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Chapter 16
The 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
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
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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 chromosomes—DNA
and protein—became candidates for the
genetic material
•
he was the one that worked on fruit flies
and took 2 years to naturally get 1 mutant
white eyed fly;
•
He bred them and ended up determining
that some traits were sex linked
•
And that other traits were carried on other
types of chromosomes
•
BTW, he was a Nobel Prize in 1933 for his
contribution to genetics
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
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
Non pathogenic is
the “rough” version
pneumococcus
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“smooth” version is the
Pathogenic
• Griffith mixed:
– heat-killed remains of the pathogenic strain
– living cells of the harmless strain,
• 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
• Up to this time they
thought the genetic
factor could be protein or
nucleic acids
• Scientists didn’t believe
it was nucleic acids
because they were so
simple (they didn’t know
how nucleic acids
worked)
Griffith Continued
1. Inject live nonpathogenic= live mouse
2. Inject pathogenic= mouse dies
3. Inject heat killed= mouse alive
4. Inject heat killed pathogenic and live
5. Nonpathogenic= dead mouse
What happened? The live nonpathogenic
Took up the DNA from the dead pathogenic
Incorporated it into its own genome, started
Making copies of the pathogenic genome
And killed the mice
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Oswald Avery experiment
• What they did: broke open
the heat killed pathogenic
bacteria
• Ran 3 different experiments,
each was designed to
inactivate the 3 likely “agents
of transformation”
– RNA -they added Rnase
to inactivate it
– DNA –they added DNAse
to inactivate it
– Protein- they added
protease to inactivate it
• He then tested each for their ability to transform
live nonpathogenic bacteria into pathogenic
• They determined that DNA was the only one that
caused the transformation (how did this
experiment work?)
• Many biologists remained skeptical, mainly
because little was known about DNA
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
Animation: Phage T2 Reproductive Cycle
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
– Remember: phages (or
bacteriophages)
• Are viruses that infect
bacteria
T2 happens to like to
infect E. coli
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• To determine the source of genetic material in the
phage,
– they designed an experiment showing that only
one of the two components of T2 enters an E.
coli cell during infection
• DNA or protein They concluded that the
injected DNA of the phage provides the
genetic information
Fig. 16-4-1
EXPERIMENT
Phage
Radioactive
protein
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Radioactive
DNA
Batch 2:
radioactive
phosphorus (32P)
Batch 1 was labeled with radioactive sulfur which
would incorporate Into the protein portion of the phage
(the amino acid cysteine has sulfur in it)
Batch 2 had radioactive phosphorus the binds to the DNA
Inside The phage
Fig. 16-4-2
EXPERIMENT
Phage
Empty
protein
Radioactive shell
protein
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Phage
DNA
Radioactive
DNA
Batch 2:
radioactive
phosphorus (32P)
Mechanically disrupted the
Membranes by the use of a
Homogenizer- fancy word for
blender
Fig. 16-4-3
The proteins are lighter than the DNA
So the proteins float while DNA will
Make a pellet; they found that the radiolabeled DNA
Was found in the bacteria but not any of the radioLabeled protein because the bacteria “ingested” the viral DNA
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
For the curious minded:
• Proteins were recognized as a biological molecule
in the 18th century (egg albumin, fibrin, wheat
gluten, and blood serum albumin were
distinguishable forms at this time)
• They were recognized by their ability to coagulate
or flocculate (come out of suspension) based on
heating or the addition of acid
• Frederic Sanger was the 1st to correctly sequence
a protein: insulin
• And won a Nobel for this in 1958
And how we found out about nucleic acids
• Friedrich Miescher wanted to
isolate the protein
components of leukocytes
(white blood cells).
• Miescher made
arrangements for a local
surgical clinic to send him
used, pus-coated patient
bandages
• once he received the bandages, he
planned to wash them, filter out the
leukocytes, and extract and identify the
various proteins within the white blood
cells.
• he came across a substance from the
cell nuclei that had chemical properties
unlike any protein,
– -a much higher phosphorous
content
– No sulfur
– resistance to proteolysis (protein
digestion) he found something new
More FYI
Mature red blood cells do not have a nucleus;
probably because up to 1/3 of their total cell weight is
taken up by hemoglobin so they can carry more
oxygen
White blood cells do have nuclei since they are only
needed at specific sites and do not recirculate
What DNA is made of:
How to sequence DNA
• Purines are double
ringed structures=
adenine and guanine
• Pyrimidines are single
ringed= cytosine and
thymine and uracil
Uracil is only found in
RNA as a replacement
for thymine
• 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
• Chargaff knew the percentages but didn’t know
the reason for the %s: the double helix
• In humans for example the four bases show up
in these percentages:
– A= 30.3%
G=19.5%
– T=30.3%
C=19.9%
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Animation: DNA and RNA Structure
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The percentages are going to vary by species
• Remember, the more you have in common
biochemically with another species the more
closely related you are to each other (this is part of
the biochemistry link we talked about in evolution)
Fig. 16-5
Sugar–phosphate
backbone
5 end
Nitrogenous
bases
5’ means the 5th
carbon
of the 5 carbon sugar
Thymine (T)
Adenine (A)
Cytosine (C)
3’ means the 3rd carbon
of The 5 carbon sugar
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
• Up to this point scientists knew it had 4
nitrogenous bases, a phosphate acid and a
sugar
• They didn’t know the configuration
But they thought in was a triple strand (Linus
Pauling)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
• (Varying accounts, of course, about this story) She
did a majority of the work involved in discovering
the form of DNA
• Frank and Wilkins didn’t really work together, they
both ran separate research groups that were both
working on the structure of DNA
• They got off on a bad note: he initially thought she
was a research assistant, and not a peer
• Realizing his mistake didn’t really solve anything
• Because of this there was tension between the two
• Wilkins, withouth Rosalind knowing, showed
Watson a photo that Rosalind Frank had taken
• Franklin’s X-ray crystallographic images of DNA
enabled Watson to deduce that DNA was helical
Fig. 16-6
(a) Rosalind Franklin
(b) Franklin’s X-ray diffraction
photograph of DNA
• 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
Animation: DNA Double Helix
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Watson and Crick thought the phosphate/sugar
backbone was really on the inside and the
nitrogenous bases were on the outside- so they
had to flip their idea inside out
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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 (A or G) with a
pyrimidine (C or T) 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
Fig. 16-8
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
Watson Crick and Wilkins
Watson/Crick published first so got all the glory while
Rosalind Franklin was published as a support
article in the same edition of Nature.
Watson/Crick/Wilkins were given the Nobel Prize in
Physiology or medicine in 1962
Rosalind Franklin got
Ovarian cancer at the age
Of 37 died.
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
Animation: DNA Replication Overview
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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- magically
they thought)
• the dispersive model (each strand is a mix of
old and new like a patchwork)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-10
Parent cell
(a) Conservative
model
(b) Semiconservative model
(c) Dispersive
model
First
replication
Second
replication
• Messelsohn and Stahl Experiment
• Experiments by Matthew Meselson and
Franklin Stahl supported the semiconservative
model
• Their experiment:
– They labeled the nucleotides of the old
strands with a heavy isotope of nitrogen
(they cultured e. coli in the presence of the
heavy isotope)
• They also cultured e coli with the normal
isotope of nitrogen
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• They then processed each batch of e. coli and
mixed them in culture together
• They centrifuge them (with cesium chloride, which
is about the same weight as DNA) to make sure
that the nitrogen-15 would actually separate out at
as lower band than the nitrogen-14
The real experiment:
• E. coli grown for 14 generations in the nitrogen-15
(NH4Cl was the nitrogen source)
• They prepared the E. coli and separated some out
to be grown in the nitrogen-14 (so old nitrogen-15
e. coli moved to a new medium with nitrogen-14)
• Sampled the broth every 20 minutes (the
replication time for e. coli)
• Generation 0= all heavy
• Next generations= neither heavy nor light
• 2 generations= half light, half intermediate
• After more generations= more light than
intermediate
• The first replication produced a band of hybrid
DNA, eliminating the conservative model(why?)
• A second replication produced both light and
hybrid DNA, eliminating the dispersive model
and supporting the semiconservative model
• https://www.youtube.com/watch?v=ZTMyNmZr
cwA
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-11a
EXPERIMENT
1 Bacteria
cultured in
medium
containing
15N
2 Bacteria
transferred to
medium
containing 14N
RESULTS
3 DNA sample
centrifuged
after 20 min
(after first
application)
4 DNA sample
centrifuged
after 20 min
(after second
replication)
Less
dense
More
dense
Fig. 16-11b
CONCLUSION
First replication
Conservative
model
If this were true, then they would
Have had light and heavy, and no
Intermediate, over time they would
Have had more light and less heavy
Semiconservative
model
Dispersive
model
Second replication
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
https://www.youtube.com/watch?v=8kK2zwjRV0
M (12:59)
Crash course biology lesson 10
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
Animation: Origins of Replication
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-12a
Remember: bacteria reproduce through binary fission (exact method not known) but the
Circular DNA must Replicate before the cell can divide
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
Fig. 16-12b
THIS IS HAPPENING DURING THE S PHASE OF INTERPHASE
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
• At the end of each replication bubble is a
replication fork, a Y-shaped region where
new DNA strands are elongating
• They elongate in BOTH directions at the same
time BUT the new strand is only made in the 5’
to 3’ end
• There are free nitrogenous bases floating
around in the cytoplasm
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-13
Single-strand binding protein binds to and
stabilizes single-stranded DNA until it can be used as a
template know this
Primase adds a few
Single-strand binding
Topoisomerase corrects “overwinding” proteins
RNA nucleotides to start
the replication
ahead of replication forks by breaking,
swiveling, and rejoining DNA strands have to
Know this for the exam
3
Topoisomerase
5
RNA
primer
5
3
5
Helicase
Helicases are enzymes
that untwist the double
helix at the replication forks (breaks the h
bonds) have to know this for the exam
3
• DNA polymerases (gotta know this enzyme)
cannot initiate synthesis of a polynucleotide;
they can only add nucleotides to the 3 end
• The initial nucleotide strand is a short RNA
primer called primase about 5-10 nucleotides
long adds RNA nucleotides one at a time using
the parental DNA as a template
• Then new DNA can start being added to the 3’
end of the RNA primer
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
• Each nucleotide that is added to a growing DNA
strand is a nucleoside triphosphate
The base would be an A, T,
C, G
• For instance, 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
• This is a COUPLED EXERGONIC REACTION
that helps drive the polymerization reaction
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-14
The bases are held together by H bonds, the Phosphate is attached to the phosphate backbone by a phosphodiester
Bond courtesy of the DNA polymerase
New strand
5 end
Sugar
5 end
3 end
T
A
T
C
G
C
G
G
C
G
C
T
A
A
Base
Phosphate
Template strand
3 end
3 end
DNA polymerase
A
Pyrophosphate 3 end
C
Nucleoside
triphosphate
5 end
C
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
• The template strand is being read in the 3’ to 5’
direction
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Along one template strand of DNA, the DNA
polymerase synthesizes a leading strand
continuously, moving toward the replication fork
• (your book shows a leading strand and lagging
strand on the same template side; remember:
you have the origin of replication that are all
over; so, depending on what side of the
replication fork you are on you are either
lagging or leading))
Animation: Leading Strand
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-15
Overview
Origin of replication
Leading strand
Lagging strand
Primer
Lagging strand
Leading strand
Overall directions
of replication
Origin of replication
3
5
RNA primer
5
“Sliding clamp”
3
5
Parental DNA
DNA poll III
3
5
DNA pol 3 replaces the
RNA primer (takes over for and
Starts to place nucleotides);
It is moved along by a protein
Called the “sliding clamp”
Then DNA pol 2 digests
Away the primer and
Replaces it with the proper
Nucleotides to fill the
gap
5
3
5
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
3
5
5
3
5
• 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
• It joins the phosphate backbones of all the
Okazaki fragments into a continuous DNA
strand
Animation: Lagging Strand
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-16
Overview
Origin of replication
Lagging strand
Leading strand
Lagging strand
2
1
Leading strand
Overall directions
of replication
3
5
5
Template
strand
3
RNA primer
3
5
3
1
5
3
5
Okazaki
fragment
3
1
5
3
5
2
3
5
2
3
3
5
1
3
5
1
5
2
1
3
5
Overall direction of replication
Fig. 16-16a
Overview
Origin of replication
Leading strand
Lagging strand
Lagging strand
2
1
Leading strand
Overall directions
of replication
Fig. 16-16b1
3
Template
strand
5
5
3
Fig. 16-16b2
3
Template
strand
3
5
5
3
RNA primer
1
5
3
5
Fig. 16-16b3
3
Template
strand
3
5
5
3
RNA primer
3
1
5
5
Okazaki
fragment
3
1
5
3
5
Fig. 16-16b4
3
5
5
Template
strand
3
3
RNA primer
3
1
5
5
3
5
Okazaki
fragment
3
1
5
3
5
2
1
3
5
Fig. 16-16b5
3
5
5
Template
strand
3
3
RNA primer
3
1
5
5
3
1
5
3
5
2
3
3
5
Okazaki
fragment
1
3
5
5
3
5
2
1
Fig. 16-16b6
3
5
5
Template
strand
3
3
RNA primer
3
1
5
5
3
1
5
3
5
2
3
3
5
Okazaki
fragment
3
5
1
5
3
5
2
1
5
3
1
2
Overall direction of replication
3
5
Table 16-1
You have to
Know these!
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
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
Animation: DNA Replication Review
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
Cuts the “problem” on two side
DNA
polymerase
DNA pol replaces the mistake with
The correct nucleotides
DNA
ligase
DNA ligase seals the DNA
together
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 end (because no way to
replace the primer), so repeated rounds of
replication produce shorter DNA molecules
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-19
5
Leading strand
Lagging strand
Ends of parental
DNA strands
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
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 (supposedly non coding areas)
• 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
Fig. 16-20
1 µm
• 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 cellscompensating for the shortening
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
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
Animation: DNA Packing
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
• Chromatin is organized into fibers
• 10-nm fiber
– DNA winds around histones to form
nucleosome “beads”
– Nucleosomes are strung together like beads
on a string by linker DNA
• 30-nm fiber
– Interactions between nucleosomes cause the
thin fiber to coil or fold into this thicker fiber
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 300-nm fiber
– The 30-nm fiber forms looped domains that
attach to proteins
• Metaphase chromosome
– The looped domains coil further
– The width of a chromatid is 700 nm
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Most chromatin is loosely packed in the
nucleus during interphase and condenses prior
to mitosis
• Loosely packed chromatin is called
euchromatin –the normal kind we talked about
• During interphase a few regions of chromatin
(centromeres and telomeres) are highly
condensed into heterochromatin –will appear
clumpy
• Dense packing of the heterochromatin makes it
difficult for the cell to express genetic
information coded in these regions
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Histones can undergo chemical modifications
that result in changes in chromatin organization
– For example, phosphorylation of a specific
amino acid on a histone tail affects
chromosomal behavior during meiosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-22
RESULTS
Condensin and
DNA (yellow)
Outline
Condensin
of nucleus (green)
Normal cell nucleus
DNA (red at
periphery)
Mutant cell nucleus