Читать книгу Cell Biology - Stephen R. Bolsover - Страница 66

Deoxyribonucleic acid carries the genetic information encoded in the sequence of the four bases – guanine, adenine, thymine, and cytosine. 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 messenger RNA( mRNA). The DNA code is transferred to mRNA by a process known as transcription (Chapter 5). The mRNA code is then translated into a sequence of amino acids during protein synthesis (Chapter 6). This is the central dogma of molecular biology: DNA makes RNA makes protein.

Retroviruses such as the 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.

We do not yet know the exact number of genes that encode messenger RNA in the human genome. The current estimate is 19 116. Table 3.1 compares the number of predicted messenger RNA genes in the genomes of different organisms. In each organism, there are also a small number of genes (about 100 in humans) that code for ribosomal RNAs and transfer RNAs. The roles these three types of RNA play in protein synthesis is described in Chapter 6.