In news, books, TV-shows ASF it’s DNA this and chromosome that. Can someone break this down for me? There are chromosomes, DNA, genes (hopefully not Jordache). Which fits with what? Are genes a part of the DNA which is part of the chromosome?
And no, I’m not asking someone to do my homework. I’ve been out of college since '84. Just curious and ignorant.
DNA is a chemical, made up of tinker-toy-like sequences of a few basic parts (nucleotides). The DNA exists as long, long strands, which get wound around various proteins; the resulting bundles of DNA and protein are called chromosomes.
Genes are units of genetic code; the code is written in DNA, and genes are the sentences (or messages). The genes instruct your cellular machinery in making proteins. The dictum when I was in school was “one gene codes for one protein,” but this is probably a simplistic. There are many genes on each chromosome, and a given gene shows up on the same chromosome in everybody.
How much more do you want?
Okay, to repeat some stuff that Nametag said (IMO, multiple explanations in different words often help), and add some stuff of my own.
DNA is the term for the molecule. It can be used to describe any length, any structure. Organisms have one or more very long strands of DNA in either a linear or ciucular shape, each of which is called a chromosome. DNA is essentially a blueprint, in that it doesn’t DO anything itself. Particular parts of your DNA are translated into RNA, and then sometimes into proteins. Mostly it is the protein (sometimes RNA) which performs all the work: structure, movement, sythesis, enzymes, DNA-copying, all the other things that a cell needs. A gene consists of the particular part of a chromosome that codes for something useful, and the parts that control when and where that gene is activated (“expressed”, in biospeak). For instance, alcohol dehydrogenase gene includes the DNA sequence that has the code that tells the cell how to make the alcohol dehydrogenase protein, and the bits that tell a cell that only liver cells should be expressing it.
The “one gene=one protein” dogma is a bit oversimplified, but generally true. Most genes code for basically one protein, but often different bits can be spliced out. A few genes code for two or more genuinely different proteins, this is very rare in animals, but common in bacteria and viruses. Some proteins (depends on how you define this) require more than one gene to form them.
A chromosome contains hundreds to thousands of genes, but more DNA that is not part of any gene. Some of this non-genic DNA appears to have a role in the physical structure of the chromosome, about a third of it appears to be “parasitic” DNA (dead retroviruses and the like), much of it has no obvious function. There are generally-accepted arguements that involve this functionless DNA, and other less accepted arguements, I’d be glad to go into it, but I suspect that’s more than you want to know.
I’ll type for hours about genetics, this is what I do for a living, plus I think it’s REALLY COOL. Post what more you want to know (if anything) and I’ll go on. (and on, and on.)
cheers,
mischievous
This is a good start. Thanks guys.
It’s just that when i watch a show like C.S.I. it’s always “Let’s check for DNA” as sort of a tech shorthand saying - we really don’t want to explain this, but by throwing the DNA thing around casually, we can get away with it.
When trying to understand physics, chemistry or biology, I need to visualize it, to bring some sort of order.
Let me do an analogy, to see if I got what you said:
Chromosomes are the bigger building blocks of a house - walls, floor, roof - DNA is the wiring that runs electricity and water and what have you to make it liveable - and genes are the bluprints for the whole thing?
Am I grossly stupid? 
No, you aren’t stupid. 
DNA is the blueprint. It contains all the information necessary to create proteins. One DNA helix may contain data for many, many genes but only small sections may be copied at any given time.
Chromosomes are what we call DNA and its associated proteins when they are all jumbled about together in the nucleus, living it up at the taxpayers expense.
A gene is a segment of the DNA that contains the information for the making of a specific protein. The size of the gene is highly variable. So, the gene is a specific section of the blueprint that only relates to one particular aspect of the house, be it wiring, a chimmney or a ballroom.
All of this is in somewhat simplified terms, of course.
This would be a better version of your analogy:
The DNA would be the paper and ink of a blueprint.
A Chromosome would be a page of the blueprint made of DNA.
A gene would be the drawing of a structure (a wall for instance) on the page.
A gene is the instruction for making something specific.
A chromosome is a long string of genes bundled up in a package.
A gene is written as a sequence of DNA, therefore a chromosome is a just a longer package of DNA.
Another thing to consider when thinking about this is that a cell is a very small entity, let’s say around 100 microns (1x10^-6 meters) in diameter. The length of a DNA molecule could measure up to several centimeters.
How do you fit this long molecule into a tiny cell?
You twist and compress it into a structure that will fit into a cell. This would be the chromosome which, as pointed out above, is composed of DNA wrapped around protein components.
Maybe another analogy would be a floppy disk. You wouldn’t want to store the string of 1s and 0s of binary code on a sheet of paper.
The code is nicely store on the disk, which is the packaging. The files stored as 1s and 0s could be thought of genes, which are read by the drive in the computer somewhat like the genes are read by the cell’s transcriptional ‘machinery’.
These are really good explanations!
One implied question hasn’t been answered. That “C.S.I. check for DNA” refers to identification by DNA samples. Every part of a person’s body has DNA, and anything left behind (commonly blood or semen) can be analyzed. Since most genes come in several versions (think blood types, or light fixtures for the blueprint model), the lab can test a crime-scene sample for the combination of gene versions it has, and match it against a sample from a suspect. If anything doesn’t match, the suspect is innocent; if everything matches, he’s probably guilty. Of course it’s possible that someone else has the same combination of genes as the suspect, but the odds are very slim.
Just to toss this out if it will help:
DNA is a chemical name: it referes to “deoxyribose nucleic acid” - it is a molecule called ribose (a sugar, actually!), which is missing an oxygen group (specifically, -OH, an alcohol) attached to it, hence it is “deoxy”. It is “nucleic” because it is found in the cell nucleus, and it is an “acid” because, well, it is acidic in the sense that it is often dissociated from a hydrogen (what pH is measured by).
Thank might be more confusing than its meant to be, but basically its just a chemical name for a LONG strand of molecules, which are found attached in a specific order.
RNA is similar: it is ribose nucleic acid. The only difference is that it has that OH group at the position that DNA doesn’t have it at, but that makes a whole lot of difference in how it functions.
In humans, there are 46 bundles of DNA, called chromosomes. These 46 bundles are often expressed as 23 pairs, since there is basically a duplicate of each in each of your cells. They are not identical, though, since one of each came from your mother, and the other from your father.
The differences in the mother and father sets are what each gene on each chromosome codes for. This code is found in the “base pairs” of the DNA - there are four chemicals that are found in different patterns throughout the DNA, and it is the specific orders of these four chemicals which define where a gene starts and ends, and what the specific gene will do. In one case, your mother’s chromosome might contain a gene that codes for Brown eyes, and your fathers might be for Blue. You would have brown eyes as a result of this. If you have kids, then their eye colour would depend on ONE of your genes (which are basically randomly split up to make sperm and eggs) and on ONE of your spouse’s genes. If your spouse had Brown eyes, then he/she might have. like you, one gene for brown and one for blue, or both brown genes. If your child gets 2 browns, then they have brown eyes, if they get one of each, then they have brown eyes, and if they get 2 blues, they have blue eyes.
Other traits get a lot more complicated, but there are basic statistical distributions of traits.
As was said, each chromosome - each bundle of DNA - contains MANY MANY genes, all, or most, of which are essential to you functionning properly. Missing certain genes may not be life-threatening (e.g. missing a skin pigment gene might simply cause albinism), but missing an entire chromosome is, IIRC, often lethal.
The other thing about DNA, and why they talk about it so much in shows like CSI, is that it IS an incredibly valuable tool in determining whodunnit. DNA is a code for YOU, and only you (I won’t go into twins). And there are certain genes that are known to change frequently (just simply ones that can withstand slight changes across generations with no adverse effects, and ones that are basically KNOWN to be different from family to family, and person to person). So what gets done is that a DNA sample is looked at to find these particular “markers”, and they can compare the marked gene to a sample from a suspect, and see if it was them. More than 99% of the time, it will be. Genetic evidence is powerful, since although you can wear gloves, or mess up your finger prints, etc, you cannot mess up your DNA sequence, since it is found in each and every cell of your body, whether or not it is expressed there.
Ok, I think I’ve babbled on long enough.
Oh, and that “more than 99%” thing - its a random number, I haven’t acutally looked at stats, so don’t ask me for a cite! Basically, identical markers in sample and suspect often mean that the suspect is the one who the sample came from…
DNA is a material – a molecule. You’ve no doubt seen pictures of the DNA double helix. It may be easier to imagine it untwisted as a long, straight ladder. Each rung of the ladder is one unit of information – it codes one of four possible values: A, G, C, or T. These values are the letters of a language that codes the recipe for building an organism.
There are a lot of rungs on our ladder – three billion in a human. That’s a long ladder, and, as moocher mentioned, it is too long to fit linearly inside a cell nucleus. So the ladder is chopped up into smaller units. In humans, the DNA ladder is chopped into 46 shorter ladders. These shorter units are still long and unwieldly, so they are wrapped up and coiled around various structural protiens, like thread around a spool. This short, manageable unit of DNA and protien is a chromosome, and we humans have 46 of them in every cell.
Now a gene is a stretch of DNA on a chromosome that does something – it is a portion of the coded “language” that gives one instruction. To first approximation, it is a length of DNA that codes for the construction of one protien. Say one that makes your eyes brown or one that makes your hair black.
Recall that the human DNA ladder has three billion rungs divided between 46 chromosomes. So that’s an average of about 65 million rungs (or “letters” in the code) per chromosome. It is (very roughly) estimated that there are somewhere on the order of 100,000 genes in this DNA, or about 2,000 genes per chromosome. And each gene is about 3,000 DNA “rungs” or “letters” in length. These 3,000 rungs code for about 1,000 amino acids (the building blocks of protiens) that are strung together to build one specific protien, hemoglobin in your blood, say.
To your analogies: from a physical point of view, DNA is like concrete. It is the material from which the buildings are constructed. The entire thing is concrete, and each smaller part is concrete. Chromosomes are like seperate buildings, and genes are like the smaller blocks of which those buildings are composed. From an information point of view, DNA is the language of the blueprints. Imagine a shelf of books called “How to Make a Human.” The complete DNA of the person is the entire shelf of books, and the words in the books are printed in the language of DNA. The shelf has 46 volumes (chromosomes), so that it is not all just one unwieldy tome. And each volume has 2,000 pages (genes), each page with the instructions on how to make one protien.
One last thing: the analogy of the blueprint is a good one for understanding the fundamentals of the genetic code, however it does lead to one common misunderstanding. The genetic code is not a one-to-one map of the resulting organism. There is no gene for your right index finger, or your left pinkie toe, as one would infer from a blueprint where every part of the building is represented in an exact for. Rather, the DNA code is more like a recipe – it is the series of instructions for a developmental process that results in a human. There is nothing in a recipe that says, “There will be a walnut at coordinates (x,y),” just as there is not a gene that says “grow nose here.” Rather, the recipe says, “Add walnuts, stir, cook at 425 for half an hour,” and the genetic code says, “At this step in the development, create so much of this protien.” Richard Dawkins sums it up in his excellent book “The Blind Watchmaker,” an excerpt of which can be found here.
-b
Genetics 101: http://k9ped.com/genetics101.pdf
Also, having an extra gene is not at all good. There are many genetic diseases caused by an extra chromosome (which contains many, many genes, as previously pointed out) or by defects in a single gene. More than 1500 conditions have been identified as single-gene defects: cleft lip, congenital heart disease, clubfoot, epilepsy, etc. Many conditions involve an extra chromosome: Downs Syndrome (Trisomy 21 - 3 chromosomes numbered 21 instead of the usual 2), Trisomy 18 (marked hyperplasia of muscle, fat, brains, etc., and such a child does not survive long), Trisomy 13. In addition, many genetic conditions are caused not by missing a whole chromosme but by an arm of a chromosome.
True, barbituate. I wasn’t confident enough in my knowledge of any diseases to really discuss it much…I was thinking of mentioning Downs Syndrome, but couldn’t remember if it was chromosome 21 or 16 - its a confusion I have always made, and I don’t know why! Also, the only example I could think of of missing a chromosome are XO females, which are viable, but likely sterile. I know that YO and I think YY "males"are complerely inviable, as the Y chromosome tends to regulate the X one, but is lacking a lot of necessary information that the X does have, which is why males need to be XY (in mammals, at least).
Genes found on the Y chromosome:
Hey, heard of google?
Actually, missing any one of the autosomal chromosomes (i.e. not the X or Y) is invariably pre-natallly lethal in humans. As pointed out above, having an extra chromosome 21 produces Down’s Syndrome, an extra 18 or 13 produces syndromes that involve early childhood lethality (often within a month of birth). An extra copy of any other chromosme is invariably fatal pre-natally. Since only extra copies of 21, 18, and 13 were ever found in babies, it used to be thought that these chromosomes had something unusual about them, but when people started testing spontaneous miscarriages for extra chromsomes, they found that extra or deleted copies of all chromosomes occur naturally, but only these three survive to term.
Extra sex chromosomes are less of a problem. The Y chromosome contains very few genes and the fetus seems to mostly compensate for the excess. The X chromosome has lots of genes, but in most tissues all but one copy is shut down. However, there are still some detrimental effects of extra Y and X chromosomes. Increasing numbers of either lead to dramatically increased chances of mental retardation, and (last I heard) it was thought that increasing numbers of Y chromosomes lead to increases in hyperactivity, poor socialization, and violent behavior. (This has been questioned, and I don’t know the results.) I’m not sure of the effects on fertility, you might want to ask a medical professional.
Nitpick: rjk, not all of the cells in your body have DNA, and many secretions have very little. Sweat, saliva, urine, have very few cells, so one needs a fair amount for testing. Blood has lots of cells, but red blood cells lose their DNA in the process of maturation, so the only useful cells are the rarer white blood cells. Also, the cells that make up the outer layer of your skin are dead, and many will have lost or degraded their DNA. This is not to say that modern techniques can’t pick up a useful number of cells from astonishingly small samples, but having a sample does not necessarily mean that you’ve got DNA.
KarlGrenze, what do you mean by antigene genes?
mischievous, blathering on
(I did warn you)
I just wanted to add that scotth draws perfect analogies.
mischievous
Since it’s late at night and I can access this Board quickly (a rare occasion :)), I just want to emphasize to **Gaspode ** that an amazing “thing” is that every cell in the body contains all 23 pairs of chromosomes, which means that every cell contains a complete set of genes. The cells engage in specification very early after the sperm and egg unite. This is the basis for the debate over using “stem cells” from embryos. We are learning that cells even after the embryo stage may learn to express genes that have long been silent. However, not as completely and sometimes no. There are three types of stem cells, depending upon their ability to express genes. The embryo, of course, is able to fully express all its genes because the cells have not undergone specialization. There are two other types of stem cells, which can engage in only partial specialization, but I have forgotten the names for them.
How specialization occurs is not really known, AFAIK, but location is important. Location, location, location. Apparently the location of a gene determines different chemical signals it receives.
Ah, sorry for that, I meant genes that code for proteins that identify the cells as part of the body (like ABO proteins in the blood). And like I said, that is a possibility, neither the teacher nor the book said that was completely accurate.