How it works
A chromosome diagram labeled with its parts: one copied chromosome shaped like an X, then a zoom into the chromatin fiber, the nucleosome beads and the histones the DNA wraps round. Start with the 8 main parts.
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Why it looks like an X
Before a cell divides it copies each chromosome. The two copies, sister chromatids, stay joined at the centromere, so the chromosome looks like an X. The centromere splits each chromatid into a short p arm and a long q arm, and a telomere caps each end.
How the DNA fits
The DNA double helix winds round cores of eight histones to make nucleosomes, strung together by linker DNA. The beads coil into a 30 nm chromatin fiber, which folds up again into the chromosome.
- p for petite: the p arm is always the short one.
Questions and answers
What is the difference between a chromosome and a chromatid?
A chromatid is one copy of the DNA. After copying, a chromosome is two sister chromatids joined at the centromere. Once they separate in anaphase, each chromatid counts as a chromosome of its own.
What is the difference between the centromere and the kinetochore?
The centromere is the pinched region of the chromosome itself. The kinetochore is a protein structure built on it, one on each chromatid, where the spindle fibers attach.
What does a telomere do?
It caps the end of a chromosome with repeated DNA and proteins, which keeps the end from being worn away and stops the cell treating it as a break.