The very first thing that I wish to explain here, is that there are two basic types of cells amongst the living organisms on planet earth. The first group is known as prokaryotic cells, and this article will for the most part not concern them. The most familiar prokaryotic cells are bacteria, and they are characterized by being primitive cells, without much internal organization and without organelles or a cell nucleus. It is the second group of cells, the eukaryotic cells, that this article is largely about. These are the cells of virtually all the life forms with which we are all familiar, like animals, plants and fungi. Their cells are in general larger and more complex than those of prokaryotes, and they contain their genetic material in a nucleus, a small, globular body surrounded by a membrane, in the cell. It is the contents of this nucleus that forms the focus of this article.

As you might know, the genetic material of the cell, also known as DNA (deoxyribonucleic acid), contains the blueprint of how to construct the cell. In eukaryotic cells, the DNA is organized in the form of a number of chromosomes, which are sections of tightly coiled DNA of varying length. When cells divide, the DNA in the chromosomes coil up more tightly, so that the chromosomes become more easily visible under a microscope as X-shaped structures. Any given species of organism has a specific, fixed number of these chromosomes.

Since the chromosomes contain the information necessary to construct a cell, it stands to reason that when cells multiply, they need to make copies of the chromosomes to pass on to their offspring. In this article, I am not going to go into the details of the nature of the genetic material or how exactly it is copied. Instead, I shall keep the discussion mostly on the level of the chromosomes themselves, and the changes they undergo during cell multiplication.

In all organisms that reproduce sexually, half of the cell's chromosomes come from the mother, and the other half from the father. For instance, you have 46 chromosomes in each of your cells, 23 of which come from your father, and 23 from your mother. Since this is an inconveniently large number, I am going to use as my example an imaginary organism with only 4 chromosomes, 2 from its mother and 2 from its father. Let's call this little creature a "boonk".

As with all creatures, the boonk originally started life as a single cell. In order to grow, that single cell somehow had to turn into the millions that now make up the boonk's body. A cell can do this by dividing into two cells, which each in turn divides and so on, until the necessary number of cells is achieved. This division of cells, known as mitosis, is at the base of a lot of biology, and is one of the most fundamental processes in nature.

Let us first take a look at the boonk's four chromosomes. I am going to call them A, a, B and b. Chromosomes A and B come from his father, and a and b from his mother. Chromosomes A and a are visually identical: they have the same length and cannot be distinguished from one another by just looking at them. Such chromosomes are known as homologous chromosomes, and not only A and a, but also B and b, are homologous.

Every chromosome contains entities known as genes, which contain the coded information to construct specific proteins. The interactions between these proteins then construct and maintain the organism's cell and body. As you can imagine, the details of this are very complex and have not been fully worked out for even simple cells, but in some cases, there is a fairly simple correspondence between possessing a specific form of a gene, and possessing a corresponding physical attribute. For example, a specific gene could correspond with eye color.

Now an interesting thing about two homologous chromosomes is that not only are they visually the same, they also contain corresponding genes in corresponding physical places on the chromosome, known as loci (singular locus). For example, if chromosome A has a gene for eye color on a specific locus on the chromosome, then chromosome a will also have a gene specifying eye color on the corresponding locus. These genes code for the same thing, but they are not necessarily identical. The gene on A might code for blue eyes, while the one on a might code for green eyes. Which eye color is actually displayed in the boonk depends on interactions between these two genes that might range from quite simple to quite complex.

Now let's take a look at the actual process of cell division. Before division, the chromosomes become thicker and shorter, and the nucleus loses its membrane, so that the contents of the nucleus mixes with that of the rest of the cell. It is in this phase that the chromosomes become visible as X-shaped bodies in the cell. Since A and a, and B and b are visually identical, we'll see two pairs of chromosomes in the cell. Assuming that B and b are somewhat smaller than A and a (the chromosomes in any cell are typically of varying sizes), we can expect to see two large X's and two small x's floating around in the cell, and this is in fact the case. If you divide any of the chromosomes lengthwise in two, you are left with two pieces, looking like a > and a <. Each of these pieces is known as a chromatid, and they are connected to each other by a centromere to form the X of the chromosome.

The four chromosomes now align themselves with their centromeres along the middle of the cell. From opposite poles of the cell, thin threads known as microtubules begin to extend towards the chromosomes, eventually attaching themselves to the centromeres. The microtubules then begin to pull the two chromatids of each chromosome towards opposite sides of the cell, breaking the two halves apart at the centromere. A new cell membrane begins to extend inward from the sides of the cell, eventually dividing it in two, each of the two daughter cells containing one chromatid of each original chromosome.

A question now presents itself: since each chromosome has been divided into two, does this not mean that each daughter cell has now lost some of the original genes in the chromosome? The answer is no, because the two chromatids in any chromosome are identical copies of each other. We have seen that while two homologous chromosomes might be visually the same, they are not necessarily identical. But the two chromatids of a chromosome are in fact truly identical in every way: each chromosome contains two copies of the same information. Before the two daughter cells formed during mitosis can divide again, they make copies of their chromatids in order to form new chromosomes, and usually the cells also enlarge.

Up to now, we have seen how cells can multiply themselves, so that the organisms they are part of can grow. But what happens when organisms reproduce? We all know (hopefully!) that reproduction involves a male and female cell meeting and fusing together to form a single cell, known as a zygote.
. . But in light of what we know about chromosomes, this raises a problem: if two boonk cells with four chromosomes each fuse, they will form a zygote with eight chromosomes. When this zygote grows up to become a mature boonk, and it wants to reproduce, it will involve fusing two cells with eight chromosomes each, forming a sixteen chromosome zygote. And so on. But in fact, the chromosome number of all organisms remains constant across the generations, and even our imaginary boonk is no exception. So how does this happen? We shall take a closer look at this question in the second part of this article.

When I first introduced the boonk, I mentioned that two of its four chromosomes (A and B) are from its father, while the other two (a and b) are from its mother. But the boonk's parents were also boonks, so they also must have had four chromosomes in each of their cells. How did they manage to each pass along only half that number to their offspring? The answer is that when reproductive cells form, they do so by means of a different form of cell division known as meiosis, in which the number of chromosomes is halved. This explains how the chromosome number of organisms can remain constant.

The reproductive cells, namely the sperm and egg cells, originate from specialized mother cells in an organism's reproductive organs. A meiotic division is broadly similar to mitosis. The DNA also coils up more tightly, and the chromosomes become visible as the same X-shaped bodies we encountered in mitosis.
. . The difference comes in when the chromosomes align themselves in the middle of the cell prior to division. As we have seen in the discussion of mitosis, during a normal mitotic division, the chromosomes all line up with their centromeres along the cell's equator. In meiosis, homologous chromosomes line up on opposite sides of the cell's equator. Microtubules now attach to the centromeres of the chromosomes, and the chromosomes themselves, instead of the chromatids, are pulled away from each other. Two chromosomes end up in one half of the cell, and in the other, after which the two halves separate to form two cells with only half the number of chromosomes of the original. Since each of these chromosomes still consist of two identical chromatids, each of the two daughter cells will undergo a further division, but a normal mitotic one, so that we end up with four cells, all of which have two chromosomes instead of the original four.

A further feature of meiosis is noteworthy. The boonk has the chromosomes A, a, B and b. As mentioned before, it got A and B from its father, and the other two from its mother. Each of its reproductive cells will have two of these chromosomes, but when the chromosomes separate during meiosis, they do so independently. That is, its reproductive cells will not only have the chromosome arrangements AB and ab. The arrangements Ab and aB are equally possible. In this way, meiosis not only reduces the chromosome number, it also randomly reshuffles genetic combinations. And the independent sorting of chromosomes during meiosis is not the only way in which this reshuffling of genes takes place. When the homologous chromosomes line up facing each other, they often also exchange little sections of their DNA, leading to an even further random shuffling of the genes. The advantage of this is that new genetic combinations are created in this way, leading to more variability in the offspring. This in turn means that should the environment in which the organism lives change, there is a greater chance that at least some of the new variants will be able to survive.

Meiosis also helps to explain why different species can usually not successfully interbreed. Two very different species, say a cat and a dog, will not be able to interbreed at all, even if they tried. (Which they don't in the first place, because they are too different to have any sexual interest in each other). But not only their behavior is incompatible, their reproductive cells will not fuse either, and even if they do, such a cross would have two sets of genes that are so different that they will not be able to work together to produce a viable organism. So there are quite a number of mechanisms that serve to keep different species apart. The science fiction scenario where humans and aliens can interbreed is indeed nothing more than science fiction.

But two closely related species can sometimes interbreed and produce viable offspring. The mule, the offspring of a horse and a donkey, is perhaps the most familiar example. But as is also well known, mules are sterile, and cannot produce little mules. The reason for this is that meiosis is necessary for the formation of reproductive cells, and for meiosis to occur, homologous chromosomes need to line up on either side of a cell, after which they will move apart. When two different species cross, they pass on two different sets of chromosomes, so that the resulting organism does not have homologous pairs of chromosomes. This causes meiosis to fail, so that viable reproductive cells are not formed.

And that concludes this article. The twin processes of mitosis and meiosis can be tricky to understand at first. But they lie at the base of a whole lot of modern biology, and the study is well worth the effort.

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