Lesson 1:
Before I begin the lesson I just want to state that you must have some knowledge of a cell's structure, including its organelles, in order to go on.
A cell is the fundamental form of life on this planet. There are some organisms that are unicellular or single-celled and there are others, like us, that have many cells (multicellular). A cell must duplicate itself. There are two ways for a cell to do this, mitosis and meiosis. These two forms of duplication sound similair and are kind of similair, but they have very different results.
When a cell divides itself by mitosis, it creates two "daughter" cells which are exact copies of itself. Mitosis is divided up into five different sections, interphase, prophase, metaphase, anaphase, and telophase (IPMAT).
Interphase is really just the cell when it is undergoing normal functions like duplicating its organelles and chromosomes.
In prophase the nuclear envelope begins to dissolve leaving the double chromosomes bare. Each separate chromosome is now called a chromatid. These are attached to each other at the centromere (see figure). Spindle fibers are being formed between two pairs of centrioles which lie at the poles of the cell. Some of them attach to the cell's chromosomes. These fibers are what determines the direction in which the cell will divide.
The next stage is metaphase. During this stage, the chromosomes simply move to the center of the cell. The chromosomes are arranged in such a way that they are perpendicular to the spindle fibers and they are lined up single file.
While anaphase is beginning the chromosomes separate, leaving two chromatids floating away from each other, pulled by the spindle fibers in opposite directions. The fibers pull the chromosome strands (previously chromatids) to the opposite poles of the cell.
The chromosomes gather together at the opposite poles during the final event, telophase. New nuclear envelopes form around each group of chromosomes, which means that the cell now has two new nuclei. The two nuclei contain a complete set of chromosomes that are identical to those of the parent cell. In the meantime the other organelles move to the opposite ends of the cell, then the cytoplasm divides (this is also called cytokinesis) into two roughly equal parts.
Animal and plant cells are structured differently. For example animal cells have centrioles and plant cells do not. Plant cells have a cell wall (which allows them to grow very tall and stay sturdy) and animal cells don't. This means that mitotic division has to be a little different for plants, but that doesn't mean that it is very different. The spindle fibers are formed inside the plant cell's cytoskeleton. The major difference between plant and animal cell mitosis is that during the final phase in the plant cell, a cell plate forms across the middle of the cell, and this plate divides the cell in two.
When organisms reproduce sexually, two cells meet and fuse to eventually become the offspring. These two cells are called sex cells, or gametes. One is supplied by the male and the other is supplied by the female, so logically, one is the male gamete (the sperm) and the other is the female gamete (the egg or ovum).
Why are these cells the cells of choice for sexual reproduction? What makes them different? The difference is that these cells only contain half the total chromosomes of the species. For example, our species has 46 chromosomes. Within these chromosomes is the genetic information that makes us what we are. A human gamete has only 23 chromosomes. When an egg and a sperm unite, they form a zygote, which has all 46 chromosomes. This works out nicely since now the offspring will have half of the genetic information of its mother, and half of its father.
Meiosis is the process in which sex cells are formed. It is similair to mitosis but obviously must have some very major differences in order to achieve the sperm or egg, rather than the two identical daughter cells. It is seperated into two sections, each of which have four different parts. The sections of meiosis are as follows: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, and finally telophase II.
During the beginning of prophase I, each chromosome is a double structure consisting of two chromatids joined at the centromere (like mitosis). The nuclear envelope begins to disapear making the chromosomes more distinct. A subprocess known as crossing-over goes on as well. Matching chromosomes (remember that the chromosomes have replicated) will come close together and sometimes they may twist around each other and matching parts of the chromatids may brake off and exchange places, thereby mixing up the genetic information, allowing for a little more genetic diversity. Finally towards the end of prophase I, the chromosomes make their way towards the equator of the cell. The centrioles in animal cells in the meantime are moving towards the opposite poles of the cell and start creating spindle fibers which attach themselves to the chromosomes.
In metaphase I the chromosomes have arrived at the center of the cell. The chromosomes then line up in pairs. This is different from mitosis when they line themselves up single file.
During anaphase the matching chromosomes in each pair move towards opposite poles of the cell. The chromosome is still a double structure consisting of two chromatids joined at the centromere.
As the chromosomes begin to approach the poles of the cell, telophase I begins, and the cell splits into two. Now each new cell only has half as many chromosomes as a body cell (they can be called haploid and the body cells can be called diploid. These terms are represented n, and 2n, respectively). This is the first meiotic division, but wait, there's more! A whole second division that happens after the first! This second meiotic division is going on in both of the haploid cells at the same time.
During prophase II nothing really goes on, except that those little old centrioles are at work, doing what they do so well.
Then ofcourse comes metaphase II, the double stranded chromosomes again move to the equator of the cell, only this time they are lined up single file, just like mitosis. There are also no matching chromosome pairs.
During anaphase II the centromeres divide, and the two chromatids of each chromosome start moving towards opposite poles (by the way, they are now reffered to as chromosomes).
As the chromosomes gather at the poles of the cell, new nuclear envelopes form around them. Then once again, cytokinesis occurs, and wa la, we've got four haploid cells.
Meiosis obviously occurs in both males and females. Males get sperm because of it, and females get eggs. But there is a difference.
Males will usually get all four of the cells that result from meiosis to be sperm, but females usually only end up with one egg. The
reason for this is that an egg needs a lot more cytoplasm than a sperm, so when it splits, it throws half of the chromosomes into
something called a polar body. This is a very small cell which has very little in it aside from the chromosomes. The next meiotic division
results in another polar body. Usually the two polar bodies disintegrate, and the one full sized egg remains.
On to Lesson 2 -- Genes