Download Replication can then occur in either direction along the strand

Chapter 8
• DNA: The Molecule of Heredity
• NUCLEIC ACID – your “4th” macromolecule!
• Nucleic Acid – “a polymer consisting of many
nucleotide monomers, serves as a blueprint
for proteins and, through the action of
proteins, for all cellular structures and
activities.”
• Example - DNA (and RNA)
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8.1 What Are Genes Made
of?
• Figure 8.1 Genetic differences (p. 114)
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Many human traits are
inherited, including
(a) Viggo Mortensen’s
(b) cleft chin and (b)
(c) Orlando Bloom’s
(d) smoothly rounded
(e) chin.
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8.2 What Is the Structure of
DNA?
• 8.2.1 DNA Is Composed of Four
Different Subunits, called nitrogenous
bases
– 8_UN01 Thymine and adenine (p. 114)
– 8_UN02 Cytosine and guanine (p. 114)
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phosphate
base = thymine
sugar
phosphate
base = cytosine
sugar
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phosphate
sugar
base = adenine
phosphate
sugar
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base = guanine
What is a nucleotide?
• Phosphate + sugar + nitrogenous base,
makes up the “sugar phosphase
backbone”
• Nitrogenous bases (DNA) are A, T, G,
C (in RNA they are w/ U, uracil instead
of tyamine)
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8.2 What Is the Structure of
DNA?
• 8.2.2 A DNA Molecule Contains Two
Nucleotide Strands
– Figure 8.2 The Watson-Crick model of
DNA structure (p. 115)
– Base pairing (complementary)
– In spiral (double helix)
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A
T
T
C
G
G
C
C
C
G
G
A
A
T
T
C
G
A
T
T
T
A
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A
A
A
C
G
A
T
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T
A
T
What to notice
•
•
•
•
•
Spiral
Backbone
Base pairing
Nitrogenous bases
Sugar location
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T
C
A
G
G
C
C
G
A
T
C
G
A
T
T
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A
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8.3 How Does DNA Encode
Information?
• Figure E8.1 The discovery of DNA (p.
116)
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James Watson and Francis Crick with a model of
DNA structure
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Discovery of DNA
• DNA molecules were know over 100 years
ago, chromosomes were known in the
1940’s, but they thought that the 20 amino
acids of proteins made up the genes
• 3 levels of study: Griffith (bacteria, 1928);
Hershey and Chase (bacteriophage, 1952)
then Watson and Crick (the
STRUCTURE…as you’ve seen…)
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Griffith (bacteria, 1928);
•
2 types of bacteria: harmless, non-pneumonia
carrying bacteria, and disease carrying strains.
He heated the disease carrying bacteria and
denatured the disease and then chopped it up.
These (deactivated bacterial parts) were then
mixed with the non-diseased bacteria and the
diseased strain CAME BACK! He further noted
that when these newly infected bacteria
reproduced, they passed on the disease (i.e. it
was INHERITED!). This is an example of
bacterial transformation.
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Hershey and Chase, 1952
•
•
DNA was the genetic material of a bacteriophage
(T2), a “virus that attacks bacteria.”
EXPERIMENT: Using E. coli they labeled
protein “phages” with radioactive sulfur (yellow)
and DNA “phages” with radioactive phosphorus
(green). OVERHEAD 10.1a. These were then
each allowed to infect bacteria. Shake mixture
to separate loose pieces of phages that had not
infected (entered) the E. coli. Centifuge,
separates the heavier bacteria into a pellet and
leaves the phages in the liquid. Test for
radioactivity between the pellet and the liquid.
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Result…
• They found that:
• In the PROTEIN (sulfur) containing liquid:
The radioactivity remained in the liquid.
• In the DNA (phosphorus) containing liquid:
The radioactivity was in the pellet (bacteria)
not the liquid. OVERHEAD 10.1c They then
took these bacteria out of the liquid and
allowed them to lyse (reproduce) and they
reproduced bacteria with radioactive
phosphorus inside – it was INHERITED!
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Now…back to DNA
•
•
•
•
•
•
5 nitrogenous bases
Adenine, Thymine (DNA only), Cytosine,
Guanine and Uracil (RNA only).
AUGC in RNA (Ribonucleic acid). Sugar =
ribose. {{RNA is the genome of most viruses
and functions mainly in humans for protein
synthesis. Usually single stranded.}}
ATGC in DNA (Deoxyribonucleic acid). Sugar =
deoxyribose.
A and G = Purines, T and C = Pyrimidines (In
DNA)
Now…to replication…
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8.4 How Is DNA Copied?
• 8.4.1 Why Does DNA Need to Be
Copied?
• 8.4.2 DNA Is Copied Before Cell
Division
• 8.4.3 DNA Replication Produces Two
DNA Double Helixes, Each with One
Original Strand and One New Strand
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free nucleotides
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One DNA
double helix.
DNA replication
Two identical DNA
double helixes, each
with one parental
strand (blue) and one
new strand (pink).
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Replication summary
•
•
•
Base pairing is needed
Complimentary strands (like in a photograph –
the negative image would serve as a template
for the replicated molecule).
Process: (1) Strands separate, (2) Become
templates for new base pairing to occur, (3)
Nucleotides come in to line up, 1 by 1, along the
strand. (4) Enzymes (DNA polymerase and
ligase) link the nucleotides to the strands, (5)
New (identical) molecule formed = “Daughter
DNA.”
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8.5 What Are the
Mechanisms of DNA
Replication?
• Figure 8.4 Details of DNA replication
(p. 119)
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replication forks
DNA helicase
DNA helicase
replication bubble
DNA polymerase #1
DNA
polymerase #2
DNA polymerase #1
continues along parental
DNA strand
continuous synthesis
DNA polymerase #2
leaves
DNA
polymerase #3
DNA polymerase #3
leaves
DNA
polymerase #4
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DNA ligase joins
daughter DNA strands
together
replication forks
DNA helicase
DNA helicase
replication bubble
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DNA polymerase #1
DNA
polymerase #2
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DNA polymerase #1
continues along parental
DNA strand
continuous synthesis
DNA polymerase #2
leaves
DNA
polymerase #3
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DNA polymerase #3
leaves
DNA
polymerase #4
DNA ligase joins
daughter DNA strands
together
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summary
•
Replication can then occur in either direction along
the strand, located by “replication bubbles.”
OVERHEAD 10.5a : This speeds the process up and a
lot of these can go on, until they all form one
(identical) daughter strand. Also, remember, these
can run in either direction because the base pairs are
complimentary, so the sugar phosphate backbone is
opposite on each strand. OVERHEAD 10.5b. One
side of the backbone will be 3’ to 5’, while the other
will go 5’ to 3’. {3’ = Carbon attached to -OH, 5’ =
carbon attached to phosphate}. NUCLEOIDES are
only added to the 3’ end (so they “grow” in only 1
direction, the 5’ to 3’ way).
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8.5 What Are the Mechanisms
of DNA Replication?
• 8.5.1 DNA Helicase Separates the
Parental DNA Strands
• 8.5.2 DNA Polymerase Synthesizes
New DNA Strands
• 8.5.3 DNA Ligase Joins Together
Segments of DNA
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enzymes
•
•
DNA polymerase: link nucleotides to the
growing strand. This enzyme is also
responsible for “proofreading” and
removing any nucleotides that are
incorrectly trying to base pair. And repair
too.
DNA ligase: ties smaller synthesized
pieces together to form 1 DNA strand
through covalent bonding. And it is also
responsible for repair.
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8.4 How Is DNA Copied?
• 8.4.4 Proofreading Produces Almost
Error-Free Replication of DNA
• 8.4.5 Mistakes Do Happen
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