on activity, ie. constitutive heterochromatin that is tightly condensed in virtually all cell types and facultative heterochromatin which varies between cell types and/or developmental stages. Other methods of characterizing types of DNA suggest that there are sequences of DNA that can occur in may copies in the genome. These types of sequences can be repeated only once in the genome or they can occur 10’s of thousans of times or more in genomes. Sequences can be categorized into: Unique-sequence DNA, present in one or a few copies per genome. Moderately repetitive DNA, present in a few to 105 copies per genome Highly repetitive DNA, present in 105–107 copies per genome Observations repetitive DNA sequences as described above have been known for decades, and initially it was shown that Prokaryotic DNA was mostly unique-sequence DNA, and Prokaryotes had little or no repetitive sequences. However, Eukaryotes have a mix of unique and repetitive sequence types of DNA. Unique-sequence DNA includes most of the genes that encode proteins, and Euchromatin is rich in unique-sequence DNA. Repetitive-sequence DNA includes the moderately and highly repeated sequences. They may be dispersed throughout the genome or clustered in tandem repeats. Heterochromatin is rich in moderate and highly repetitive DNA. Human DNA contains about 65% unique sequences while unque sequence DNA makes up a much lower percentage of the genome of organisms that have unexpectedly large genomes (C-values) that were discussed earlier in this section. As Watson and Crick were solving the structure of DNA, they realized the general mechanism by which the molecule could be copied and maintain fidelity in copying the DNA molecule. From that beginning, interest in understanding the duplication of the DNA molecules of a cell became a subject of investigation, and led to a number of Nobel Prize awards. However, understanding DNA replication was critical to the development of the technologies needed for molecular genetics and ultimately genomic biology research.
Telomerase, an enzyme containing both protein and RNA, includes an 11-bp RNA sequence used to synthesize the new telomere repeat DNA. Using an RNA template to make DNA, telomerase functions as a reverse transcriptase called TERT (telomerase reverse transcriptase). The 3’-end of the telomerase RNA contains the sequence 3’-CAUC, which binds the overhang on the chromosome, positioning telomerase to complete its synthesis of the GGGTTAG telomere repeat. Additional rounds of telomerase activity lengthen the chromosome by adding telomere repeats. Ends of telomere DNA usually loop back to form a D-loop. After telomerase adds telomere sequences, chromosomal replication proceeds in the usual way. Any shortening of the chromosome ends is compensated for by the addition of the telomere repeats. Telomere length may vary, but organisms and cell have characteristic telomere lengths, resulting from many levels of regulation of telomerase. Mutants affecting telomere length have been identified, and data shortening of telomeres eventually leads to cell death. Loss of telomerase activity results in limited rounds of cell division before the cell death. In cells the genetic information carried in the DNA nucleotide sequence becomes functional information that gives characteristics to cells ultimately specifying traits. This conversion of DNA sequence information into functional information begins with the creation of cellular RNAs from one of the two strands of DNA sequence. This process is called transcription. The mechanism by which these cellular RNAs are transcribed