Cell Biology MCQ question and answer with solution | Biology Gk MCQs

DNA (Deoxyribonucleic acid) uses four "letters" or bases to write its instructions: adenine (A), guanine (G), cytosine (C), and thymine (T).
RNA (Ribonucleic acid) is very similar, but it has a slight change in one of its letters.

Let's look at the options:
* Option A: thymine: DNA does have thymine (T). RNA doesn't use thymine.
* Option B: uracil: RNA uses uracil (U) instead of thymine. DNA doesn't have uracil.
* Option C: adenine: Both DNA and RNA have adenine (A).
* Option D: guanine: Both DNA and RNA have guanine (G).

The key difference is that DNA uses thymine (T), while RNA uses uracil (U).

Therefore, the correct answer is B: uracil.

Explanation:
Let's break down each statement about Prophase I of meiosis:

Statement 1: Progressive condensation and coiling of chromatin fibres occur at Leptonema stage
This statement is correct. Leptonema is the first stage of prophase I, and it's when the chromosomes start to condense and become visible.

Statement 2: Two homologous chromosomes each derived from one parent during sexual reproduction come together and form pairs during Zygonema stage
This statement is also correct. The pairing of homologous chromosomes is called synapsis, and it happens during Zygonema. These paired chromosomes are called bivalents.

Statement 3: Crossing over takes place during Diplonema stage
This statement is incorrect. Crossing over (the exchange of genetic material) actually occurs during the Pachynema stage which comes after Zygonema, even though the visual evidence of crossing over (chiasmata) is seen during Diplonema. Diplonema is when the homologous chromosomes begin to separate but remain connected at chiasmata.

Statement 4: The nucleolus and nuclear envelope disappear and a spindle begins to be formed at the end of Diakinesis stage
This statement is correct. Diakinesis is the final stage of prophase I, and it's when the nuclear envelope breaks down, the nucleolus disappears, and the spindle fibers start to form in preparation for metaphase I.

Therefore, statements 1, 2, and 4 are correct.

Correct Answer: Option C: 1, 2 and 4 are correct