problems
for the linear DNA of eukaryotic chromosomes
The usual replication machinery provides no
way to complete the 5+ ends, so repeated
rounds of replication produce shorter DNA
molecules
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-19
Ends of parental
DNA strands
5+
Leading strand
Lagging strand
3+
Last fragment
Previous fragment
RNA primer
Lagging strand
5+
Parental strand
3+
Removal of primers and
replacement with DNA
where a 3+ end is available
5+
3+
Second round
of replication
5+
New leading strand 3+
New lagging strand 5+
3+
Further rounds
of replication
Shorter and shorter daughter molecules
Eukaryotic chromosomal DNA molecules have
at their ends nucleotide sequences called
telomeres
Telomeres do not prevent the shortening of
DNA molecules, but they do postpone the
erosion of genes near the ends of DNA
molecules
It has been proposed that the shortening of
telomeres is connected to aging
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
21
Fig. 16-20
1 m
If chromosomes of germ cells became shorter
in every cell cycle, essential genes would
eventually be missing from the gametes they
produce
An enzyme called telomerase catalyzes the
lengthening of telomeres in germ cells
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The shortening of telomeres might protect cells
from cancerous growth by limiting the number
of cell divisions
There is evidence of telomerase activity in
cancer cells, which may allow cancer cells to
persist
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
22
Concept 16.3 A chromosome consists of a DNA
molecule packed together with proteins
The bacterial chromosome is a doublestranded, circular DNA molecule associated
with a small amount of protein
Eukaryotic chromosomes have linear DNA
molecules associated with a large amount of
protein
In a bacterium, the DNA is supercoiled and
found in a region of the cell called the nucleoid
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Chromatin is a complex of DNA and protein,
and is found in the nucleus of eukaryotic cells
Histones are proteins that are responsible for
the first level of DNA packing in chromatin
Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-21a
Nucleosome
(10 nm in diameter)
DNA
double helix
(2 nm in diameter)
H1
Histones
DNA, the double helix
Histones
Histone tail
Nucleosomes, or beads
on a string (10-nm fiber)
23
Fig. 16-21b
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
Replicated
chromosome
(1,400 nm)
30-nm fiber
Looped domains
(300-nm fiber)
Metaphase
chromosome
24