The helical structure of protein is stabilised by

Consider the following statements:
Statement-$I$: The primary structure of a protein represents its amino acid sequence.
Statement-$II$: The $\alpha$-helix and $\beta$-pleated sheet structures of a protein represent its tertiary structure.

In which of the following is $Keratin$,a fibrous protein,absent?

Assertion $(A)$: The denaturation of proteins can destroy all $1^{\circ}, 2^{\circ}$ and $3^{\circ}$ protein structures.
Reason $(R)$: Curdling of milk is due to denaturation of proteins.
The correct option among the following is

Which of the statements about "Denaturation" given below are correct?
$A$. Denaturation of proteins causes loss of secondary and tertiary structures of the protein
$B$. Denaturation leads to the conversion of double strand of $DNA$ into single strand
$C$. Denaturation affects primary structure which gets distorted

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