Biology Lecture No.13: Gene Structure & Expression
Monday February 27 , 2012
Elysia & The Nature Of DNA:
-Most proteins involved in the Electron Transport System and the Calvin Cycle are coded by genes that are no longer in
the chloroplast. Many however, are still coded by genes in the chloroplast.
-Chloroplast genomes are circular and are expressed in the typically prokaryotic environment inside the organelle.
Chloroplasts also have their own sophisticated system of gene expression complete with its own RNA polymerase.
-It is interesting to note that Elysia has genes that are transcribed and translated from 3 distinct genomes that are found
in the nucleus, mitochondria, and chloroplasts.
-All life is similar in the sense that DNA is found as the information storage centre in all life forms. There is information to
be transcribed and translated in all DNA sequence.
The Structure & Significance Of RNA:
-mRNA does not simply appear in a linear manner (with a ribbon shape). mRNA can base-pair with itself and thus yield a
secondary or tertiary structure. In this structure there is also valuable information.
-The secondary structure of mRNA can influence its subsequent translation. An example of this is a riboswitch which has
a great deal of secondary structure and intended function for that reason.
-tRNA is also known to base-pair with itself. Moreover, ribosomal RNA frequently base-pairs with itself and is catalytic as
a result of its secondary structure. The catalytic/enzymatic activity actually lives in the RNA portion of the ribosome, and
not in the part consisting of protein.
The Prokaryotic/Mitochondrial/Chloroplastic Gene Structure:
-It is essential to understand that genes do not necessarily always code for proteins like DNA polymerase as one would
think. For example, some genes actually code for RNA as a final product.
-The promoter is a sequence of DNA that attracts the attention of RNA polymerase. Such polymerases that transcribe
DNA always read the sequence from the 3’ to 5’ end. All polymers that are synthesized are done so from the 5’ to 3’ end.
-If a polymerase reads the bottom strand of one gene that does not necessarily mean that another gene will have the
same strand read by the polymerase. Each gene has a defined stand to be transcribed (either the top or bottom) and it is
independent of the template strand in other genes.
-This difference in gene transcription has to do with the location of its respective promoter. Unless the location of the
promoter is verified (either top or bottom strand), it is very uncertain as to which strand will be read in that gene.
Elements Of Transcription V.s. Translation:
-Start codons have nothing to do with the transcription of genes, but rather they are the first codons to be translated.
-Stop codons also have nothing to do with transcription, but are rather the codons that end translation and re