Final answer:
In viruses, the step of the central dogma that is not always followed is transcription, due to mechanisms like reverse transcription in retroviruses and the lack of genetic material in prions.
Step-by-step explanation:
The part of the central dogma which is not always followed in viruses is transcription. The central dogma of molecular biology, as proposed by Crick, suggests that genetic information flows from DNA to RNA to proteins. However, certain viruses do not follow this pathway. For example, retroviruses like HIV contain RNA as their genetic material and use an enzyme called reverse transcriptase to convert their RNA into DNA inside a host cell, which is then integrated into the host's genome and can be transcribed into mRNA and translated into proteins. This step deviates from the traditional central dogma, as it involves RNA to DNA transfer, contrary to the usual DNA to RNA process.
Furthermore, in the case of prions, which are infectious agents causing neurodegenerative diseases, these entities do not follow the central dogma as they do not contain DNA or RNA. Instead, prions replicate by inducing misfolding in normal proteins.