Page 354 - Applied Probability

Appendix A: Molecular Genetics in Brief
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The processing of premessenger RNA involves several steps. In eukary-
otes (organisms with a well-defined nucleus for housing chromosomes), the
exons of a typical gene are interrupted by noncoding sequences. These in-
tervening sequences (or introns) must be spliced out of the premessenger

RNA. A cap involving a methylated guanine is also added to the 5 end
of the RNA, and a poly(A) tail involving about 200 adenines is added to
the 3 end. These additions assist in stabilizing the RNA and binding it to

the ribosomes. After messenger RNA is transported to the exterior of the
nucleus, it is threaded through a ribosome like a magnetic tape through
the head of a tape player. Transfer RNA molecules bring the appropri-
ate amino acids into place for addition to the growing chain of the protein
encoded by the messenger RNA. Figure A.1 summarizes the flow of infor-
mation from gene to protein.
protein
DNA pre mRNA mRNA
Stored Replicate Edited Active
Information Information Information Product

FIGURE A.1. Information Flow from Gene to Protein

In the eukaryotic cell, transcription is initiated in the nucleus by a large
complex of proteins that binds to the DNA upstream or at the start of a
gene. The central player in the transcription complex is one of three RNA
polymerases. RNA polymerase I transcribes ribosomal RNA genes, RNA
polymerase II transcribes genes encoding messenger RNA, and RNA poly-
merase III transcribes small RNAs such as transfer RNA [6]. The broad role
assumed by RNA polymerase II is critically dependent on a great number
of accessory proteins known as transcription factors. Many transcription
factors are small proteins that are bound constantly to the central poly-
merase, while others float more freely around the nucleus and bind to more
or less specific patterns in DNA. As scouts for the polymerase, transcription
factors collectively accentuate or repress transcription at specific genes, de-
pending on the needs of the cell. The stretches of DNA recognized by these
proteins are known as binding domains, binding motifs, or simply binding
sites. Promoter domains occur near the start of a gene and enhancer do-
mains more distally. Although the minimal set of promoters necessary for
gene transcription is fairly well understood, repeated or shared promoter
elements are suspects in the precise choreography of tissue and time specific
gene expression.