In Lederberg's replica plating experiment,what should be used to obtain streptomycin-resistant strains?

$A$ mutant microorganism that is unable to synthesize a compound required for its growth,but is able to grow if the compound is provided,is known as a/an ...... .

Read the following four statements ($1$ to $4$):
$1$. In transcription,uracil pairs with adenosine.
$2$. Regulation of $lac$ operon by a repressor is referred to as positive regulation.
$3$. The human genome contains approximately $50,000$ genes.
$4$. Haemophilia is a sex-linked recessive disease.
How many of the above statements are correct?

Distinguish between: $m-RNA$ and $t-RNA$.

Which one of the following pairs is not correctly matched?

Study the given figure of a nucleosome and select the option that correctly identifies the parts $A, B$ and $C$.