Biological Classification

Answer (d) Transduction

Answer (c) Anabaena

Many cyanobacteria are free living and filamentous such as Aulosira, Tolypothrix, Nostoc, etc. These cyanobacteria have some specialised and colourless cells, called heterocysts which are the sites of nitrogen fixation.

In the DNA double helix, the base-pairing rules are governed by hydrogen bonds between nitrogenous bases. The correct base pairings in DNA are:

• Adenine (A) pairs with Thymine (T) through two hydrogen bonds.
• Guanine (G) pairs with Cytosine (C) through three hydrogen bonds.

Thus, the correct base pairing is: Adenine pairs with Thymine, and Guanine pairs with Cytosine.

The incorrect options are:

• Option (1): Adenine does not pair with Cytosine. Instead, Adenine pairs with Thymine.
• Option (2): Thymine does not pair with Guanine. Instead, Thymine pairs with Adenine.
• Option (4): Cytosine does not pair with Uracil. Cytosine pairs with Guanine in DNA, whereas Uracil is found in RNA, where it pairs with Adenine.

Thus, the correct answer is Option (3).

The production of curd from milk involves the fermentation of lactose (milk sugar) into lactic acid by specific bacteria. Lactobacillus acidophilus, along with other lactic acid bacteria such as *Lactobacillus delbrueckii* subsp. *bulgaricus* and *Streptococcus thermophilus*, is commonly used in this process. These bacteria convert lactose into lactic acid, which lowers the pH, causing milk proteins (casein) to coagulate and form curd.
The other options have different roles:
- Saccharomyces cerevisiae: A yeast used in baking (bread production) and alcohol fermentation (e.g., wine, beer).
- Aspergillus niger: A fungus used in the production of citric acid and certain enzymes, not curd.
- Penicillium notatum: A fungus used in the production of the antibiotic penicillin, not involved in curd formation.
Thus, the microorganism used in the production of curd from milk is *Lactobacillus acidophilus*.

A prime element is one that is fundamental and cannot be broken down further into simpler units. Elements like carbon, hydrogen, and nitrogen are considered prime elements because they are basic and cannot be decomposed. The option (d) refers to something that can be factored, which disqualifies it from being a prime element.