Eukaryotic Information Flow

Each of the three fractions contain a number of sequences that are sometimes called junk and can represent, for example, viruses that found their way into DNA in the past but were inactivated, leading to the fact that these sequences remain in the genome, but never express themselves. All of this DNA must be highly condensed. The DNA in each chromosome is a single molecule, on the order of several centimeters in length the total DNA in a eukaryotic cell is as much as three meters long. This DNA must be condensed so as to fit into a nucleus that is about 10 5 meters (lOvm) in diameter. The condensed structure of eukaryotic DNA is called chromatin. 

---

The flow of genetic information involves biosyntheses of DNA, RNA, and proteins, known as replication, transcription, and translation, respectively. DNA replication in prokaryotes (Nossal, 1983) and eukaryotes (Campbell, 1985) are very similar, though eukaryotic replication is more complex (DePamphilis, 1996). A... 

Figure 1 (a) The flow and the regulation of genetic information, (b) The processing of a eukaryotic mRNA transcript. AJG, the methionine start codon Ofif, open reading frame UTR, untranslated region. 

This flow of information depends on the genetic code, which defines the relation between the sequence of bases in DNA (or its mRNA transcript) and the sequence of amino acids in a protein. The code is nearly the same in all organisms a sequence of three bases, called a codon, specifies an amino acid. There is another step, between transcription and translation, in the expression of most eukaryotic genes, which are mosaics of nucleic acid sequences called introns and exons. Both are transcribed, but introns are cut out of newly synthesized RNA molecules, leaving mature RNA molecules with continuous exons. The existence of introns and exons has crucial Implications for the evolution of proteins. 

---