DNA Structure and Biomolecules Analysis

Let's carefully examine each statement provided in the question regarding DNA structure and other biomolecules to determine which ones are correct.

Statement A: B form DNA & A form DNA Properties

• Statement A says: "B form of DNA has ~10 base pairs/turn and A form of DNA has ~2.3Å helix rise per base pair".
• B-form DNA is the most common form found under physiological conditions. It has approximately 10 base pairs per helical turn.
• A-form DNA is a different helical structure that DNA can adopt, typically under dehydrated conditions. In A-form DNA, the helix rise per base pair is approximately 2.3 Å (or 0.23 nm). The helix diameter is also larger in A-form than B-form.
• Both parts of Statement A are consistent with standard biochemical knowledge about DNA structures.
• Therefore, Statement A is correct.

Statement B: DNA Groove Widths

• Statement B says: "Both the A and B form of DNA have wider major groove and narrow minor groove".
• In B-form DNA, there is a wider major groove and a narrower minor groove. This is correct for B-DNA.
• However, in A-form DNA, the major groove is narrow and deep, while the minor groove is wide and shallow.
• Since the statement claims "Both" forms have the same groove characteristics (wider major, narrow minor), and this is only true for B-DNA, Statement B is incorrect.

Statement C: Cellulose Structure

• Statement C says: "The crystalline nature of cellulose is brought about by α (1 → 4) linkage between the glucose subunits."
• Cellulose is a linear polysaccharide made up of glucose units.
• The glucose subunits in cellulose are linked by β (1 → 4) glycosidic bonds, not α (1 → 4) bonds. α (1 → 4) linkages are found in starch (amylose) and glycogen.
• The β (1 → 4) linkage allows cellulose chains to form strong inter-chain hydrogen bonds, leading to the formation of crystalline microfibrils, which contribute to its structural rigidity and crystalline nature.
• Since the linkage type mentioned is incorrect, Statement C is incorrect.

Statement D: Lipid Double Bonds and Fluidity

• Statement D says: "The double bonds in natural lipids are always cis, which provides fluidity to the plasma membrane."
• Natural unsaturated fatty acids, common components of membrane lipids, typically contain double bonds in the cis configuration.
• Cis double bonds cause kinks in the fatty acid chains, preventing them from packing together as tightly as saturated fatty acids.
• This reduced packing density increases the fluidity of the lipid bilayer in the plasma membrane, especially at lower temperatures.
• Trans double bonds can occur but are less common in natural lipids (though found in some foods due to processing) and lead to straighter chains, reducing fluidity.
• Statement D accurately describes the common configuration of double bonds in natural lipids and their effect on membrane fluidity.
• Therefore, Statement D is correct.

Identifying Correct Combinations

Based on our analysis:

• Statement A is Correct.
• Statement B is Incorrect.
• Statement C is Incorrect.
• Statement D is Correct.

The correct statements are A and D.

Therefore, the combination representing the correct statements is A and D.