WEEK 8 (DNA Replication) Flashcards
Except in some viruses, DNA serves as the ____________________ in all living organisms on Earth
genetic material
According to the Watson-Crick model, DNA exists in the form of what?
a right-handed double helix
Describe the DNA structure
The strands of the double helix are antiparallel and are held together by hydrogen bonding between complementary nitrogenous bases
What does the structure of DNA provide the means of?
storing and expressing genetic information
Describe the study that became the first evidence that DNA could serve as the genetic material
Fred Griffith was studying STREPTOCOCCUS PNEUMONIAE (a bacterium that causes pneumonia). The experiment involved an injection into mice of living IIR (avirulent) [not pathogenic] cells combine with heat-killed IIIS (virulent) cells. Griffith showed that heat-killed, infectious bacteria can transform harmless, living bacteria into pathogenic ones.
Later, researchers prepared an extract from the disease-causing S strain of pneumococci and showed that the “transforming principle” that would permanently change the harmless R-strain pneumococci into the pathogenic S strain is DNA
What does the Watson-Crick model predict?
It predicts that genetic continuity between parental and progeny cells is maintained by semiconservative replication of DNA
Describe the Semiconservative replication of DNA
Semiconservative replication uses each strand of the parent double helix as a template, and each newly replicated double helix includes one “old” and one “new” strand of DNA
What does the complementarity of DNA strands allow?
It allows each strand to save as a template for synthesis of the other
What are the three possible modes by which replication could be accomplished?
- Semi-conservative
- Conservative
- Dispersive
In 1958, Matthew Meselson and Franklin Stahl published the results of what?
an experiment providing strong evidence that semiconservative replication is the mode used by bacterial cells to produce new DNA molecules
Describe what happened in the Meselson-Stahl experiment
E-coli DNA was uniformly labelled with 15N nitrogenous bases by being grown in a 15N-labeled medium. This medium was then added to a 14N-labeled medium and the two cells were left to replicate. With the prediction of conservative replication, two distinct bands was expected; however, after the first generation only one intermediate band occurred between 14N and 15N. After two cell divisions, DNA samples showed two density bands- one intermediate and one lighter band corresponding to the 14N position in the gradient. After the third generation, the proportion of the lighter band increased.
This was evidence of semiconservative replication
Describe the Taylor-Woods-Hughes experiment
The Taylor-Woods-Hughes experiment demonstrates the semiconservative mode of replication of DNA in root tips of Vicia faba
1) An unlabelled chromosome proceeds through the cell cycle in the presence of 3H-THYMIDINE. As it enters mitosis, both sister chromatids of the chromosome are labelled by AUTORADIOGRAPHY.
2) After a second round of replication, this time in the absence of 3H-THYMIDINE, only one chromatid of each chromosome is expected to be surrounded by grains. (except where a reciprocal exchange has occurred between sister chromatids)
the expectation was upheld and was evidence of semiconservative replication
What are the two experiments that lead to the general acceptance of the Semiconservative mode of replication
Meselson-Stahl experiment & Taylor-Woods-Hughes experiment
Where does DNA replication begin?
at the origin of replication
Define Replicon
A replicon is the length of DNA that is replicated following one initiation event at a single origin
Why is the rate of fork movement slower in eukaryotes than in prokaryotes?
It may be due to the difficulties in replicating DNA through the more complex chromatin structure of eukaryotic chromosomes
DNA molecules in the process of being replicated contain Y-shaped junctions called ______________________
replication forks
How many replication forks are formed at each replication origin?
2
Describe what happens at the origin of replication
1) DNA molecules in the process of being replicated contain Y-SHAPED JUNCTIONS called REPLICATION FORKS. Two replication forks are formed at each replication origin.
2) At each fork, a replication machine moves along the DNA, opening up the two strands of the double helix and using each strand as a template to make a new daughter strand
3) The two forks move away from the origin in OPPOSITE DIRECTIONS, unzipping the DNA double helix and replicating the DNA as they go
Why is DNA replication in bacterial and eukaryotic chromosomes termed ‘bidirectional’?
Since the two forks move away from the origin in opposite directions, unzipping the DNA double helix and replicating the DNA as they go
What are the 7 key issues that must be resolved during DNA replication?
- unwinding of the helix
- reducing increased coiling generated during unwinding
- synthesis of a primer for initiation
- discontinuous synthesis of the second strand
- removal of the RNA primers
- joining of the gap-filling DNA to the adjacent strand
- proofreading
Why can only one strand of DNA serve as a template for continuous DNA synthesis?
The two strands of a double helix are ANTIPARALLEL to each other since one runs in the 5’ to 3’ direction and the other has the opposite 3’ to 5’ polarity. Since DNA polymerase III synthesises DNA in only the 5’ to 3’ direction, synthesis along an advancing replication fork occurs in one direction on one strand and in the opposite direction on the other. As a result, as the strands unwind and the replication fork progresses down the helix, only one strand can serve as a template for continuous DNA synthesis.
What is the name of the newly synthesised DNA?
Leading strand
What results in the discontinuous DNA synthesis of the lagging strand?
As the fork progresses, many points of initiation are necessary on the opposite DNA template, resulting in discontinuous DNA synthesis of the lagging strand
