Which one of the following is the correct structure of cytosine?

Observe the structures of the following compounds:
$1$. $2$-chlorobutane
$2$. Glycine $(NH_2CH_2COOH)$
$3$. Alanine $(CH_3CH(NH_2)COOH)$
$4$. Styrene $(C_6H_5CH=CH_2)$
$5$. Menthol
The total number of structures/compounds which possess asymmetric carbon atoms is $....$

$A$ tetrapeptide $x$ on complete hydrolysis produced glycine $(Gly)$,alanine $(Ala)$,valine $(Val)$,and leucine $(Leu)$ in equimolar proportions. The number of possible tetrapeptide sequences involving each of these amino acids exactly once is:

What is the number of amino acids present in a single turn of an $\alpha$-helix of a protein?

Which synthesis was done by Stanley Miller?

Find the isoelectric point $(pI)$ of the given amino acid: $HOOC-CH_2-CH_2-CH(NH_3^+)-COOH$. Given: $pK_{a1} = 2$,$pK_{a2} = 4$,$pK_{a3} = 9$.