Question

Briefly explain the differences between F⁺, F⁻, Hfr, and F′ cells.

Generalized transduction by phage P1 of four E. coli genes (a, b, c, and d) shows the following percentages of cotransduction: a -- d 51%, d -- c 2%, b -- d 12%, a -- b 40%, a -- c 19%, c -- b 79%.

d a b c

a b c d

b d a c

b a c d

A wild-type strain (h⁺r⁺) of phage T2 was mixed with a double mutant (hr) for host range (h) and rapid lysis (r) and used with a high multiplicity of infection to infect E. coli B. The progeny from the cross were titered (counted) on a mixture of E. coli B and E. coli B/2 strains. The following plaques were scored:          hr,320; h⁺r,110; hr⁺,106; h⁺r⁺,290. What is the approximate recombination frequency for these two loci?

	16 %
	40 %
	24 %
	26 %

Briefly outline the history of our knowledge of the structure of DNA until the time of Watson and Crick. Which do you think were the principle contributions and developments?

Answer 1

F⁺ cells are cells that contain F plasmid.Here the F⁺ bacterial cell possesses F factor as a plasmid independent of the bacterial genome.They act as a donor.

F^- cells do not contain F factor and act as the recipients.

Hfr bacteria possess the complete F episome integrated into the bacterial genome.An Hfr cell can transfer a portion of the bacterial genome.They are high frequency donors.

F^’ cells (F prime) contain F plasmid carrying some bacterial genes.They act as donors in conjugation.They are formed by the incorrect incorporation of the F factor in the chromosome of Hfr bacterium.

Option 1 d a b c

Cotransduction frequency is inversely proportional to the distance between two genes.

From this data we can understand that c is closer to b and a is closer to d and b.Also c is far from d.

So the answer is d a b c.

Remaining options are wrong due to wrong arrangements.

Option 4 - 26%

Here hr and h⁺r⁺ are parental progeny and h⁺r and hr⁺ are recombinant progeny

Recombination frequency is the percentage of recombinant progeny.

Number of recombinant progeny = 110+106=216

Total number of progeny =826

Recombination frequency (%) =Number of recombinant progeny /Total number of progeny*100

=216/826*100=26%

DNA was discovered in 1869 by Friedrich Miescher.

1866- Gregor Mendel suggest that characteristics are passed down from generation to generation.

1869- Friedrich Miescher identified the 'nuclein' later become known as DNA.

1881- Albrecht Kossel isolated 5 nitrogen bases (A,T,G,C and U).

1900- Chromosomal theory of inheritance .Also Mendel's theories are rediscovered by Hugo devries,Carl Correns and Erich Tcshermak.

1944- Avery outlined DNA as the transforming principle.

1944-1950- Ewin Chargaff discovered that DNA is responsible for heridity.Formulate Chargaff's rules.

1940-Barbara McClintock discovered jumping genes.

1952-Roslind Franklin photographs crystallized DNA fibers.

1953-Watson and Crick discover the double helix structure of DNA.