DNA and RNA Structure: Nucleotides, Base Pairing, and Differences

Structure of DNA and RNA

Understandings:

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.

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 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.

Applications and skills:

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.