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

2. Describe the structure of DNA
Learning Targets:
Describe the structure of DNA
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
Describe the function of telomeres
Compare a bacterial chromosome and a eukaryotic chromosome
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3. Phenomenon termed transformation
DNA a timeline
The genetic role of DNA began with research by Frederick Griffith in 1928
He worked with two strains of a bacterium, one pathogenic and one harmless
When heat-killed remains of the pathogenic strain were mixed with living cells of the harmless strain some cells became pathogenic
Phenomenon termed transformation
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4. Griffith EXPERIMENT RESULTS
Mixture of heat-killed S cells and living R cells
Griffith EXPERIMENT
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. DNA a timeline continued
In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA
Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria
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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6. Evidence That Viral DNA Can Program Cells
More evidence for DNA as the genetic material came from studies of viruses that infect bacteria
Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research
Animation: Phage T2 Reproductive Cycle
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7. Evidence That DNA Is the Genetic Material
In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2
They concluded that the injected DNA of the phage provides the genetic information
Animation: Hershery-Chase Experiment
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8. Chase and Hershey EXPERIMENT
Empty protein shell
Radioactivity (phage protein) in liquid
Radioactive protein
Phage
Bacterial cell
Batch 1: radioactive sulfur (35S)
DNA
Phage DNA
Centrifuge
Radioactive DNA
Pellet (bacterial cells and contents)
Figure 16.4 Is protein or DNA the genetic material of phage T2?
Batch 2: radioactive phosphorus (32P)
Centrifuge
Radioactivity (phage DNA) in pellet
Pellet

9. Evidence That DNA Is the Genetic Material
It was known that DNA is a polymer of nucleotides( Sugar, Phosphate, Base)
In 1950, Erwin Chargaff showed DNA composition varies from species to species
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
Animation: DNA and RNA Structure
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10. Sugar–phosphate backbone 5 end Sugar (deoxyribose) 3 end
Nitrogenous bases
Thymine (T)
Adenine (A)
Figure 16.5 The structure of a DNA strand
Cytosine (C)
Phosphate
DNA nucleotide
Sugar (deoxyribose)  end
Guanine (G)

11. Building a Structural Model of DNA
After most biologists became convinced that DNA was the genetic material, the challenge was to determine how its structure accounts for its role
Rosalind Franklin produced a picture of the DNA molecule using a technique called X-ray crystallography
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12. Building a Structural Model of DNA
Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical
The X-ray images also enabled Watson to deduce 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
Animation: DNA Double Helix
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13. 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm 5 end
Figure 16.7 The double helix
3 end
0.34 nm
5 end

14. Watson and Crick’s contribution
determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)
The Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C
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15. Adenine (A) Thymine (T) Guanine (G) Cytosine (C)
Figure 16.8 Base pairing in DNA
Guanine (G)
Cytosine (C)

16. Base Pairing to a Template Strand
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
Animation: DNA Replication Overview
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17. Base Pairing to a Template StrandC G C G C G C G T A T A T A T A A T A T A T A T G C G C G C G C
Parent molecule
Separation of strands
“Daughter” DNA molecules, each consisting of one parental strand and one new strand
Figure 16.9 A model for DNA replication: the basic concept

18. Animation: Origins of Replication
DNA Replication
Replication begins at special sites called origins of replication
More than a dozen enzymes and other proteins participate in DNA replication
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
Animation: Origins of Replication
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19. Origins of replication in E. coli
Origin of replication
Parental (template) strand
Daughter (new) strand
Replication fork
Double-stranded DNA molecule
Replication bubble
0.5 µm
Two daughter DNA molecules
Figure Origins of replication in E. coli and eukaryotes

20. Origins of replication in eukaryotes
Origin of replication
Double-stranded DNA molecule
Parental (template) strand
Daughter (new) strand
0.25 µm
Bubble
Replication fork
Figure Origins of replication in E. coli and eukaryotes
Two daughter DNA molecules

21. DNA Replication
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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22. The initial nucleotide strand is a short RNA primer
DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3 end
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
The initial nucleotide strand is a short RNA primer

23. DNA Replication
An enzyme called primase can start 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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24. 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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25. Synthesizing a New DNA Strand
Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate
dATP supplies adenine to DNA and is similar to the ATP of energy metabolism
The difference is in their sugars: dATP has deoxyribose while ATP has ribose
As each monomer of dATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate
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26. Nucleoside triphosphate
New strand 5 end
Template strand 3 end
5 end
3 end
Sugar A T A T
Base
Phosphate C G C G G C G C
DNA polymerase
3 end A T A
Figure Incorporation of a nucleotide into a DNA strand T
3 end C
Pyrophosphate C
Nucleoside triphosphate
5 end
5 end

27. Antiparallel Elongation
The antiparallel structure of the double helix (two strands in opposite directions) affects replication
DNA polymerases add nucleotides only to the free 3end of a growing strand; therefore, a new DNA strand can elongate only in the 5 to 3direction
Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork
Animation: Leading Strand
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28. Overall directions of replication
Overview
Origin of replication
Leading strand
Lagging strand
Primer
Lagging strand
Leading strand
Overall directions of replication
Origin of replication 3 5
RNA primer 5
“Sliding clamp” 3 5
DNA poll III
Parental DNA
Figure Synthesis of the leading strand during DNA replication 3 5 5 3 5

29. Animation: Lagging Strand
To elongate the other new strand, called 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
Animation: Lagging Strand
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30. Overall directions of replication
Overview
Origin of replication
Leading strand
Lagging strand
Lagging strand 2 1
Leading strand
Figure 16.6 Synthesis of the lagging strand
Overall directions of replication

31. Figure 16.6 Synthesis of the lagging strand3 5 5 3
Template strand
Figure 16.6 Synthesis of the lagging strand

32. Figure 16.6 Synthesis of the lagging strand3 5 5 3
Template strand 3 5
RNA primer 3 1 5
Figure 16.6 Synthesis of the lagging strand

33. Figure 16.6 Synthesis of the lagging strand3 5 5 3
Template strand 3 5
RNA primer 3 1 5 3
Okazaki fragment 5 3 1 5
Figure 16.6 Synthesis of the lagging strand

34. Figure 16.6 Synthesis of the lagging strand3 5 5 3
Template strand 3 5
RNA primer 3 1 5 3
Okazaki fragment 5 3 5 1 3 5 3 2 1 5
Figure 16.6 Synthesis of the lagging strand

35. Figure 16.6 Synthesis of the lagging strand3 5 5 3
Template strand 3 5
RNA primer 3 1 5 3
Okazaki fragment 5 3 5 1 3 5 3 2 1 5 5 3
Figure 16.6 Synthesis of the lagging strand 3 5 2 1

36. Figure 16.6 Synthesis of the lagging strand3 5 5 3
Template strand 3 5
RNA primer 3 1 5 3
Okazaki fragment 5 3 5 1 3 5 3 2 1 5 5 3
Figure 16.6 Synthesis of the lagging strand 3 5 2 1 5 3 3 1 5 2
Overall direction of replication

38. Single-strand binding protein Overall directions of replication
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

39. The DNA Replication Complex
The proteins that participate in DNA replication form a large complex, a “DNA replication machine”
Animation: DNA Replication Review
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40. 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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41. Nuclease DNA polymerase DNA ligase
Figure Nucleotide excision repair of DNA damage
DNA ligase

42. 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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43. Figure 16.19 Shortening of the ends of linear DNA molecules5
Ends of parental DNA strands
Leading strand
Lagging strand 3
Last fragment
Previous fragment
RNA primer
Lagging strand 5 3
Parental strand
Removal of primers and replacement with DNA where a 3 end is available 5 3
Second round of replication
Figure Shortening of the ends of linear DNA molecules 5
New leading strand 3
New lagging strand 5 3
Further rounds of replication
Shorter and shorter daughter molecules

44. Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called telomeres
Telomeres do not prevent the shortening of DNA molecules, but they do 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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45. If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce
An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells
The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions
There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist
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46. Chromosome makeup
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
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47. Animation: DNA Packing
Chromosome makeup
Chromatin is a complex of DNA and protein, and is found in the nucleus of eukaryotic cells
Histones are proteins that are responsible for the first level of DNA packing in chromatin
For the Cell Biology Video Cartoon and Stick Model of a Nucleosomal Particle, go to Animation and Video Files.
Animation: DNA Packing
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48. Chromosome makeup DNA, the double helix Histones
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)

49. Chromosome makeup 30-nm fiber Looped domains (300-nm fiber)
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

50. Loosely packed chromatin is called euchromatin
Chromosome makeup
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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51. Chromosome modifications
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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