Deoxyribonucleic acid carries the genetic information encoded in the sequence of the four bases—adenine, guanine, cytosine, and thymine. The information in DNA is transferred to its daughter molecules through replication (the duplication of DNA molecules) and subsequent cell division. DNA directs the synthesis of proteins through the intermediary molecule RNA. The DNA code is transferred to RNA by a process known as transcription (Chapter 6). The RNA code is then translated into a sequence of amino acids during protein synthesis (Chapter 8). This is the central dogma of molecular biology: DNA makes RNA makes protein.
Retroviruses such as human immunodeficiency virus, the cause of AIDS, are an exception to this rule. As their name suggests, they reverse the normal order of data transfer. Inside the virus coat is a molecule of RNA plus an enzyme that can make DNA from an RNA template by the process known as reverse transcription.
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