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Lecture 3

BIO 1130 Lecture Notes - Lecture 3: Threonine, Lysine, Plasmid

Course Code
BIO 1130
Jon Houseman

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Key questions Unit 3
8. If I have seen further than others, it is by standing up on the shoulders of giants. Isaac
This quote by Isaac Newton addresses the nature of scientific inquiry and suggests that the new
science discoveries are built upon the previous work of other scientists.
a) Describe the Watson and Crick model of DNA. They (with help of others) deduced the double
helix structure, and how the bases bond together.
b) Choose one of the following scientists : Levene, Franklin, or Chargaff, and show how their
work was used to assist Watson and Crick. P.A. Levene - was a biochemist who analyzed DNA
and found that it contained adenine, guanine, thymine, cytosine, deoxyribose, and a phosphate
29. What chemical features of DNA and protein permitted Hershey and Chase to prove that
DNA was the only molecule that carried hereditary information? Protein contains sulfur but
very little phosphorus while DNA contains phosphorus but very little sulfur, so Hershey and
Chase injected bacteriophages into a organism. When they injected the bacteriophages with
sulfur, it did not show the genetic material as a protein. But when they injected the
bacteriophages with phosphorus, they obtained a reaction called radioactivity from the
organism. So our genetic material is considered DNA (deoxyribosenucleicacid). You could
consider the radioactivity portion of the process as a chemical feature involving sulfur and
30. For the extraction of DNA activity that you did in the lesson, describe two methods that you
used to separate the strawberry DNA from the rest of the cell. I mechanically mashed up my
strawberry to break cell walls , used a few drops of dishwashing detergent to dissolve away
membranes, used a few drops of pineapple juice as a protease to dissolve away proteins and
used some ethanol to precipitate the DNA
31. Write the DNA strand that is complementary to the strand shown below. Be sure to mark
ends of your strand appropriately.
5'-TACGGGCCCTTAGCTTTA-’ given DNA strand
3'-ATGCCCGGGAATCGAAAT-5' (complement DNA strand)
32. Is the DNA code the same in every somatic cell in the human body? Explain your answer.
Somatic cells are body cells which are important part of the body's multicellularity and function.
So they should have same DNA code in order to maintain a balance in the functioning of the
body. And except sex cells (gamates) all the cells of human body (i.e. somatic cells) have the
same DNA code. Another reason for this is mitosis. As body cells arise from mitosis, the division
is conservative and doesn't involve any process of inducing variation in the cells, hence all the
cells originating from it have the same DNA code.
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33. In Meselson and Stahl experiment, what pattern of banding would you expect to find in the
centrifuge tubes after two generations in the regular nitrogen medium, if DNA replication were,
in fact, dispersive? Two rounds of dispersive replication would produce four DNA molecules,
each of which would contain mostly light nitrogen and some heavy nitrogen. There would be a
single density
34. In one sentence for each, explain the role of the following enzymes in DNA replication :
a) Helicase : DNA is double stranded. In order to replicate both strands, the strands have to be
unwound from each other first. Helicase unwinds the 2 DNA strands from each other.
b) Gyrase : As helicase unwinds the DNA strands, each strand itself begins to coil, like a curl.
This is called a positive supercoil, which gives a lot of strain to the molecules. To relieve the
strands of this strain, Gyrase changes the positive supercoils into negative supercoils.
c) DNA polymerase : DNA is essentially a chain of nucleotides. DNA polymerase forms a chain of
the nucleotides to form a DNA strand.
d) DNA ligase : When a DNA strand is being replicated, there are nucleotides that may not have
any complements, and so the strand of DNA is not continuous. To fix these discontinuities,
ligase (through a complex mechanism) fills in these breaks within the strand with the proper
nucleotides to yield a continuous strand of DNA..
35. Explain why the activation of the enzyme telomerase in somatic cells can potentially lead to
cancer in those cells while its activation in reproductive cells does not lead to cancer. Cancer
can be defined as having two characteristics: 1) uncontrolled cell growth and 2) invasion of
other tissues (metastasis).
Telomeres get shorter with each cell division until they signal the cell to stop dividing - this gives
a cell a finite lifespan (approximately 50 cell divisions). The enzyme telomerase elongates
telomeres, but it's nearly absent in most of our cells. Reproductive cells are one exception to
this rule. Sperm, for example, are continuously produced anew. You can imagine that if their
telomeres were always getting shorter and never being elongated, young men would rapidly
become infertile as the sperm cells reached the ends of their lifetimes. This would be pretty
catastrophic for us as a species! So, sperm are one of the types of cells in our body that have
higher levels of telomerase.
All cells accumulate mutations (changes in their DNA) over time. In fact, the rate of somatic
mutation is approximately 1 mutation in 1,000,000 cell divisions. The longer a cell is allowed to
continue dividing, the more mutations build up, and the greater chance that some of these
mutations will adversely affect the normal checks and balances (this is one of the reasons why
the overall risk of cancer is greater as you get older). Limiting a cell's reproductive lifespan helps
to minimize this problem; however, telomerase upregulation alone is not sufficient to cause
cancer. Cancer cells harbor mutations that make them disregard other cells telling them it's too
crowded, signals from within saying there are too many internal problems, etc. If telomerase is
off, these cells are most likely to divide a few times and then drop out of the population when
their telomeres get too short. If telomerase gets turned on after a cell has already lost control,
then we have the conditions for uncontrolled growth.
So, reproductive cells are not undergoing uncontrolled cell growth, and therefore are not
cancer. Their growth, while less limited than most somatic cells, is carefully stewarded by
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