Chapter 16: The Molecular Basis of Inheritance
The elongation of the leading strand during DNA synthesis______?
depends on the action of DNA polymerase
In a nucleosome, the DNA is wrapped around ____?
histone proteins
What do single strand binding proteins do?
they hold the DNA strands apart
What is the role of DNA polymerase in DNA proofreading and repair?
to proofread each nucleotide before it is added, removes incorrect pairing
What is the purpose of topoisomerase?
to relieve strain caused by unwinding of DNA
What is the role of repair enzymes in DNA proofreading and repair?
to remove and replace incorrectly paired nucleotides from replication errors
What did Oswald Avery determine to be the transforming factor?
DNA
Name the 5 nitrogenous bases and if they are purine, pyrimidine and found in DNA or RNA
1. Adenine is a purine found in both DNA and RNA 2. Guanine is a purine found in both DNA and RNA 3. Cytosine is a pyrimidine found in both DNA and RNA 4. Thymine is a pyrimidine found in only DNA 5. Uracil is a pyrimidine found in only RNA
Distinguish between the leading strand and lagging strand during DNA replication***
3' end of original strand is leading, 5' of original strand is lagging. https://www.youtube.com/watch?v=TEQMeP9GG6M
How did Watson and Crick's model explain the basis for Chargaff's rules?
A and T pair together, C and G pair together.
Explain telomere erosion and the role of telomerase ***
After a certain number of replications, telomeres shorten (causes aging). Telomerase prevents shortening and restores telomeres to original length in germ cells, not somatic cells.
Explain the experiment of Meselson and Stahl
Bacterium with 15N DNA and 14N DNA were separately centrifuged. When allowed to replicate, hybrid dNA was created that weighed a mass intermediate to the 15N and 14N DNA.
Why are cancer cells immortal but most body cells have a limited life span?
Cancer cells often have telomerase, body cells do not. This protects cancer cells from their telomeres shortening and limiting their number of possible divisions. They can divide infinitely
What are the two chemical components of chromosomes?
DNA and protein
What is the basis for the difference in how the leading and lagging strands of DNA molecules are synthesized?
DNA polymerase can join new nucleotides only to the 3' end of the growing strand.
Why are RNA primers necessary for replication?
DNA polymerases cannot initiate replication, they can only add nucleotides to the 3' end
What is the semiconservative model of replication?
During DNA replication, the strand split and each side is translated. This creates 2 new strands half old DNA and half new DNA
What is the role of ligase in DNA proofreading and repair?
It fills the gaps that nuclease leaves when DNA is excised
Explain Oswald Avery's experiment
He experimented with RNA, DNA, and protein. One was inactivated in each sample and injected into live bacteria. Only those with active DNA were transformed
What are Chargaff's rules? How did he arrive at them?
His rule states that Adenine only pairs with Thymine and that Guanine only pairs with Cytosine. He discovered that bases may vary between species but somehow the number of adenines was always equal to the number of thymines, same with cytosine and guanine.
What does DNA polymerase do ?
It adds DNA nucleotides to the new strand of DNA
How does a bacteriophage destroy a bacterial cell?
It attaches to the outside of the cell and injects its own DNA, which results in transforming the infected cell
Why is a thymine dimer? How might it occur? How is it repaired?
It is covalent linking between adjacent thymine bases. Can be caused by ultraviolet radiated. It is repaired by DNA repair enzymes, possibly in a nucleotide excision repair.
What does helices do?
It untwists and separates the strand of DNA
Who built the first model of DNA and shared the 1962 Nobel Prize for the discovery?
James Watson and Francis Crick
Who performed the experiments that elucidated the correct mechanism of DNA replication?
Matthew Meselson and Franklin Stahl in the late 1950s
Why does the shortening of DNA not affect prokaryotes?
Most of them have circular DNA and can access the 3' end of it at all times
Why did researches originally think that protein was the genetic material?
Proteins had been discovered as macromolecules and were known to have great variety and specificity. Little was known about nucleic acids
Distinguish between the structure of pyrimidines and purines. Explain why adenine only bonds with thymine
Purines are Adenine and Guanine. Pyrimidines are cytosine and thymine. Purines and pyramidines always bond to each other. G-C = 3 h bonds. A-T= 2 bonds
Structurally what is the difference between purines and pyrimidines?
Purines have a double sugar, pyrimidines have a single sugar
What was the role of Rosalind Franklin in the discovery of the double helix?
She first x-rayed DNA, discovered the x shaped form that revealed the double helix
Which direction does the lagging strand get added to in terms of the replication fork?
The lagging strand is being elongated in the direction away from the replication fork.
Distinguish between virulent and nonvirulent strains of streptococcus pneumoniae studied by Frederick Griffith
The virulent strain caused disease and the nonvirulent was harmless. Somehow in his experiment, the killed sample of the virulent strain infected the nonvirulent strand when the killed pathogenic strain was mixed with the harmless cells.
What are okazaki fragments? How are they welded together?
They are segments added to the lagging strand of DNA synthesis. They are welded together with DNA ligase
How did Hershey and Chase "label viral DNA and viral protein so that they could be distinguished?
They designed their experiment to test if DNA or protein was inherited. Radioactive sulfur was used in proteins and radioactive phosphorus was used in DNA
Describe the means by which Hershey and Chase established that only the DNA of a phage enters an E. coli cell. What conclusions did these scientists draw based on these observations?
They had two separate bacteriophages, one had radioactive protein in it, the other radioactive DNA. In the infected cells, only the radioactive DNA showed up. They concluded that DNA was what was used to transform cells.
How did Meselson and Stahl create heavy DNA for their experiments?
They used isotopes of nitrogen, 15N and 14N. The heavier isotope caused differences when centrifuged.
How do telomeres act as a buffer zone?
When the end is reached in replication, there is a small fragment that cannot be replicated since there is no 3' end to be added to. The telomeres are long regions of DNA that do not code for genes, they are repetitions of a short sequence. These areas are where DNA replication must stop each time, each replication resulting in shorter and shorter telomeres.
What does a T2 bacteriophage look like?
a bacteriophage is a virus that infects bacteria
Define transformation
a change in genotype and phenotype due to the assimilation of external DNA
List the three components of a nucleotide
a nitrogenous base, deoxyribose, phosphate group
Explain what is meant by 5' and 3' ends ****
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What is considered the backbone of DNA? the rungs?
backbone is the sugar and phosphate, nitrogenous bases are the rungs
What is the difference between heterochromatin and euchromatin?
heterochromatin is interphase chromatin, it shows up as irregular clumps under a microscope. the chromosomes are very tightly wound. euchromatin is true chromatin, it is less compact and more dispersed
Make a detailed list of the steps that occur in the synthesis of a new strand
https://www.youtube.com/watch?v=TEQMeP9GG6M Helicase splits the strands apart. single strand binding proteins attach to prevent the DNA from rebinding. topoisomerase helps relieve strain by binding the parental strands ahead of the replication fork. Primes synthesizes RNA primers that attach to the DNA. RNA bind complimentary, instead of thymine, uracil is used. DNA polymerase III then begin to add complimentary base pairs after the primer. Each nucleotide added is a nucleoside triphosphate, composed of a sugar a base pair and three phosphate groups. When the two base pairs bind, two phosphates are released. This serves as an energy source for polymerization. This addition of base pairs can only occur on the 3' end of the new strand. A leading strand and a lagging strand form since they run antiparallel. The leading strand is continually added to. The lagging strand must be unwound a certain amount before new base pairs can be attached. because of this, there are multiple segments of RNA primers. The replication is added to in okazaki fragments, not base pair by base pair like in the leading strand. DNA ligase adds these fragments, acting as glue. After replication has occurred, DNA polymerase I comes in and replaces the RNA primers with DNA base pairs.
What does primase do?
it synthesizes the RNA primer
What does DNA ligase do?
joins DNA fragments together
The spontaneous loss of amino groups from adenine in DNA results in hypoxanthine, and uncommon base, opposite thymine. What combination of proteins could repair such damage?
nuclease, DNA polymerase, DNA ligase
What is the direction of synthesis of the new strand?
nucleotides can only be added to the 3' end of the new strand DNA. Meaning it starts at its 5' and adds until it reaches the 3'
E coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations. DNA extracted from these cells is centrifuged, what density distribution of DNA would you expect?
one low density and one intermediate density band
What is the role of nuclease in DNA proofreading and repair?
referred to as nucleotide excision repair, it is a DNA cutting enzyme that removes damaged or mutated segments of DNA
What does DNA polymerase I do?
removes RNA primer and replaces with DNA
In his pneumonia causing bacteria and mice, Griffith found that?
some substance from pathogenic cells was transferred to nonpathogenic cells, making them pathogenic
What do we mean when we say the two strands of DNA are antiparallel?
the end of one strand matches with the beginning the other, seen by the 5' end lining up with the others 3' end
Define the origins of replication
the sequence where replication begins in S phase.
Given that the DNA of a certain fly species consists of 27.3% adenine and 25.5% guanine, use Chargaff's rule to determine the percentages of thymine and cytosine.
thymine= 27.3% cytosine = 25.5%
What is mismatch repair?
when enzymes other than DNA polymerases remove and replace incorrectly paired nucleotides
