Question

The RNA : DNA ratio increases from pregnancy to lactogenesis. It increases, respectively from 1 : 1 to

• A. 1 : 2
• B. 2 : 1
• C. 2 : 1
• D. 1 : 3

Hint:

Mammary Biochemical Indices:
DNA content = Measure of Total Cell Number.
RNA:DNA ratio = Measure of Protein Synthetic Activity per cell (Rises from 1:1 to 2:1 during lactation).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Mammary cellular metabolic activity index: mammary DNA content reflects total cell population (hyperplasia), while the RNA:DNA ratio reflects per-cell protein synthetic and translational activity (hypertrophy).
Key Formula or Approach:
\[ \text{Mammary Tissue: } \frac{\text{RNA}}{\text{DNA}} \approx 1:1 \text{ (Pregnancy / Quiescent)} \xrightarrow{\text{Lactogenesis Initiation}} 2:1 - 3:1 \text{ (Active Lactation)} \]

Step 2: Detailed Explanation:
In mammary gland biochemical physiology:
- DNA content is a constant marker of cell number ($6.0\text{ pg DNA/nucleus}$ in cattle), measuring mammary tissue cellularity/growth.
- RNA content measures ribosomal machinery and transcription of milk protein genes (caseins, lactalbumin).
- During pregnancy, the mammary epithelial RNA : DNA ratio is approximately 1 : 1 (cellular proliferative phase).
- With the onset of lactogenesis and active lactation, intense transcription and translation boost cytoplasmic ribosomal RNA content, increasing the RNA : DNA ratio to 2 : 1 (and up to $3:1$ at peak lactation), reflecting a two-fold increase in per-cell metabolic synthetic activity.

Step 3: Final Answer:
Hence, the RNA:DNA ratio increases from 1:1 to 2:1, matching option (B).