Unit 5: DNA Replication Flashcards
State the
Base Pairing Rule
In DNA, the nitrogenous bases always pair:
A with T
C with G
Explain why the bases pair the way they do
- Purines and pyrimidines always bind together to maintain the proper diameter of the DNA molecule. Guanine and adenine are purines; thymine and cytosine are pyrimidines
- The specific purine and pyrimidine binding is based on hydrogen bonding. Adenine and thymine each have two sites for hydrogen bonding; Guanine and cytosine each have three sites for hydrogen bonding
Define
Purine
Nitrogenous bases with two rings
(Guanine and adenine)
Define
Pyrimidine
Nitrogenous bases with one ring
(Cytosine and thymine)
Describe
Meselson & Stahl’s Experiment
This is the one that showed DNA replication is semi-conservative
Two isotopes of nitrogen used: N-15 (“heavy”) and N-14 (“light”)
Original DNA made with N-15 was replicated in a solution containing nucleotides made with N-14
After one round of replication, the daughter DNA molecules were intermediate between N-14 and N-15 weights.
*This indicated that each molecule was composed of one new strand (of N-14) and one original strand (of N-15)
After two rounds of replication, half of the daughter DNA molecules were intermediate and the other half were light.
*This indicated that each molecule contained one strand from the previous generation (N-14 or N-15) and one new strand (N-14)
List and briefly describe
Possible replication mechanisms
Include correct and incorrect
Conservative: New molecules of DNA contain two completely new or two completely original strands
Semi-conservative: New molecules of DNA contain one new and one original strand
Dispersive: New molecules of DNA contain two strands, each a mix of original and new parts
Generally describe the steps of DNA replication
Two strands of DNA are separated
Each strand acts as a template for building a new strand
Compare and contrast prokaryotic and eukaryotic replication
Prokaryotes have one origin of replication and replication proceeds in both directions outward until the whole (circular) chromosome is done
Eukaryotes have multiple origins of replication and proceeds until each replication area (replication bubble) fuses together
Same general mechanism / enzymes involved
Where does DNA replication begin?
Origin of replication
Describe the
Area of DNA that is being replicated
Origin of replication: the point where replication starts
The area where the two strands are separated is called the replication bubble
Each bubble has two sides that are actively being replicated; these are replication forks
List
Enzymes needed in DNA replication
DNA ligase
DNA polymerase
Helicase
Primase
Topoisomerase
Describe the role of
Helicase
In DNA replication, separates two strands of DNA by breaking hydrogen bonds between complementary nitrogenous bases
“Unzips your genes”
Describe the role of
Topoisomerase
Helicase untwists the two strands of DNA, causing additional stress ahead of the replication fork
This enzyme cuts the DNA and allows it to untwist to relieve some of the stress caused from the helicase, then re-joins the cut regions
Describe the role of
Primase
Adds short segments of complementary RNA to single strands of DNA
This allows DNA polymerase to bind
Describe the role of
DNA polymerase
Adds DNA nucleotides onto the 3’ end of a growing DNA strand, using the existing strand of DNA as a template (note: Do not confuse this with “template strand,” which is important in transcription)
What is required for DNA polymerase to do its thing?
Primer
A strand of DNA to use as a template
Free nucleotides
Describe the role of
DNA ligase
Makes a covalent bond between Okazaki fragments
Why does DNA replication result in a lagging strand?
- Two strands of DNA are antiparallel
- This means helicase moves along one strand from 5->3 and moves along the other strand from 3->5
- DNA replication builds from 5’ -> 3’, meaning that it “reads” the DNA strand from 3’ -> 5’
- On one strand, the DNA polymerase follows behind the helicase like two train cars - this is the leading strand
- On the other strand, the DNA polymerase is moving in the opposite direction of the helicase, making short fragments of DNA rather than one continuous strand
Which strands are original? Which ones represent the “new” DNA?
(see image)
Which is the leading strand (top or bottom) and why?
Top is leading, as new DNA is being made continuously
Label the ends of DNA with 5’ and 3’
(see image)
