Читать книгу Astrobiology - Charles S. Cockell - Страница 109

The question arises: How do we go from the DNA information storage system to making proteins and other molecules needed by the cell for growth and reproduction?

This extraordinary transformation is accomplished in two stages. First, DNA is used to make a similar molecule called ribonucleic acid, RNA, the structure of which was described in the last chapter. This process is called transcription. RNA is similar to DNA except for three features: (i) the thymine base is replaced by uracil (U), (ii) it has a ribose sugar with a hydroxyl group (–OH) in the 2′ position, which is not present in DNA (DNA is deoxy-ribose), (iii) it generally (but not always) is single-stranded. DNA and RNA are similar in that they both contain a base, sugar, and phosphate in each structural unit.

The first step in reading the DNA is to make a copy of the sequences of the bases, or a “complementary” copy of the DNA. This copy is made out of RNA. The DNA molecule unwinds along a small part of its length (about 15 base pairs long) called the transcription bubble (Figure 5.9). At this point on the DNA, an RNA polymerase binds. RNA polymerase is a protein with five subunits and a sigma factor, a small specialized protein that recognizes particular promoters on the DNA. Promoters are sequences of DNA that correspond to the beginning of the sequences of DNA that are to be decoded. In colloquial language, the promoters tell the RNA polymerase: “Start decoding the DNA into an RNA strand here.” The RNA polymerase reads along the DNA strand, generating an RNA molecule, called messenger ribonucleic acid or mRNA. This mRNA molecule is sometimes called the mRNA transcript or primary transcript. The production of the mRNA strand occurs from the 3′ to the 5′ end of the DNA strand.

Figure 5.9 The transcription of DNA into mRNA.

In prokaryotes, this process occurs in the cytoplasm of the cell. In eukaryotes, it occurs in the cell nucleus. In eukaryotic cells, the RNA transcript must be transported out of the cell nucleus into the cytoplasm. The mRNA strand so produced can then act as the template for protein synthesis.