Preview - Exploring Dna Replication - Eths

Challenge 4: Completely Replicate DNA

EXPLORE THE BIOLOGY:

Nucleotides are made of 3 parts: a sugar attached to a phosphate group and a nitrogen-containing base.  Human DNA is made of MANY nucleotides (about 3.2 billion base pairs) and there are different types of nucleotides because they are made with different types of bases. 

T = THYMINE G = GUANINE C = CYTOSINE A = ADENINE

DNA-strand-length:  30                              Speed:  Normal

Enzyme labels: checked                             Substitutions:  unchecked

Nucleo labels: unchecked                           Free-nucleosides:  50                   Time-limit:  None

Play with the model to answer the following questions: 

Questions

Please answer the questions below.

Question 5:  Can polymerase attach any nucleotide, anywhere on a DNA strand?

(Hint:  Try out different bases at different locations; note what works and what doesn’t.)

Question 6:  Which nucleotides (colors, labels) will pair together?  (Turn NUCLEO-LABELS on)

Question 7:  Why do you think only certain nucleotides will pair up, but others won’t? 

(Hint:  Review your answer to Question 4 in CHALLENGE #3 on the page 4.)

Reflection Question 1:

Before scientists had discovered that nucleotides form the base pairs you observed with the model today, one biochemist, Edwin Chargaff, discovered something unexpected.  He was studying DNA samples from humans, other animals and bacteria.  Chargaff discovered that, in every sample, the percent of cytosine was about the same as the percent of guanine AND the percent of adenine was about the same as the percent of thymine.  This observation became known as Chargaff’s Rule.

Explain how Chargaff’s observation relates to the base pairings you saw today.

The percentages of A & T and C & G were about the same, but not exactly equal.  For example, in a sample of human DNA, Chargaff noted that the percentages of nucleotides were: Adenine = 30.9%    Thymine = 29.4%    Guanine = 19.9%    Cytosine = 19.8%

Why do you think the percentages of A & T and C & G are not EXACTLY the same? 

Reflection Question 2:

In this model, you moved enzymes and nucleotides around to replicate DNA.  But, in real, living cells, no tiny person is there to move enzymes and nucleotides into position to replicate DNA.   

So, how do you think enzymes and nucleotides “know” where to go to create new DNA?  How do enzymes “know” what to do?  How does polymerase “know” which nucleotide is the right one to add to a growing DNA strand? 

Notes

These notes will appear on every page in this lesson so feel free to put anything here you'd like to keep track of.