Transcript chapter8_Sections 4-7.ppt

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 8
DNA Structure and Function
(Sections 8.4 - 8.7)
Albia Dugger • Miami Dade College
8.4 Discovery of DNA’s Structure
• James Watson and Francis Crick’s discovery of DNA’s
structure was based on many years of research by other
scientists
DNA’s Building Blocks
• A DNA nucleotide has a five-carbon sugar, three phosphate
groups, and one of four nitrogen-containing bases
• How the four nucleotides — adenine (A), guanine (G),
thymine (T), and cytosine (C) — are arranged in DNA was a
puzzle that took over 50 years to solve
DNA Nucleotides
• Each nucleotide has three phosphate groups, a deoxyribose
sugar, and a nitrogen-containing base after which it is named
• Biochemist Phoebus Levene identified the structure of these
bases and how they are connected in DNA in the early 1900s
Four DNA Nucleotides
Four DNA Nucleotides
Fig 8.7a, p. 128
Four DNA Nucleotides
adenine (A)
deoxyadenosine triphosphate
guanine (G)
deoxyguanosine triphosphate
Fig 8.7a, p. 128
Four DNA Nucleotides
Fig 8.7b, p. 128
Four DNA Nucleotides
thymine (T)
deoxythymidine triphosphate
cytosine (C)
deoxycytidine triphosphate
Fig 8.7b, p. 128
ANIMATION: Subunits of DNA
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Chargaff’s Rules
• 1950s: Erwin Chargaff made two discoveries:
• Chargaff’s first rule: A = T and G = C (amounts of thymine
and adenine in all DNA are the same, as are amounts of
cytosine and guanine)
• Chargaff’s second rule: Proportions of adenine and
guanine differ among the DNA of different species
Watson and Crick
• American biologist James Watson and British biophysicist
Francis Crick suspected that the DNA molecule was a helix
• They argued about the size, shape, and bonding
requirements of the four DNA nucleotides
Rosalind Franklin
• Biochemist Rosalind Franklin had also been working on the
structure of DNA, using x-ray crystallography to calculate the
size, shape, and spacing between any repeating elements of
the molecules
• She made the first clear x-ray diffraction image of “wet” DNA –
the form that occurs in cells
First Image of DNA
• Rosalind Franklin and her xray diffraction image of DNA
The First DNA Model
• Watson and Crick used
Franklin’s information to
build a model of the
DNA helix
Structure of DNA
• 3’ carbon of a sugar is
joined by a phosphate
group to 5’ carbon of
next sugar, forming 2
sugar-phosphate
backbones running in
opposite directions
• Inside are paired bases:
A to T, and G to C
Structure of DNA
2-nanometer
diameter
0.34 nanometer
between each
base pair
3.4-nanometer
length of each
full twist of the
double helix
H
H
H
P
H
H
H
H
d
H
P
5
3
H
H
H
H
H
H
H
P
5P
H2C O
5 P3
H2C O
3
5 P
H
H2C O
H
4
1
3 2
HO
H
H
H
d
d
3
d5
3
O 5
CH2
d
3
O 5
CH2
2 d
34
1 O 5
d CH2
Fig 8.8b, p. 129
ANIMATION: DNA close up
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DNA’s Base-Pair Sequence
• The order in which one base pair follows the next varies
tremendously among species (Chargaff’s second rule)
• Variations in base sequence are the source of life’s diversity
Key Concepts
• Structure of DNA
• A DNA molecule consists of two long chains of nucleotides
coiled into a double helix
• Four kinds of nucleotides make up the chains: adenine,
thymine, guanine, and cytosine
• The order of these bases in DNA differs among individuals
and among species
ANIMATION: DNA structure
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Animation: Structure of DNA
8.5 Fame and Glory
• Watson and Crick reviewed Franklin’s x-ray diffraction image
with crystallographer Maurice Wilkins, and used Franklin's
unpublished data to build their DNA model
• In 1962, Watson, Crick, and Wilkins received the Nobel Prize
for the discovery of the structure of DNA
• Franklin didn’t get the Nobel Prize – she died at age 37 of
ovarian cancer, possibly related to x-ray exposure
8.6 DNA Replication and Repair
• The order of nucleotide bases in a strand of DNA – the DNA
sequence – is genetic information
• Descendant cells must get an exact copy of DNA
• When the cell reproduces, it must contain two sets of
chromosomes: one for each of its future offspring
• DNA duplicates itself by DNA replication
Key Terms
• DNA sequence
• Order of nucleotide bases in a strand of DNA
• DNA replication
• The process by which a cell duplicates its DNA before it
divides
Steps in DNA Replication
1. The two strands of a DNA molecule are complementary:
nucleotides match up according to base-pairing rules (G to C,
and T to A)
2. DNA helicase breaks the hydrogen bonds that hold the double
helix together, and the two DNA strands unwind
Steps in DNA Replication (cont.)
3. Each parent strand serves as a template for assembly of a
new DNA strand from nucleotides, according to base-pairing
rules – DNA polymerase assembles a complementary strand of
DNA on each of the parent strands
4. DNA ligase seals any gaps that remain between bases of the
“new” DNA, so a continuous strand forms
Key Terms
• DNA polymerase
• DNA replication enzyme
• Uses a DNA template to assemble a complementary
strand of DNA
• DNA ligase
• Enzyme that seals gaps in double-stranded DNA
DNA Replication
DNA
Replication
1 The two strands of a DNA
molecule are complementary:
their nucleotides match up
according to base-pairing rules
(G to C, T to A).
2 As replication starts,
the two strands of DNA
unwind at many sites
along the length of the
molecule.
3 Each parent strand
serves as a template for
the assembly of a new
DNA strand from
nucleotides, according
to base-pairing rules.
4 DNA ligase seals any gaps that
remain between bases of the
“new” DNA, so a continuous
strand forms. The base sequence
of each half-old, half-new DNA
molecule is identical to that of the
parent.
Fig 8.10, p. 130
DNA
Replication
1 The two strands of a DNA
molecule are complementary:
their nucleotides match up
according to base-pairing
rules (G to C, T to A).
2 As replication starts,
the two strands of DNA
unwind at many sites
along the length of the
molecule.
3 Each parent strand
serves as a template for
the assembly of a new
DNA strand from
nucleotides, according
to base-pairing rules.
4 DNA ligase seals any gaps that
remain between bases of the
“new” DNA, so a continuous
strand forms. The base sequence
of each half-old, half-new DNA
molecule is identical to that
Stepped Art
of the parent.
Fig 8.10, p. 130
ANIMATION: DNA replication details
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Semiconservative Replication of DNA
• A parental DNA strand serves as a template for assembly of a
new strand of DNA
• The two parental DNA strands stay intact, and a new strand is
assembled on each of the parental (old) strands
• Each new DNA molecule that forms consists of one old strand
and one new strand
Semiconservative Replication of DNA
Semiconservative Replication of DNA
old
new
new
old
Fig 8.11, p. 131
DNA Synthesis
• Each DNA strand has an unbonded 5’ carbon at one end and
an unbonded 3’ carbon at the other:
• DNA synthesis occurs in the 5’ to 3’ direction
DNA Synthesis
• Only one of the new
DNA strands can be
assembled in a single
piece – the other forms
in short segments called
Okazaki fragments
DNA Synthesis
The parent DNA
double helix unwinds
in this direction.
Only one new
DNA strand is
assembled
continuously.
The other new
DNA strand is
assembled in
many pieces.
Gaps are
sealed by
DNA ligase.
Fig 8.12, p. 131
Proofreading
• A DNA molecule is not always replicated correctly – luckily,
DNA polymerases proofread their own work
• DNA repair mechanisms correct mismatches by reversing
the synthesis reaction to remove a mismatched nucleotide,
then resuming synthesis in the forward direction
• When proofreading and repair mechanisms fail, an error
becomes a mutation
Key Terms
• DNA repair mechanism
• Any of several processes by which enzymes repair
damaged DNA
• Example: Enzymatically excising and replacing any
damaged or mismatched bases before replication begins
• mutation
• Permanent change in the sequence of DNA
Key Concepts
• DNA Replication
• Before a cell divides, it copies its DNA so that each of its
descendants gets a full complement of hereditary
information
• Newly forming DNA is monitored for errors, most of which
are corrected
• Uncorrected errors may be perpetuated as mutations
ANIMATION: Semidiscontinuous DNA
Replication
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ANIMATION: Semi Conservative
Replication
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Animation: DNA Replication
3D ANIMATION: DNA replication
8.7 Duplicating Existing Mammals
• Various technologies are involved in cloning:
• Reproductive cloning technologies produce genetically
identical individuals (clones)
• Somatic cell nuclear transfer (SCNT) fuses an adult cell
with an enucleated egg
• Therapeutic cloning produces embryos that are used for
stem cell research
Reproductive Cloning
• Cloning can refer to a laboratory method by which
researchers copy DNA fragments
• It can also refer to interventions in reproduction that result in
an exact genetic copy of an organism
• reproductive cloning
• Artificial twinning and any other technology that yields
genetically identical individuals
Somatic Cell Nuclear Transfer
• To clone an adult, scientists first transform one of its
differentiated cells into an undifferentiated cell by turning its
unused DNA back on
• somatic cell nuclear transfer (SCNT)
• Method of reproductive cloning in which a researcher
removes the nucleus from an unfertilized egg, then inserts
into the egg a nucleus from an adult body cell
Somatic Cell Nuclear Transfer
Somatic Cell Nuclear Transfer
A A cow egg is held in place by
suction through a hollow glass
tube called a micropipette. DNA is
identified by a purple stain.
Fig 8.13a, p. 132
Somatic Cell Nuclear Transfer
B Another micropipette punctures
the egg and sucks out the DNA. All
that remains inside the egg’s
plasma membrane is cytoplasm.
Fig 8.13b, p. 132
Somatic Cell Nuclear Transfer
C A new micropipette prepares to
enter the egg at the puncture site.
The pipette contains a cell grown
from the skin of a donor animal.
Fig 8.13c, p. 132
Somatic Cell Nuclear Transfer
D The micropipette enters the
egg and delivers the skin cell to
a region between the cytoplasm
and the plasma membrane.
Fig 8.13d, p. 132
Somatic Cell Nuclear Transfer
E After the pipette is withdrawn,
the donor’s skin cell is visible
next to the cytoplasm of the egg.
The transfer is now complete.
Fig 8.13e, p. 132
Somatic Cell Nuclear Transfer
F The egg is exposed to an electric
current. This treatment causes the
foreign cell to fuse with and empty its
nucleus into the cytoplasm of the egg.
The egg begins to divide, and an
embryo forms. After a few days, the
embryo may be transplanted into a
surrogate mother.
Fig 8.13f, p. 132
ANIMATION: How Dolly was created
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Cloned Animals
• Champion Holstein dairy cow Nelson’s Estimate Liz (right)
and her clone, Nelson’s Estimate Liz II (left)
Therapeutic Cloning
• Researchers are already using SCNT to produce human
embryos for research, a practice called therapeutic cloning
• The researchers harvest undifferentiated (stem) cells from the
cloned human embryos
• therapeutic cloning
• Use of SCNT to produce human embryos for research
purposes
Key Concepts
• Cloning Animals
• Several methods are now commonly used to produce
clones of adult animals for research and agriculture
• The techniques are far from perfect, and the practice
continues to raise serious ethical questions
Golden Clones (revisited)
• Cloned animals tend to have problems:
• The first cloned mammal, Dolly the sheep, had old-age
problems when she was only two
• Clones may be overweight or have enlarged organs
• Cloned mice develop lung and liver problems, and almost
all die prematurely
• Cloned pigs tend to limp and have heart problems: One
never did develop a tail or, even worse, an anus