Lesson 1 — DNA Structure


Antiparallel DNA Strands
The two strands of nucleotides in the double-helix run antiparallel. This means the strands are parallel but opposite in linear or rotational direction.

The deoxyribose sugar in a DNA molecule has five carbons in its structure. To identify them individually, scientists have numbered each carbon in the sugar in a counterclockwise fashion. The carbon that connects to the phosphate group is the fifth carbon or 5'. The carbon that connects to a hydroxyl group (-OH) is the third carbon or 3'. Careful consideration of the two DNA strands results in noticing that one strand runs from 5' to 3' and the other runs from 3' to 5'.

Because the nucleotides differ in sizes, the two strands must run in opposite alignment to allow hydrogen bonds to hold together the complementary base pairs. Look at the nucleotide molecules on the right. You will notice that cytosine and thymine are smaller than guanine and adenine. Cytosine and thymine are pyrimidines, and guanine and adenine are purines. (The classification of the nitrogenous bases are included in the data booklet provided during your exams.)

Although the two DNA strands are parallel to each other, they run in opposite directions; this is called antiparallel. Read more about the structure of DNA in your textbook on pages 628 and 629.


Try This

Building DNA Gizmo

Use the following Building DNA Gizmo 

activity to build DNA molecules virtually. Follow the steps outlined in the Student Guide to help you complete it. You may stop at the "DNA Replication" section because this is a topic of Lesson 2. Be sure you can answer the questions in the guide as you move through each step.


DNA strands are antiparallel. National Human Genome Research Institute.  Public Domain.

Watch This

Now that you have learned the history of how DNA was discovered, consider in detail the structure of the DNA molecule. Because DNA is a 3D molecule, you might visualize the structure more readily by watching a video. The following video (12 minutes) shows the structure of the DNA molecule. Give attention to the double-helix shape and the arrangement of the deoxyribose sugars, phosphates, and nitrogen bases in the molecule.

The relationship between DNA, chromosomes, and cells is reviewed in the video. As an extra, you can watch students perform a lab in which they extract and isolate DNA from calf thymus cells.





 

 

©Alberta Education. DNA Structure and Replication: Duplicating the Code (0:00-12:12); Series 32 LearnAlberta.ca

 


Self-Check

The following self check will help you review the concepts concerning DNA Structure.

1.    If the double helix structure is unwound, which molecules make up the horizontal “rungs” of the ladder? Which molecules make up the vertical “rails” of the ladder?
2.    Of the nitrogen – based molecules present in the DNA of an animal, 14.5% are cytosine molecules. What percentage of thymine would be present in this animal’s DNA?
3.    How is DNA structurally different than RNA?
4.    Explain how complementary base pairing maintains a constant width in a DNA molecule.
5.    What would the sequence of nucleotides in the complementary strand of DNA be for the following sequence – ATTCGCC?

1.    The bases are found with hydrogen bonds between them on the horizontal rungs of the ladder, and the phosphate and sugar groups alternate up the vertical rails of the ladder.
2.    If 14.5% is cytosine, then 14.5% must be guanine, since they pair in the double – helix. This leaves a total of 71% - so half (35.5%) must be adenine and the other half (35.5%) must be thymine.
3.    RNA contains the base uracil (U) instead of thymine (T), and RNA is usually single stranded. RNA contains the sugar ribose, and DNA has deoxyribose.
4.    The nitrogenous bases are different sizes and shapes. Adenine and guanine have a double – ring structure, while thymine and cytosine have a single – ring structure. The A-T and C-G pairs are called “complementary base pairs” and maintain a constant width of three rings within the DNA molecule. 
5.    TAAGCGG