1. The Molecular Basis of Inheritance
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Chapter 16
The Molecular Basis of Inheritance

2. DNA: The Molecule of Life
Early in the 20th century, the identification of the molecules of inheritance was a major challenge to biologists
When T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material
The key factor in determining the genetic material was choosing appropriate experimental organisms
The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them
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3. EVIDENCE THAT DNA CAN TRANSFORM BACTERIA - Frederick Griffith (1928)
Griffith worked with two strains of a bacterium, one pathogenic and one harmless
He mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic
He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA
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4. EXPERIMENT RESULTS Mixture of heat-killed S cells and living R cells
Living S cells (control)
Living R cells (control)
Heat-killed S cells (control)
RESULTS
Figure 16.2 Can a genetic trait be transferred between different bacterial strains?
Mouse dies
Mouse healthy
Mouse healthy
Mouse dies
Living S cells

5. Further Evidence - Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944)
Follow up on Griffith’s experiment:
Purpose: To figure out the molecule responsible for the tranformation
What was done:
Homogenized heat-killed bacteria
Extracted the cellular components
Isolated the molecule of interest by selectively inactivating the others
Tested the sample for the ability to transform live pathogen
Conclusion: Only DNA worked in transforming harmless bacteria into pathogenic bacteria
Many biologists remained skeptical, mainly because little was known about DNA
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6. ADDITIONAL EVIDENCE THAT DNA IS THE GENETIC MATERIAL – Erwin Chargaff (1950)
Known: DNA is a polymer of nucleotides, made of a nitrogenous base, a sugar, and a phosphate group
From biochemical analyses, Chargaff found that DNA composition varies from species to species. Ex. Humans have 30.3% Adenine while E.coli has 26%
This evidence of diversity made DNA a credible candidate for the genetic material
Regularity in the ratios of the bases within a species was observed.
Adenines and Thymines almost equal
Guanines and Cytosines same
Chargaff’s rules: 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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7. Sealing Evidence – Alfred Hershey and Martha Chase (1952)
Studied bacteriophages
Bacteriophages are viruses that infect bacteria.
Made of proteins and DNA
Insert the genetic material into the bacterium during an infection
Question: What was inserted? The Protein or the DNA?
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8. Conclusion: The injected DNA of the phage provides
the genetic information

9. 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 used X-ray crystallography to study molecular structure
Franklin produced a picture of the DNA molecule using this technique
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10. Franklin’s X-ray crystallographic images of DNA enabled Watson & Crick to deduce:
That DNA was helical
The width of the helix and the spacing of the nitrogenous bases
The width suggested that the DNA molecule was made up of two strands, forming a double helix
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11. 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm 5 end
Figure 16.7 The double helix
For the Cell Biology Video Stick Model of DNA (Deoxyribonucleic Acid), go to Animation and Video Files.
For the Cell Biology Video Surface Model of DNA (Deoxyribonucleic Acid), go to Animation and Video Files.
3 end
0.34 nm
5 end
(a) Key features of DNA structure
(b) Partial chemical structure
(c) Space-filling model

12. At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width
Instead, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray
***Also consistent with Chargaff’s Rule
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14. CONCEPT 16.2: MANY PROTEINS WORK TOGETHER IN DNA REPLICATION & REPAIR
Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material
Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication
In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
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15. Other suggested models were:
Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand
Other suggested models were:
the conservative model (the two parent strands rejoin) and
the dispersive model (each strand is a mix of old and new)
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16. Experiments by Matthew Meselson and Franklin Stahl supported the semi-conservative model
They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope
The first replication produced a band of hybrid DNA, eliminating the conservative model
A second replication produced both light and hybrid DNA, eliminating the dispersive model and supporting the semi- conservative model
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17. 3 EXPERIMENT 1 Bacteria cultured in medium containing 15N 2
Bacteria transferred to medium containing 14N
RESULTS 3
DNA sample centrifuged after 20 min (after first application) 4
DNA sample centrifuged after 40 min (after second replication)
Less dense
More dense
CONCLUSION
First replication
Second replication
Conservative model
Figure Does DNA replication follow the conservative, semiconservative, or dispersive model?
Semiconservative model
Dispersive model

18. The copying of DNA is remarkable in its speed and accuracy
More than a dozen enzymes and other proteins participate in DNA replication
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19. DNA REPLICATION: A CLOSER LOOK
Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble”
A eukaryotic chromosome may have hundreds or even thousands of origins of replication
Replication proceeds in both directions from each origin, until the entire molecule is copied
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20. 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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21. Single-strand binding proteins
Fig
Primase
Single-strand binding proteins 3
Topoisomerase 5 3
RNA primer
Figure Some of the proteins involved in the initiation of DNA replication 5 5 3
Helicase

22. Getting Started
DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3 end
The initial nucleotide strand is a short RNA primer
The enzyme, primase, starts an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template
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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23. Synthesizing a New DNA Strand
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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24. Each nucleotide that is added to a growing DNA strand is a deoxyribose nucleoside triphosphate
dATP supplies adenine to DNA and is similar to the ATP of energy metabolism
The difference is in their sugars
As each monomer of dATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate
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25. Antiparallel Elongation
The antiparallel structure of the double helix (two strands oriented in opposite directions) affects replication
DNA polymerases add nucleotides only to the free 3end of a growing strand
Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork
To elongate the other new strand, the lagging strand, DNA polymerase 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 joined together by DNA ligase
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26. Overall directions of replication
Fig
Overview
Origin of replication
Leading strand
Lagging strand
Lagging strand 2 1
Leading strand
Overall directions of replication 3 5 5 3
Template strand 3
RNA primer 3 5 1 5
Okazaki fragment 3 5 3 1 5 5 3 3
Figure 16.6 Synthesis of the lagging strand 2 1 5 3 5 3 5 2 1 5 3 3 1 5 2
Overall direction of replication

27. Table 16-1

28. Single-strand binding protein Overall directions of replication
Fig
Overview
Origin of replication
Leading strand
Lagging strand
Leading strand
Lagging strand
Single-strand binding protein
Overall directions of replication
Helicase
Leading strand 5
DNA pol III 3 3
Primer
Primase 5
Parental DNA 3
Figure A summary of bacterial DNA replication
DNA pol III
Lagging strand 5
DNA pol I
DNA ligase 4 3 5 3 2 1 3 5

29. 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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30. 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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31. Ex. Of telomere sequence in humans: TTAGGG repeated 100 to 1000 times
Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called telomeres
Ex. Of telomere sequence in humans: TTAGGG repeated 100 to 1000 times
Telomere sequence are not genes
Telomeres do not prevent the shortening of DNA molecules,
They postpone the erosion of genes near the ends of DNA molecules
It has been proposed that the shortening of telomeres is connected to aging
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32. Telomerase catalyzes the lengthening of telomeres in germ cells
If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce
Telomerase catalyzes the lengthening of telomeres in germ cells
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33. CONCEPT 16.3 A CHROMOSOME CONSISTS OF A DNA MOLECULE PACKED TOGETHER WITH PROTEINS
The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein
Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein
In a bacterium, the DNA is “supercoiled” and found in a region of the cell called the nucleoid
In Eukaryotes:
Nucleus contains Chromatin - a complex of DNA and protein
Histones: proteins that are responsible for the first level of DNA packing in chromatin
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34. Nucleosomes, or “beads on a string” (10-nm fiber)
Fig a
Nucleosome
(10 nm in diameter)
DNA double helix (2 nm in diameter) H1
Histone tail
Histones
Figure 16.21a Chromatin packing in a eukaryotic chromosome
DNA, the double helix
Histones
Nucleosomes, or “beads on a string” (10-nm fiber)

35. Looped domains (300-nm fiber) Metaphase chromosome
Fig b
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
Figure 16.21b Chromatin packing in a eukaryotic chromosome
Replicated chromosome (1,400 nm)
30-nm fiber
Looped domains (300-nm fiber)
Metaphase chromosome

36. Chromatin Organization
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
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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37. Loosely packed chromatin is called euchromatin
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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