Chromosome
- CBSE · NCERT Class 10, Ch 8 Heredity
- ICSE Class 10, Unit 1 Basic Biology
A chromosome is a thread-like body in the nucleus, made of DNA and protein, that carries a cell's genes; body cells have them in pairs.
Inside the nucleus, DNA sits as a loose tangle. When a cell gets ready to divide, it coils up into separate rods you can see under a microscope. These rods are chromosomes. Each one carries many genes. A body cell has two copies of each chromosome, one from the mother and one from the father.
A germ-cell, a sperm or an egg, takes just one chromosome from each pair. A special kind of cell division, meiosis, makes sure of this. When two germ-cells join, the full number comes back.
A small example
Humans have 23 pairs, or 46 chromosomes in a body cell. A sperm or an egg carries 23. At fertilisation, 23 + 23 = 46 again. Before a cell divides, each chromosome copies itself. The two copies, called chromatids, stay joined at the centromere.
Where marks go
In CBSE, Question 6 of the 2026-27 Science sample paper says an apple plant's male gamete has 17 chromosomes. The board's Science marking scheme for 2026-27 gives its 1 mark to 17 in the female gamete and 34 in the adult's cells. It is a choice question, so there is no part credit.
In ICSE, the 2025 Biology paper asked for the point where two chromatids join. CISCE's report on the 2025 ICSE Biology paper says candidates were not clear about "similar sounding terms like centromere/chromatid/chromosome/centrosome". The same paper wanted "double helix", the twisted two-strand shape of DNA. The report names "spiral structure" and "ladder" among the answers given instead. Learn each word with the exact part it names.
Sources
Facts last checked against these sources on 30 September 2026.
- Science, Class X (2026-27), Chapter 8: Heredity · NCERT
- Science (086) Marking Scheme, Class X 2026-27 - Question 6 · CBSE Academic Unit
- Science (086) Sample Question Paper, Class X 2026-27 - Question 6 · CBSE Academic Unit
- ICSE Biology syllabus, Examination Year 2028 edition (January 2026), Class X unit 1 · CISCE
- Analysis of Pupil Performance, ICSE Biology 2025 (October 2025) - Questions 2(iv) and 4(i) · CISCE
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