I know I should have learned this stuff way back in grade school, but I never quite got it.
What’s a chromosome? How many DNA strands are in a chromosome? Do chomosome pairs correspond to DNA pairs which correspond to gene pairs? What’s bigger, a gene or a DNA strand? Is DNA a single (or double) molecule, or a bunch of them?
When explaining, pretend I’m four.
When a Mommy and a Daddy love each other very much, Daddy puts a special seed into Mommy’s tummy, it meets with Mommy’s egg and it grows into a baby.
So he puts the seed in her mouth and she swallows it? Interesting.
But seriously, folks…
Hey, I can answer some of these!
A DNA strand. A gene is part of a DNA strand.
DNA is a single molecule, but it’s an enormously long molecule. I’ve heard that if you it straightened out could be measured in meters.
*What’s a chromosome?
Basically a long piece of DNA with genes and other stuff (things that control DNA copying, “junk DNA” that might have some unknown purpose etc.) on it.
*How many DNA strands are in a chromosome?
one piece of double stranded DNA. Now the X shaped things that you see in pictures represent two chromosomes that are joined to each other at the center of the X.
*Do chomosome pairs correspond to DNA pairs which correspond to gene pairs?
By DNA pairs, do you mean the two sides of the “ladder”? If so, then no. Each chromosome is made up of double stranded DNA (the ladder and rungs). Chromosome pairs do correspond to gene pairs though. If you find gene A on chromosome 1, you will also find some version of gene A on chromosome 1* (which is chromosome 1’s pair.
So since we have 23 pairs of chromosomes, we have 46 (23*2) strands of double DNA, or 92 half-strands?
Per cell?
Yes, basically. To take it a litte further: The chromosomal DNA is found in the nucleus of each cell. Red blood cells don’t have a nucleus, so no strands of DNA in them. Germ cells (eggs and sperm) should only have 23 strands. Our cells also have another source of DNA, which is found in the part of the cell called the mitochondria. The DNA in the mitochondria is not part of the 23 pairs of chromosomes, so there’s a little bit more that has to be added to your total.
DNA is the way in which organisms encode their genetic makeup. The term is used to describe both the whole package, as in the whole DNA within a cell, or it is used to describe the basic building blocks, as in the DNA required to make a gene.
From the ground up, DNA is a funny helix structure with four variations, A C T G (and some times U). As a comparison, we use 26 letters to make up our alphabet, those letters make words, the words make a sentence, the sentences make a thought.
DNA arranges itself into a string of those letters to form a gene. It is the gene that is responsbile for the functions of our body. For example, there is a gene that makes the protein insulin, insulin then lets your body use sugar. For every function within your body, there is a corresponding gene or genes.
A chormosome is then a collection of genes, along with a lot of filler. The chromosome gets coiled up tightly, the way you might see on a site like Wikipedia.
A collection of chromosomes then make up the genetic material of said organism.
Humans have 23 pairs of chromosomes (diploid), that is, we have 46 chromosomes. One set came from your mother, the other from your father. Each organism will have a different number of chromosomes; dogs have 78, a fruit fly as 8. Some things like wheat will use a triploid (three sets), freaky things like mushrooms just keep collecting the stuff.
Hope that helps a bit. Its kind of a BIG topic.
OK, I’m starting to get it. It’s a big complicated topic, but the basics are pretty easy to understand.
So is a gene more than just one rung on the ladder?
Also, when a mommy and daddy love each other very much, and commence the boinking like bonobos, what do they contribute to the offspring? Half of chromosome 6, for instance, with everything it contains? For instance, let’s say (this is way oversimplified) that chromosome 6A contains genes for blond hair and blue eyes, while chromosome 6B contains genes for black hair and brown eyes. Is it impossible for the father to contribute genes for blond hair and brown eyes? Do hair and eye color come as a package set, depending on which chromosome half is contributed?
Well, U is only in RNA, not DNA.
Yes.
Yeah, 46 chromosomes in most humans, and those 46 chromosomes are exactly duplicated (other than an occasional mutation) in the nucleus of almost every cell. As noted above, each chromosome is a really long strand of DNA.
As Omega notes, sperm and egg cells typically just have 23 chromosomes, one half of each pair. But which pair? That’s up to chance, so for a human, that’s 2^23 different potential sperm or eggs s/he could produce. (Is my math right?) So each child is a combination of 23 of dad’s chromosomes and 23 of mom’s, but that child’s siblings could conceiveably share no DNA with him if they happened to get an exact opposite set. What is astondingly more likely, of course, is that the sibs will share some chromosomes and not share others.
These chromosomes build the body through proteins. Which proteins get built is a funcion of what the DNA says. As you know, DNA is a ladder. The rungs of the ladder are made up of four nucleotides (just a part of the DNA molecule) - cytosine, guanine, thymine, and adenine. Each rung of the ladder is a bonded pair of two of the nucleotides, one that’s woven into the “left” side of the ladder and one that’s woven into the “right.” Because of the way these four nucleotides are shaped, they only bond with one of the other four. So if one side of a rung has a C, the other side of the rung is always G (and vice versa), and if one side of a rung is T, the other side is always A.
This is the elegant thing about DNA. If you split the molecule into two pieces down the center of the rungs, you get two half-strands, each of which contains all the information of the whole. Because if a half-strand has a G rung, you know the other half strand has a C at that position, etc. This is how DNA can self-replicate – a cell splits the chromosomes in half and then builds a full molecule from each half.
This is also how DNA builds the body. Your cells do this by translating the information in your DNA into proteins. Each gene is the code for a protein. The way it works is that your cell splits the DNA of a chromosome (actually it’s done pieces at a time) into a half of a ladder. The cell then builds a matching molecule of RNA, which is a very similar molecule to DNA (but with an extra oxygen atom on the side of the ladder “above” each rung, IIRC – hence ribonucleic acid vs. deoxyribonucleic acid). RNA is geometrically similar to DNA and understands the same four-letter language. (Well, it uses uracil instead of thymine, but it doesn’t much matter because U, like T, only bonds to A.)
Once the RNA is done, it moves out of the nucleus and the DNA gets zipped back up into a pair. The RNA goes out into the cell where the protein-making machinery is. Proteins are long strings of amino acids. Amino acids are complex molecules which have a variety of different shapes (20, in fact) but essentially all of them have two identical branches – the amine group and the acid group. These two groups are designed to bond with each other, so you can make a big long string of amino acids, each one’s amine group connected to its neighbor’s acid group all the way down the row. Proteins are these very long strings. (I believe a protein must be at least 50 animo acids long, but most of them are hella longer.)
So you’ve got your RNA molecule with its list of ACGGUCUAAGU, or whatever. Each three-letter sequence of RNA codes for one particular amino acid. So a given strand of RNA is a recipe for making a particular protein, and only that protein. It is proteins which make up the structures of the body, and it is proteins that do the work of growing, fueling, and maintaining it. Each protein, because of the geometry of the amino acids that make it up, balls up into a particular shape when it is built. These shapes interact in specific ways with other proteins with their own unique shapes as well as other stuff they encounter (oxygen, e.g.). The particular interactions, drivin by the particular shapes of all that stuff, is how the body is created and operates. Since the operation of a protein is a function of its shape, and its shape is a function of the amino acids making it up, and the amino acids are a function of the RNA that ordered them up, and the RNA is a function of the DNA it was built from, the various operations of your body can be traced back to those ACTG code letters in your DNA.
The thing is, there are occasional transcription errors when you unzip a DNA molecule to copy it. If this happens, say, in a sperm cell that later fertilizes an egg, that error will become part of the child’s DNA code – since all the nuclear DNA in the child as it grows is a copy of a copy of a copy of a copy (etc.) of the original DNA in its parent’s sperm and egg. Often these mutations don’t do anything. Sometimes they cause problems. And, of course, sometimes they are beneficial, making that individual more likely to survive than his counterparts without the mutation. And that means he’s more likely to have kids (because you can’t have kids if you’re dead), which means he may pass that mutation on to his children (if they happen to get the chromosome that has the mutation, and not its pair). And that is evolution.
–Cliffy
Okay slight correction. DNA never has a U. DNA is always ATCG. The building blocks of RNA are AUCG.
A gene is much more than just one rung on a ladder. Genes are made up of a combination several As Gs Ts and Cs. On the DNA ladder, An A on one ladder leg will only bind to a T on the other ladder leg. A C only binds tightly to a G. So an AG pairing and a CG pairing make up one rung. Several of the rungs make up a gene. All of the rungs in a gene might not be right next to each other. When it is time for a gene to be used, a protein cuts out the pieces of DNA that are separating sections of the gene. Each gene is a code that tells the body to make a specific protein.
Example: Mommy’s body has two copies of chromosome 6 called sister chromosomes. One from her mom, and one from her dad. We’ll call them C6A and C6B. Lets say that C6A calls for black hair and brown eyes*. C6B codes for blond hair and blue eyes. During egg production, C6A and C6B bind to each other. This is the X shaped thing that you often see. While the two copies of C6 are connected to each other, they swap some genes. So the egg might get new chromosome C6C, which calls for brown hair and blue eyes, or C6D which codes for blond hair and brown eyes. The chances of the egg getting a chromosome that hasn’t swapped genes at all are incredibly low. Now if both sister chromosomes have the same hair color gene, no one will notice that a swap has occurred.
The same process happens with Daddy’s sperm. Baby get’s one chromosome 6 from mom, and one from dad. Baby’s C6 #1 is a mixture of mommy’s C6 #1 and #2. Baby’s C6# is a mixture of daddy’s C6 #1 and C6#2. Both parents are donating and entire chromosome to the child. If a parent donates more than one copy of each chromosome, or less than a full chromosome, then bad things can happen. For example, Downs syndrome results when baby accidently gets two copies of chromosome 21 from one parent and one copy from another, and ends up with *three * C21’s instead of the normal two.
Aaand I see that some of my post is just a repeat of what others have said. Well can’t hurt to say it more than once:)
Oh, I forgot to deal with the asterisk in my post from this sentence “say that C6A calls for black hair and brown eyes*.”
This is all very simplified since hair color and eye color are determined by more than one gene. IIRC, eye color is controlled by three seperate genes.
Yes. Proteins are made up of long strings of amino acids, and each amino acid is coded by a three-letter sequence. Since a gene is the strand of DNA that codes for a specific protein, genes are hundreds or thousands of rungs long.
Remember that each human has 23 pairs. So each parent doesn’t contribute half a chromosome – he contributes one of each of the 23 chromosomes – half of a pair.
So, to kind of use your example, let’s say the dad has brown eyes. You look at him, you think “That dude’s eyes are brown.” If you looked at the spot for the eye-color gene on his DNA, he’d have two versions, called alleles-- one allele on one of his chromosomes and another one in the same spot on that chromosome’s pair. He got one of these from his mom and one from his dad.
Here’s what might be happening. (I don’t know anything about eyes specifically and don’t know if this is actually happening, although I’ve heard eye color in particular is more complicated than this.) One of these alleles codes for brown eyes by making a protein that colors irises brown. The other one codes for blue eyes by making no protein at all. Let’s say the materials that make the iris are naturally blue. So it takes extra protein to color it. (Again, this is by way of example, I don’t think this is actually true.)
Becasue he had one brown-coding allele, his eyes look brown. He marries a brown-eyed woman. Unbeknownst to either of them, she also has one brown-coding chromosome and one nothing/blue-coding chromosome. They then have four children.
Each child’s chromosomes are half mom’s and half dad’s. In every sperm, Dad has a 50-50 chance of passing on his copy of chromosome 6 with the brown allele vs. the nothing/blue one. In every egg, Mom has the same chances. So for any kid, each pairing has a 25% chance. (Dad’s A and Mom’s A, Dad’s A and Mom’s B, Dad’s B and Mom’s A, or Dad’s B and Mom’s B all have equal chance.) Of the four kids, by chance, they each have a different one of these sets.
Kid 1 has a brown allele from both Mom and Dad. Kid 2 has brown from Mom, blue from Dad. Kid 3 has blue from Mom, brown from Dad. And Kid 4 has blue alleles from both parents. So we look at the kids. In a pure dominant/recessive relationship (which I’m pretending this is, even though I think eye color isn’t really), Kids 1, 2 and 3 all look to have the same color eyes. In actuality, Kid 1 is pupming out twice the brown coloring because both members of his pair are making it, but to all appearances, his eyes look the same as Kid 2 and 3’s. Kid 4, on the other hand, has none of that brown-coloring protein, so his eyes are blue.
But let’s look at the next generation. Kid 4 will, by definition, contribute only blue-eyed alleles to his offspring, and since blue is recessive, his children’s eye color will be determined by whatever their mom gives them. Kid 1 will always contribute brown and (assuming hypothetically that brown or blue are the only choices) therefore all his kids will have brown eyes, although they might be homozygous like he is (two brown alleles) or heterozygous like Kid 2 and 3 (differing alleles). Kids 2 and 3 each have one of each. Even though they have brown eyes, they could pass on the blue allele to heir kids, just like the dad did to them. And maybe their kids are all bron-eyed. And maybe their’s too. And theirs. BUT, if all those matings were with partners that contributed the brown allele, the recessive blue gene might possibly have survived by being passed down by chance each time as the recessive gene. And so one day a guy who’s had all brown-eyed ancestors for 5 generations marries a brown-eyed girl but their son might have blue eyes.
As for the package set business, I don’t believe that hair and eye color are on the same chromosome. And things are complicated because chromosomes actually swap genetic material with their pair-mates. But ignoring that for a second, yes, genes do come in packages, because if you get the eye-color gene from one end of your paternal grandmother’s version of chromosome 6, you’re going to also get (say) her flat feet gene from the other end.
–Cliffy
Oh, let me also make the simplification caveat – in my case it’s because I don’t actually know anything about the complicating factors. (I was a Philosophy major for pete’s sake!)
–Cliffy