Teacher's Notes

 

NSES

 

The national standard that will be addressed in this lesson includes the understanding of the process of DNA replication as a semi-conservative mechanism.  This will facilitate and enhance the learning of DNA replication and build upon their previous knowledge of DNA structure to construct an ever-growing concept to see the role of DNA in individuality.

 

DESE  

The DESE frameworks that will be addressed in this lesson are those that describe that DNA contains the hereditary information and encodes for the information for all cell function.  DNA synthesis is essential during cell replication in order to ensure that two copies of the genetic material are passed along to each of the new cells.  Coding errors in the amino acid sequence can occur during DNA synthesis causing random mutations. 

 

The Concept:   

This lesson will describe details of and manipulation of a DNA molecule to represent replication. We will examine the rules of base pairing (complementary) through experimental analysis proving semi-conservative replication (exploration activity). We will:

Exploration activities:  

For the first activity, the students will be directed to go to the  Build a DNA Molecule activity and follow the directions to complete this exercise. While working with the molecule via computer, the students will be directed to construct a “live” DNA molecule by hand using a paper model.  We will follow through the exploration exercise described here to construct our DNA molecule.  We will follow steps 1, 2, and 3. (This same model will then be used for the students to demonstration comprehension of replication later in exploration)

The students will then be required to explore the DNA animation of replication found at the following website: http://www.ncc.gmu.edu/dna/repanim.htm.  The students will read through the description of DNA replication, take notes on important concepts, and then view the animation.  Immediately following this brief demonstration, the students will manipulate their own DNA replication with the paper model just constructed to better understand the semi-conservative mechanism of replication.  This part of the exercise is a continuation of the DNA molecule construction.  We will follow steps 4 and 5 to complete this exercise.  

Description of Data: 

The students will explore and collect information regarding fundamental process of DNA replication, which includes the “unzipping” of the helix and the complimentary base pairing to form the new strand of DNA.  Students will draw and label a portion of a DNA strand focused around the replication fork and all parts will be labeled and described.  The term that must be included are: replication fork, Helicase, RNA primase, DNA polymerase, leading strand, lagging strand, Okazaki fragments, DNA ligase.  

 

Once the students complete their own diagram of the replication fork, they will proceed to the following website to ensure they depicted the fork accurately:  http://chem-mgriep2.unl.edu/replic/fork.html

 

Concept Introduction:

 

Within the nucleus of every cell are long strings of DNA, the code that holds all the information needed to make and control every cell within a living organism.

DNA, which stands for deoxyribonucleic acid, resembles a long, spiraling ladder. It consists of just a few kinds of atoms: carbon, hydrogen, oxygen, nitrogen, and phosphorus. Combinations of these atoms form the sugar-phosphate backbone of the DNA -- the sides of the ladder, in other words.

Other combinations of the atoms form the four bases: thymine (T), adenine (A), cytosine (C), and guanine (G). These bases are the rungs of the DNA ladder. (It takes two bases to form a rung -- one for each side of the ladder.)

A sugar molecule, a base, and a phosphate molecule group together to make up a nucleotide. Nucleotides are abundant in the cell's nucleus. Nucleotides are the units which, when linked sugar to phosphate, make up one side of a DNA ladder.

During DNA replication, special enzymes move up along the DNA ladder, unzipping the molecule as it moves along. New nucleotides move in to each side of the unzipped ladder. The bases on these nucleotides are very particular about what they connect to. Cytosine (C) will "pair" to guanine (G), and adenine (A) will "pair" to thymine (T). How the bases are arranged in the DNA is what determines the genetic code.

When the enzyme has passed the end of the DNA, two identical molecules of DNA are left behind. Each contains one side of the original DNA and one side made of "new" nucleotides.  

Application:  

The Meselson and Stahl experiment would be the perfect conclusion of this lesson.  We can use the following website to understand how the proof was discovered that supports that DNA is replicated according to the semi-conservative mechanism:  http://www.dnaftb.org/dnaftb/20/concept/index.html     

History:  

For students to get an understanding of the history of this scientific concept, they will review the timeline from 1955-1959 found at  www.dnai.org

Francis Crick and James Watson - the two solved the structure of DNA. The classic paper was published in Nature in April 1953.   

Matthew Meselson and Franklin Stahl experimentally proved Watson and Crick’s model of semi-conservative replication.  They invented a new technique called density gradient centrifugation, which uses centrifugal force to separate molecules based on their densities. Their "classic" paper was published in 1958 and their experiment has been called "one of the most beautiful experiments in biology."   

Related Websites:

http://www.ncc.gmu.edu/dna/repanim.htm

http://www.dnaftb.org/dnaftb/20/concept/index.html

http://www.dnai.org  

http://chem-mgriep2.unl.edu/replic/fork.html

 

References:

http://www.ncc.gmu.edu/dna/repanim.htm

http://www.riverdeep.net/

Curtis, H. & Barnes, N. S., Biology of Cells, Worth Publishers, Inc., 1989.  

 

 

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