chapter 16 the molecular basis of inheritance. fig. 16-1 in 1953, james watson and francis crick...
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Chapter 16
The Molecular Basis of Inheritance
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Fig. 16-1
•In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA
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Fig. 16-2
Living S cells (control) Pathogenic
Living R cells (control)
Heat-killed S cells (control)
Mixture of heat-killed S cells and living R cells
Mouse diesMouse dies Mouse healthy Mouse healthy
Living S cells
RESULTS
EXPERIMENT Frederick Griffith in 1928
Transformation,
now defined as a change in genotype and phenotype due to assimilation of foreign DNA
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•In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA
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Hershey and Chase
• 1952, studying T2 virus infecting Escherichia coli– Bacteriophage or phage
• Phage coat made entirely of protein• DNA found inside capsid
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Fig. 16-4-1
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactive protein
Radioactive DNA
Batch 1: radioactive sulfur (35S)
Batch 2: radioactive phosphorus (32P)
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Fig. 16-4-2
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactive protein
Radioactive DNA
Batch 1: radioactive sulfur (35S)
Batch 2: radioactive phosphorus (32P)
Empty protein shell
Phage DNA
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Fig. 16-4-3
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactive protein
Radioactive DNA
Batch 1: radioactive sulfur (35S)
Batch 2: radioactive phosphorus (32P)
Empty protein shell
Phage DNA
Centrifuge
Centrifuge
Pellet
Pellet (bacterial cells and contents)
Radioactivity (phage protein) in liquid
Radioactivity (phage DNA) in pellet
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Chargaff’s rules
• 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
• 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
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Fig. 16-5Sugar–phosphate
backbone
5 end
Nitrogenous
bases
Thymine (T)
Adenine (A)
Cytosine (C)
Guanine (G)
DNA nucleotide
Sugar (deoxyribose)
3 end
Phosphate
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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
• 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
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Fig. 16-6
(a) Rosalind Franklin (b) Franklin’s X-ray diffraction photograph of DNA
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Fig. 16-7
(c) Space-filling model
Hydrogen bond 3 end
5 end
3.4 nm
0.34 nm
3 end
5 end
(b) Partial chemical structure(a) Key features of DNA structure
1 nm
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• DNA is– Double stranded– Helical– Sugar-phosphate
backbone– Bases on the
inside– Stabilized by
hydrogen bonding– Base pairs with
specific pairing
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• AT/GC or Chargoff’s rule– A pairs with T
– G pairs with C
• Keeps with consistent
• 10 base pairs per turn
• 2 DNA strands are complementary– 5’ – GCGGATTT – 3’
– 3’ – CGCCTAAA – 5’
• 2 strands are antiparallel– One strand 5’ to 3’
– Other stand 3’ to 5’
•two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior
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• Space-filling model shows grooves– Major groove
• Where proteins bind
– Minor groove
Fig. 16-7b
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Replication
• 3 different models for DNA replication proposed in late 1950s– Semiconservative– Conservative– Dispersive
• Newly made strands are daughter strands
• Original strands are parental strands
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Fig. 16-10
Parent cellFirst replication
Second replication
(a) Conservative model
(b) Semiconserva- tive model
(c) Dispersive model
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Fig. 16-11EXPERIMENT
RESULTS
CONCLUSION
1 2
43
Conservative model
Semiconservative model
Dispersive model
Bacteria cultured in medium containing 15N
Bacteria transferred to medium containing 14N
DNA sample centrifuged after 20 min (after first application)
DNA sample centrifuged after 40 min (after second replication) More
dense
Less dense
Second replicationFirst replication
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Fig. 16-11a
EXPERIMENT
RESULTS
1
3
2
4
Bacteria cultured in medium containing 15N
Bacteria transferred to medium containing 14N
DNA sample centrifuged after 20 min (after first application)
DNA sample centrifuged after 20 min (after second replication)
Less dense
More dense
a rare heavy form (15N)
a common light form (14N)
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Fig. 16-11b
CONCLUSION
First replication Second replication
Conservative model
Semiconservative model
Dispersive model
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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
• Origin of replication– Site of start point for replication
• Bidirectional replication– Replication proceeds outward in opposite directions
• Bacteria have a single origin
Eukaryotes require multiple origins
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Fig. 16-12Origin of replication Parental (template) strand
Daughter (new) strand
Replication fork
Replication bubble
Two daughter DNA molecules
(a) Origins of replication in E. coli
Origin of replication Double-stranded DNA molecule
Parental (template) strandDaughter (new) strand
Bubble Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
0.5 µm
0.25 µm
Double-strandedDNA molecule
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Fig. 16-12a
Origin of replication Parental (template) strand
Daughter (new) strand
Replication fork
Replication bubble
Double-stranded DNA molecule
Two daughter DNA molecules
(a) Origins of replication in E. coli
0.5 µm
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Fig. 16-12b
0.25 µm
Origin of replication Double-stranded DNA molecule
Parental (template) strandDaughter (new) strand
Bubble Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
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Topoisomerase
Helicase
PrimaseSingle-strand binding proteins
RNA primer
5 5
53
3
3
DNA helicaseBinds to DNA and travels 5’ to 3’ using ATP to separate strand and move fork forward
DNA topoisomerase: Relives additional coiling ahead of replication forkSingle-strand binding proteins: Keep parental strands open to act as templates
DNA polymerasescannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3 endPrimerase:The initial nucleotide strand is a short RNA primer
•The primer is short (5–10 nucleotides long), and the 3 end serves as the starting point for the new DNA strand
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• At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating
• Helicases are enzymes that untwist the double helix at the replication forks
• Single-strand binding protein binds to and stabilizes single-stranded DNA until it can be used as a template
• Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
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Fig. 16-14
A
C
T
G
G
G
GC
C C
C
C
A
A
AT
T
T
New strand 5 end
Template strand 3 end 5 end 3 end
3 end
5 end5 end
3 end
Base
Sugar
Phosphate
Nucleoside triphosphate
Pyrophosphate
DNA polymerase
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DNA polymerases
• 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
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Fig. 16-15
Leading strand
Overview
Origin of replicationLagging strand
Leading strandLagging strand
Primer
Overall directions of replication
Origin of replication
RNA primer
“Sliding clamp”
DNA poll IIIParental DNA
5
3
3
3
3
5
5
5
5
5
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Fig. 16-15a
Overview
Leading strand
Leading strandLagging strand
Lagging strand
Origin of replication
Primer
Overall directions of replication
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Fig. 16-15b
Origin of replication
RNA primer
“Sliding clamp”
DNA pol IIIParental DNA
3
5
5
5
5
5
5
3
3
3
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Fig. 16-16Overview
Origin of replication
Leading strand
Leading strand
Lagging strand
Lagging strand
Overall directions of replication
Template strand
RNA primer
Okazaki fragment
Overall direction of replication
12
3
2
1
1
1
1
2
2
51
3
3
3
3
3
3
3
3
3
5
5
5
5
5
5
5
5
5
5
53
3
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Fig. 16-16a
Overview
Origin of replication
Leading strand
Leading strand
Lagging strand
Lagging strand
Overall directions of replication
12
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Fig. 16-16b1
Template strand
5
53
3
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Fig. 16-16b2
Template strand
5
53
3
RNA primer 3 5
5
3
1
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Fig. 16-16b3
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
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Fig. 16-16b4
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
12
3
3
5
5
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Fig. 16-16b5
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
12
3
3
5
5
12
3
3
5
5
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Fig. 16-16b6
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
12
3
3
5
5
12
3
3
5
5
12
5
5
3
3
Overall direction of replication
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Fig. 16-17
OverviewOrigin of replication
Leading strand
Leading strand
Lagging strand
Lagging strandOverall directions
of replication
Leading strand
Lagging strand
Helicase
Parental DNA
DNA pol III
Primer Primase
DNA ligase
DNA pol III
DNA pol I
Single-strand binding protein
5
3
5
5
5
5
3
3
3
313 2
4
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Table 16-1
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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
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Fig. 16-18
Nuclease
DNA polymerase
DNA ligase
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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 ends, so repeated rounds of replication produce shorter DNA molecules
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Fig. 16-19
Ends of parental DNA strands
Leading strand
Lagging strand
Lagging strand
Last fragment Previous fragment
Parental strand
RNA primer
Removal of primers and replacement with DNA where a 3 end is available
Second round of replication
New leading strand
New lagging strand
Further rounds of replication
Shorter and shorter daughter molecules
5
3
3
3
3
3
5
5
5
5
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Telomeres and aging
• Body cells have a predetermined life span
• Skin sample grown in a dish will double a finite number of times– Infants, about 80 times– Older person, 10 to 20 times
• Senescent cells have lost the capacity to divide
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• Progressive shortening of telomeres correlated with cellular senescence
• Telomerase present in germ-line cells and in rapidly dividing somatic cells
• Telomerase function reduces with age
• Inserting a highly active telomerase gene into cells in the lab causes them to continue to divide
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Telomeres and cancer
• When cells become cancerous they divide uncontrollably
• In 90% of all types of human cancers, telomerase is found at high levels
• Prevents telomere shortening and may play a role in continued growth of cancer cells
• Mechanism unknown
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Fig. 16-20
1 µm
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Fig. 16-21a
DNA double helix (2 nm in diameter)
Nucleosome(10 nm in diameter)
Histones Histone tailH1
DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)
Concept 16.3 A chromosome consists of a DNA molecule packed together with proteins
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Fig. 16-21b
30-nm fiber
Chromatid (700 nm)
Loops Scaffold
300-nm fiber
Replicated chromosome (1,400 nm)
30-nm fiber Looped domains (300-nm fiber)
Metaphase chromosome
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• 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
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• 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
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• Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis
• Loosely packed chromatin is called euchromatin
• During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin
• Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions
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• 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
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Fig. 16-22
RESULTS
Condensin and DNA (yellow)
Outline of nucleus
Condensin (green)
DNA (red at periphery)
Normal cell nucleus Mutant cell nucleus
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You should now be able to:
1. Describe the contributions of the following people: Griffith; Avery, McCary, and MacLeod; Hershey and Chase; Chargaff; Watson and Crick; Franklin; Meselson and Stahl
2. Describe the structure of DNA
3. Describe the process of DNA replication; include the following terms: antiparallel structure, DNA polymerase, leading strand, lagging strand, Okazaki fragments, DNA ligase, primer, primase, helicase, topoisomerase, single-strand binding proteins
4. Describe the function of telomeres
5. Compare a bacterial chromosome and a eukaryotic chromosome
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Fig. 16-UN5