of Gene Expression

Figure 3-5 Flow of information from DNA to RNA to protein for a hypothetical gene with three exons and two introns. Within the exons, purple indicates the coding sequences. Steps include transcription, RNA processing and splicing, RNA transport from the nucleus to the cytoplasm, and translation.


The primary RNA transcript is processed by addition of a chemical “cap” structure to the 5′ end of the RNA and cleavage of the 3′ end at a specific point downstream from the end of the coding information. This cleavage is followed by addition of a polyA tail to the 3′ end of the RNA; the polyA tail appears to increase the stability of the resulting polyadenylated RNA. The location of the polyadenylation point is specified in part by the sequence AAUAAA (or a variant of this), usually found in the 3′ untranslated portion of the RNA transcript. All of these post-transcriptional modifications take place in the nucleus, as does the process of RNA splicing. The fully processed RNA, now called mRNA, is then transported to the cytoplasm, where translation takes place (see Fig. 3-5).

Translation and the Genetic Code

The key to translation is a code that relates specific amino acids to combinations of three adjacent bases along the mRNA. Each set of three bases constitutes a codon, specific for a particular amino acid (Table 3-1). In theory, almost infinite variations are possible in the arrangement of the bases along a polynucleotide chain. At any one position, there are four possibilities (A, T, C, or G); thus, for three bases, there are 43, or 64, possible triplet combinations. These 64 codons constitute the genetic code.


The Genetic Code


Stop, Termination codon.

Codons are shown in terms of mRNA, which are complementary to the corresponding DNA codons.

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Nov 27, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on of Gene Expression

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