question_answer

Show that Darwin's natural selection is based on natural observation, with an example for each.

OR

Explain Meselson and Stahl's experiment to prove semiconservative DNA replication.

Answer:

(i) The resources of the nature are limited, so the size of population of a species is limited.

(ii) The size of the population is fixed, except for some seasonal fluctuations.

(iii) The each and every member of a population show variation for every character.

(iv) It is found in microbial or bacterial population growing in a culture that if everybody in a population reproduce with maximum capacity, the population will grow exponentially.

(v) The population size in an ecosystem is limited-due to competition among individuals for limited resources and only those fit individual utilise the resources at the cost of other to reproduce.

Darwin suggested that nature creates some pressure on the population which eliminate some individual and some better adapted survive. This natural pressure at each and every generation create some small variation in a population accumulation of which for many generations had lead to origin of new species.

OR

Matthew Meselson and Franklin Stahl performed the following experiment in 1958:

(a) They grew E. coli in a medium containing  \[^{15}N{{H}_{4}}Cl\] (\[^{15}N\] is the heavy isotope of nitrogen) as the only nitrogen source for many generation. The result was that \[^{15}N\] was incorporated into newly synthesized DNA (as well as other nitrogen containing compounds). This heavy DNA molecule could be distinguished from the normal DNA by centrifugation in a cesium chloride (CsCI) density gradient (Please note that \[^{15}N\] is not a radioactive isotope, and it can be separated from \[^{14}N\] only based on densities)

(b) Then they transferred the cells into a medium with normal \[^{14}N{{H}_{4}}Cl\]and took samples at various definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices. The various samples were separated independently on CsCI gradients to measure the densities of DNA.

(c) Thus, the DNA that was extracted from the culture of 1st generation after the transfer from\[^{15}N\] to \[^{14}N\]medium [that is after 20 minutes] had a hybrid or intermediate density. DNA extracted from the culture after another generation that is after 40 minutes] was composed of equal amounts of this hybrid DNA and of 'light' DNA.