DNA Structure and Function

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• Chapter 8
• DNA Structure and Function

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Eukaryotic Chromosomes

• The DNA in a eukaryotic cell nucleus is organized
  as one or more chromosomes that differ in length
  and shape
• Chromosome
• A structure that consists of DNA and associated
  proteins
• Carries part or all of a cells genetic
  information

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Chromosome Organization

• During most of the cells life, each chromosome
  consists of one DNA strand.
• When the cell prepares to divide, it duplicates
  all of its chromosomes, so that both offspring
  get a full set
• Each duplicated chromosome has two DNA strands
  (sister chromatids) attached to one another at
  the centromere

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centromere
one chromatid
its sister chromatid
a chromosome (unduplicated)
a chromosome (duplicated)
p134
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Chromosome Structure

• A duplicated, condensed chromosome consists of
  two long filaments bunched into a characteristic
  X shape
• Each filament consists of a coil of DNA wrapped
  around spools of proteins called histones
• Each DNA-histone spools is a nucleosome, the
  smallest unit of chromosomal organization in
  eukaryotes
• Stretched out end to end, the DNA molecules in a
  human cell would be about 2 meters (6.5 feet)
  long. That is a lot of DNA to fit into a nucleus
  that is less than 10 micrometers in diam- eter!
  Proteins structurally organize the DNA and help
  it pack tightly into a small nucleus.

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The DNA molecule consists of two strands twisted
into a double helix
DNA molecule
Zooming in on chromosome structure. Tight packing
allows a lot of DNA to fit into a very small
nucleus.
Figure 8-2b p134
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5
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Figure 8-2c2 p134
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Chromosome Number

• The total number of chromosomes in a eukaryotic
  cell (chromosome number) is characteristic of the
  species human body cells have 46 chromosomes
• Human body cells have two of each type of
  chromosome their chromosome number is diploid
  (2n)
• A karyotype shows how many chromosomes are in an
  individual cell, and reveals major structural
  abnormalities

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A karyotype. This one shows 22 pairs of autosomes
and a pair of X chromosomes.
Figure 8-3a p135
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Chromosome Numbers
Source http//plantcellbiology.masters.grkraj.org
/html/Plant_Cell_Genetics1-Chromosomes.htm
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Autosomes and Sex Chromosomes

• In a diploid organism, one chromosome in a
  chromosome pair is inherited from the mother and
  one from the father
• All except one pair of chromosomes are autosomes
  pairs of chromosomes with the same length,
  shape, and centromere location
• Pairs of sex chromosomes differ between females
  and males human females have two X chromosomes
  (XX) human males have one X and one Y chromosome
  (XY)

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Stepped Art
Figure 8-3b p135
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The Discovery of DNAs Functions

• Investigations that led to our understanding that
  DNA is the molecule of inheritance reveal how
  science advances

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Discovery of DNA

• 1800s Johannes Miescher found DNA
  (deoxyribonucleic acid) in nuclei, though its
  function was unknown

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Griffiths Experiments

• Pneumococci has two general formsrough (R) and
  smooth (S). The virulent S form, when injected
  subcutaneously into mice succumbed to pneumonia
  and died within a couple of days. The avirulent R
  form that does not prompt pneumonia.
• When Griffith injected heat-killed S into mice,
  as expected no disease ensued. When mice were
  injected with a mixture of heat-killed S and live
  R, however, pneumonia and death ensued. The live
  R had transformed into Sand replicated as such.

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Griffiths Experiments
Summary of results from Fred Griffiths
experiments. The hereditary material of harmful
Streptococcus pneumoniae cells transformed
harmless cells into killers. 1 Mice injected with
live cells of harmless strain R do not die. Live
R cells in their blood. 2 Mice injected with live
cells of killer strain S die. Live S cells in
their blood. 3 Mice injected with heat- killed S
cells do not die. No live S cells in their
blood. 4 Mice injected with live R cells plus
heat-killed S cells die. Live S cells in their
blood.
1
2
3
4
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Griffiths Experiments

• Early 1900s Griffith transferred hereditary
  material from dead cells to live cells
• Mice injected with live R cells lived
• Mice injected with live S cells died
• Mice injected with killed S cells lived
• Mice injected with killed S cells and live R
  cells died live S cells were found in their
  blood

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Avery and McCarty Find the Transforming Principle

• 1940 Avery and McCarty separated deadly S cells
  (from Griffiths experiments) into lipid,
  protein, and nucleic acid components
• When lipids, proteins, and RNA were destroyed,
  the remaining substance, DNA, still transformed R
  cells to S cells
• Conclusion DNA is the transforming principle

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Confirmation of DNAs Function

• 1950s Hershey and Chase experimented with
  bacteriophages (viruses that infect bacteria)
• Protein parts of viruses, labeled with 35S,
  stayed outside the bacteria
• DNA of viruses, labeled with 32P, entered the
  bacteria
• Conclusion DNA, not protein, is the material
  that stores hereditary information

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Bacteriophages
The HersheyChase experiments. Alfred Hershey
and Martha Chase tested whether the genetic
material injected by bacteriophage into bacteria
is DNA, protein, or both. The experiments were
based on the knowledge that proteins contain more
sulfur (S) than phosphorus (P), and DNA contains
more phosphorus than sulfur.
DNA inside protein coat
tail fiber
hollow sheath
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Stepped Art
Figure 8-6 p137
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The Discovery of DNAs Structure

• James D. Watson and Francis Cricks discovery of
  DNAs structure was based on 150 years of
  research by other scientists

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The Discovery of DNAs Structure

• DNA structure was discovered through the work of
  many scientists.
• One crucial piece of evidence came from X-ray
  crystallography.
• A purified substance can be made to form
  crystals the pattern of diffraction of X rays
  passed through the crystallized substance shows
  position of atoms.
• Rosalind Franklin
• Prepared crystallographs from uniformly oriented
  DNA fibersher images suggested a spiral model

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The Short Story of Rosalind Franklin

• In science, as in other professions, public
  recognition does not always include everyone who
  contributed to a discovery
• Rosalind Franklin was first to discover the
  molecular structure of DNA, but did not share in
  the Nobel prize which was given to Watson and
  Crick.
• Franklin died of cancer at age 37 probably caused
  by extensive exposure to x-rays during her work

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Rosalind Franklin and Her X-Ray Diffraction
Image of DNA
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X-Ray Crystallography Helped Reveal the Structure
of DNA
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DNAs Building Blocks

• Nucleotide
• A nucleic acid monomer consisting of a
  five-carbon sugar (deoxyribose), three phosphate
  groups, and one of four nitrogen-containing bases
• DNA consists of four nucleotide building blocks
• Two pyrimidines thymine and cytosine
• Two purines adenine and guanine

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Four Kinds of Nucleotides in DNA
adenine (A) deoxyadenosine triphosphate, a purine
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Four Kinds of Nucleotides in DNA
guanine (G) deoxyguanosine triphosphate, a purine
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Four Kinds of Nucleotides in DNA
thymine (T) deoxythymidine triphosphate, a
pyrimidine
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Four Kinds of Nucleotides in DNA
cytosine (C) deoxycytidine triphosphate, a
pyrimidine
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DNA Structure Reflects Its Role as the Genetic
Material
Chargaffs rule

• In 1950 Erwin Chargaff found that in the DNA from
  many different species
• Amount of A amount of T
• Amount of C amount of G
• Or, the abundance of purines the abundance of
  pyrimidines Chargaffs rule.

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• Francis Crick and James Watson used model
  building and combined all the knowledge of DNA to
  determine its structure.
• Franklins X-ray crystallography convinced them
  the molecule was helical.
• Modeling also showed that DNA strands are
  anti-parallel.

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• Watson and Crick suggested that
• Nucleotide bases are on the interior of the two
  strands, with a sugar-phosphate backbone on the
  outside.
• Per Chargaffs rule, a purine on one strand is
  paired with a pyrimidine on the other.
• These base pairs (A-T and G-C) have the same
  width down the helix.

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Watson and Crick Model of DNA
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Franklin, Watson and Crick

• Rosalind Franklins research in x-ray
  crystallography revealed the dimensions and
  shape of the DNA molecule an alpha helix
• This was the final piece of information James
  Watson and Francis Crick needed to build their
  model of DNA

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Watson and Cricks DNA Model

• A DNA molecule consists of two nucleotide chains
  (strands), running in opposite directions and
  coiled into a double helix
• Base pairs form on the inside of the helix, held
  together by hydrogen bonds (A-T and G-C)

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DNAs Base-Pair Sequence

• Bases in DNA strands can pair in only one way A
  always pairs with T G always pairs with C
• The DNA sequence (sequence of bases) is the
  genetic code that varies between species and
  individuals

one base pair
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Base Pairs in DNA Can Interact with Other
Molecules
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DNA Structure Reflects Its Role as the Genetic
Material

• Four key features of DNA structure
• (see next slide)
• It is a double-stranded helix of uniform
  diameter.
• It is right-handed.
• It is antiparallel.
• Outer edges of nitrogenous bases are exposed in
  the major and minor grooves.

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DNA Is a Double Helix
Grooves. The strand backbones are closer together
on one side of the helix than on the other. The
major groove occurs where the backbones are far
apart, the minor groove occurs where they are
close together and the grooves twist around the
molecule on opposite sides. Certain proteins
bind to DNA to alter its structure or to regulate
transcription (copying DNA to RNA) or replication
(copying DNA to DNA). It is easier for these DNA
binding proteins to interact with the bases on
the major groove.
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Structure of DNA
0.34 nanometer between each base pair
2-nanometer diameter
3.4-nanometer length of each full twist of the
double helix
Figure 8-8b p139
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• Surfaces of A-T and G-C base pairs are chemically
  distinct.
• Binding of proteins to specific base pair
  sequences is key to DNAprotein interactions, and
  necessary for replication and gene expression.

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• DNA has three important functions
• double-helical structure is essential
• Storage of genetic informationmillions of
  nucleotides base sequence encodes huge amounts
  of information
• Precise replication during cell division by
  complementary base pairing
• Expression of the coded information as the
  phenotypenucleotide sequence is transcribed into
  RNA and determines sequence of amino acids in
  proteins

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DNA Replication

• DNA replication is the energy - intensive process
  by which a cell copies its DNA
• A cell copies its DNA before it reproduces
• Each of the two DNA strands in the double helix
  is replicated
• DNA replication requires many enzymes, including
  DNA polymerase, and other molecules

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DNA Replication

• A cells genetic information consists of the
  order of nucleotide bases (the DNA sequence) of
  its chromosomes
• Descendant cells must get an exact copy of that
  information
• Each chromosome is copied entirely the two
  chromosomes that result are duplicates of the
  parent molecule

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Semiconservative DNA Replication

• Each strand of a DNA double helix is a template
  for synthesis of a complementary strand of DNA
• One template builds DNA continuously the other
  builds DNA discontinuously, in segments
• Each new DNA molecule consist of one old strand
  and one new strand (semiconservative replication)

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Primers for DNA Polymerase

• There are several types of DNA polymerases
• All types require a primer in order to initiate
  DNA synthesis
• Primer
• A short, single strand of DNA or RNA that is
  complementary to a targeted DNA sequence

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Enzymes of DNA Replication

• DNA helicase breaks hydrogen bonds between DNA
  strands
• Topoisomerase untwists the double helix
• DNA polymerase joins free nucleotides into a new
  strand of DNA
• DNA ligase joins DNA segments on the
  discontinuous strand

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Discontinuous Replication

• DNA polymerases attach a free nucleotide only to
  the 3' end of a DNA strand (not the 5' end)
• Only one of the two new strands of DNA can be
  synthesized continuously during DNA replication
• Synthesis of the other strand occurs in segments,
  in the direction opposite that of unwinding
• DNA ligase joins segments into a continuous
  strand of DNA

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DNA Replicates Semiconservatively

• Semiconservative replication means that each
  parental strand serves as a template for a new
  strand.
• Conservative replication would show that the
  intact parental DNA (both strands) serves as a
  template.
• Evidence from radioactively-labeled strands
  supports semiconservative replication.

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DNA Replicates Semiconservatively

• Two steps in DNA replication
• The double helix is unwound, making two template
  strands available for new base pairing.
• New nucleotides form base pairs with template
  strands and linked together by phosphodiester
  bonds. Template DNA is read in the 3'-to-5'
  direction.

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DNA Replicates Semiconservatively

• During DNA synthesis, new nucleotides are added
  to the 3' end of the new strand, which has a free
  hydroxyl group (OH).
• Deoxyribonucleoside triphosphates (dNTPs), or
  deoxyribonucleotides, are the building blockstwo
  of their phosphate groups are released and the
  third bonds to the 3' end of the DNA chain (see
  next slide). A bond forms between the 3' carbon
  on the end of a DNA strand and the phosphate of a
  new nucleotides 5' carbon.

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Each New DNA Strand Grows by the Addition of
Nucleotides to Its 3' End
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DNA Replicates Semiconservatively

• DNA replication begins with the binding of a
  large protein complexthe pre-replication
  complexto a specific site on the DNA molecule.
• The complex contains DNA polymerase, which
  catalyzes addition of nucleotides.
• The complex binds to a region on the chromosome
  called the origin of replication (ori).

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DNA Replicates Semiconservatively

• When the pre-replication complex binds to ori,
  the DNA unwinds and replication proceeds in two
  directions.
• The replication fork is the site where DNA
  unwinds to expose bases.
• Eukaryotic chromosomes are linear and have
  multiple origins of replication, which speed up
  replication. (Compare the next two slides)

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The Origin of DNA Replication Prokaryote
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The Origin of DNA Replication Eukaryote
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DNA replication. Two double-stranded DNA
molecules form one strand of each is parental
(old), and the other is new, so DNA replication
is said to be semiconservative. Green arrows show
the direction of synthesis for each strand. The
Y-shaped structure of a DNA molecule undergoing
replication is called a replication fork. 1 As
replication begins, many initiator proteins
attach to the DNA at certain sites in the
chromosome. Eukaryotic chromosomes have many of
these origins of replication DNA replication
proceeds more or less simultaneously at all of
them. 2 Enzymes recruited by the initiator
proteins begin to unwind the two strands of DNA
from one another. 3 Primers base-paired with the
exposed single DNA strands serve as initiation
sites for DNA synthesis. 4 Starting at primers,
DNA polymerases ( green boxes) assemble new
strands of DNA from nucleotides, using the parent
strands as templates. 5 DNA ligase seals any gaps
that remain between bases of the new DNA, so a
continuous strand forms. 6 Each parental DNA
strand ( blue ) serves as a template for assembly
of a new strand of DNA ( magenta ).
1
initiator proteins
topoisomerase
2
helicase
3
primer
4
DNA polymerase
5
DNA ligase
6
Figure 8-9 p140
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DNA Replicates Semiconservatively

• DNA replication begins with a short primera
  starter strand.
• The primer is complementary to the DNA template.
• Primasean enzymesynthesizes DNA one nucleotide
  at a time.
• Primase catalyzes the synthesis of a short RNA
  (or DNA in some organisms ) segment called a
  primer complementary to a DNA template.
• Primase is important in DNA replication because
  no known DNA polymerases can initiate the
  synthesis of a DNA without an initial RNA or DNA
  primer .
• DNA polymerase adds nucleotides to the 3' end.

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• Topoisomerases are enzymes that regulate the
  overwinding or underwinding of DNA.
• As DNA Helicase moves along the DNA opening the
  replication fork the DNA can become over twisted
  or coiled preventing the further movement of the
  enzyme. In order to help overcome this
  topological problem caused by the double helix,
  topoisomerases bind to either single-stranded or
  double-stranded DNA and cuts the phosphate
  backbone of the DNA.
• This break allows the DNA to be untangled or
  unwound, and, at the end of these processes, the
  DNA backbone is resealed again.

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DNA Forms with a Primer
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DNA Replicates Semiconservatively

• DNA polymerases are larger than their substrates,
  the dNTPs, and the template DNA.
• The enzyme is shaped like an open right handthe
  palm brings the active site and the substrates
  into contact.
• The fingers recognize the nucleotide bases.

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DNA Polymerase Binds to the Template Strand
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DNA Polymerase Binds to the Template Strand
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DNA Replicates Semiconservatively

• A single replication fork opens up in one
  direction.
• The two DNA strands are antiparallelthe 3' end
  of one strand is paired with the 5' end of the
  other.
• DNA replicates in a 5'-to-3' direction.

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DNA Replicates Semiconservatively

• One new strand, the leading strand, is oriented
  to grow at its 3' end as the fork opens.
• The lagging strand is oriented so that its
  exposed 3' end gets farther from the fork.
• Synthesis of the lagging strand occurs in small,
  discontinuous stretchesOkazaki fragments.

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DNA Replicates Semiconservatively

• Each Okazaki fragment requires its own primer,
  synthesized by the primase.
• DNA polymerase adds nucleotides to the 3' end,
  until reaching the primer of the previous
  fragment.
• A different DNA polymerase then replaces the
  primer with DNA.
• The final phosphodiester linkage between
  fragments is catalyzed by DNA ligase.

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The Lagging Strand Story (Part 1)
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The Lagging Strand Story (Part 2)
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The Lagging Strand Story (Part 3)
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DNA Replicates Semiconservatively

• DNA polymerase works very fast
• It is processiveit catalyzes many sequential
  polymerization reactions each time it binds to
  DNA
• Okazaki fragments are added to RNA primers to
  replicate the lagging strand.
• When the last primer is removed no DNA synthesis
  occurs because there is no 3' end to extenda
  single-stranded bit of DNA is left at each end.
• These are cut after replication and the
  chromosome is slightly shortened after each cell
  division.

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DNA Replicates Semiconservatively

• Telomeres are repetitive sequences at the ends of
  eukaryotic chromosomes.
• These repeats prevent the chromosome ends from
  being joined together by the DNA repair system.
• Telomerase contains an RNA sequenceit acts as a
  template for telomeric DNA sequences.
• Telomeric DNA is lost over time in most cells,
  but not in continuously dividing cells like bone
  marrow and gametes.

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Telomeres

• The DNA molecules in eukaryotic chromosomes are
  linear i.e., have two ends.
• The DNA molecule of a typical chromosome contains
  a linear array of genes (encoding proteins and
  RNAs) interspersed with noncoding DNA.
• Included in the noncoding DNA are long stretches
  that make up the centromere and stretches at the
  ends of the chromosome, the telomeres.
• Telomeres keep the ends of the chromosomes from
  accidentally becoming attached to each other.

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Telomeres

• Replication of linear chromosomes results in a
  special problem.
• DNA polymerase can only synthesize a new strand
  of DNA as it moves along the template strand in
  the 3' gt 5' direction. This is fine for the 3'
  gt 5' strand of a chromosome as the DNA
  polymerase moves uninterrupted from an origin of
  replication until it meets another bubble of
  replication or the end of the chromosome.
• Synthesis of the 5' gt 3' strand is
  discontinuous. DNA polymerase synthesizes
  sections of complementary strand (Okazaki
  fragment) followed by a DNA ligase stitching the
  Okazaki fragments together.
• This continues until close to the end of the
  chromosome where there is no longer enough
  template to continue forming Okazaki fragments.
  So the 5' end of each newly-synthesized strand
  cannot be completed. Thus each of the daughter
  chromosomes will have a shortened telomere.
• It is estimated that human telomeres lose about
  100 base pairs from their telomeric DNA at each
  mitosis. At this rate, after 125 mitotic
  divisions, the telomeres would be completely
  gone.
• Is this why normal somatic cells are limited in
  the number of replications and thus cell
  divisions.

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Telomeres and Telomerase (Part 1)
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DNA Replicates Semiconservatively

• DNA polymerases can make mistakes in replication,
  but most errors are repaired.
• Cells have two major repair mechanisms
• Proofreadingas DNA polymerase adds nucleotides,
  it has a proofreading function and if bases are
  paired incorrectly, the nucleotide is removed.
• Mismatch repairafter replication other proteins
  scan for mismatched bases missed in proofreading,
  and replace them with correct ones.

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DNA Repair Mechanisms (Part 1)
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DNA Repair Mechanisms

• DNA repair mechanisms correct most replication
  errors
• DNA polymerases proofread DNA sequences during
  DNA replication and repair damaged DNA
• When proofreading and repair mechanisms fail, an
  error becomes a mutation a permanent change in
  the DNA sequence

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Mutations Are Heritable Changes in DNA

• Mutations are changes in the nucleotide sequence
  of DNA that are passed on from one cell, or
  organism, to another.
• Mutations occur by a variety of processes.
• Errors that are not corrected by repair systems
  are passed on to daughter cells.

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Mutations Are Heritable Changes in DNA

• Mutations are of two types
• Somatic mutations occur in somatic (body)
  cellspassed on by mitosis but not to sexually
  produced offspring.
• Germ line mutations occur in germ line cells that
  give rise to gametes. A gamete passes a mutation
  on at fertilization.

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Mutations Are Heritable Changes in DNA

• Most genomes include genes and regions of DNA
  that are not expressed
• Genes are transcribed into RNAs, for translation
  into amino acid sequences or into RNAs with
  catalytic functions.
• The coding regions of a gene contain sequences
  within the transcribed region that are
  translated.
• Genomes also contain regions of DNA that are not
  expressed.

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Mutations Are Heritable Changes in DNA

• At the molecular level there are two categories
  of mutations
• A point mutation results from the gain, loss, or
  substitution of a single nucleotide.
• Chromosomal mutations are more extensivethey may
  change the position or cause a DNA segment to be
  duplicated or lost.

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Mutations Are Heritable Changes in DNA

• Chromosomal mutations
• Deletionsresult in the removal of part of the
  genetic material and can have severe or fatal
  consequences.
• Duplicationshomologous chromosomes break in
  different places and recombine with wrong
  partners one may have two copies of segment and
  the other may have none

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Chromosomal Mutations
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Mutations Are Heritable Changes in DNA

• Chromosomal mutations
• Inversionsresult from breaking and rejoining,
  but segment is flipped
• Translocationssegment of DNA breaks off and is
  inserted into another chromosome this can lead
  to duplications and deletions

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Chromosomal Mutations
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Mutations Are Heritable Changes in DNA

• Mutations are caused in two ways
• Spontaneous mutations occur with no outside
  influence, and are permanent.
• Induced mutations are due to an outside agent, a
  mutagen.

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Mutations Are Heritable Changes in DNA

• Induced mutationcaused by mutagens
• Chemicals can alter nucleotide bases (e.g.,
  nitrous acid can cause deamination)
• Some chemicals add other groups to bases (e.g.,
  benzopyrene adds a group to guanine and prevents
  base pairing). DNA polymerase will then add any
  base there.

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Environmental Causes of Mutations

• Ionizing radiation (gamma rays, x-rays, most UV
  light)
• Knocks electrons out of atoms
• Breaks chromosomes into pieces that get lost
  during DNA replication
• Creates free radicals in tissues
• UV light (320-400 nm)
• Forms pyrimidine dimers that kink the DNA strand
• Causes skin cancer

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thymine dimer
An example of a pyrimidine dimer. This type of
DNA damage can be caused by exposure to light
with a wavelength shorter than about 400 nm.
Pyrimidine dimers result in mutations because
they interfere with DNA replication.
Figure 8-12 p142
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Environmental Causes of Mutations

• At least fifty-five carcinogenic (cancer-causing)
  chemicals in tobacco smoke transfer small
  hydrocarbon groups to the nucleotide bases in DNA
• Many environmental pollutants are converted by
  the body to other compounds that bind
  irreversibly to DNA, causing replication errors
  that lead to mutation

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• Mutations can have benefits
• Provide the raw material for evolution in the
  form of genetic diversity
• Diversity may benefit the organism immediatelyif
  mutation is in somatic cells
• May cause an advantageous change in offspring

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Animal Cloning

• Various reproductive interventions produce
  genetically identical individuals

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Cloning

• Clones
• Exact copies of a molecule, cell, or individual
• Occur in nature by asexual reproduction or embryo
  splitting (identical twins)
• Reproductive cloning technologies produce an
  exact copy (clone) of an individual

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What is cloning?

• Reproductive cloning technologies produce clones
  genetically identical individuals
• The DNA inside a living cell contains all the
  information necessary to build a new individual
• Somatic cell nuclear transfer (SCNT) is a
  reproductive cloning technology in which nuclear
  DNA of an adult donor is transferred to an egg
  with no nucleus the hybrid cell develops into an
  embryo that is genetically identical to the adult
  donor
• Therapeutic cloning uses SCNT to produce human
  embryos for research

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Reproductive Cloning Technologies

• Somatic cell nuclear transfer (SCNT)
• Nuclear DNA of an adult is transferred to an
  enucleated egg
• Egg cytoplasm reprograms differentiated (adult)
  DNA to act like undifferentiated (egg) DNA
• The hybrid cell develops into an embryo that is
  genetically identical to the donor individual

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Somatic Cell Nuclear Transfer (SCNT)
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A Clone Produced by SCNT
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Therapeutic Cloning

• Therapeutic cloning uses SCNT to produce human
  embryos for research purposes
• Researchers harvest undifferentiated (stem) cells
  from the cloned human embryos
• Such research may ultimately lead to treatments
  for people who suffer from fatal diseases

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The Cloning Controversy

• Few cloned mammal embryos result in a live birth
  many of the clones that survive have serious
  health problems
• One problem is, the DNA in adult cells is
  controlled differently than the DNA in embryonic
  cells
• Perfecting methods for cloning animals brings us
  closer to the possibility of cloning humans, both
  technically and ethically