The total length of DNA in one copy of the human genome is about 1 m, and the diameter of the double helix is about 2 nm. Nucleotides in a DNA double helix are stacked (see Figure 5–4B) at an interval of 0.34 nm. If the DNA were enlarged so that its diameter equaled that of an electrical extension cord (5 mm), how long would the extension cord be from one end to the other (assuming that it is completely stretched out)? How close would the bases be to each other? How long would a gene of 1000 nucleotide pairs be?

Figure 5–4B

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(A) in 3' cytosine H H-C C thymine CH3 H-C T N. C hydrogen bond OH-N NH-N G C-N. C=N OH-N H H 1 nm guanine H N-HIIN A sugar- phosphate backbone adenine C N H 3' in 0.34 nm O=P-O 5' end (B) 3' end HO T G hydrogen bond bases G 5' end A G OH 0=4-0- 3' end sugar phosphodiester bonds