Genes

(Lodish et al., 2000, Section 9.1, 10.1 )

A gene is the entire nucleic acid sequence that is necessary for synthesis of a functional protein or RNA.  This includes the encludes both the coding region and the regulatory domains.

A cistron is the genetic unit that encodes a single polypeptide.

An operon is a transcriptional unit.  In prokaryotes, these are often polycistronic while in eukaryotes they are monocistronic.

Prokaryotic Genes (Fig. 9.1a)

Often polycistronic.
Control regions often short and adjacent to 5' end of operon.
Eukaryotic Genes (Fig. 9.1b)
Monocistronic.
Can have multiple control regions operating adjacent or distant, both upstream and downstream of operon.
Splicing and polyadenylation can generate alternative forms of mRNA. (Fig. 9.2)
Expression of genes is according to the following scheme:
DNA
    |
    |    (transcription, splicing, modification)
   V
RNA
    |
    |    (translation)
   V
Protein
    |
    |    (folding, modification, localization)
   V
Functional Protein
Activity

Since it is energitically expensive to make RNA and proteins, expression of such products only when needed conserves a cell's resources.

The lactose operon is a good example of gene regulation (Fig.  10.2).

The location on the DNA where RNA polymerase binds is called the promoter site.

The locations on the DNA where proteins bind and regulate transcription are called an operator sites.

Proteins binding at operator sites that facilitate transcription are called positive regulators.
Proteins binding at operator sites that prevent transcription are called negative regulators.
Transcription composed of 4 steps:


Quiz

References:
Lodish, H., Berk, A., Zipursky, S.L., Matsudaira, P., Baltimore, D., Darnell, J., 2000, Molecular Cell Biology, 4th Ed., W.H. Freeman and Company, NY, New York.  ISBN 0-7167-3136-3.