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Chapter 16
The Molecular Basis of
Inheritance
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Edited by L. Bridge
2011
DNA, the substance of inheritance
– Is the most celebrated molecule of our time
• Chromosomes are composed of atoms
arranged into molecules = field of “Molecular
Genetics"
• Hereditary information is encoded in the
chemical language of DNA and reproduced in
all the cells of your body
• It is the DNA “program” that directs the
development of many different types of traits
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
• What was "The Language of Life”? early
studies asked
• Proteins? (made up of 20 different AA’s) or
DNA (made of only 4 bases)? No one was
sure back then...
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The Nature of DNA: history
• 1869- DNA First isolated by Swiss
biologist, Friedrich Miescher, called it
“nuclein”
–
His procedure:
–
http://www.dnalc.org/view/16003-Miescher-and-the-isolation-of-DNA.html
• 1914- Robert Feulgen found DNA
staining with fuchsin (red dye)
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The Nature of DNA
• 1920- P.A. Levene biochemically broke down DNA
into:
– 5-carbon sugar (deoxyribose)
– Phosphate
– 4 Nitrogen bases
adenine/guanine= purines;
thymine/cytosine= pyrimidines
• Each unit = "Nucleotide“
• At the time, Levene
(mistakenly) assumed a 1:1:1:1
ratio between the bases
Figure 16.5
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sugar-phosphate
backbone
5 end
O–
5
O P O CH2
O 1
O– 4 H
H
H
H
2
3
H
O
O P O CH2 O
O–
H
H
H
H
H
O
O P O CH2 O
O–
H
H
H
H
H
O
5
O P O CH2
O 1
O– 4 H
H
Phosphate H
H
3 2
H
OH
Sugar (deoxyribose)
3 end
Nitrogenous
bases
CH3
O
H
N
N
H
O
Thymine (T)
H
N
H
N
N
N
N
H
H
Adenine (A)
H H
H
N H
N
N
O
Cytosine (C)
H
N
N
N
O
N H
N H
H
Guanine (G)
DNA nucleotide
Erwin Chargaff (1905-2002)
• Analyzed the base composition of DNA from a number of
different organisms:
In 1947, Chargaff
reported:
• DNA “Base Pairing
Rules”
• DNA composition varies
from one species to the
next
http://www.dnai.org/text/mediashowcase/in
dex2.html?id=569
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The Search for the Genetic Material: Scientific Inquiry
• The role of DNA in heredity
– Was first worked out by studying bacteria and
the viruses that infect them
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Evidence That DNA Can Transform Bacteria
• 1928, Frederick Griffith was studying Streptococcus
pneumoniae
– A bacterium that causes pneumonia in mammals
• He worked with two strains of the bacterium
– A pathogenic strain (S) and a nonpathogenic (R)
strain
Streptococcus pneumoniae (pneumococci) growing
as colonies on the surface of a culture medium. Left:
The presence of a capsule around the bacterial cells
gives the colonies a glistening, smooth (S)
appearance. Right: Pneumococci lacking capsules
have produced these rough (R) colonies. (Research
photographs of Dr. Harriet Ephrussi-Taylor, courtesy
of The Rockefeller University.)
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“Sugar-coated microbes”
• Griffith found that when he mixed heat-killed
remains of the pathogenic strain
– With living cells of the nonpathogenic strain, some of
these living cells became pathogenic
EXPERIMENT Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they
have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule
and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below:
Living S
(control) cells
Living R
Heat-killed
(control) cells (control) S cells
Mixture of heat-killed S cells
and living R cells
RESULTS
Mouse dies
Mouse healthy
Mouse healthy
Mouse dies
Living S cells
are found in
blood sample.
Figure 16.2
CONCLUSION Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an
unknown, heritable substance from the dead S cells.
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The “Transformation Factor”
From Griffith’s work,
O.T. Avery:
• 1943, Avery teamed up with MacCleod and
McCarty (Rockefeller Institute), and showed
that DNA was the genetic material found in
extracts from killed virulent bacteria that could
make the living , harmless bacteria into the
virulent type, and called the phenomenon
“transformation”
• defined as a “change in genotype and phenotype due to
the assimilation of external DNA by a cell”
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Further Evidence for DNA
Alfred Mirsky –
• Isolated DNA in the late 1940’s
• Found that all somatic cells (of a species) contain
same amount of DNA (gametes contain 1/2)
• This evidence of molecular diversity among
species
– Made DNA a more credible candidate for the
genetic material
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Evidence That Viral DNA Can Program Cells
• Additional evidence for DNA as the genetic
material came from studies of a virus that infects
bacteria
• Viruses that infect bacteria, bacteriophages
are widely used as tools by researchers in
molecular genetics
Phage
head
DNA
Bacterial
cell
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100 nm
Tail
Tail fiber
The Bacteriophage Experiments
Max Delbrück and Salvador Luria
• Developed a simple model system using phage for
studying how genetic information is transferred to host
bacterial cells.
• Studied a type of bacterial virus called the T phage
that consists of a protein coat containing DNA.
• 7 different phages attack E.coli bacteria, named T1 T7
• Life cycle of the T2 phage narrated animation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Alfred Hershey and Martha Chase
• Performed the famous "blender
experiment“ showing that DNA, rather
than protein, is the genetic material
of a phage known as T2
•
Advantages: small, cheap, easy to maintain in lab,
replicated in 25 minutes
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Hershey and Chase
•
Viruses made of: (1) protein coat
“capsid” ; and (2) DNA
•
The question was, which part
carried the info?
•
Labeled phage DNA with 32P, and the capsid
protein with 35S.
•
Next allowed the labeled phage to begin to infect
a bacterial colony.
•
In the infection process, the phage land on the
bacterial wall and then inject their genetic
material into the host cell.
•
Hershey allowed this to occur, but then at the
crucial moment he whirred them in a Waring
Blender, which he had discovered produced just
the right shearing force to tear the phage
particles from the bacterial walls while not
rupturing the bacteria.
•
Upon examining the bacteria, Hershey found that
only phage DNA, no detectable protein, had
been inserted into them.
•
By process of elimination, they found it was
indeed DNA that was inserted into the host cell,
and thus was the “information” molecule.
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The Nobel Prize in Physiology or Medicine 1969
Max Delbrück, Alfred D. Hershey, and Salvador E. Luria
"for their discoveries concerning the replication
mechanism and the genetic structure of viruses"
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Building a Structural Model of DNA: Scientific Inquiry
• Once most biologists were convinced that DNA
was the genetic material, the challenge was to
determine how the structure of DNA could
account for its role in inheritance
http://www.pbs.org/wgbh/nova/genome/dna.html
What must DNA do?
1. Replicate to be passed on to the next
generation
2. Store information
3. Undergo mutations to provide genetic diversity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Linus Pauling
• As a result of studying X-ray photographs and
constructing molecular models, Linus Pauling
(and Robert Cory, in 1951) proposed that the
protein structures were either in the form of an
alpha helix or the beta pleated sheet.
• the only person who has won two undivided
Nobel Prizes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Maurice Wilkins and Rosalind Franklin
Were using a technique called X-ray
crystallography to study molecular structure
• Rosalind Franklin
– Produced a picture of the DNA molecule using
this technique
(a) Rosalind Franklin
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(b) Franklin’s X-ray diffraction
Photograph of DNA
Overview: Life’s Operating Instructions
In 1953, James Watson and Francis Crick
shook the world
– With an elegant double-helical model for the
structure of deoxyribonucleic acid, or DNA
Figure 16.1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Watson and Crick deduced that DNA was a
double helix
5end
3end
P
T
S
P
S
P
G
T
purine base
pyrimidine base
phosphate
A
T
A
P
C
S
S
5end
P
P
T
P
A
A
S
4
P
T
5
S
P
C
3end
1
S
3
3
1
4
S
2
5
G
S
P
S
C
P
2
A
P
T
S
O
4
S
C
3end
a. Double helix
b. Ladder structure
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C 1
S
C
3
P
5
C
2
deoxyribose
5end
c. One pair of bases
DNA Structure
• The nitrogenous bases
Figure 16.8
H
– Are paired in specific
combinations: adenine with
N
thymine, and cytosine with
Sugar
guanine
– Dictated by the structure of
the bases
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O
H
N
H
CH3
N
N
N
O
Sugar
Thymine (T)
Adenine (A)
H
• Each base pair forms a different
number of hydrogen bonds
– Adenine and thymine form
two bonds, cytosine and
guanine form three bonds
N
N
O
N
N
Sugar
N
H
H
N
N
N
N
N
H
Guanine (G)
H
O
Sugar
Cytosine (C)
DNA Structure
Was composed of
two antiparallel
sugar-phosphate
backbones, with
the nitrogenous
bases paired in
the molecule’s
interior
5 end
O
OH
Hydrogen bond
P
–O
3 end
OH
O
O
A
T
O
O
O
CH2
P
–O
O
O–
O
H2C
O
–O
P
O
O
G
O
C
O
O
CH2
P
O
P
H2C
O
O
C
O
G
O
O
O
CH2
P
–O
O–
O
O
O–
O
P
H2C
O
O
A
O
T
O
CH2
OH
3 end
O
O–
P
O
Figure 16.7b
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(b) Partial chemical structure
O
5 end
Narrated
animation of
DNA structure
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
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Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
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animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
DNA Replication
• Concept 16.2: Many proteins work
together in DNA replication and
repair
• Since the two strands of DNA are
complementary
– Each strand acts as a template for
building a new strand in replication
–
http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/animatio
n16.html
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Base Pairing to a Template Strand
• In DNA replication
– The parent molecule unwinds, and two new
daughter strands are built based on basepairing rules
T
A
T
A
T
A
C
G
C
G
C
T
A
T
A
T
A
A
T
A
T
A
T
G
C
G
C
G
C
G
A
T
A
T
A
T
C
G
C
G
C
G
T
A
T
A
T
A
T
A
T
A
T
C
G
C
G
C
A
G
(a) The parent molecule has two
complementary strands of DNA.
Each base is paired by hydrogen
bonding with its specific partner,
A with T and G with C.
(b) The first step in replication is
separation of the two DNA
strands.
Figure 16.9 a–d
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(c) Each parental strand now
serves as a template that
determines the order of
nucleotides along a new,
complementary strand.
(d) The nucleotides are connected
to form the sugar-phosphate
backbones of the new strands.
Each “daughter” DNA
molecule consists of one parental
strand and one new strand.
DNA replication is semiconservative
– Each of the two new daughter molecules will
have one old strand, derived from the parent
molecule, and one newly made strand
Parent cell
http://highered.
mcgrawhill.com/site
s/007243731
6/student_vi
ew0/chapter
14/animatio
ns.html#
Conservative
model. The two
parental strands
reassociate
after acting as
templates for
new strands,
thus restoring
the parental
double helix.
Semiconservative
model. The two
strands of the
parental molecule
separate,
and each functions
as a template
for synthesis of
a new, complementary strand.
Dispersive
model. Each
strand of both
daughter molecules contains
a mixture of
old and newly
synthesized
DNA.
First
replication
Second
replication
(a)
(b)
(c)
Figure 16.10 a–c
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Experiments performed
by Meselson and
Stahl
Supported the
semiconservative
model of DNA
replication (1957)
Meselson and Stahl experiment (1957)
EXPERIMENT
Matthew Meselson and Franklin Stahl cultured E. coli bacteria for several generations
on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15N. The bacteria
incorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium with
only 14N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be
lighter than the parental DNA made in the 15N medium. Meselson and Stahl could distinguish DNA of different
densities by centrifuging DNA extracted from the bacteria.
1
Bacteria
cultured in
medium
containing
15N
2
Bacteria
transferred to
medium
containing
14N
RESULTS
3
DNA sample
centrifuged
after 20 min
(after first
replication)
4
DNA sample
centrifuged
after 40 min
(after second
replication)
Less
dense
Figure 16.11
More
dense
The bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flask
in step 2, one sample taken after 20 minutes and one after 40 minutes.
CONCLUSION: Meselson and Stahl concluded that DNA replication follows the semiconservative
model by comparing their result to the results predicted by each of the three models in Figure 16.10.
The first replication in the 14N medium produced a band of hybrid (15N–14N) DNA. This result eliminated
the conservative model. A second replication produced both light and hybrid DNA, a result that eliminated
the dispersive model and supported the semiconservative model.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sit
es/dl/free/0072437316/120076/bio23.swf::How%20Nu
cleotides%20are%20Added%20in%20DNA%20Replic
ation
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Getting Started: Origins of Replication
• The replication of a DNA molecule
– Begins at special sites called origins of
replication, where the two strands are
separated
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• A eukaryotic chromosome
– May have hundreds or even thousands of
replication origins
Origin of replication
1 Replication begins at specific sites
where the two parental strands
separate and form replication
bubbles.
Bubble
Parental (template) strand
Daughter (new) strand
0.25 µm
Replication fork
2 The bubbles expand laterally, as
DNA replication proceeds in both
directions.
3 Eventually, the replication
bubbles fuse, and synthesis of
the daughter strands is
complete.
Two daughter DNA molecules
(a) In eukaryotes, DNA replication begins at many sites along the giant
DNA molecule of each chromosome.
Figure 16.12 a, b
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(b) In this micrograph, three replication
bubbles are visible along the DNA of
a cultured Chinese hamster cell (TEM).
Elongating a New DNA Strand
•
Elongation of new DNA at a replication fork
–
•
Is catalyzed by enzymes called DNA polymerases, which add nucleotides to the 3 end of a
growing strand
The Energetics of DNA Replication
Powered by extra phosphates (triphosphates) ex: adenine is added to DNA strand
as"deoxyadenosine triphosphate " (dATP) and guanine added as "deoxyguanosine triphosphate"
(dGTP)
–
As the phosphate bonds are made, to attach the nucleotide to the DNA molecule, the extra
phosphates are removed (thus releasing energy to power the process)
New strand Template strand
3 end
5 end
Sugar
Phosphate
OH
Figure 16.13
3 end
5 end
A
Base
T
A
T
C
G
C
G
G
C
G
C
A
T
A
C
OH
P P
3 end
Pyrophosphate
Nucleoside
triphosphate5 end
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
C
2 P
5 end
Antiparallel Elongation
• How does the antiparallel structure of the
double helix affect replication?
• DNA is “built” in a
5’ to 3’ direction
• DNA polymerases
add nucleotides
Only to the free
3end of a
growing strand
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Leading and Lagging strands
• Along one template strand of DNA, the leading strand
– DNA polymerase III can synthesize a complementary
strand continuously (one nucleotide at a time), moving
toward the replication fork
To elongate the other new strand of
DNA, the lagging strand
DNA polymerase III 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 then joined together by
DNA ligase (enzyme)
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• Synthesis of leading and lagging strands during
DNA replication
The whole process, animated
http://highered.
mcgrawhill.com/olcweb/
cgi/pluginpop.c
gi?it=swf::535::
535::/sites/dl/fre
e/0072437316/
120076/micro04
.swf::DNA%20R
eplication%20F
ork
1 DNA pol Ill elongates
DNA strands only in the
5
3 direction. 3
5
Parental DNA
5
3
Okazaki
fragments
2
1
DNA pol III
3 The other new strand, the
lagging strand must grow in an overall
3
5 direction by addition of short
segments, Okazaki fragments, that grow
5
3 (numbered here in the order
they were made).
Template
strand
3
Leading strand
Lagging strand
2
Template
strand
Figure 16.14
3
5
2 One new strand, the leading strand,
can elongate continuously 5
3
as the replication fork progresses.
1
DNA ligase
Overall direction of replication
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4 DNA ligase joins Okazaki
fragments by forming a bond between
their free ends. This results in a
continuous strand.
Priming DNA Synthesis
• DNA polymerases
cannot initiate the
synthesis of a
polynucleotide
– They can only add
nucleotides to the 3
end
• The initial nucleotide
strand
– Is an RNA or DNA
primer
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Only one primer is needed for synthesis of the
leading strand
– But for synthesis of the lagging strand, each
Okazaki fragment must be primed separately
– View animations of the whole process here:
http://www.mcb.harvard.edu/Losick/images/TromboneFINALd.swf
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1
Primase joins RNA nucleotides
into a primer.
3
5
5
3
Template
strand
RNA primer
3
5
3
DNA pol III adds DNA nucleotides to the
primer, forming an Okazaki fragment.
2
5
3
1
After reaching the next
RNA primer (not shown),
DNA pol III falls off.
Okazaki
fragment
3
3
5
1
5
4
After the second fragment is
primed. DNA pol III adds DNA
nucleotides until it reaches the
first primer and falls off. 5
3
5
3
2
5
1
DNA pol 1 replaces the
RNA with DNA, adding to
the 3 end of fragment 2.
5
3
6
5
1
DNA ligase forms a bond
between the newest DNA
and the adjacent DNA of
fragment 1.
5
3
Figure 16.15
3
2
7
The lagging strand
in this region is now
complete.
3
2
1
Overall direction of replication
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5
Other Proteins That Assist DNA Replication
• Helicase, topoisomerase, single-strand binding
protein
– Are all proteins that assist DNA replication
Table 16.1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Link to animations of processes discussed in this chapter
• A summary of DNA replication
Overall direction of replication
1 Helicase unwinds the
parental double helix.
2 Molecules of singlestrand binding protein
stabilize the unwound
template strands.
3 The leading strand is
synthesized continuously in the
5 3 direction by DNA pol III.
DNA pol III
Lagging
Leading
strand Origin of replication strand
Lagging
strand
OVERVIEW
Leading
strand
Leading
strand
5
3
Parental DNA
4 Primase begins synthesis
of RNA primer for fifth
Okazaki fragment.
5 DNA pol III is completing synthesis of
the fourth fragment, when it reaches the
RNA primer on the third fragment, it will
dissociate, move to the replication fork,
and add DNA nucleotides to the 3 end
of the fifth fragment primer.
Replication fork
Primase
DNA pol III
Primer
4
DNA ligase
DNA pol I
Lagging
strand
3
2
6 DNA pol I removes the primer from the 5 end
of the second fragment, replacing it with DNA
nucleotides that it adds one by one to the 3 end
of the third fragment. The replacement of the
last RNA nucleotide with DNA leaves the sugarphosphate backbone with a free 3 end.
Figure 16.16
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1
3
5
7 DNA ligase bonds
the 3 end of the
second fragment to
the 5 end of the first
fragment.
The DNA Replication Machine as a Stationary Complex
• The various proteins that participate in DNA
replication
– Form a single large complex, a DNA replication
“machine”
• The DNA replication machine
– Is probably stationary during the replication
process
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Proofreading and Repairing DNA
• DNA polymerases proofread newly made DNA
– Replacing any incorrect nucleotides
– http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/anim
ation18.html
• In mismatch repair of DNA
– Repair enzymes correct errors in base pairing
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• In nucleotide excision repair
– Enzymes cut out and replace damaged
stretches of DNA
1 A thymine dimer
distorts the DNA molecule.
2 A nuclease enzyme cuts
the damaged DNA strand
at two points and the
damaged section is
removed.
Nuclease
DNA
polymerase
3 Repair synthesis by
a DNA polymerase
fills in the missing
nucleotides.
DNA
ligase
Figure 16.17
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4 DNA ligase seals the
Free end of the new DNA
To the old DNA, making the
strand complete.
Telomeres
• Eukaryotic chromosomal DNA molecules
– Have at their ends nucleotide sequences,
called telomeres, that postpone the erosion of
genes near the ends of DNA molecules
Figure 16.19
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1 µm
Telomeres: the ends of the chromosomes
• If the 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
– In somatic cells, that MITOTICALLY divide, telomeres
are thought to be involved in cell aging, as the ends
wear down (like the wick on a candle)
– http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/animation1
9.html
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