Draw the general structure of pyrimidine and number the atoms. Name the source of each atom during biosynthesis.

Here are the answers to Section B:

PART I

1. (i) Draw the general structure of pyrimidine and number the atoms there in.
The general structure of pyrimidine is a six-membered heterocyclic aromatic ring containing two nitrogen atoms at positions 1 and 3. The atoms are numbered clockwise starting from the nitrogen at position 1.

\begin{tikzpicture}[scale=0.8] % Atoms \node (N1) at (0,0) {N}; \node (C2) at (1,0) {C}; \node (N3) at (2,0) {N}; \node (C4) at (2.5,0.866) {C}; \node (C5) at (1.5,1.732) {C}; \node (C6) at (0.5,1.732) {C}; % Bonds \draw (N1) -- (C2); \draw (C2) -- (N3); \draw (N3) -- (C4); \draw (C4) -- (C5); \draw (C5) -- (C6); \draw (C6) -- (N1); % Double bonds (standard representation) \draw[double] (N1) -- (C2); \draw[double] (N3) -- (C4); \draw[double] (C5) -- (C6); % Numbering \node at (-0.3,0) {1}; \node at (1,-0.3) {2}; \node at (2.3,0) {3}; \node at (2.8,0.866) {4}; \node at (1.5,2.032) {5}; \node at (0.2,2.032) {6}; \end{tikzpicture}

(ii) For each atom in the structure, 1(i) above, name its source during biosynthesis.

• N1, C4, C5, C6: Derived from aspartate.
• C2, N3: Derived from carbamoyl phosphate (which is formed from HCO$_{3}^{-}$ and NH$_{3}$).

(iii) List the (in their order) leading to the formation of Uridine monophosphate.
The de novo synthesis of Uridine Monophosphate (UMP) involves the following steps:

1. Carbamoyl phosphate synthesis: Bicarbonate (HCO$_{3}^{-}$) and ammonia (NH$_{3}$) combine with 2 ATP to form carbamoyl phosphate, catalyzed by *carbamoyl