DNA Replication Flashcards
What are the three landmark studies which led to DNA being identified as the molecule of heredity?
- Frederick Griffith’s transformation experiment (1928)
- Avery-MacLeod-McCarty follow-up experiment (1944)
- Hershey-Chase Bacteriophage experiment (1952)
What did Frederick Griffith do in his famous transformation experiment? What is transformation? What was the key finding from his study?
He worked with pathogenic and harmless strains of the pneumococcus bacteria. When he mixed heat-killed pathogenic strains with living cells, some became pathogenic.
Transformation- the process of changing the genotype and phenotype through assimilation of foreign DNA
The study suggested that an unknown foreign molecule was capable of transferring a pathogenicity factor b/w two strains
What contribution did Avery, McCarty, and MacLeod make to our understanding of DNA? In other words, what was the experiment they performed? What are DNAses, RNAses, and proteases?
They repeated Griffith’s experiment, but treated the purified material from inactivated bacteria with DNAses, RNAses, or proteases
DNAses- enymes which destroy dna
RNAses- enzymes which destroy rna
Protases- enzymes which destroy proteins
The experiment found that only DNAses were abel to destroy the transformative substance; thus DNA was responsible for the pathogenicity factor
What are bacteriophages? Why are they a great model to address the question of whether DNA or proteins control heredity? What did Alfred Hershey and Martha Chase do in their study? What key finding came from their study?
Bacteriophages- viruses that infect specific strains of bacteria
The b-phages were tagged with a radioisotope, showing that only DNa, not protein, entered the e. coli bacteria during infection
This experiment, confirmed the finding of the previous two, that DNA was the genetic material of the cell
What are Chargaff’s Rules? What did Chargaff discover and how did he discover it?
- The amount of A, T, C, and G nucleotides varies in the DNA from species to species.
- In each species, the number of A = T and C = G
Chargaff examined nucleotide ratios in different species
What were the key contributions that Rosalind Franklin and Watson and Crick made to our understanding of DNA structure?
Rosalind Franklin’s X-ray crystal of a strand of DNa provided the helical foundation for the structural model proposed by Watson and Crick
What are the three main components of a nucleotide? What specifically is different b/w DNA and RNA nucleotides? What is attached at the 1’, 2’, 3’, and 5’ positions of a nucleotide?
- Pentose sugar group- contains five carbons number 1’ - 5’ from the nitrogenous base attachment point
- Phospahte group- forms bonds b/w nucleotides
- Nitrogenous bases- basis for the genetic code and links strands together, 5 types
DNA’s 2’ does not have a hydroxyl [OH] group; RNA’s 2’ does
1’ - nitrogenous base attachment point
2’ - OH group in RNA; H in DNA
3’ - OH group
5’ - Phosphate group attachment point
Describe how DNA directionality is determined and how the directionality of each strand of the double helix compares?
The strands of the double helix are antiparallel to one another; one strand runs 3’ - 5’ and the other runs 5’ - 3’
What is meant by the term “complementary sequence”? What nitrogenous bases pair together and which are considered purines vs. pyrimidines? Where are hydrogen bonds located and how does hydrogen bond formation relate to the term “annealing”?
The two strands of the double helix are complementary due to their respective nucleotide sequences.
A bonds with T, they share two H-bonds; C bonds with G, they share three H-bonds
A and G are purines; T and C and pyrimidines- purines and pyrimidines pair up to ensure equivalent distances b/w strands for each “step”
Annealing = binding
What is the central dogma of biology? Where do the main events of the central dogma occur in prokaryotes and eukaryotes?
Central dogma: DNA is used as a template to make RNA which is used to make protein; the order cannot reverse
In prokaryotes, replication occurs in the cytoplasm; in eukaryotes, replication takes place in the nucleus
When does DNA replication occur in the cell cycle?
During the S phase of interphase
What are the possible models proposed as a mechanism for DNA replication? Which is accurate?
Conservative- one double helix is entirely old and the other is entirely new
Semiconservative- each double helix has one strand of the old DNA and one strand of new DNA
Dispersive- each DNA molecule is a random mix of old and new DNA
Semiconservative is accurate
What was the Meselson-Stahl experiment? What was concluded?
Meselson and Stahl used DNA label with radioactive nitrogen to prove that the semiconservative model was correct
What in an origin of replication? Why is it usually A/T rich?
The origin of replication is an A/T rich site where DNA replication begins
A/T rich b/c less H-bonds to break
What is a replication bubble and replication fork? What happens at the fork?
Rep. bubble- a “bubble” formed by the separating strands
Rep. fork- the junctions where the bubble meets double stranded DNA; the location where new DNA is synthesized
