Long Answer TypeExplain how does:
(a) a primary succession start on a bare rock and reach a climax community?
(b) the algal bloom eventually choke the water body in an industrial area?The following is the illustration of the sequence of ovarian events (a to i) in a human female.
i) Identify the figure that illustrates ovulation and mention the stage of oogenesis it represents.
(ii) Name the ovarian hormone and the pituitary hormone that have caused the above mentioned event.
(iii) Explain the changes that occur in the uterus simultaneously in anticipation.
(iv) Write the difference between c and h.
(v) Draw a labeled sketch of the structure of a human ovum prior to fertilization.
How does the megaspore mother cell develop into 7-celled, 8 nucleate embryo sac in an angiosperm? Draw a labeled diagram of a mature embryo sac.
What is the inheritance pattern observed in the size of starch grains and seed shape of Pisum sativum? Workout the monohybrid cross showing the above traits. How does this pattern of inheritance deviate from that of Mendelian law of dominance?
State the aim and describe Messelson and Stahl experiment.
Messelson and Stahl in 1958 performed and experiment to prove that the DNA replicates in a semi-conservative fashion.
Experimental proof
(1) They grew E.coli in a medium containing 15NH4Cl (15N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. As a result, 15N was incorporated into
the newly-synthesized DNA. This heavy DNA could be distinguished from the normal by centrifugations in CsCl (cesium chloride) density gradient.
(2) Then, they transferred the E. coli cells to a medium with normal 14NH4Cl and the DNA samples were extracted at various time intervals. The DNA samples taken were double-stranded helix. The various samples were separated on the basis of CsCl gradients for measuring the density of DNA.
(3) Since E.coli divides in 20 min, they extracted DNA from the culture one generation after the transfer from N15 to N14 medium after 20 min. After 40 minutes, the DNA of the second generation was extracted from the 14NH4Cl medium and was found to have equal amounts of hybrid and light DNA. Thus it was proved that DNA replication is semiconservative since the samples contained both N15 and N14 strands that is both hybrid and light.
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