The important reason to know about meiosis is to understand why/how every individual organism is different if reproduction is sexual, and that this difference constitutes the flow of genetic information within the ecosystem.
We start with Mitosis for comparison:
Mitosis is the process of cell division that results in identical daughter cells (clones). The daughter cells are identical because:
1. First each chromosome in the cell makes an exact copy of itself by the process of DNA Replication but the two identical copies remain connected, as often diagramed so:
The chromosomes (diagrammed unrealistically on the left) are unwound, incredibly long, and invisible to the microscope while they are doing their functions, but after they finish copying themselves they wind up tight and get ready for the cell to divide. At this point they become so thick that they are visible microscopically, and in order to identify them, people who like to look at chromosomes have agreed on a numbering system. For this example, let’s pretend the green chromosome is number one chromosome, still attached to its exact copy, all wound up inside a cell and ready to line up on the central plane of division. The purple chromosome we will name number four. This is a mouse, with 20
different kinds of chromosomes, but we don’t want to show all 20
in our example so we are showing only one chromosome one and one chromosome four. Maybe it looks to you like two of each chromosome, because the exact copy has already been made. But until the exact copies become separated, we say it is one chromosome of each kind.
2. Then the nucleus breaks down;
3. Then the chromosomes all line up individually along the plane of division. This is where the cell will pinch itself apart to make two cells;
4. While the chromosomes are lining up, microtubules come from each end of the cell, so that, for example, the chromosome number 4 has microtubules attached to each side of the chromosome and to their source at each end of the cell. And also the chromosome number one.
And then the microtubules simply pull the chromosome’s exact copies apart from each other, and the cell divides, on the plane of division, which is now between chromosome one and its exact copy — and between chromosome four and its exact copy, and all the other chromosomes. So the two cells each get an exact copy of each chromosome because of the way the chromosomes duplicate, then line up separately and are pulled, one duplicate to each end, by the microtubules.
HOWEVER. It’s not quite that simple. (You say you are not surprised?) I may have omitted to say that chromosomes of most higher organisms come in pairs. So in fact this mouse has 20 different kinds of chromosomes (1, 2, 3, 4, 5, 6, 7 etc.) in each of its body cells, and it has two of each kind. Two of chromosome number one and two of chromosome number two, and so on. One chromosome number one came from the father in the process of fertilization. The other chromosome number one came from the mother in the same process of fertilization. The two are not identical, as I will explain in the next post, but they do have the same genes in the same sequence encoded along their lengths. So all the number one chromosomes have the same genes, all the number two chromosomes have the same genes, but they are different from number one, etc. Some kinds of plants have even more copies, but we will stick with the basic principles of how these all get sorted out in meiosis.
So in our mouse example, every body cell has 20 different kinds of chromosomes that are numbered sequentially (according to their size), and every body cell has two of each kind (total 44 chromosomes). The other way to say the same thing is: Every body cell has two complete sets of chromosomes (20 in each set, one of each kind), one that came from the father and one that came from the mother. The condition of having two sets of chromosomes is known as diploid (di = two). The chromosomes that come from the father are homologous to those that come from the mother, but they are not identical. Homologous is defined below. Before that we need to take a closer look at sexual reproduction, because it is responsible for the variability among individual organisms, and the variability is responsible for the flow of information in the whole earth ecosystem.
The body cells all have two complete sets of chromosomes, but the sex cells have only one set. The sperm carries one complete set of chromosomes, and so does the egg. That means, in this example, the sperm has one of chromosome number one (it is haploid) and the egg also has one of chromosome number one (also haploid) so that when the egg and sperm fuse together the result is a normal fertilized egg (a zygote) that is diploid and can give rise to body normal body cells by the process of mitosis. Haploid means one/half the number of sets of chromosomes that are normal for that species.
Fertilization is the process of fusion of gametes. A gamete in mice is either an egg (from the mother) or a sperm (from the father). More formally, a gamete is a haploid sex cell, and meiosis is the process of cell division that results in haploid gametes. Every gamete is haploid so that every new fertilized egg can be normal.
So in a mouse, each of the body cells is diploid, meaning it has two sets of chromosomes. The sex cells, eggs or sperms, are haploid, so they each have one complete set of chromosomes.
It is the forever cycle of life; it is the foundational reality that permits evolution and the flow of information within the ecosystem. But before we get to that dramatic finale, the next question is:
How does that clever cell manage to sort out all those 20 pairs of chromosomes that are copied in every body cell of the mouse — so that each gamete gets one complete set — no more and no less? And isn’t that what we started out to talk about?
There are two tricks to meiotic cell division that make the results different from mitotic cell division:
1. The starter cell goes through two different sequential sorts of division during meiotic cell division, instead of one as in mitotic cell division;
2. The chromosomes line up along the plane of division in pairs, rather than individually, during the first meiotic cell division.
So here is the diagram, beginning with a normal mouse cell that has two sets of chromosomes. We are showing only chromosomes one (the green one) and chromosomes four (the purple one). All the other 20 pairs of chromosomes are similarly sorting themselves out at the same time.
Filed under: Bare Bones Ecology, Chapter 03-Information Flow | Leave a Comment »