Expression of oncogenes-oncogenes are cancer-producing genes. they are closely related to normal cellular genes and are often tyrosine kinases, growth factors, or receptors for growth factors. the expression of oncogenes leads to the translation and eventual transcription of the protein product of the oncogene. thus, template-directed rna synthesis but not dna synthesis occurs during the expression of oncogenes. in contrast, template-directed dna synthesis rather than rna synthesis occurs during the repair of thymine dimers, the polymerase chain reaction, the functioning of the replication fork, and the growth of rna tumor viruses. in the final stages of the repair of thymine dimers, once the dimer has been excised, dna polymerase i enters the gap to carry out template-directed synthesis. in functioning of the replication fork, dna polymerase iii holoenzyme carries out synthesis of dna during replication. template-directed dna synthesis is required for the growth of rna tumor viruses (retroviruses). once released into the host cytoplasm, retroviral rna synthesizes both the positive and minus strands of dna, using reverse transcriptase. this unique enzyme catalyzes the initial rna-directed dna synthesis, hydrolysis of rna, and then dna-directed dna synthesis. the newly formed viral dna duplex integrates into the host cell dna prior to transcription. in this form, the retrovirus is inherited by daughter host cells. the polymerase chain reaction is a method of amplifying the amount of dna in a sample or of enriching particular dna sequences in a population of dna molecules. in the polymerase chain reaction, oligonucleotides complementary to the ends of the desired dna sequence are used as primer for multiple rounds of template-directed dna synthesis.
