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DNA Structure MJ MLIB

Page history last edited by Michael Murphy 13 years, 7 months ago


A.  Learning Objectives

In this lab, students will:

  • Students will analyze ‘alien’ DNA and compare and contrast its structure to DNA found on Earth (analysis).
  • Students will analyze the method of replication in ‘alien’ DNA and contrast it with the process on Earth (analysis).
  • Students will apply their understanding of the structure of DNA to calculate the number of specific molecules found in a given segment of DNA (application).
  • Students will summarize a scientific seminar on the topic of telomeres and human health (knowledge, analysis, and applicaiton).


B. Textbook Correlation: 

Please review Chapter 11:  Nucleic Acid Structure and Replication when preparing for the lab.


C. Structure and Replication of DNA - Alien DNA:   The following was adapted from  Brooker et al. Biology.  2nd Edition, McGraw-Hill (2009).

NASA’s ‘Exobiology Branch’ (http://exobiology.nasa.gov/) supports research to increase knowledge on the origin, evolution, and distribution of life in the universe. An area of current research seeks to understand what life might look like beyond Earth.  It is possible that the biochemistry and biological processes of extraterrestrial life are quite different than those on Earth.  One of the areas of interest to researchers in this field is the genetic information system that extraterrestrial life may have evolved.  Imagine that you are a member of a research team focusing on genetic information systems in samples collected from Mars by a probe that has just returned to Earth. 


Initial analysis reveals the presence of eukaryotic, unicellular life in a sample.  Additional analysis indicates the presence of double-stranded nucleic acids similar, although not identical, to DNA found in eukaryotic organisms on Earth.  The alien nucleic acids consist of two strands of nucleotides bonded together, with the sugar-phosphates on the outside of the structure.  Six nitrogen bases located on the interior of the double-stranded nucleic acid are identified as:  isoAdenine (iA), Uracil (U), isoCytosine (iC), isoThymine (iT), Adenine (A) and betaGuanine (bG). Data also suggests a complementary base-pairing between nitrogen bases of the two strands with the following base paring rules:

  • iA  complementary with U
  • iT  complementary with A
  • bG complementary with iC


The entire exercise can be downloaded using this link.  Please answer the questions and upload your discussion videos in the space below.  Please restate the question and include your answer and your rationale for the questions provided in the .pdf document above.



1. 95 isoAdenine nucleotides

2. You cannot determine because you need to kniow how many isoThymine there are or how many betaGuanine

3. 320 phosphate groups

4. Meselson and Stahl Experiment. In this experiment, the biologists' took nitrogen isotope N15 and placed the cells to reproduce in that environment. This caused the cells to use N15 in the cells nitrogenous bases. Then they took the alien DNA out of the radioactive nitrogenous environment and into a normal nitrogenous environment. After a generation, they looked at the weight of each DNA molecule in the newly produced alien cells. The scientists would leave the cells to reproduce for another generation in order to see whether the cells divided by conservative, semiconservative, or dispersive. After the third generation, they blend the cells and put them in a centrafuge to examine the different weights of the DNA molecules. Here are the conditions for each of the methods. Conservative, there would be one heavy strand with N15 and the other strands will be light strands with N14 in the nitrogenous bases. In semiconservative, we will expect half of the strands to be half heavy (mix of N15 and N14 strands) and half will be light strands. If they were dispersive, the strands willl all be of the same weight, which is completely unique to this process.


Question 1.

Question 2.


1.117 Guanin groups

2. 410 phosphate groups

3. 0 Uracil groups





D.  Telomeres


A telomere is a region at the ends of eukaryotic chromosomes where a specialized form of DNA replication occurs.  The telomere contains a short nucleotide sequence that os repeated several hundred timers in a row.  An enzyme, telomerase, catalyzes the replication of the telomeres.  They add the telomeres to the 3' end of DNA. An RNA primer is added to the 5' end to create a complementary strand. 


If telomeres were never developed by our bodies, a linear chromosome will become progressively shorter after each round of DNA replication.  You might be asking yourself what is so bad about that. Well, when DNA gets shorter, it loses nucleotides that help code for proteins.  This is very bad.  Our body created telomeres to help with the lose of nucleotides after DNA replication.  The lose of nucleotides aids in the aging process. 


"Eukaryotic cells typically have a finite number of mitotic divisions they can undergo before they stop dividing and die. Telomerase activity may be linked to aging (underactive enzymes) and cancer (overactive enzymes). UC San Francisco presents Mini Medical School. In this edition, join Elizabeth Blackburn from UCSF's Department of Biochemistry and Biophysics. Examine what bits of DNA dubbed telomeres tell us about cell division, and how solving this part of the molecular puzzle may help develop new tools to fight cancer and diseases of aging."


Your assignment will be to write a three-page paper (1.5 spaced, calibri font, normal margins) summarizing the contents of the video.  Imagine you are writing this summary for someone who cannot view the video themselves.  Take notes as the lecture is proceeding and pay close attention to some of the research data for your summary. You may copy and paste the contents of this paper into your page or upload it as a .pdf document.


John Tamanas <- Click to download pdf.

Mike Murphy <- Click to download PDF.

Mike Murphy

YouTube plugin error





Comments (4)

Derek Weber said

at 12:57 am on Dec 16, 2010

Nice avatars. That was sweet.

Mike you have to make your .pdf public so I can open.

Michael Murphy said

at 4:18 am on Dec 17, 2010

i believe i fixed it now. It should open in my public folder.

Michael Murphy said

at 6:37 am on Dec 17, 2010

i tried two different ways to link my essay. if it doesn't work i will upload the whole essay. i will not be available this weekend though since i will be in vermont.

Derek Weber said

at 11:30 pm on Dec 20, 2010

Got it, thanks Mike.

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