<P> RNA - dependent DNA polymerases are a specialized class of polymerases that copy the sequence of an RNA strand into DNA . They include reverse transcriptase, which is a viral enzyme involved in the infection of cells by retroviruses, and telomerase, which is required for the replication of telomeres . For example, HIV reverse transcriptase is an enzyme for AIDS virus replication . Telomerase is an unusual polymerase because it contains its own RNA template as part of its structure . It synthesizes telomeres at the ends of chromosomes . Telomeres prevent fusion of the ends of neighboring chromosomes and protect chromosome ends from damage . </P> <P> Transcription is carried out by a DNA - dependent RNA polymerase that copies the sequence of a DNA strand into RNA . To begin transcribing a gene, the RNA polymerase binds to a sequence of DNA called a promoter and separates the DNA strands . It then copies the gene sequence into a messenger RNA transcript until it reaches a region of DNA called the terminator, where it halts and detaches from the DNA . As with human DNA - dependent DNA polymerases, RNA polymerase II, the enzyme that transcribes most of the genes in the human genome, operates as part of a large protein complex with multiple regulatory and accessory subunits . </P> <Table> <Tr> <Td> </Td> </Tr> <Tr> <Td> </Td> </Tr> </Table> <P> A DNA helix usually does not interact with other segments of DNA, and in human cells, the different chromosomes even occupy separate areas in the nucleus called "chromosome territories". This physical separation of different chromosomes is important for the ability of DNA to function as a stable repository for information, as one of the few times chromosomes interact is in chromosomal crossover which occurs during sexual reproduction, when genetic recombination occurs . Chromosomal crossover is when two DNA helices break, swap a section and then rejoin . </P>

What are three types of human cells that contain dna