Theme 1c Flashcards
How was semiconservative DNA discovered
Meselson and Stahl experiment
-track parental and newly synthesized DNA strands over several generations with nitrogen isotopes
-nitrogen isotopes are incorporated into the DNA molecules via nitrogenous bases
What does the banding pattern look like for semiconservative DNA and for conservative or dispersive DNA (original DNA is grey and replicated DNA is red)
-Semiconservative: after 2 rounds of replication there are 4 strands, 2 that are fully red and 2 that are half grey and half red
-Conservative: 3/4 strands are red and 1 is fully grey
-Dispersive: all strands are both red and grey
Where on the DNA molecule are nucleotides added
To the 3’ OH end so synthesis occurs in the 5’ to 3’ direction
Where does the energy for the formation of new phosphodiester bonds come from
Hydrolysis of pyrophosphate
What is DNA polymerase and it’s properties
-synthesizes new DNA strand only in the 5’ to 3’ direction
-travels/reads the template strand in the 5’ to 3’ direction
-has a single active site that can catalyze 4 different reactions (dATP, dCTP, dGTP and dTTP)
What is the order of the 8 DNA replication enzymes and what do they do
1) helicase: unwinds double helix by breaking h-bonds
2) primase: synthesizes RNA primers fro DNA polymerase
3) single stranded binding proteins: stabilizes ssDNA before replication by preventing reannealing so that the strands can serve as a template
4) DNA topoisomerase/gyrase: removes super coils that form ahead of the replication fork
5) DNA polymerase lll: synthesizes DNA by adding nucleotides to the new DNA strand
6) DNA polymerase l: removes RNA primer and fills the gaps with DNA
7) sliding clamp: attaches DNA pol lll to DNA template, replication is more efficient
8) DNA ligase: joins the ends of DNA segments by forming phosphodiester bonds
Continuous vs discontinuous DNA replication
Leading strand is synthesized continuously
Lagging strand is synthesized discontinuously
Describe DNA replication in bacterial chromosomes (3 steps)
1) initiation: unwinding and separation of two template strands at the origin of replication site forming a replication bubble with 2 forks
2) elongation: simultaneous synthesis of the two new DNA strands from the template strands for the template strands from DNA polymerase
3) termination: DNA replication stops when a termination site is reached
Does DNA have more A-T bonds or more C-G bonds and why
A-T bonds because there are less h-bonds to break
What is the polymerase chain reaction (PCR)
DNA replication and amplification in a test tube
What are the 4 steps for PCR
1) start with template DNA and denature at 95C
2) anneal the primer using bacterial polymerase to withstand high temp (50-60C)
3) extend the strand at 72C (now have 2 DNA copies)
4) repeat for 30 cycles (have 2^30 molecules)
What are telomeres for
-solution to end replication problem
- telomeres add a non coding single-stranded DNA added to the 3’ end of chromosomes
-telomeres will be worn away after each replication instead of chromosomes
-when the telomere region is gone, the cell stops dividing
What is the telomere hypothesis
-since many cancers involve mutations that activate the telomere gene to negate limitations of rapid cell division, if we could come up with a vaccine that kills cancer cells that express the telomerase gene it could stop cancer cells from replicating
-telomerase is an enzyme that restores shortened telomere, present in gametes and stem cells (babies, older people have shorter telomeres)
What is the DNA replication error rate
1/10,000,000,000 nucleotides because of DNA repair mechanisms
How does DNA polymerase proofread DNA
DNA pol lll can detect mistakes and can remove and replace the incorrect nucleotides
Reduces error rate to 1/10,000,000
