1

mRNA

codon translation |

growing polypeptide chain incoming tRNA

mRNA

growing polypeptide chain incoming tRNA

codon translation |

(Figure 2-17). In bacteria, the same enzyme makes each of the major RNA classes (ribosomal, transfer, and messenger), using appropriate segments of chromosomal DNA as their templates. Direct evidence that DNA lines up the correct ribonucleotide precursors came from seeing how the RNA base composition varied with the addition of DNA molecules of different AT/GC ratios. In every enzymatic synthesis, the RNA AU/GC ratio was roughly similar to the DNA AT/GC ratio [Table 2-2).

FIGURE 2-16 Diagram of a polyribosome. Each rtbosome attaches at a start signal at the 5' end of an mRNA chain and synthesizes a pcfypeptide as it proceeds abng the molecule Several nbosomes may be attached to one inRNA molecule at one time, the entite assembty is calied a polyribosome

growing ribosorne polypeptide

5 "start^

complete polypeptide release ribosorne subunits released

template strand non-template strand site of nucleotide addition to growing R(\JA strand

FIGURE 2-17 Enzymatic synthesis of RNA upon a DNA template, catalyzed by RNA polymerase.

During transcription, only one of the two strands of DNA is used as a template to make RNA. This makes sense, because the messages carried by the two strands, being complementary bul not identical, are expected to code for completely different polypeptides. The synthesis of RNA always proceeds in a fixed direction, beginning al the 5' end and concluding with the 3'-end nucleotide (see Figure 2-17).

By this time, there was firm evidence for the postulated movement of RNA from the DNA-containing nucleus to the ribosome-contaming cytoplasm. By briefly exposing cells to radioactively labeled precursors, then adding a large excess of unlabeled amino acids (a "pulse chase" experiment), mRNA synthesized during a short time window was labeled. These studies showed that mRNA is synthesized in the nucleus. Within an hour, most of this RNA had left the nucieus to be observed in the cytoplasm (Figure 2-18).

Establishing the Genetic Code

Given the existence of 20 amino acids but only four bases, groups of several nucleotides must somehow specify a given amino acid. Groups of two, however, would specify only Ifi (4 X 4) amino acids. So from 1954, the start of serious thinking about what the genetic code might be like, most attention was given to how triplets (groups of three) might work, even though they obviously would provide more permutations (4 s< 4 x 4) than needed if each amino acid was specified by only a single triplet. The assumption of colinearity was then

TA s l E 2-2 Comparison of the Base Composition of Enzymatkalfy Synthesized RNAs with the Base Composition of Their Double-Helical DNA Templates

TA s l E 2-2 Comparison of the Base Composition of Enzymatkalfy Synthesized RNAs with the Base Composition of Their Double-Helical DNA Templates

Source of DNA Template

0 0

Post a comment