Which of the following statements about chromosomes is correct?

Analyze the question: We need the one true statement about chromosomes.

Concept: Chromosomes are thread-like structures of DNA and protein found in the nucleus of eukaryotic cells. Each species has a characteristic, fixed chromosome number (e.g., humans 46, maize 20, fruit fly 8). This constancy is maintained from generation to generation through mitosis and meiosis, and fertilisation.

Evaluate each option:

• A. All the chromosomes of a species are the same shape. False. Chromosomes differ in length and in centromere position (metacentric, submetacentric, acrocentric, telocentric), so they are not all the same shape.
• B. The number present in a species is constant. True. Every normal member of a species has the same diploid number of chromosomes in its body cells, and the gametes have the haploid number.
• C. They are neatly arranged in a cytoplasm. False. Chromosomes are located in the nucleus (nucleoplasm), not the cytoplasm. They are only arranged neatly at the equator during metaphase, and that happens within the dividing cell, not "in the cytoplasm" as a general statement.
• D. They bear ribosomes on their outer membranes. False. Chromosomes have no membrane. Ribosomes are attached to the rough endoplasmic reticulum.

Answer: B. The chromosome number is constant for a species.

Exam tip: Remember "constant number, not constant shape." Whenever you see a question on chromosomes, think of the fixed number per species (diploid 2n, haploid n), and remember that they are found in the nucleus and have no membrane.