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19 Cards in this Set
- Front
- Back
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How do you replicate the lagging (displaced) strand? |
Because DNA polymerase III moves in a 3'-5' direction only, it must move away from the replication fork in order to synthesize DNA in a 5'-3' direction. Therefore, on the lagging strand, new primase and polymerase are required for each fragment due to the discontinuous nature of replication. This leaves the replication fork assymetric. |
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What are Okazaki fragments? |
The approximately 100 ntd fragments created as the lagging strand is discontinuoisly synthesized. |
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How is the daughter strand of the lagging strand completed? |
RNase H removes the primers, DNA polymerase I fills in the gaps left by the primers with ntds, and DNA ligase seals the backbone of the links together. |
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Which DNA strand remains single stranded longer during replocation? |
The lagging strand. |
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Why is the leading strand left with an overhanging end? |
Because DNA pol. I needs a primer to attach, once the last primer is removed from the lagging strand, it has nothing to attach to in order to complete replication of the strand. |
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How do cells prevent DNA from becoming shorter after each replocation? |
Telomeres and Telomerase. Telomeres all have a 6 base repeat. |
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How does Telomerase work? |
Telomerase contains its own RNA primer. It uses that to bind to the end of the leading strand overhang, which allows polymerase to add repeats to the end of the leading strand. Then, DNA polymerase on the lagging strand is able to synthesize the repeats without losing any genes. While the lagging strand still isn't completely replicated, it is at least as long as it was before replication. |
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In what cells is telomerase activity high? In what cells is it low? |
Germ cells and cancer cells. Somatic cells. |
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What is senescence? |
Since telomerase isn't active in somatic cells, telomeres become shorter over your lifetime. When this happens, it triggers senescence: the loss of a cell's ability to divide and proliferate. It leads to cell death and is a defence against cancer. |
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What are tumorous cells able to do? |
Bypass senescence in part by abnormal production of telomerase, leading to somewhat longer telomeres. They bypass the checkpoint that causes cells to die when their telomeres are too short, and thus proliferate uncontrollably. |
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What are some of the DNA repair mechanisms? |
1. Proofreading function of DNA polymerase to prevent replication errors 2. Excision Repair 3. DNA break repair |
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What is depurination? |
A chemical reaction that cleaves n-glycosidic bonds holding purines (A, G) to the 1' Carbon of the pentose sugar. |
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What is deamination? |
The spontaneous loss of an amino group (NH2) from a cytosine to produce uracil (not normally found in DNA) |
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What kind of damage can ultraviolet radiation cause? |
It promotes the covalent linking of two adjacent bases, forming a dimer, which can mess up the DNAs ability to be replicated. |
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What are the three basic steps to DNA repair? |
1. Damaged DNA recognized (DNA polymerase can recognize distorted hydrogen bonds) and removed (nucleases cleave covalent bonds of damaged nucleotides to remove them, leaving a small gap). 2. Repair DNA polymerase binds to the 3'-hydroxyl end of cut DNA strans and fills gap with correct ntd based on complementary strand. 3. DNA Ligase repairs the nick in the backbone. Remember: step 1 uses a series of different enzymes, each specialized for removing different types of DNA damage. The second two steps are basically always the same. |
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What is mismatch repair? |
A backup system that catches rare mistakes when the proofreading mechanism fails. Makes errors go from approx. 1 in 10⁷ to 1 in 10⁹. It can also recognize which strand has the error. |
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What is nonhomologous end joining? |
A strategy for repairing double stranded breaks when you can't rely on the redundant copy providing a template for the damaged strand.
Quick and dirty. Involves quickly bringing together, trimming. And rejoining the two broken ends. However, this results in a loss of information at the site of repair, which can potentially **** up a gene. |
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What is homologous recombination? |
A mechanism that allows ds breaks to be repaired flawlessly. It uses undamaged, duplicates or homologous chromosomes to guide the repair. Often occurs during meiosis and results in and exchange of genetic material between maternal an paternal homologs. |
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Mutations of what alleles can increase the probability of cancer? |
Alleles of DNA repair genes, which are in charge of maintaining repair systems. This increases frequency of mutagenesis, and therefore cancer. |
