How do chromosomes relate to DNA? Why do they exist at all, if all genetic information is stored in a single strand of DNA within the cell? I read that chromosomes only form when cells are about to divide – if so, why is this necessary and what determines how many chromosomes are formed?
In: 18
So, to give a (maybe decent?) analogy of a book, DNA would be the words, and the chromosome would be the physical book in its totality.
Life has evolved based around DNA, and as such, preserving and organising this “source code” is of the utmost importance. One way that DNA is organised is via chromosomes. You have a lot of information within your cells–IIRC, it’s about 6 ft of DNA in every cell of your body (excepting haploids). As a 5’4″ person myself, my DNA simply wouldn’t fit if it weren’t supercoiled into chromatin/chromosomes. So this is one way that they relate–chromsomes are essentially an ordered way to store DNA in our cells.
Since chromosomes are a structure that contains DNA, something else must be present in chromosomes, right? Chromosomes also contain a large amount of protein that allows for the coiling of DNA, which are called “histones.” These are basically “spools” for our linear DNA to wrap around. It is also important to note that humans do /not/ have a single stand of DNA in our cells. We have 46 chromosomes (92 during division and 23 in a haploid cell) which are each distinct structures. To return to the analogy, the nucleus is a library for our 46 books, each of which contain distinct words/information.
Chromosomes are only necessary at division for very physical reasons. We mentioned that DNA is coiled around histones–this structure alone is called “chromatin” and roughly looks like beads on a string. This DNA is coiled, but not to the degree of chromosomes. Why? Because, in an active cell that is not dividing, you want to be able to access your DNA. A book wouldn’t be very useful if you could never open it (I didn’t account for chromatin in this analogy, so try and bear with me). The lesser coils of chromatin allow for your cells to get the proteins that they need from your DNA source code, and also regulate what segments of DNA can be accessed. You wouldn’t want your cornea cells acting like your lung cells, and chromatin coiling helps physicochemically prevent this from happening.
Now, when your cell is ready to divide, this requires a lot of energy and direction. Your cell is no longer “working” in a sense because it is busy continuing on the cycle of life. As such, the DNA needs to be replicated, meaning you’ll have 2x the amount (12 ft) of “stuff” in a confined space. This is mitigated by supercoiling into chromosomes, which are more tightly packed than the chromatin. These denser strictures also allow for easier attachment to the centrioles, which physically pull the chromosomes apart to different poles of the cell. Search “anaphase mitosis” for more information on this process.
Chromosomal number determination is formed based on…evolution. I don’t have a good answer for you other than “46 chromosomes provided advantages to us and we haven’t mutated anything better quite yet”, but 46 chromosomes just happens to be how human genomes evolved. Other organisms have varying amounts of chromosomes.
I think I got everything.
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