Green Chemistry and the Ten Commandments

Chap. 9. The Biosphere 237

strands are represented by dashed lines. Because of their structures that make hydrogen
bonding possible, adenine on one strand is always hydrogen-bonded to thymine on the
opposite strand and guanine to cytosine. During cell division the two strands of DNA
unwind and each generates a complementary strand for the DNA of each new cell. Figure
9.6 shows a representation of two complementary strands of DNA in which thymine and
adenine from the two strands are hydrogen-bonded together. When guanine is opposite
cytosine, the two bases are bonded by three hydrogen bonds.

Continuation

of  strand  A

Continuation

of  strand  A

Continuation

of  strand  B

Continuation

of  strand  B

Hydrogen    bonds   between

bases   on  adjacent    strands

marked  with    asterices

Adenine base

Thymine base

*

* C C

C C

O

C

N C

N

C C

O

H

H

CH 3

O

H

H

H H

H

C

C C

C C

O H

H

N

N

N

C

N

C N

C

C C H

H O

H

H C H

H

P

O

O

O

O-

H

O-

O

O

O

P

H H

H

H

Figure 9.6. Representation of two nucleotides in two adjacent strands of DNA showing hydrogen bonding
between the bases thymine and adenine. These two bases bonded together by hydrogen bonds constitute
a base pair.

In organisms with eukaryotic cells, DNA is divided into units associated with protein
molecules called chromosomes. The number of these varies with the organism; humans
have 23 pairs of chromosomes, a total of 46. The strands of DNA in chromosomes,
in turn, are divided into sequences of nucleotides, each distinguished by the nitrogen-
containing base in it. These sequences of nucleotides give directions for the synthesis of
a specific kind of protein or polypeptide. (Polypeptide is a general term for polymers of
amino acids; proteins are the relatively long-chain polypeptides.) These specific groups
of nucleotides, each of which has a specific function, are called genes. When a particular
protein is made, DNA produces a nucleic acid segment designated mRNA, which goes out
into the cell and causes the protein to be formed through a process called transcription
and translation (the gene is said to be expressed). Whereas the number of chromosomes
in an organism is now easy to determine, the number of genes is a matter of debate and