3.40 Nucleic Acid Classification
A:All biochemical compounds contain carbon, hydrogen, and oxygen; and proteins as well as nucleic acids contain
nitrogen. Phosphorus is the only element that is found in nucleic acids alone.
Nucleotides
Nucleic acids consist of chains of small molecules called nucleotides, which are held together by covalent bonds.
The structure of a nucleotide is shown in theFigure3.77. Each nucleotide consists of:
- a phosphate group, which contains phosphorus and oxygen (PO 4 ).
- a sugar, which is deoxyribose (C 5 H 8 O 4 ) in DNA and ribose (C 5 H 10 O 5 ) in RNA.
- one of four nitrogen-containing bases. (A base is a compound that is not neither acidic nor neutral.) In DNA,
the bases are adenine, thymine, guanine, and cytosine. RNA has the base uracil instead of thymine, but the
other three bases are the same.
FIGURE 3.77
The Double Helix
RNA consists of just one chain of nucleotides. DNA consists of two chains. Nitrogen bases on the two chains
of DNA form hydrogen bonds with each other. Hydrogen bonds are relatively weak bonds that form between a
positively charged hydrogen atom in one molecule and a negatively charged atom in another molecule. Hydrogen
bonds form only between adenine and thymine, and between guanine and cytosine. These bonds hold the two chains
together and give DNA is characteristic double helix, or spiral, shape.
You can see the shape of the DNA molecule in theFigure3.78. Sugars and phosphate groups form the “backbone”
of each chain of DNA. The bonded bases are called base pairs. Determining the structure of DNA was a huge
scientific breakthrough. You can read the interesting story of its discovery and why it was so important at the URL
below.
http://nobelprize.org/educational/medicine/dna_double_helix/readmore.html
Q:Compare the structure of DNA to a spiral staircase. What part of the molecule do the stair steps represent?
A:The steps represent the base pairs.