Why must a cell be made 'competent' in biotechnology experiments? How does calcium ion help in doing so?

How do leydig's cells help in spermatogenesis?

What does 'competent' refer to in component cells in transformation experiments?

Why DNA cannot pass through the cell membrane? Explain. How is a bacterial cell made 'competent' to take up recombnant DNA from the medium?

What do you understand by elevation and angular distance between two stars ? How does angular measurement help us to determine. Size of astronomical objects and distances.

Calcium ion concentration in blood affects muscles contraction. Does it lead to tetany in certain cases? How will you correlate fluctuation in blood calcium with tetany?

Write the electron dot structure for sodium and chlorine atoms. How do these form a chemical bond? Name the type of bond so formed. Why does compound so formed has high melting point?

Answer the following questions: In a single slit diffraction experiment, the width of the slit is made double the original width. How does this affect the size and intensity of the central diffraction band?

When calcium metals is added to water, the gas evolved does not catch fire but the same gas evolved on adding sodium metals to water catches fire. Why is it so?