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A diagram of four chromosomes, each with its centromere in a different place, is one of the most redrawn figures in Class 11 biology. Questions on it are short, but students lose marks by confusing acrocentric with telocentric, or by miscounting kinetochores. This post shows how to classify a chromosome from its arm lengths, how to count its parts at each stage, and why acrocentric chromosomes come up again in human genetics.
Where is this in the syllabus?
The classification sits in section 8.5.10, Nucleus, of the NCERT Class 11 Biology chapter Cell: The Unit of Life (Reprint 2026-27), and its exercise 14 asks for it directly. The NEET (UG) 2026 syllabus lists "Nucleus-nuclear membrane, chromatin, nucleolus" under Cell Structure and Function but does not name centromere types, and the CBSE 2026-27 Biology curriculum lists "nucleus". The NCERT text is unchanged, so treat it as examinable.
What does NCERT say about the centromere?
According to NCERT, every chromosome, visible only in dividing cells, has a primary constriction called the centromere. Disc-shaped structures called kinetochores sit on its sides, and the centromere holds the two chromatids together. The NHGRI glossary entry for centromere adds that it divides the chromosome into a short arm (p) and a long arm (q), and is where the spindle fibres attach during cell division.
The four types:
Metacentric: centromere in the middle, two equal arms.
Sub-metacentric: centromere slightly away from the middle, one shorter and one longer arm.
Acrocentric: centromere close to one end, one extremely short and one very long arm.
Telocentric: centromere at the end.
NCERT adds that some chromosomes have a non-staining secondary constriction at a constant place, which makes a small fragment called the satellite.
Worked example: classify from arm lengths
A student measures four chromosomes in a photograph (sample numbers). For each, find the arm ratio (long arm ÷ short arm) and the share of the total length taken by the short arm.
p = 2.4 µm, q = 2.6 µm. Ratio = 1.08; short arm = 2.4/5.0 = 48%. Nearly equal arms: metacentric.
p = 1.5 µm, q = 3.5 µm. Ratio = 2.33; short arm = 30%. Clearly unequal, but the short arm is substantial: sub-metacentric.
p = 0.4 µm, q = 4.6 µm. Ratio = 11.5; short arm = 8%. One extremely short arm: acrocentric.
p = 0 µm, q = 5.0 µm. No short arm at all: telocentric.
NCERT uses words, not numbers, for the boundaries. Cytogeneticists use fixed arm-ratio ranges. Those come from a 1964 proposal by Levan, Fredga and Sandberg, written because different authors had used terms such as metacentric and acrocentric inconsistently. For NEET and school exams, the four descriptions above are what you need.
How many centromeres and kinetochores are there?
Take a human cell with 46 chromosomes in metaphase of mitosis.
Each chromosome has two chromatids, so there are 92 chromatids.
Each chromosome has one centromere: 46 centromeres.
NCERT places kinetochores on the sides of the centromere, one per chromatid: 92 kinetochores.
At anaphase the centromeres split, giving 92 chromosomes, each with one chromatid and one kinetochore.
Count chromosomes by centromeres, never by arms. A metacentric chromosome has two equal arms, but it is still one chromosome. Our guide to counting through mitosis and meiosis applies the same rule stage by stage.
Why does the centromere position change the shape at anaphase?
The spindle pulls at the centromere, so the centromere leads and the arms trail behind. Picture each type moving towards a pole:
Metacentric: two equal arms trail, so it looks like a V.
Sub-metacentric: unequal arms trail, giving an L or J.
Acrocentric: a tiny arm and a long one, close to a straight rod.
Telocentric: a single arm pulled from its tip, a straight rod.
Working it out from the pull is safer than memorising the letters.
Where do acrocentric chromosomes matter in humans?
A 2023 Nature study by Guarracino and colleagues describes the short arms of the human acrocentric chromosomes 13, 14, 15, 21 and 22. These short arms share large similar regions, including ribosomal DNA repeats, and the study links some of these regions to the breakpoints of Robertsonian translocations, in which two acrocentric chromosomes join. Chromosome 21 is one of them; NCERT's Principles of Inheritance chapter notes that Down's syndrome results from an extra copy of it. Our post on chromosomal disorders and pedigree charts covers the syndromes NEET names.
Which mistakes cost marks?
Calling a chromosome with a tiny short arm telocentric. That is acrocentric.
Counting arms as chromosomes.
Counting one kinetochore per chromosome at metaphase. There is one per chromatid.
Confusing centromere and kinetochore. The centromere is the constriction; kinetochores are the disc-shaped structures on its sides.
Mixing up the satellite and the centromere. The satellite sits beyond a secondary constriction, not the primary one.
A practice routine
Draw the four types from memory, label p and q, and check against NCERT Figure 8.13.
Invent five pairs of arm lengths and classify each, writing the ratio first.
For a cell with 2n = 8, write chromosomes, chromatids, centromeres and kinetochores at G1, metaphase and anaphase of mitosis.
Explain to someone else why a telocentric chromosome looks like a rod at anaphase.
If the parts of the nucleus still blur together, the Fermi Concept Library has every concept in biology, each with its own lesson and its place in the map, so you can see the centromere in context with chromatin and the cell cycle.
Start by drawing Figure 8.13 from memory tonight, then check every label against the book.
