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Chapter 15: Molecular Basis of
Inheritance
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 Search for the Genetic
Material
• When T. H. Morgan’s group showed that
genes are located on chromosomes, the two
components of chromosomes—DNA and
protein—became 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
DNA is Genetic Material
• 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
Mixture of
heat-killed
Living S cells Living R cells Heat-killed
S cells and
(control)
(control)
S cells (control) living R cells
EXPERIMENT
RESULTS
Mouse dies Mouse healthy Mouse healthy Mouse dies
Living S cells
Frederick Griffith (1928)
-
Gave mice the bacterium that causes pneumonia (2 strains)
- Smooth strain killed the mice (DEADLY)
- Rough strain did not kill the mice (HARMLESS)
- Killed smooth strain with heat, did not kill the mice
-
Mixed the harmless strain and the heat-treated deadly strain
He found that the harmless rough strain, TRANSFORMED into
a deadly form.
He discovered the TRANSFORMING principle which is now
DNA
Oswald Avery(1944)
-
Wanted to test if protein was the transforming factor
- Treated Griffith’s mixture of heat-treated deadly strain and live
harmless strain with protein-destroying enzymes.
- The bacterial colonies grown from the mixture were still
transformed
- Then treated the mixture with DNA destroying enzymes.
- The bacterial colonies failed to transform.
- CONCLUSION: DNA IS THE GENETIC MATERIAL OF THE
CELL
Additional Evidence
• Viruses: small infectious agent that can only
reproduce in living cells
• More evidence for DNA as the genetic material
came from studies of bacteriophages
Bacteriophages- viruses that infect bacteria
Fig. 16-3
Phage
head
Tail
sheath
Tail fiber
Bacterial
cell
100 nm
DNA
Alfred Hershey and Martha
Chase(1952)
-
-
Conducted experiments using viruses to be sure DNA was the
transforming factor, not protein.
A virus is DNA wrapped in a protein. They reproduce by infecting a
living cell with genetic materal. A virus that infects bacteria is a
bacteriophage.
Concluded that the phage’s DNA entered the bacterial cell during
infection but the proteins did not.
Fig. 16-4-3
EXPERIMENT
Phage
Empty
Radioactive protein
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
• 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
Sugar–phosphate
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\
• 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
• 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
– 2 strands, that are twisted
Nucleotides
Monomers of DNA- nucleotides
Made up of:
1.Pentose Sugar
2.Phosphate group
3.Nitrogenous Base
Nucleotides join together
• Joined together by covalent bonds that connect sugar of
one nucleotide to the phosphate group of the next
• The repeating pattern of sugar-phosphate is called a
sugar-phosphate “backbone”
Fig. 16-7a
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
Fig. 16-7b
(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
Antiparallel- run side by side in opposite
directions
• 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
• 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
Purine:
Adenine and Guanine
2 fused rings
Pyrimidine:
Thymine and Cytosine
1 ring
Fig. 16-UN1
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data
Fig. 16-8
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
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
Base Pairing to a Template
Strand
How does DNA act as a template?
• 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. 16-9-1
A
T
C
G
T
A
A
T
G
C
(a) Parent molecule
Fig. 16-9-2
A
T
A
T
C
G
C
G
T
A
T
A
A
T
A
T
G
C
G
C
(a) Parent molecule
(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
• 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)
Fig. 16-10
Parent cell
(a) Conservative
model
(b) Semiconservative model
(c) Dispersive
model
First
replication
Second
replication
DNA Replication
• The copying of DNA is remarkable in its speed
and accuracy
• More than a dozen enzymes and other proteins
participate in DNA replication
Getting Started
1. 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
Fig. 16-12
Origin of
replication
Parental (template) strand
Daughter (new) strand
Doublestranded
DNA molecule
Replication fork
Replication
bubble
0.5 µm
Two
daughter
DNA
molecules
(a) Origins of replication in E. coli
Origin of replication
Double-stranded DNA molecule
Parental (template) strand
Daughter (new) strand
0.25 µm
Bubble
Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
Fig. 16-12b
Origin of replication Double-stranded DNA molecule
Parental (template) strand
Daughter (new) strand
0.25 µm
Bubble
Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
Fig. 16-13
Primase
Single-strand binding
proteins
3’
Topoisomerase
5’
3’
5’
Helicase
5’
RNA
primer
3’
2. At the end of each replication bubble is a
replication fork, a Y-shaped region where
new DNA strands are elongating
Enzymes/Proteins Involved in the beginning….
• 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
tp://www.youtube.com/watch?
=EYGrElVyHnU
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
Synthesizing a New DNA Strand
• Enzymes called DNA polymerases catalyze
the elongation of new DNA at a replication fork
(adding new nucleotides)
• The rate of elongation is about per second in
bacteria and 50 per second in human cells500
nucleotides
• Along one template strand of DNA, the DNA
polymerase synthesizes a leading strand
continuously, moving toward the replication fork
• DNA polymerases can only add nucleotides to a FREE
3’ end
• THERE IS NO FREE 3’ end on lagging strand
• Uses a short “primer” to initiate synthesis
• The initial nucleotide strand is a short RNA primerused on lagging strand
• 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 (5–10 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
Fig. 16-13
Primase
Single-strand binding
proteins
3’
Topoisomerase
5’
3’
5’
Helicase
5’
RNA
primer
3’
• 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
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
Leading strand
3
DNA pol III
3
Parental DNA
Primer
5
Primase
3
DNA pol III
Lagging strand
5
4
DNA pol I
3 5
3
2
DNA ligase
1
3
5
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
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
5
Ends of parental
DNA strands
Leading strand
Lagging strand
3
Last fragment
Previous fragment
RNA primer
Lagging strand
5
3
Parental strand
Removal of primers and
replacement with DNA
where a 3 end is available
p://www.youtube.com/watch?
http://www.youtube.com/watch?
AJNoTmWsE0s
v=x1zw6uRxKYU
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
• 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
• 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)
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