Silly high-school biology questions here. What exactly is the relationship between DNA, genes, and chromasomes? How much DNA is in a single adult-human cell? Is it all identical, or are there different strands which code for different things?
Well, when you get around to doing a google search, you should spell it chromosomes.
Every human cell (except mature red blood cells) have fourty six chromosomes. Each chromosome is a strand of DNA, that’s wound around some proteins. We have 23 seperate kinds of chromosomes, and two copies (not exact) of each kind, except that in normal men, there is one x and one y instead of two copies of x or y. Genes are pieces of the DNA that have a code that tell how to make a protein. Different strands of DNA code for different things, which is why we need all of the chromosomes. So genes are little sequences of DNA, and chromosomes are strands of DNA.
Here’s a thread that we covered this in.
And X and Y are both chromosomes, I don’t know if that’s clear from the post. hope it helped.
OK, I think I’m getting it.
A ‘codon’ is one of those triplets you hear about that codes for an amino acid.
A sequence of codons that code for all of the amino acids making up a specific protein are a ‘gene’? Some untold-number of these genes are contained on a chromosome, and we have 46 chromosomes in each cell. So all of the DNA in a cell is not in one long strand - there are 46 individual strands of DNA? Or is there more to a chromosome than just DNA?
I’ve also read things like “mitochondria & ribosomes contain DNA” - presumably this is their own DNA which the mitochondria & ribosomes use for their own nefarious purposes - it’s not part of any chromosome? But since it codes for a protein it could be called a gene?
OK, let’s start at the beginning.
DNA is a two stranded molecule that consists of 4 nucleotide bases (A-T, C-G) and a sugar-phosphate backbone. In a chromosome, 1 molecule of DNA is wrapped and compressed very tightly (around many different proteins, including histones), but needs to become unwound to be copied or to be read. As said above, each human cell (except mature gametes) contains 23 pairs of chromosomes – 22 “autosomes” and 1 pair of sex chromosomes. In a male, XY. Female, XX. 46 chromosomes in every cell – the genome of a human. These are very long molecules – if fully unwound and put end-to-end IIRC, around a meter of DNA per cell. Mitochondria and chloroplasts (in plants) also contain little chromosomes, encoding some enyzmes necessary for their processing (their genomes have mostly been incorporated into the main nuclear genome through evolution). Ribosomes contain RNA scaffolding, but this functions more as a structural and functional component rather than a information-carrying one.
Each chromosome contains genes. These encode proteins, which are made by unwinding and copying one of the two strands of DNA into mRNA in a process called transcription. The T base of the DNA becomes U in the mRNA. The mRNA contains a header and a footer, and a coding region in the middle. The coding region consists of 3 base codons. This coding region (usually) begins with an AUG codon, coding for methionine, and ends with a stop codon (UAG, UGA, UAA). In between are codons which encode amino acids, which is how the ribosomes know which amino acid to insert. Ribosomes read the coding region and make the encoded protein in a process called translation.
Back to the DNA in the nucleus. Let’s look at the structure of genes. To generalize, in prokaryotes (bacteria), it is pretty simple. There is some dead space, which contains stuff like binding sites for proteins which activate or repress downstream genes, and then the coding regions. These are directly copied into mRNA, and ribosomes bind and translate the mRNA as it is being transcribed off the genome. In eukaryotes (everything else), genes are not so simple. The coding regions are split up into exons and introns. Both exons and introns are copied into an immature mRNA, and then another set of nuclear machinery called the splicing apparatus splices out the introns to make a mature mRNA (along with some other steps) – exons plus a header and a footer (now similar to prokaryotic mRNA). This is another level of genetic regulation, as some genes can be made into slightly different proteins depending on how they are spliced. In eukaryotes, the mature mRNA is exported into the cytoplasm, where ribosomes translate it.
The definition of gene is not so simple, especially in eukaryotes. When we talk about genes, we include not only the transcribed region, but also the regulatory regions outside – these tell the transcription machinery where and when to start and to end. This is a very, very complicated process (I got a PhD in it) called transcriptional regulation. It turns on genes such as insulin in the pancreas but not in the skin. Or keratin in the skin but not in the liver. Or genes involved in specifying an eye in the developing retina but not in the neighboring brain (or in the fly, the neighboring antenna). Let me remind you that this is only one level of regulation in the cell, though – splicing, nuclear export, mRNA breakdown, translation, post-translational modification, protein distribution, and protein breakdown are all highly regulated as well.
But transcriptional regulation is really cool. Unfortunately, the regulatory elements of a gene are usually pretty poorly defined, and often well separated from the transcripted sequence. Genes overlap, repress and activate their neighbors, etc. etc. So sometimes, especially in the higher eukaryotes, we struggle a lot to tell what is in a gene and what is outside a gene.
So that’s my life so far.
Just to blow your mind a bit, there are approximately 20 trillion cells in a human body. So there is approximately 20 trillion meters (2 x 10[sup]13[/sup]) of DNA in your body – about 1/500 of a light-year.
Thanks for the readable claification.