IB Biology - Curriculum Notes 

2.6 Structure of DNA and RNA 


∑ - The nucleic acids DNA and RNA are polymers of nucleotides.

Nucleotides are the monomers of the polymer DNA.
DNA nucleotides are made up of 3 components; a phosphate group (PO4-3), a pentose sugar, and a nitrogenous base.
The phosphate, sugar and base are linked by covalent bonds
In DNA and RNA each nucleotide is linked to the next nucleotide between the phosphate of one and the pentose sugar of the other nucleotide

∑ - DNA differs from RNA in the number of strands present, the base composition and the type of pentose.

Sugar is deoxyribose (carbon 2 - no oxygen attached)
Sugar is ribose (carbon 2 has an –OH attached)
Nitrogenous bases are guanine, adenine, cytosine and thymine
Nitrogenous bases are guanine, adenine, cytosine and uracil
Double-stranded molecule
Single-stranded molecule

∑ - DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

DNA is double-stranded and shaped like a ladder, with the sides of the ladder made out of repeating phosphate and deoxyribose sugar molecules covalently bonded together. The two strands are antiparallel to each other.
The rungs of the ladder contain two nitrogenous bases (one from each strand) that are bonded together by hydrogen bonds.
The nitrogenous bases match up according to the Chargaff’s Rules in which adenine always bonds to thymine, and guanine always bonds with cytosine. These bonds are hydrogen bonds.
These base pairs, A-T and G-C, are considered to be complimentary.
Guanine and cytosine are held together by 3 hydrogen bonds.
Adenine and thymine are held together by 2 hydrogen bonds.

β - Applications and skills:

β - Application: Crick and Watson’s elucidation of the structure of DNA using model making.

Go to http://www.dnai.org/a/index.html and click on Finding the Structure.

Then click on the menu button players

Watch the interviews for Erwin Chargoff, Rosalind Franklin and Watson and Crick
Write a one page summary on how Watson and Crick used models to discover the structure of DNA 

B - Skill: Drawing simple diagrams of the structure of single nucleotides of DNA and RNA, using circles, pentagons and rectangles to represent phosphates, pentoses and bases.

Draw and label your own simple structure of DNA using circles for phosphates, pentagons for pentose sugar and rectangles for bases.

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