Gene=Enzyme, RNA Transcription,
Beadle and Tatum (1940's): One Gene, One Enzyme
(figures from Freeman, Biological Science)
Genes specify proteins. A defective protein leads to an inherited disorder such as PKU.
Using genetic and biochemical approaches with bread mold (Neurospora crassa), Beadle and Tatum devised the first hypothesis about the functional nature of genes.
The first step for expression of any gene function is transcription of RNA.
A famous investigator of the mechanisms of
and translation of genetic information is Kenyon alumnus
information on his research on gene
in blood cell development:Lodish
Homeostatic regulation of Epo synthesis and secretion.
A drop in the pressure of oxygen in the blood, such as is caused by ascension to a high altitude, or a major bleeding episode, or anemia, causes certain cells in the kidney to transcribe and translate erythropoietin (Epo) and release it into the blood. Epo activates transcription of other genes in blood cell precursors to develop into red cells, thus increasing the oxygen carrying capacity of the blood. When the oxygen pressure in the blood returns to normal, Epo production returns to its low basal levels.Mechanism of Transcription
Ribonucleic acid (RNA) is like DNA except:
from T.M. Terry, U Connecticut
Some RNA molecules themselves are the final functional product of the gene from which they are transcribed. These include:
All classes of RNA molecule need to be
copied, from the DNA gene.
(1) Promoter sequencewhere RNA polymerase (RNA Pol) recognizes where a gene starts.
(2) Sigma factor joins core RNA polylmerase to bind promoter (in bacteria). Eukaryotes use transcription factors instead.
(3) Melting of the DNA helix so that the RNA polymerase can bind to exposed bases.
(1) Sigma factor (or transcription factors, in eukaryote) comes off.
(2) RNA Pol now binds non-specifically, and loosely, to the DNA; it glides through the helix, while it sequentially adds rNTPs. Removal of pyrophosphate (PPi) provides energy.
(3) The rNTPs have to be complementary to the template strand of DNA. The coding strand of DNA (same sequence as RNA transcript) does not actually participate in the reaction.
The details of regulation of
transcription--how the molecules "know" where a gene begins and ends,
and when and where to express it--are very complex and
interesting. We will get into this more after Spring Break.
In prokaryotes and eukaryotes:
Note: In high school you probably learned that translation by ribosomes does "not" happen in the nucleus. Recent research, however, shows that 15% of translation occurs by ribosomes in the nucleus! The other 85% occurs in the cytoplasm, after the mRNA leaves the nucleus.
The Genetic Code
How do 64 different codons produce 20 different amino acids?
Evolution of the Code
The code evolved at random, in that there is no direct chemical connection between, say, GGG and Glycine. BUT--the code appears to have evolved along certain lines for logical reasons. The two most "fundamental" amino acids are Gly and Ala, in biochemical pathways and in natural occurence in prebiotic systems. Both are specified by G/C pairing at the first two positions--the strongest possible interaction. Early life, under high-heat conditions, would have needed extra strong codon-anticodon pairing. The first code may even have been a two-base code. For more evidence and speculation, http://www.evolvingcode.net/