Which of the following statements about chromosomes is correct?

Analyze the question: We need the true statement about chromosomes. Option B appears to have typos ("bear number" is most likely "the number present in a species is constant").

Concept: Chromosomes are thread-like structures made of DNA and protein (histones), found in the nucleus of eukaryotic cells. They carry genes. Each species has a characteristic, fixed chromosome number in its body (somatic) cells, for example:

• Humans: 46 (2n)
• Maize: 20
• Fruit fly: 8

This constancy is maintained from generation to generation by mitosis (body cells) and meiosis followed by fertilisation (gametes combine to restore the full number).

Checking each option:

• A. All chromosomes of a species are the same in shape: Wrong. Chromosomes differ in length and centromere position (metacentric, acrocentric, and so on).
• B. The number present in a species is constant: Correct. Chromosome number is characteristic of a species.
• C. They are neatly arranged in the cytoplasm: Wrong. Chromosomes are located in the nucleus, not the cytoplasm. They only line up neatly at the equator during metaphase, and this happens after the nuclear membrane breaks down.
• D. They bear ribosomes on their outer membranes: Wrong. Chromosomes have no membrane. Ribosomes attach to the rough endoplasmic reticulum.

**Answer: B.

Tip: Remember "Constant Number, Characteristic of the species." Also, chromosomes have no membrane, so any option that says they have one can be eliminated immediately.