Download AP Biology, Chapter 16 The Molecular Basis of Inheritance Life`s

AP Biology, Chapter 16
The Molecular Basis of Inheritance
Life’s Operating Instructions
16.1 DNA is the genetic material
Intro
The Search for the Genetic Material: Science Inquiry
Intro
1. Explain why researchers originally thought protein was the genetic material.
Chromosomes were known to be composed of DNA and protein
Hypothesized characteristics of the genetic material
Had to be specific in determining traits
Had to be heterogeneous for transmitting a variety of traits
Characteristics of protein
Enzymes, at least were know to be very specific in function
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Proteins were known to come in many forms
Evidence That DNA Can Transform Bacteria
2. Summarize the experiments performed by scientists that provided evidence
that DNA can transform bacteria.
Frederick Griffith
Transformed Pneumococcus
Experimental: heat-killed smooth + live rough  killed
Controls
Live smooth  killed
Live rough  didn't
Heat-killed smooth  didn't
Heat-killed rough  didn't
Conclusion: a substance transferred the smooth trait
Oswald Avery, Maclyn McCarty, and Colin MacLeod
Purified components of the smooth bacteria
Only DNA could transform
Evidence That Viral DNA Can Program Cells
3. Summarize the experiment that provided evidence that viral DNA can
program
cells.
Alfred Hershey and Martha Chase
System: T2 bacteriophage of E. coli
Differential labeling
T2 grown in radioactive sulfur had labeled protein
T2 grown in radioactive phosphorus had labeled DNA
Experiment
Infect
A short time later knock phage off with a blender
Centrifuge to separate knocked off phage and host cells
Results
Radioactive protein stayed on the outside of the host
Radioactive DNA entered the host
Conclusion: DNA carried traits for viral replication
Additional Evidence That DNA is the Genetic Material
4. Summarize the findings of Chargaff that provided evidence that DNA is the
genetic material.
Erwin Chargaff
Analyzed DNA from a variety of organisms
Results
Amounts of ATGC varied
But A=T and G=C
Conclusion: DNA not so simple, specifically varies by species
Building a Structural Model of DNA: Scientific Inquiry
5. Explain how Watson and Crick deduced the structure of DNA and describe the
evidence they used. Explain the significance of the research of Rosalind Franklin.
Known covalent structure
Sugar-phosphate backbone
One nitrogen base attached to each sugar
From Rosalind Franklin's X-ray crystallography
Helix
Constant width of 2 nm
One turn every 3.4 nm
Bases spaced every 0.34 nm
Open questions
Number of strands
Bases outside or inside
Watson and Crick built models
Two strands
Hydrophobic bases inside
Strands run in opposite directions; 3'5' and 5'3'
AT and GC pairs give a constant 2 nm width
Also satisfies Chargaff's rule (equilvalencies)
6. Describe the structure of DNA. Explain the "base-pairing rule" and describe its
significance.
Double stranded helix
Sugar-phosphate backbones
AT and GC pairs held by hydrogen bonds
Opposing sequences are complementary; given one  the other
16.2 Many proteins work together in DNA replication and repair
Intro
The Basic Principle: Base Pairing to a Template Strand
5. Describe the semiconservative model of replication and the significance of the
experiments by Matthew Meselson and Franklin Stahl.
Three models
Conservative: original DNA stays intact
Dispersive: original is spread at each replication
Semiconservative: half of the original makes half of the new DNAs
Experiment
Completely label DNA with heavy 15N
Switch to light 14N medium
Grow, sample, purify DNA, determine density in an ultracentrifuge
Results
Original heavy band is replaced by a half-heavy band
Half-heavy band then splits off an equal light band
Interpretation: semi-conservative
DNA Replication: A Closer Look
Intro
Getting Started
6. Describe the process of DNA replication. Note the structure of the many
origins of replication and replication forks and explain the role of DNA
polymerase.
DNA sequences where replication begins = origins of replication
Appear as a bubble in EM
Origin is essential for DNA to be copied
Replication forks are at the ends of the bubbles
Steps
Helicases unwind the original double helix
Single-stranded binding proteins stabilize the unwound parental
DNA
Matching bases line up
The leading strand is synthesized continuously, 5'3'
The lagging strand is synthesized in pieces, 3'5'
DNA polymerase checks for the correct base and covalently
attaches them
Synthesizing a New DNA Strand
7. Explain what energy source drives the polymerization of DNA.
Nucleotide precursors are their triphosphates: dGTP, dTTP, dATP, dCTP
Like ATP, their carry energy
Antiparallel Elongation
8. Define "antiparallel" and explain why continuous synthesis of both strand is
not possible.
Nucleotides are added onto the 3' end of the growing strand
The 5'3' strand can be continuously synthesized
New nucleotides matching 3'5' original strand cannot be added onto
the 5' end
The DNA Replication Complex
9. Distinguish between the leading strand and the lagging strand.
Leading strand (new with 3' end toward the fork)
No problem
3 phosphates on 5' end of d_TP react with the 3' hydroxyl
Catalyzed by DNA polymerase
Lagging strand (new with 5' end toward fork)
10. Explain how the lagging strand is synthesized and even though DNA
polymerase can add nucleotides only to the 3' end.
Primase makes a series of short RNA primers
DNA polymerase adds DNA bases onto the primer's 3' end
The short DNA sections are Okazaki fragments
DNA polymerase removes primers and fills in the gaps
Fragments are joined by DNA ligase
11. Explain the roles of DNA ligase, primer, primase, helicase, and the singlestrand binding protein.
DNA ligase heals single-stranded breaks
Primers are short, temporary sequences that allow DNA polymerase to
work
Primase is an RNA polymerase that makes short primers
Helicase breaks the sugar-phosphate backbone and allows unwinding
Single-strand binding protein stabilizes the separated strands
Proofreading and Repairing DNA
12. Explain the roles of DNA polymerase, mismatch repair enzymes, and nuclease in
DNA
proofreading and repair.
DNA polymerase recognizes non-AT,GC pairs
Backs up removing them
Reverses and resumes adding bases
Mismatch repair enzymes
Recognize damaged bases in DNA
Cut the sugar-phosphate backbone
Nuclease removes nucleotides in the damaged area
DNA polymerase fills in the gap
Ligase heals the sugar-phosphate backbone
Evolutionary Significance of Altered DNA Nucleotides
13. How would organisms and species be affected if DNA was either replicated too
accurately or not accurately enough?
Too accurately
No new traits
Species wouldn’t be able to generate new adaptations
Not accurately enough
Cells resulting from mitosis would be too different
Traits would change too fast for evolution to have an effect
Replicating the Ends of DNA Molecules
14. Describe the structure and function of telomeres. Explain the significance of
telomerase to healthy and cancerous cells.
Telomeres are chromosome ends; a 3' and a 5' end
Leading strand cannot be completed, primer remains
Telomeres shorten with each replication
Contain repeated TTAGGG = extra DNA
We can lose some at each replication without effect
As we age the telomeres get shorter
Telomerase has built-in primer, can add more TTAGGG
Present in germ cell lines
And present in cancer cells
16.3 A chromosome consists of a DNA molecule packed together with proteins
15. Compare the structure and organization of prokaryotic and eukaryotic genomes.
Prokaryotes
Several million base pairs
Circular chromosome
Attached proteins control and pack it
Eukaryotes
Billions of base pairs
2 to hundreds of linear chromosomes
Larger amounts of protein control and pack it
16. Describe the current model for progressive levels of DNA packing.
Mitotic chromosomes
Nucleosomes are the 11 nm beads on a string
Folded into a 30-nm chromatin fiber
30-nm fibers form looped domains attached to protein scaffold
Looped domains coil and fold
Interphase
Same up to looped domains
Looped domains attached to inside surface of nuclear envelope
17. Explain how histones influence folding in eukaryotic DNA.
Nucleosomes = DNA wound around clusters of 8 histones with space in between
Add histone H1, nucleosomes fold into a 30-nm chromatin fiber
18. Distinguish between heterochromatin and euchromatin.
More condensed, non-transcribed = heterochromatin
Less condensed, transcribed = euchromatin