eli5: What do chromosomes do?

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eli5: What do chromosomes do?

In: Biology

Chromosomes are mostly DNA, with some other components mixed in to hold it all together in a very, very tightly wound shape. DNA is the cell’s source of genetic material. In essence, chromosomes and DNA tell organisms what to be and how to act on a very, very small scale. They tell your skin cells to be skin cells and act like skin cells, and they tell your nerves to be nerves and act like nerves, etc.

Chromosomes are the highest order of organization of DNA. This is easier to explain [visually](https://www.creativebiomart.net/images/researcharea-chromosome-structure-proteins-2.png). DNA curls up into a tight spiral. That spiral curls up into a bigger spiral. *That* spiral curls into an even bigger spiral clump, and *those* spiral clumps bunch up into chromosomes. This is to keep the DNA organized and keep it from getting tangled.

You can imagine the long strings of DNA swirling around in the nucleus like headphone cords in your pocket, getting ridiculously tangled up. DNA is pretty fragile, so all this tangling would end up breaking it – not to mention making it impossible for the enzymes that read DNA to do their job.

The chromosome structure also makes it easier to separate the DNA cleanly in gametes (sperm and eggs). They’re packets of DNA that can be easily moved around cells during meiosis, which creates haploid gamete cells.

Different chromosomes are responsible for a lot of different *genes*, which are the sequences of DNA bases that code for complete proteins or further RNA instructions. A single chromosome has many, *many* genes inside. Although genes are a little different from person to person, they are still organized into the same chromosomes: if you have brown hair or red hair, the genes will be found on the same chromosome (or couple of chromosomes) in roughly the same place in the DNA sequence.

The best example of this is probably the sex chromosomes, X and Y. There are a lot of other genes coded on those (well, mostly just the X because the Y is tiny and doesn’t do much). The genes that have instructions to build the red-light sensitive cone cells in your eyes are found on the X chromosome, so they don’t *only* code for biological sex. But biological sex is always coded on the X and Y chromosomes (in humans).