8.2 Study Guide Flashcards
Explain the difference between the leading and lagging strands during DNA replication.
The leading strand is added on continuously and is built towards the beginning of the replication fork. The lagging strand is added on discontinuously, as it is built towards the end of the replication fork and, thus, must pause construction and begin again repeatedly.
What are Okazaki fragments, and where are they found during DNA replication? Which protein is responsible for mending and merging them into one continuous strand?
Okazaki fragments are the separated pieces of the lagging strand, which must be built in fragments due to being built into the dead-end of the replication fork. The protein ligase is responsible for merging these fragments together once construction is complete.
Why is DNA replication categorized as a ‘semi-discontinuous’ process? What is the cause of its ‘semi-discontinuousness’?
DNA replication is categorized as a semi-discontinuous process because it is literally semi, or half, continuous and semi-discontinuous (I assume they just had to use one or the other, continuous or discontinuous, for the name). The construction of its leading strand is continuous while the construction of its lagging strand is discontinuous, and this is caused by the orientation of the two DNA strands placed antiparallel to each other.
Which proposed model of DNA replication has been proven to be correct?
Through Meselson and Stahl’s experiments on DNA replication, the proposed ‘Semiconservative Model’ was proven to be correct.
Explain the difference between the functions of DNA Polymerase III and DNA Polymerase I.
While both DNA Polymerases bind new DNA nucleotides to split DNA strands during DNA replication, DNA Polymerase III adds the nucleotides starting at set RNA Primers and building in continuous strands, while DNA Polymerase I removes the small sections of RNA Primers and replaces them with DNA nucleotides after DNA Polymerase III has done its job.
Why is primase required for DNA replication? What does it do?
Primase is required for DNA replication because it binds RNA Primers temporarily to separate DNA strands, which allow new DNA nucleotides to be bonded to the strands beginning at the Primers. Without it, DNA replication could not be carried out.
True or False: Helicase’s function in DNA replication is to unwind the two DNA strands in DNA molecules so that they can be broken apart.
False. Helicase’s function in DNA replication is to break the hydrogen bonds between the already unwound DNA molecules, splitting them into two separate pieces. A different protein is responsible for unwinding DNA.
Place the following steps of DNA replication in chronological order:
1. RNA Primers are placed onto the two strands to allow DNA nucleotides to be added.
2. The DNA Primers are removed and replaced with more DNA nucleotides.
3. Hydrogen bonds between DNA bases are broken, splitting the DNA into two strands.
4. Any remaining gaps or breaks caused by errors in construction are mended and the new DNA strands are completed, leaving two whole DNA molecules, each identical to the first, as products.
5. DNA nucleotides are added, originating at their connections to the DNA Primers.
- Hydrogen bonds between DNA bases are broken, splitting the DNA into two strands.
- RNA Primers are placed onto the two strands to allow DNA nucleotides to be added.
- DNA nucleotides are added, originating at their connections to the DNA Primers.
- The DNA Primers are removed and replaced with more DNA nucleotides.
- Any remaining gaps or breaks caused by errors in construction or discontinuous construction are mended and the new DNA strands are completed, leaving two whole DNA molecules, each identical to the first, as products.
What did Meselson and Stahl’s experiments aim to determine/prove? How did they do so?
Meselson and Stahl’s experiments aimed to determine which of three leading proposed models for DNA replication was accurate or correct. They did so by stimulating DNA replication first in a solution with only a heavy isotope of nitrogen, N15, and then moving the replicated DNA into a solution with only a lighter isotope of nitrogen, N14, and observing and recording the products of replication. This observation allowed them to see how nucleotides with different isotopes of nitrogen would be dispersed and split during replication. Their results showed that the Semiconservative Method of DNA replication, in which DNA builds whole replicated strands onto the two original strands of DNA, and nothing else, to replicate a DNA molecule, was correct.
In which direction (relative to the carbons of deoxyribose) is DNA built in? Which direction does each the leading and the lagging strand go in?
DNA is built in the direction of its 5’ carbon atoms moving towards its 3’ carbon atoms. The leading strands are built towards the helicase proteins at each end of the replication bubble, while the lagging strands are built away from the helicase proteins, and thus must be built in pieces whenever the DNA Polymerase III molecules come into contact with the adjacent strands within the bubbles.
