Draw the structure of chromosome and label the parts.

Question 76 of 97

Q. Draw the structure of chromosome and label the parts.

Tamil Nadu DgeTamil Nadu HSC (DGE) Board 2018Subjective· 5mImportance★★★★★

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Concept understanding — Chromosome Morphology

Chromosome Morphology: What a Chromosome Actually Looks Like

Think of a chromosome as a tightly packed instruction manual. During most of a cell's life, that manual is loose and spread out — that's chromatin, the relaxed, stringy form of DNA wrapped around histone proteins. But when it's time for the cell to divide, the cell needs to move those instructions safely into two new cells. So it coils and condenses the chromatin into compact, rod-like structures we call chromosomes.

If you look at a condensed chromosome under a microscope, you see a distinct shape. It looks like two identical sticks joined at a pinched point. That pinched point is the centromere.

The Centromere: The Grip Point

The centromere is not just a visual feature — it's the functional anchor. During cell division, spindle fibres attach to a protein structure on the centromere called the kinetochore. The kinetochore is the actual "handle" that the spindle fibres grab to pull the chromosome apart. Without the centromere and kinetochore, the chromosome can't move, and division fails.

The centromere's position along the chromosome is fixed for that chromosome. And that position determines the chromosome's morphology — its shape and arm lengths.

The Four Types: Where the Centromere Sits

The centromere divides the chromosome into two arms: a short arm (called the p arm, from "petite") and a long arm (called the q arm, from "queue", French for tail). Depending on where the centromere lies, we get four categories:

Type	Centromere Position	Arm Lengths	Shape
Metacentric	Exactly in the middle	p arm = q arm	V or X shape
Sub-metacentric	Slightly off-centre	p arm < q arm	L shape
Acrocentric	Very close to one end	p arm very short, q arm very long	J shape
Telocentric	At the very end	Only one arm visible	I shape

Important

In humans, telocentric chromosomes do not exist. The closest we have are acrocentric chromosomes (like chromosomes 13, 14, 15, 21, 22), which have such tiny p arms that they're often hard to see.

Visualising the Difference

Draw a straight line. Mark a point on it:

Metacentric: point at the centre. The two halves are equal. When pulled apart during anaphase, the chromosome looks like an X.
Sub-metacentric: point a bit to one side. One arm is noticeably longer. The chromosome looks like an L during movement.
Acrocentric: point very near one end. One arm is a stub, the other is long. The chromosome looks like a J.
Telocentric: point at the very end. Only one arm exists. The chromosome looks like a straight I.

Why This Matters …

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Figure — Labelled diagram of a single metaphase chromosome (metacentric form)

A chromosome's labelled parts are: two sister chromatids, the centromere/kinetochore, two arms, telomeres, and (in some) a secondary constriction with a satellite.

Since an actual figure cannot be produced here, the parts a student should draw and label on a typical (metaphase) chromosome are:

Chromatid -- each chromosome, as seen at metaphase, is made up of two identical, longitudinally arranged strands called sister chromatids, produced by DNA replication in the S-phase and held together until anaphase.
Centromere (primary constriction) -- the constricted region at which the two sister chromatids are joined; it is the point of spindle-fibre attachment and divides the chromosome into two arms.
Kinetochore -- a disc-shaped proteinaceous structure present on the centromere, to which spindle fibres actually attach during cell division.
Chromosome arms -- the two segments on either side of the centromere, conventionally labelled the short arm (p arm) and the long arm (q arm); the position of the centromere relative to the arms determines whether the chromosome is metacentric (centromere in the middle), sub-metacentric, acrocentric, or telocentric (centromere near one end).
Telomere -- the specialised, protective terminal region at each free end of a chromosome arm; it prevents the chromosome ends from fusing with other chromosomes and from enzymatic degradation. …

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Q.Draw the structure of chromosome and label the parts.

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