A codon consists of the three contiguous bases in an mRNA molecule that specify the addition of a specific amino acid to the growing amino acid chain as the ribosome moves along the mRNA (23). The process by which the enzyme RNA polymerase makes an RNA molecule complementary to the sequence of the template strand of the DNA is known as transcription. This transcribed message then undergoes a process know as translation in which a ribosome makes a protein by "reading" the
Conjugation in Paramecium
FIGURE 1 Schematic of sexual reproduction in the protozoan Paramecium aurelia (re-drawn from p70 in W. Clark 1996. Sex and the Origin of Death, Oxford). (A) Two genetically different protozoa, each with macronucleus (large oval in individual protozoa) and a micronucleus (small oval in protozoa). (B) The two protozoa fuse in the first step of conjugation, and the macronuclei and micronuclei move to opposite ends of the cell. (C) Each micronucleus divides once by meiosis, and (D) the daughter micronuclei each divide again, to produce four haploid micronuclei. (E) three of the four haploid microcuclei disappear. (F) The remaining micronucleus divides once more, to produce two identical micronuclei, and then (G) the two conjugants exchange one micro-nucleus. (H) the two haploid micronuclei fuse to produce a single diploid micronucleus. (I) The new micronuclei each direct the production of a new macronucleus; the old macronucleus begins to disintegrate. (J) the two protozoa disengage, and the nuclei assume their starting positions in the cell. The exconjugates are now genetically identical to one another, but genetically different from either of the two starting cells. Each will go on to produce genetically identical daughters by simple fission.
sequence of an mRNA codon by codon. The codon restriction theory of aging is based on the hypothesis that the accuracy of translation is impaired with aging (24). Experimental evidence in support of this theory includes studies from Ilan and Patel (25) who reported alterations in mRNA and corresponding synthetases during the development period in a Tenebrionid beetle from the free-living nematode Turbatrx aceti (26) and in the fly Drosophila melanogaster where it was shown that the efficiency of some synthetases of old flies is only 50% as that of the young flies (27). The fetal rat liver contains six isoacceptor for tRNA (tyr) whereas the adult has only three (28).
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