26.4. Nucleic Acids http://www.ck12.org
double helix. One aspect of this structure is that each nitrogenous base on one of the DNA strands must be paired up
with another base on the opposite strand (Figure26.25). Each adenine base is always paired with a thymine, while
each cytosine is paired with a guanine. The bases fit together perfectly from one strand to the other and are also held
together by hydrogen bonds. The A-T pairing contains two hydrogen bonds, while the C-G pairing contains three
hydrogen bonds. The ends of each strand are labeled either 3’ or 5’ based on the numbering of the deoxyribose sugar
ring.
FIGURE 26.25
Each strand of a DNA molecule consists of a backbone of phosphate
and deoxyribose, to which is attached one of four bases: adenine (A),
thymine (T), guanine (G), or cytosine (C). Each strand is then paired to a
complimentary strand in such a way that A is paired with T and G is paired
with C. The bases are held in place via hydrogen bonds. The double
helical structure of DNA is shown on the right.The Genetic Code
Each particular organism contains many protein molecules that are specific to that organism. The particular base
sequence of DNA is responsible for the production of all of the different proteins that are present in each and
every living thing that has ever inhabited Earth. How does that work? Cells use the unique sequence of DNA
bases to decide which proteins to synthesize. A gene is a segment of DNA that carries a code for making a specific
polypeptide chain. The cell essentially decodes the DNA in order to make whatever peptides and proteins are needed
by that organism.
The genetic code works as a series of three-letter codes. Each sequence of three letters, called a triplet, corresponds
to one of the twenty common amino acids. The triplets are read by the cell, one after the other, in the process of
protein synthesis. Listed below (Table26.2) are all of the possible triplets and the amino acid that result from each
three-letter code.
TABLE26.2: DNA Triplet Codes for Amino Acids
1st Base: A 1st Base: G 1st Base: T 1st Base: C
AAA Phe GAA Leu TAA Ile CAA Val
AAG Phe GAG Leu TAG Ile CAG Val
AAT Leu GAT Leu TAT Ile CAT Val
AAC Leu GAC Leu TAC Met CAC Val
AGA Ser GGA Pro TGA Thr CGA Ala
AGG Ser GGG Pro TGG Thr CGG Ala
AGT Ser GGT Pro TGT Thr CGT Ala
AGC Ser GGC Pro TGC Thr CGC Ala
ATA Tyr GTA His TTA Asn CTA Asp
ATG Tyr GTG His TTG Asn CTG Asp
ATT End GTT Gln TTT Lys CTT Glu