GRE Subject Test: Biochemistry, Cell, and Molecular Biology › Help with DNA Replication Processes
Which enzyme is responsible for removing the RNA primers added during DNA replication?
DNA polymerase I
DNA polymerase III
DNA ligase
Primase
In order for DNA polymerase III to lengthen the new DNA strands, RNA primers must be put in place as a template. Once the strands are done being made, the RNA primers are removed and replaced with DNA nucleotides by DNA polymerase I.
Magnesium is a necessary component of DNA polymerization, but is not found in the DNA molecule. What is the purpose of magnesium in the polymerization of DNA?
It binds to the two leaving groups during the DNA polymerization reaction
It binds nucleotides in the cytosol and helps import them into the nucleus
It is a cofactor for DNA polymerase III
None of these answers are correct
In order for a nucleotide to be added to a growing DNA molecule, two reactions must occur involving magnesium. First the 3'-OH group on the end of the growing DNA molecule is bound by magnesium and removed from the deoxyribose sugar so that the phosphate from the new nucleotide can bind in its place. Second, nucleotides exist in the nucleus as dNTPs (deoxyribose nucleoside triphosphates). This means that there are three phophates attached to the deoxyribose sugar on the free nucleotide. Only one phosphate (the primary phosphate) binds to the growing DNA molecule. Magnesium binds the other two phosphates and removes them from the dNTP so that the reaction can continue.
Why do microsatellites, such as the one that causes Huntington's disease, occur in genomes?
DNA polymerase "stutters" on repeated bases during DNA replication and adds or deletes bases
Microsatellites are caused by insertion of DNA material by viruses
Microsatellites are a result of very slow evolutionary processes
The body is better able to interpret microsatellite sequences into proteins, so they are more common in the genome
None of these
Microsatellites are generally composed of many repeated bases over and over again, which result from mistakes by DNA polymerase. Polymerase tends to create more errors in cases where many bases are repeated, often creating neutral mutations that evolve extremely quickly.
Which of the following enzymes is not involved in the process of replicating DNA in a replication bubble?
Methyltransferase
Primase
Ligase
Polymerase
Topoisomerase
The correct answer is methyltransferase, which is involved in the methylation of DNA post transcription. The other 4 enzymes are directly involved in the DNA replication bubble. Topoisomerase unwinds the replication fork, polymerase elongates new DNA strands, primase creates primers on the discontinuous 5' to 3' side of the replication bubble, and ligase joins the Okazaki fragments on the discontinous 5' to 3' side.
What is the main reason for there being both a leading and a lagging strand during DNA replication?
DNA polymerase can only read in the 3'-to-5' direction
DNA polymerase can only synthesize one strand at a time
There are not enough RNA primers to have both strands be synthesized simultaneously
Only one strand is available to be read at any given time
When the DNA helix is opened by DNA helicase, both strands are available to be read by DNA polymerase. However, since DNA polymerase can only read from 3'-to-5', one strand must be synthesized in segments (called Okazaki fragments), rather than one continuous strand. The leading strand is read in the 3'-to-5' direction away from the replication fork, while the lagging strand is read in the 3'-to-5' direction toward the replication fork. This results in a leading and a lagging strand due to the antiparallel structure of DNA.
Which of the following is true with regards to the leading strand during DNA replication?
It only requires one DNA polymerase III in order to complete the strand.
It can read the template strand in the 5' to 3' direction.
It requires the joining of Okazaki fragments.
It does not require an RNA primer.
In DNA replication, the synthesis of new strands can be accomplished in both directions. In each direction, you will have both a leading strand and a lagging strand. While both new strands require an RNA primer in order to get started, the leading strand can be continuously synthesized because the template strand is exposed in a 3' to 5' direction. As a result, only one DNA polymerase III is required for this strand.
Keep in mind that while the new strand is synthesized in a 5' to 3' direction, the template strand is read in a 3' to 5' direction. This allows for the new strand to be complementary to the template.
What 3' functional group must be free in order to add the next nucleotide during DNA synthesis?
Hydroxyl
Carboxylic acid
Phosphate
Amine
DNA synthesis requires a free 3' hydroxyl (-OH) group to add the next nucleotide base. Drugs that block DNA replication often have a modified 3' hydroxyl group, which prevents the addition of the next nucleotide and results in chain termination.