This is difficult stuff at first; it takes some study and concentration.
I am here to help you with it. Non-participants may have lots of
difficulty.
Pages 45 - 48 of the workbook should help.
gene fine structure principles
There may be more than a single mutation within a gene (x =
mutation).
We may have two genes and two mutations.The mutations may be
in different genes.
The mutations may be in the same gene.
Two mutations in a diploid may be in the same gene or in different
genes; they may be in either in cis or trans. Here we have
two mutations in two genes in trans.
Is the phenotype of this arrangement wild or mutant? It
is wild. We may call this complementation.
The secret:...In order to have a wild phenotype we must have
a non mutant gene A and a non mutant gene B; as in diagram
above.
gene fine structure principles ....as a table
Experimental work of S. Benzer
Picture of
Benzer (not on web)... Benzer2.gif
Picture of phage... Tatoo.gif
In order to study lots of mutations within genes we must have an
"organism" which reproduces rapidly and takes up little space.
Such an "organism" is the bacteriophage T4 (also see pages 36, 37
in our workbook)
animation of the work of the virus (as per the discussion of Hershey
and Chase).......General infection
kinds of infection, within a short time
one virus, one spot
lots of viruses, all bacteria get at least one virus, all die,
plate clears quickly, no spots
kinds of spots
On bacterial strain B, large spots (mutant) and small spots
(wild) .......BenzerMutantSpot.GIF
Considering bacterial strains B, S and K each mutant may be
of the type rI, rII, or rIII .....BenzerTable.GIF
Important statement: on K wild kills, rII mutant does not kill..........BenzerImportant.GIF
Two Tests for two mutations
Test for complementation (same
or different genes?)
Consider the two possibilities for two mutations in trans......BenzerComplementation4.GIF
Two mutations (1 and 2) on bacterial strain K; mutation 1 only,
mutation 2 only, and mutations 1 and 2... ............BenzerComplementation1.gif
Two possibilities for relationship between mutations 1 and
2; in same gene (do not kill bacteria) or in different genes (kill
bacteria) ........BenzerComplementation2.GIF
Test for recombination (same or different sites?)
For two mutations at different locations in a gene, recombination
may occur on bacteria B giving four kinds of chromosome .....BenzerRecombination1.gif
These four kinds of chromosome put on bacteria K .... ......BenzerRecombination2.GIF
Summary
Important items
1) Start with a plate covered with bacteria
2) two kinds of infections
a) not all bacteria get a virus, so spots appear
b) all bacteria get at least one virus, so plate may clear
quickly
3) size of spot is the phenotype
4) rII mutations do not grow on bacterial strain K
5) For two mutations, are they in the same gene or different genes
6) For two mutations in same gene, they are at the same location
or not
22 November 2004
review Benzer sheet and continue
Benzer style map
Benzer fine structure map, (not on web).......but as text Fig.
6.22
Old view of the gene
The gene is not divisible by:
mutation
recombination
function
That is, gene could have only one function, could not be divided
up by mutation and recombination
New view of the gene
The gene is divisible by function, mutation and recombination.
rII "gene" really has two functions, represented by genes A and
B; each gene has many mutation sites and recombination may occur between
any two mutation sites.
new terms by Benzer
cistron smallest unit of complete function
recon smallest unit of recombination
muton smallest unit of mutation
Book:...Weiner, J. 1999. Time, Love, Memory : A Great Biologist and His
Quest for the Origins of Behavior. Knopf, New York.
And, finally, back to the hemoglobin sheet!
Synthesis: combine DNA, proteins, and Benzer
A Benzer way of looking at these mutations