1.How does gene work? 2.Gene-protein relationship. 3.Genetic fine structure.

1.How does gene work?

Gene

• A gene is a molecular hereditary unit of all living organisms.

Gene carries all the information to build and maintain the

cell and pass genetic traits to the generations.

One-gene-One-Enzyme Hypothesis

• 1940s, George Beadle and Edward Tatum• Gene act by the production of the enzymes

and each gene is responsible for production of the single enzyme that in turn effect a single step in metabolic pathway.

One-gene-One-Enzyme Hypothesis

• Diagrammatic representation of production of arginine

One-gene-One-Enzyme Hypothesis

RESULT

• This model, which is known as one-gene-one-enzyme hypothesis, provides the first evidence of function of the gene.

• some genes were responsible for the functions of the enzymes and each gene apparently control one specific enzyme.

2.GENE-PROTEIN RELATIONSHIP

Relationship between genotype and phenotype.

Genes

Linear sequence of amino acid

Enzymes

Structural proteins

Phenotyope of the cell

Characteristics features of the organism

STRUCTURE OF PROTEIN

PROTEIN MOTIFS

• Several elements of secondary structure combine to produce a pattern, or motif, that is found in numerous other proteins.

RELATIONSHIP BETWEEN GENE MUTATION AND ALTERED PROTEINS

• To alter the protein function only the change of one amino acid is enough. Vernon Ingram, showed this 1st time in 1957 when he study the globular protein called hemoglobin molecule which transport oxygen in cells. Hemoglobin consists of 4 polypeptide chains :

• Two identical alpha chains each containing 141 amino acids.

• Two identical beta chains each containing 146 amino acids.

RELATIONSHIP BETWEEN GENE MUTATION AND ALTERED PROTEINS

Genes and cellular metabolism:

GENETIC FINE STRUCTURE

BEAD THEORY

• Structure: gene is indivisible by crossing over. Crossing over always occurs between the genes but never within them.

• Function: gene is the fundamental unit of function. Parts of gene cannot function.

• Change: gene is also treated as a fundamental unit of change or mutation. It changes from one allelic form to another. There are no smaller components within it that can be changed.

BEAD THEORY

• Seymour Benzer in 1950s showed that bead theory was not correct.

• Benzer was able to use genetic system in which extremely small level of recombination could be detected.

• The smallest units of mutation and recombination are now known to be correlated with single nucleotide pairs.

Fine structure analysis of gene

• Life cycle of bacteriophage• Plaque morphology and rII system of phageT4.• The concept of selection in genetic crosses

with bacteriophage• Deletion mapping• Destruction of bead theory

Phage T4

• T4 are viruses that infect the bacterium E. coli. The infection ends with destruction (lysis) of the bacterial cell.

• They have been enormously useful in genetic studies becausea. Viruses of two (or more) different genotypes can

simultaneously infect a single bacterium.b. The DNA molecules of one of the infecting viruses can

recombine with that of another forming recombinant molecules.

c. The huge number of viruses released from a huge number of bacterial hosts enables even rare recombination events to be detected.

T4 phage

• T4 can be used toa. detect mutations within a single geneb. speed up the process of mapping these point

mutations by the use of deletion mutants.

Selection in genetic cross

• The procedure was to infect strain B in liquid culture with two mutants to be tested

Benzer findings

• Benzer eventually found some 2000 different mutations in the rII gene. The recombination frequency between some pairs of these was as low as 0.02.The T4 genome has 160,000 base pairs of DNA extending over ~1,600 centimorgans (cM).

• So 1 cM 100 base pairs≅• So 0.02 cM represents a pair of adjacent nucleotides.• From these data, Benzer concluded that the

– smallest unit of mutation and– the smallest unit of recombination

• was a single base pair of DNA.

Mapping Point Mutations Within A Gene

• Benzer was able to speed up the mapping process by taking advantage of the discovery that some of his mutants did not have point mutations but deletions instead. In contrast to the properties of T4 viruses with point mutations, T4 viruses with deletions in rII showed

a. no recombination with other rII mutants or any other genes for that matter;

b. never back-mutated.

Deletion Mapping

Strains 1 2 3 4 5 6 1 0 0 + 0 0 0 1 and 3 do not overlap2 0 + + 0 0 must shift 4 away from 23 0 + + 0 6 must extend under 3

4 0 + + right-hand end of 4 must be removed from over 6

5 0 +left-hand end of 6 must not overlap 5 but must continue

to overlap 2. shorten right-hand end of 5∴

6 0

• Deletions can be mapped by the same procedure used for point mutations. Simply cross pairs of deletion mutants and see if they produce progeny that can grow on E. coli strain K.

• Here is a hypothetical example. Each of 6 strains of deletion mutants are crossed with each of the others.

Deletion Mapping

• From the results, one can draw a map showing the order and relative size of the deletions.

• With such a deletion map, one can now quickly map the location of point mutations

Complementation test

• In this test, E. coli strain K growing in liquid culture, was co-infected with two different rII mutants.

• each should be able to produce the gene product missing in the other — complementation.

• Benzer coined the term cistron for these genetic units of function.

REVISED BEAD THEORY

The nucleotide pair is the fundamental unit of 1. Structure2. Change3. Benzer coined the term cistron for genetic

units of function