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What's the difference between chromosomes, chromatin and chromatid.

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The difference between chromatin, chromatid and chromosome
It's easy to confuse these 3 terms! Let's try to clear things up here.

DNA, the blueprint of life, is organized into structures called chromosomes. In prokaryotic cells, chromosomes are circular, whereas in eukaryotic cells, they are linear strands. Different organisms have different numbers of chromosomes: human cells usually have 46 chromosomes, dogs have 78 chromosomes, while kangaroos have only 12 chromosomes!

This karyotype of a human male cell shows the 46 chromosomes.

When you add all these chromosomes up, each cell actually contains about 2m of DNA! And all this DNA has to fit into a tiny nucleus of 5-10um in diameter. This is like trying to stuff a piece of string 2km long (it will take you about 20 minutes to walk from one end to the other) into a tiny bead smaller than 1cm!!! To do this seemingly impossible feat, cells devised an ingenious packaging system: it wraps DNA around proteins called histones. The resulting DNA-protein complex is called chromatin.

At the beginning of cell division (S-phase), the DNA is replicated, producing two identical copies of DNA, which are connected to each other at the centromere. This replicated X-like structure is now called a sister chromatid pair. A chromatid is therefore just one of the strands.

During mitosis, the sister chromatid pair condenses further, giving rise to the fat X chromosomes that you can see in the karyotype above.

Therefore, chromosomes can be found in 3 forms: thread-like chromatin (during G1 of interphase), thread-like sister chromatids (during S-phase of interphase) and the condensed, visible form (during mitosis).
When a cell divides, the sister chromatids separate, and each daughter cell receives one of the strands. The chromatid then decondenses into a long single chromatin strand when the new cell goes into interphase.

For more information about DNA, check out this Scitable entry: DNA Is a Structure That Encodes Biological Information

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