Cells and Molecules - Brutes or Angels: Human Possibility in the Age of Biotechnology - page 18

strand serves as a template for transcription to produce an mrnA mole-
cule. Transcription follows the base pairing rules so that wherever C oc-
curs in the template DnA, G is placed in the corresponding mrnA (and
vice versa) and similarly for T and A, except that in rnA, the U base is
used instead of T.
Applying the base pairing rules, a strand of template DnA reading
TACTTACGGAAACAG will be transcribed to produce an mrnA mol-
ecule reading AUGAAUGCCUUUGUC. from left to right, this small
mrnA contains five triplet codons (AUG/AAU/GCC/UUU/GUC) that
specify five adjacent amino acids in a protein (fig. 1.7). in a functional
rnA, the last codon is a stop codon that does not specify an amino acid
but simply tells the ribosome to stop adding amino acids to the protein. 10
A protein three hundred amino acids long requires three hundred triplet
codons (nine hundred bases) in its mrnA. 11
now we turn to chromosomes to see what happens to them when cells
reproduce. We are only concerned with eukaryotic chromosomes here be-
cause the application of biotechnology to ourselves and other eukaryotes is
the main concern of this topic.
Chromosomes
near the end of the nineteenth century, decades before the 1953 discovery
of DnA's structure, cytologists used light microscopes to identify eukaryo-
tic chromosomes as dark-staining bodies near the center of dividing cells.
now, from the work of individual researchers and from international en-
deavors such as the human Genome Project (chapter 4) completed in the
early twenty-irst century, we know chromosomes much more intimately.
To appreciate how detailed knowledge about chromosomes contributes
to modern biotechnologies, we must briefly consider the relationship be-
tween four genetic terms: genome, chromosome, gene, and DNA. This series
represents a hierarchy of structural organization for an organism's genetic
material (fig. 1.8). The genome of a plant or animal consists of the DnA
in all of the chromosomes within a single cell of an organism. This DnA
occurs as long, linear (as opposed to circular), double-stranded molecules