(a) Glucose with bromine water

Bromine water is a mild oxidising agent. It oxidises only the aldehyde group $(-CHO)$ into a carboxyl group $(-COOH)$, leaving the primary alcoholic group $(-CH_2OH)$ untouched.

$$\underset{\text{glucose}}{CHO-(CHOH)_4-CH_2OH} \xrightarrow{Br_2\ \text{water}} \underset{\text{gluconic acid}}{COOH-(CHOH)_4-CH_2OH}$$

Product : gluconic acid

What this proves : the carbonyl group present in glucose is specifically an aldehyde. A ketone would not be oxidised by so mild a reagent, so this reaction distinguishes glucose from fructose in structural terms.

Compare : the stronger oxidising agent conc. $HNO_3$ attacks both end groups and gives saccharic acid.

(b) Bases in DNA and RNA

(i) Thymine — present in DNA

(ii) Uracil — present in RNA

Three bases are common to both : adenine, guanine and cytosine. Only the fourth differs.

In DNA, adenine pairs with thymine through two hydrogen bonds; in RNA, uracil takes thymine's place.