MUTATIONS
Change in the amount,content or organization of genetic material is called a mutation.  

TYPES OF MUTATIONS:

a)  POINT MUTATIONS (SINGLE BASE SUBSTITUTION): 

It is a type of small scale GENE MUTATION that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA.

They maybe:

• Transitions: replacement of a purine base with another purine or replacement of a pyrimidine with another pyrimidine.
• Transversions: replacement of a purine with a pyrimidine or vice versa.

Functional categorization:
1- Missense mutation: which results in a protein in which one amino acid is substituted for another due to base substitution.

2- Nonsense mutation: in which a stop codon replaces an amino acid codon, leading to premature termination of translation.

3- Frameshift mutation: which causes a change in the reading frame due to indels (insertions or deletions) of a number of nucleotides that is not evenly divisible by three from a DNA sequence, leading to introduction of unrelated amino acids into the protein, generally followed by a stop codon.

b)  CHROMOSOMAL MUTATIONS:
Chromosomal abnormalities involve alterations in large segments of DNA.

• Inversions: occurs when a break is rejoined to the correct chromosome but in an incorrect orientation.
• Interstitial deletions: an intra-chromosomal deletion that removes a segment of DNA from a single chromosome, thereby apposing previously distant genes
• Translocations: interchange of genetic parts from nonhomologous chromosomes.
• Insertions: when a segment from one chromosome is inserted into another chromosome.

Functional Categorization:

• Loss-of-function mutations are the result of gene product having less or no function. 
• Gain-of-function mutations change the gene product such that it gains a new and abnormal function
• Dominant negative mutations have an altered gene product that acts antagonistically to the wild-type allele. 

(IF A GENE REQUIRES ONLY ONE PERFECT ALLELE TO WORK AND THAT GENE IS HETEROZYGOUS OR PART MUTANT BUT THE DEFECTIVE PRODUCT OF ONE ALLELE ANTAGONIZES THE PRODUCT OF THE NORMAL THEN ITS CALLED NEGATIVE........)

• Lethal mutations are mutations that lead to the death of the organisms which carry the mutations.
• A back mutation or reversion is a point mutation that restores the original sequence and hence the original phenotype.

• Recessive mutations lead to a loss of function, which is masked if a normal copy of the gene is present. For the mutant phenotype to occur, both alleles must carry the mutation.
• Dominant mutations lead to a mutant phenotype in the presence of a normal copy of the gene. The phenotypes associated with dominant mutations may represent either a loss or a gain of function.)

The one gene-one enzyme hypothesis
(one gene one polypeptide synthesis is a better name since genes do not always code for enzymes but also structural proteins such as keratin of hair and collagen in our skin)
It is the idea that genes act through the production of enzymes, with each gene responsible for producing a single enzyme that in turn affects a single step in a metabolic pathway.

BEEDLE AND TATUM chose the fungus NEUROSPORA for their experiments.
Neurospora crassa had several advantages:

1.  it required a simple growth medium,
2.  it grew quickly,
3.  because of the production of ascospores during reproduction it was easy to isolate genetic mutants for analysis. 

EXPOSURE AND IDENTIFICATION OF MUTANT STRAINS:
They produced mutations by exposing the fungus to X-rays and then identified strains that had metabolic defects by growing them in minimal medium.

They had created single gene mutations that incapacitated specific enzymes and the mutant strains could be grown by adding the necessary and (formerly non-essential) nutrient to the minimal medium.

ARGININE DEFICIENCY:

In some cases it was the amino acid, Arginine which had to be added in order for the spores to grow.
Arginine is produced by Neurospora in a metabolic pathway controlled by enzymes where the product of one reaction serves as the substrate or precursor for the next.

ACETYLCHOLINE----ORNITHINE----CITRULINE----ARGINOSUCCININE-----ARGININE
                           
The following are the three mutant strains that are blocked at some stage in the above metabolic pathway.

• MUTANT 1:would grow if Ornithine,Citruline or Arginine were added to it showing that the enzyme that produced Ornithine was missing.

• MUTANT 2:would grow if C or A were added showing that the pathway was blocked due to enzyme B that produced Citruline.

• MUTANT 3:would grow if A was added to the minimal medium thus in this case enzyme D was absent and could not contribute to the formation of A.

 CONCLUSION:
They concluded that the irradiation had altered the genes in such a way that one precursor could not convert into another.
Since the addition of one enzyme seemed to be required for the production of Arginine in each mutant Beedle and Tatum argued that the each gene was producing an enzyme.

INTRODUCTION:
It was invented by the German physicist Hans Geiger (co-discoverer of the atom nucleus) and later improved by his student Walther Muller, therefore the name Geiger-Muller counter.

GEIGER MULLER COUNTER:
A Geiger counter is an avalanche detector and is used to measure and detect forms of ionizing radiation (which includes alpha α particles, beta β particles and gamma γ rays)

PRINCIPLE:
It relys on runaway multiplication of electrons.

CONSTRUCTION:
Geiger-Muller tube, is basically a chamber filled with low pressure gas(inert gas or a mix of organic vapor and halogens.)The tube contains two electrodes, the anode and the cathode, which are usually coated with graphite. The anode is represented by a wire in the center of the cylindrical chamber while the cathode forms the lateral area. One end of the cylinder, through which the radiation enters the chamber, is sealed by a mica window.
A potential difference of +1,000 volts relative to the tube is maintained between the electrodes, therefore creating a strong electric field near the wire.

WORKING:

• Ionization in the gas is caused by the entry of photon or a particular radiation coming through the mica window. 
• The positively charged ions are attracted to their appropriate electrode (i.e. cation to cathode, anion to anode) and they gain sufficient energy to eject electrons from the gas atoms as they pass through the gas. This causes more atoms to ionize. 
• Therefore electrons are produced continuously by this process and rapid gas multiplication takes place.
• The effect of "gas multiplication" is that more than one million electrons are collected by the central electrode for every ion produced in the primary absorption process.When this happens an electrical current is established between the two electrodes.

This current can then be easily collected, amplified and measured or counted and played in the form of an acoustic signal made out of clicks each of which should correspond to the detection of a single ion.

Therefore a Geiger counter is able to detect low-energy radiation because even one ionized particle produces a full pulse on the central wire.