So I’ve looked at a number of sites about this but I can’t find an answer to this. I’ve learned that a gene is a part of DNA, but what I can’t determine is if there is a fixed unit of DNA (eg all DNA “units” comprise 24 “rungs” of the double helix). Or, in other words, does a chromosome consist of one long bundle of some/a DNA? What do you call “a DNA”?
Also, related to this, is a gene a part of a DNA or does a gene have a/some DNA(s) in it?
Thank you very much
A chromosome is a single DNA molecule. If you stretched it all out it’d be quite long, but usually it’s all scrunched up into a small, tight tangle. A gene is a particular region of a chromosome.
A gene is a unit of DNA that contains the “instructions” for building a protein. (It does not have to be contiguous, it can be broken up into segments interrupted by unrelated DNA.)
A chromosome is a very long string of DNA, plus various proteins that the DNA wraps around to become more compact.
Genes are made of DNA in the way that water is made of H[sub]2[/sub]O.
So is there a “standard” strand of DNA or can they be any length, depending on their purpose or coding?
DNA is the type of molecule that genes and chromosomes are built from. Think of Lego bricks. The minimum number of Lego bricks you need to have to consider yourself having any Lego bricks is one. You can’t build anything with just one Lego brick, but it is still a Lego brick. Genes and chromosomes are what you build from Lego bricks.
DNA is a polymer that can be any length. It is made from repearing units (monomers) that each contain one of 4 bases (G,A,T,C). The sequence of bases encodes the information, analogous to the sequence of 1’s and 0’s in a computer program.
Genes are sequences of DNA of varying lengths that encode proteins. They are strung end-to-end on a chromosome. There are gaps of non-protein-coding DNA within and between genes, but the DNA polymer is continuous along the length of the chromosome.
Human chromosomes come in 24 different standard sizes. Your chromosome 13 is the same size as my chromosome 13 and Darren Garrison’s chromosome 13. Most multicellular organisms also have chromosomes in a set of standard sizes, though every species’ standard sizes are different. Bacteria, though, can have bits and pieces of DNA in any old random length at all.
Mostly. To add a little wrinkle, crossing over, a process that occurs during meiosis (meiosis creates the sperm and egg that join to form an individual) doesn’t have to be symmetrical - some parts of chromosomes exchanged in the process are different sizes.
So yes, mostly peoples’ chromosomes are the same size, but there will be some variation. Like with a lot of natural processes, uniform size/length/molecular weight/amount isn’t really common.
“Base pairs” - one unit of two nucleobases forming DNA (e.g. Adenine-Thymine or Guanine-Cytosine) is a standard of measurement for DNA length, so you might say that a particular gene is 1,000,000 base pairs long, while a chromosome might be 150,000,000 base pairs long.
BTW, chromosomes only “look like chromosomes” during cell division. During the “resting” portion of the cell cycle, the DNA is less tightly packed and is constantly being read from in order to manufacture proteins.
It’s worth mentioning that sequences of 3 base-pairs specify particular amino-acids, so for example GAA means Glutamic acid while AAA means Lysine.
Yes. This. IANABiologist, but - how a gene works- DNA is not a straight pretty string, but as comments have mentioned, tangles up into messy knots. As I understand it - The specific mess the knots make for a single gene form just the right surface shape to attract specific organic molecules, and then as they latch onto the gene they bind to other latched molecules (or pull those molecules apart) until the whole builds specifically the molecules needed for specific steps in the cellular processes. Release the molecule, repeat - a catalyst essentially. (IIRC, doesn’t DNA make RNA which them makes all the assorted chemicals which power life?) So a gene might include a bit of useless “garbage” DNA between the pieces of the gene, provided it does not interfere with the gene twisting into the proper shape to present a surface suitable for building its specific necessary molecule… but more logically, a gene would be a continuous segment of the chromosome that has a specific purpose to produce a specific chemical of the cellular process.
I think.
The garbage sections are called introns, and they’re edited out after the RNA is made. That is, a sequence of DNA is transcribed to RNA and then some enzymes come along and remove sections of the RNA (the DNA is not changed). After that, the RNA goes to be translated into a protein. Those sections that are left are called exons.
As far as genes go, there are sequences of DNA neucliotides that say “start gene here” rather than code for protein. Other sequences say “end gene”. That’s how the machinery of the cell knows where to start and end transcribing the gene.
As far as bacterial DNA, those mostly in one big loop, rather than a number of separate chromosomes. However, they also have smaller loops of DNA, called plasmids, that may contain a single or just a few genes. This is a mechanism where genes can be spread throughout a group of bacteria. If a bacterium dies, some of these plasmids get released and other bacteria can pick them up. There could be large numbers of the same plasmid in a single bacterium, so more than one other bacterium can benefit. Biologists sometimes use plasmids for their own nefarious purposes. ::evil scientist laugh::