Identify the essential amino acids from the following:
$A$) Leucine
$B$) Tyrosine
$C$) Cysteine
$D$) Histidine

The correct statements are:
$A$. Activation energy for enzyme catalysed hydrolysis of sucrose is lower than that of acid catalysed hydrolysis.
$B$. During denaturation,secondary and tertiary structures of a protein are destroyed but primary structure remains intact.
$C$. Nucleotides are joined together by phosphodiester linkage between $C_{3}$ and $C_{5}$ carbons of the pentose sugar.
$D$. Quaternary structure of proteins represents the spatial arrangement of two or more polypeptide chains.
Choose the correct answer from the options given below:

Which compound can exist in a dipolar (zwitter ion) state?

Which of the following is a molecular disease?

The dipeptide,$Gln-Gly$,on treatment with $CH_3COCl$ followed by aqueous work-up gives:

Thyroxine produced in the thyroid gland is an iodinated derivative of