appear to be concentrated in random areas along the genome, with vast expanses of noncoding DNA between. Stretches of up to 30,000 C and G bases repeating over and over often occur adjacent to gene-rich areas, forming a barrier between the genes and the "junk DNA." These CpG islands are believed to help regulate gene activity. Chromosome 1 has the most genes (2968), and the Y chromosome has the fewest (231).
             This close-up shows the four bases that make up the genetic code. The bases are thymine (T), adenine (A), guanine (G)
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