Exam 3- Chapter 13, 14, 15 Flashcards
Describe Griffith’s experiment
-Experimented with streptococcus pneumoniae
-Used virulent S strain and nonvirulent R strain
-Mice injected w/ live S cells died
-Mice injected w live R cells lived
-Mice injected with heat-killed S cells lived
-Mice injected with killed S cells and live R cells lived
Conclusion: some molecule released from S cells transformed the R cells into their virulent form– “tranfroming principle” present
Avery’s Experiment
-Identified the chemical nature of the transforming principle
-Degraded either protein, DNA, and RNA in heat killed S cells
- If proteins or RNA were destroyed, the S still transformed R cells
Conclusion: DNA is the transforming principle
Hershey-Chase experiment
-Established definitively that DNA is the hereditary molecule using E.coli and the bacteriophage T2
-Radioactively labeled DNA with 32P and proteins with 35S
-Found that labeled DNA entered the cell and was present in progeny phages
-There was also little radioactivity inside the E.coli cell for radioactive protein
Structure of a nucleotide
-5 carbon sugar (deoxyribose)
-phosphate group attached to 5’ side and 3’ side to link nucleotides together
-nucleotide (A,C,G, or T) attached to 1’ side
Purines
Adenine and Guanine
Pyrimidines
Thymine and Cytosine (also Uracil)
How many bonds do C and G form?
3
How many bonds do A and T form?
2
How many rings in A and G?
Two
How many rings in C and T?
One
What is Chargaff’s rule?
- Amount of purines=amount of pyrimidines (adenine=thymine, guanine=cytosine)
- Ratio of guanine+cytosine:adenine+thymine is species specific
What bond links adjacent nucleotides together?
Phosphodiester bond
Where is the hydroxyl end of a sugar?
The 3’ end
What did Wilkins and Franklin discover?
They used X-ray diffraction to find the double helical structure of DNA
DNA diameter
2 nm
Length between base pairs
.34 nm
Number of base pairs in one twist
10
Length of one helical twist
3.4 nm
Double-helix model
- DNA is double stranded and right-handed, forming a double helix 2 nm in diameter
- Sugar-phosphate backbones are on the outside of the helix, with base pairs on the inside
- Two strands are antiparallel
- Base pairs connect the sugar-phosphate backbones
- Base pairs lie flat and perpendicular to the long axis of the DNA molecule
- DNA has major and minor grooves
What model of replication did Watson and Crick propose for DNA replication?
Semiconservative replication
Semiconservative replication
Two parental strands of DNA unwind, and each is a template for a new strand. Each new DNA molecule has 1 parental and 1 new strand
Conservative replication
Parental strands of DNA unwind, each is a template, and parental strands pair up again. No new DNA is mixed with the old DNA
Dispersive replication
Double helix splits into segments and intersperse with old and new
Which end does DNA polymerase add on to?
3’ end only (synthesizes new strands in 5’ to 3’ direction)
Which direction does DNA polymerase read the template in?
3’ to 5’
Sliding clamp
A protein that encircles DNA and binds to the rear of DNA polymerase, anchoring the DNA polymerase to the template
What would happen without the sliding clamp?
DNA polymerase would detach only after a few polymerizations
Topoisomerase
Cuts DNA ahead of replication fork to prevent twisting
RNA primers
Synthesized by primase, gives DNA polymerase a free 3’ end to add onto
Okazaki fragments
Short lengths produced by discontinuous replication in opposite direction to unwinding, gradually as the DNA unwinds since DNA polymerase can only synthesize in the 5’ to 3’ direction (occurs on the lagging strand)
DNA polymerase III
Main polymerase- extends primer by adding DNA nucleotides
DNA polymerase I
Removes RNA primer at 5’ end of previous newly synthesized okazaki fragment, replacing it with DNA nucleotides
DNA helicase
unwinds the DNA
T or F: the lagging strand uses more than one primer
T
When does new DNA synthesis stop on the lagging strand?
When the polymerase reaches the 5’ end of the previously synthesized okazaki fragment
T or F: DNA polymerase has 5’ to 3’ exonuclease activity and 5’ to 3’ polymerizing activity
T
DNA ligase
Seals space between adjacent fragments
DNA polymerase 1
Acts on lagging strand to remove primers and replace them with DNA- has 5’ to 3’ exonuclease activity
DNA polymerase II
DNA repair
DNA polymerase III
Main replication enzyme
All three DNA polymerases have what activity
3’ to 5’ exonuclease activity (proofreading)
Why does the lagging strand loop around
So that primase can synthesize the primer for the next fragment- this loop becomes smaller as replication proceeds
Replisome components
Primosome- primase, helicase, SSB, topoisomerase
2 DNA POL III
How many replication forks happen in a bacterial chromosome?
Two, proceeding bidirectionally away from eachother
How many oris present in eukaryotes?
multiple
Why is the RNA primer a problem for linear chromosome replication?
New DNA synthesis on the 3’ to 5’ template starts with a primer and is removed, but this leaves a region at the 5’ end after it leaves. DNA polymerase cannot add onto this region . THis creates a shortened chromosome
Telomeres
Repeated DNA sequences (5’ TTAGG 3’ on the template strand. It binds to the single stranded 3’ end of the chromosome via complementary base pariing and synthesizes new telomere DNA using telomerase RNA as the template. 3’ end hangs over 5’ end
What enzyme makes telomeres?
Telomerase (stops shortening of chromosome ends by adding on telomere repeats)
Do telomeres shorten with age?
Yes
How to tell cancer cells apart from others?
Telomerase is upregulated so chromosome length is preserved, allowing for indefinite division
Nucleosomes in DNA replication
Nucleosomes first disassemble as replication fork passes, then reassembles into nucleosomes (mix of new and parental histones)
Types of repair mechanisms
- Proofreading- corrects errors made by dna polymerase during replication
- mismatch repair- occurs after proofreading
excision repair- corrects dna damage such as those caused by chemicals and radiation
What is the most common error for DNA polymerase?
base-pair mismatches
what happens if an error occurs in proofreading?
DNA polymerase reverses using 3’ to 5’ exonuclease activity to remove incorrect nucleotide, adds correct one, then resumes
Proofreadinf leaves about ___ in 107 erorrs, and mismatch repair corrects about ____%
1, 99
How does a postreplication error become corrected after proofreading?
mispaired base detected, DNA is cut on either side of the mismatch and removed, and DNA pol 1 fills in the new gab with DNA, and sealed with DNA ligase
Nonbulky damage
No DNA bulging
Base excision repair
removes erroneous base and replaces it with the correct one, is the most important fixing mechanism after proofreading
Bulky distorsions
UV light causes adjacent thymines to form dimers, which bulge out and affect DNA synthesis. Nucleotide excision repairs remove the segment and replace it with new DNA (XP) is caused by a defect in excision repair, causing sensitivity to UV light
Mutations
Replication errors and DNA damage that remain unrepaited
George Beadle and Edward Tatum hypothesis
Neurospora uses chemicals in MM to synthesize more complex molecules
Auxotrophs
Mutant strains that require a nutrient supplement in the MM to grow
Srb and Horowitz experiment
3 arginine auxotrophs- found that each of the three arg genes encoded an enzyme that controls a different step in the arginine pathway
wildtype grew on all four
arg-1 grows on mm+arginine but not on mm+ citrulline or ornithine
arg-2 grows on mm+arginine or citrulline but not on ornithine
arg-4 grows on all except mm+nothing
what was the one-gene-one enzyme hypothesis later modified to?
one gene one polypeptide
Transcription
genetic info in DNA transferred to RNA
translation
use of info encoded in RNA to assemble amino acids into a polypeptide