At the very beginning of Chapter 5, your book gets you thinking about how a cell can divide. They point out a remarkable fact-- that we all start as a single cell, the fertilized egg or zygote. This one cell has to divide enough to give rise to the trillions of cells that make up who we are!
Consider the genetic content of the zygote. You know it is the DNA inside its nucleus. In humans, we have 46 molecules of DNA inside each cell's nucleus. When that zygote divides, and then divides again and again, what does it do with its DNA? In each division, does the DNA get split up or not? If you really think about this for a while, you will realize that 46 molecules of DNA could not get split up trillions of times. In fact, every single cell needs all those molecules in order to function. So, before each cell division, a cell will have to double its DNA, making an exact copy, so that each of the future cells will have the full complement of DNA. Making a copy of all the DNA is called DNA replication.
To really get familiar with cell division requires that you learn a bit more about the DNA in a cell itself. We'll do that on this page. Then we'll get back to the actual cell division.
DNA, what is it?
We're going to get into a detailed understanding of DNA in chapter 6. But for now, you know from Chapter 4 that DNA is a nucleic acid macromolecule, typically made up of millions of nucleotides. If you think about what a DNA molecule would look like, then, it is very, very long, and very, very thin. Something so long and thin would be vulnerable to getting tangled or broken if it floated around within the cell... that is why our cells store our DNA within the nucleus!
Also, the information in the DNA, our genetic code, is actually encoded through the nucleotide sequence that makes up the DNA. If we need to read the genetic code, for example, in order to make a protein or to replicate our DNA, we have to be able to see each and every nucleotide.
When it is time for a cell to divide, it has to replicate its DNA and sort its DNA, so that each cell gets a complete set. I already mentioned the need for DNA replication. And, the sorting of the DNA is just to make sure that each future cell (called a daughter cell) ends up with a full complement of the DNA molecules, and not 2 copies of one DNA molecule and none of another. You will learn about how the sorting works as you study mitosis.
But if you think about the task of sorting out 92 molecules (2 sets of 46) of DNA, you might realize that this task could be problematic... After all, each molecule of DNA is so long and thin that it might be easily torn or broken during sorting. Also, how can 92 molecules be sorted within the tiny confines of the nuclear envelope? So, for DNA to get sorted, these problems are taken care of.
What is a chromosome?
Basically, a chromosome is molecule of DNA. However, we cannot normally see that. For cell division, DNA doesn't stay loose, long and thin. Instead, it gets packed up tightly. In fact, it gets packed up so tightly that we can even see it under the light microscope. And in your mitosis lab, you will be seeing it.
When we can see a DNA molecule is when it is packed up tightly, so that is also called a chromosome. To give you an idea of how much packing goes on, if the DNA molecule were as long as a football field and as thin as sewing thread, when it got packed up, it would be only as long as approximately 1 cm, and about 2 mm thick. Can you imagine trying to fold up such a long piece of thread into such a small bundle? You would really have to wind it up very tightly, right? Well, that is what DNA does so that it can be sorted during cell division. Therefore, when a cell is going to divide, it has to take all of its DNA and pack it up into chromosomes. The packing process is called DNA condensation.
When a cell is not dividing, it is doing everything it needs to do to stay alive. And you have seen that cells are constantly making proteins with their nucleus, ribosomes, rER and Golgi complex. So, during the rest of a cell's life, when it is not dividing, it needs to be able to access its genetic code. That means that the DNA cannot stay folded up all the time as dense chromosomes. The rest of the time, a cell has to have loose, unpacked, readable DNA. The loose form of DNA is called chromatin. So after a cell has divided, it has to unpack its DNA back into chromatin form... the unpacking is DNA decondensation.
Finally, keep in mind that the words "chromosome" and "DNA molecule" are used interchangeably. Most non-science people don't know what a "DNA molecule" mean, but would have some idea that a chromosome is something genetic. However, when the DNA is all loose, the pile of it with its associated proteins are called chromatin (even those it is chromosomes/DNA molecules that are loose).
What does a chromosome look like?
Well, that depends on when during cell division you look at it. I told you that before cell division can get going, and before DNA condensation, the DNA goes through replication. That means, when the chromosomes first become visible, there are 92 molecules of DNA, right? A significant part of the way a cell can sort these molecules is to keep the identical copies together. So after a molecule has undergone DNA replication, the two identical copies of that DNA molecule remain together. Sorting occurs as each of these sets of identical molecules are separated from one another. Therefore, at the end of cell division, each future daughter cell will have only one copy of each.
Please note that in the middle drawing above, where there are two halves to the chromosome, each half is called a chromatid. The terminology here is very difficult, because the words chromosome, chromatin, and chromatid are all so similar. Because the two chromatids are identical, they are called sister chromatids.
What is a gene?
A gene is a region on a chromosome. A specific region that contains the information to make a protein. This is shown for you in your book in figure 5-5 very nicely. Each chromosome contains hundreds or thousands of genes. In Figure 5-5, only some of the genes found on chromosome number 11 are shown. There are many others that they don't show. Of course, we can't use the information in a gene when the DNA is all bundled up in chromosome form, only when the DNA is in chromatin form, but we can still point to where along the length of the DNA the gene lies.
What are homologous chromosomes?
Now you know that there are 46 molecules of DNA in every human cell. Well, another way to talk about these molecules is by saying that they are organized in pairs. The pairs are called homologous chromosomes. That means that there are 23 homologous pairs in every human nucleus.
A homologous pair of chromosomes has nothing to do with sister chromatids. Nothing at all. Sister chromatids are within one chromosome, and they are identical.
Homologous pairs of chromosomes are not identical... just similar. Remember way back in genetics when you learned that every gene is represented by two alleles? One is allele is on one chromosome, and the other is on the homologue of that chromosome. I have tried to show this in this diagram.
Here I have depicted the skin color alleles and the height alleles on homologous chromosomes (although this isn't at all an accurate representation like you have in Figure 5-5). Your skin color alleles might be the same or different, but you will express the dominant allele.
Homologous chromosomes code for the same genes. All along the length of a chromosome are the genes it codes for. Along the length of its homologue, the same genes are coded for in the same order... but the specific allele may differ. So the allele that says that you will have dark skin is on a separate chromosome from the allele that says that you will have light skin; yet, the two separate chromosomes also code for all the same other genes (like height in this case) and are thus called homologous.
Human chromosome #11 that is shown in Figure 5-5 has the gene on it for hemoglobin-- and if it is in a slightly messed up version, it is the sickle-cell anemia allele. Human chromosome #12 is the homologue for #11. It would also have an allele for hemoglobin on it. As long as the allele on one of the two chromosomes is good, a person will not have sickle cell anemia. Do you remember learning that before?
My drawing above shows chromosomes that are in a cell that is just about to undergo cell division. I drew it that way so that you can see that the sister chromatids are not the same thing as homologous chromosomes. Sister chromatids are identical, and even have identical alleles for every gene. Homologous chromosomes are not identical-- they just code for the same genes, but do not necessarily have the same alleles. The drawing in Figure 5-5 shows a chromosome as it would look in a cell at the end of cell division.
During mitosis you will see that we won't talk at all about homologous chromosomes. Because during mitosis, we need to take a cell that has 46 chromosomes and divide it to make identical daughter cells, each with 46 chromosomes. So each daughter cell will also have homologous chromosomes within them. But we will talk about homologous chromosomes a lot more in next week's lesson on the other type of cell division.
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007