Lesson 19: DNA Replication Flashcards
What is the central dogma?
A theory stating that genetic information flows only in one direction, from DNA, to RNA, to protein, or RNA directly to protein
What does the structure of DNA suggest?
Structure of DNA suggests a method for replication. Structure determines function
What was Menselson and Stahls experiment?
In 1958, Matthew menselson and Frank stahl devised an experiment to test the various models of DNA replication. Grew bacteria in a culture with a heavy isotope of nitrogen. Moved the bacteria into media with a lighter isotope of nitrogen. DNA made in this media will be less dense than DNA made in the first media. Separate the DNA based on density.
What does the process of DNA replication require?
Many interacting proteins
What are some facts of DNA replication?
DNA replication is amazingly fast (in humans 50 nucleotides/second and in bacteria 1000 nucleotides/second). DNA replication occurs without many errors, one error in 1 to 10 billion nucelotides
What actually does the replication of DNA?
Enzymes and proteins DO replication. DNA is just a template.
Where does DNA replication occur?
In the nucleus
What gets copied?
The entire strand of DNA
What is needed prior to cell division?
One DNA strand becomes two and each original strand acts as a template for base-pairing
What is DNA replications process?
Semiconservative process: each new double helix contains one strand from the original DNA double helix and one new strand
What are the origins of replication?
Replication begins at a specific site with a specific sequence of nucleotides, called an origin of replication. Proteins attach to the origin of replication and separate the two strands of DNA, forming a bubble. Replication proceeds in both directions, making the bubble bigger until the entire chromosome is copied
What is the replication fork in DNA replication?
Each side of the bubble the DNA strands are being unwound and separated
What are the origins of replication for eukaryotes?
Multiple chromosomes that are larger than bacterial chromosomes, linear chromosomes, and multiple sites where replication begins, instead of just one
What is the helicase in the beginning of replication?
Process of replication begins when the helicase (enzyme) recognizes a specific area that starts replication. Helicase “unzips” the DNA double helix making two single strands
What are the single stranded binding proteins (SSBPs)?
Bind to the newly separated DNA strands and prevent them from re-pairing
What does the helicase do?
Protein that untwists the double helix at the replication fork
What is the topoisomerase?
Protein that breaks, swivels, and rejoins the parental DNA ahead of the replication fork, relieving the strain caused by unwinding
How does the starting of a new strand occur?
Once the DNA is unwound at an origin of replication the two parental strands serve as templates for the new strands that will be made. You need primer in order to start this process. New nucleotides are added to a free 3’ end
What is primase?
An enzyme that creates an RNA molecule (primer) that is complementary to the template DNA strand.
The short RNA molecule is 5 to 10 nucleotides long and provides a 3’ end to which new DNA nucleotides are added
What are DNA polymerases?
Enzymes that catalyses the synthesis of new DNA by adding nucleotides to a preexisting chain
How many polymerases do prokaryotes and eukaryotes have?
Prokaryotes
Bacteria have two polymerases that are involved in DNA replication (pol III and polI)
Eukaryotes
Eukaryotes have a family of three polymerases that are primarily used for replication (pol alpha, s and e)
What is DNA polymerase III?
Adds a nucleotide complementary to the template strand. Catalyses a covalent bond between the 3’ carbon of the existing DNA strand and the phosphate on the 5’ carbon of the new nucleotide
What is the only way new nucleotides can be added to a DNA strand through DNA polymerase III?
DNA polymerase III can only add new nucleotides to the 3’ end of the DNA strand. It builds in 5’ to 3’ direction but reads from 3’ to 5’
What is the leading strand?
On one template strand only on RNA primer is needed to initiate a long continuous complementary strand that forms as the replication fork progresses
What is a problem that occurs on the lagging strand but not the leading strand?
DNA polymerase III on the leading strand follows helicase
DNA polymerase III on the lagging strand needs to build in the opposite direction
What is the lagging strand?
To synthesize a new complementary strand on the other template strand, DNA polymerase III must move away from the replication fork in the 5’ to 3’ direction
What are Okazaki fragments?
A series of short DNA segments on the lagging strand
Each Okazaki fragment needs a new RNA primer
100 to 200 nucleotides long in eukaryotes, 1000 to 2000 nucleotides long in bacteria
What happens for each Okazaki fragment?
Primase makes an RNA primer
What does DNA polymerase III do on the lagging strand?
Extends the complementary strand from the primer until it reaches the previously generated primer and Okazaki fragment and then falls off
What happens after DNA polymerase III falls off of the lagging strand?
It is replaced by DNA polymerase I, which removes the RNA primer and replaces it with new DNA nucleotides
What is DNA ligase on the lagging strand?
Catalyses the covalent bond between the two Okazaki fragments
What is a replication machine?
DNA replication proteins work together to form a replication machine
What does the replication machine do?
One complex of proteins at each replication fork works to synthesize new DNA on both the leading and lagging strand. The lagging strand loops back through the replication machine to generate Okazaki fragments. The replication machine is attached to the nuclear matrix. Primase, which is associated with the replication machine, controls the rate of replication
How are mistakes repaired in DNA?
Cells have mechanisms to fix mistakes in DNA
only 1 in 10 billion nucleotides are mismatched at the end of replication (A to C or T to G). The replication machinery makes an error every 100000 nucleotides. DNA polymerases at the replication fork can “back up” and remove a nucleotide that is incorrectly paired (proofreading mechanism)
How is mismatched DNA repaired?
Cells can repair mismatched DNA
Despite the proofreading mechanism of DNA polymerase, sometimes mismatches occur in Neely synthesized DNA
Other enzymes detect the mismatch, cut out the mismatched nucleotides and fill in the gap with the correct nucleotides called mismatch repair
Mismatch repair can also make repairs in DNA that occur through environmental damage in addition to mistakes that occur in replication
