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Biology - Reproduction

Cells normally reproduce by dividing.  The division is called mitosis.  You should know that mitosis has four stages, and you have to know what goes on in each stage.  You should also know what goes on before mitosis.

Before a cell undergoes mitosis, every single chromosome in its nucleus has to reproduce itself.  That is called interphase.  It is called interphase because it takes place in between two mitotic divisions.  It happens after a previous mitotic division has occurred and before the next one occurs.  Most of the cell’s life is spent in interphase.

After interphase, each chromosome and its duplicate are joined in the middle by a centromere, so they make one discrete physical structure called a chromosome made of two chromatids.  So for example in a human cell, even though all chromosomes have been duplicated, we say that after interphase the cell still has a total of 46 chromosomes.

After interphase, the cell is ready to begin mitosis.  When the cell’s chromosomes have fully replicated, the cell is ready to begin mitosis.  The following are the four steps of mitosis observed under the microscope.

In step one or prophase, the chromosomes thicken and become visible.  Meanwhile, the centrioles move to opposite poles of the cells, and next a bunch of lines called tubules form a spindle.  Aster fibers appear.  Aster fibers form around each centriole.  Finally, the nuclear membrane begins to fall apart.

In step two or metaphase, the double-stranded chromosome line up in a column at the center of the cell.

In step three or anaphase, each double-stranded chromosome divides at the centromere.  The chromatids separate.  The separated chromatids then move toward opposite poles along the spindle.  Each chromatid has now become a single-stranded chromosome.

In step four or telophase, the cytoplasm divides.  This is called cytokinesis.  The nuclear membranes re-form and two daughter cells result, each with single-stranded chromosomes.

Now, we will talk a little about meiosis.  Meiosis is the process from which sperm and ova arise.  Ova and sperm are haploid, not diploid.  When we think about humans, we remember that ova and sperm are the only haploid cells in the body and they only arise through meiosis.  That means meiosis must somehow start with a diploid cell and produce a haploid cell.  The easiest way to understand meiosis is to compare it to mitosis and see how it is similar and how it is different.

Let us talk about the formation of sperm cells or spermatozoa.  Before meiosis begins, the diploid cell goes through interphase just as it does before it begins mitosis.  It is then called a primary spermatocyte.  In humans, the spermatogonium has 46 single-stranded chromosomes, and the primary spermatocyte has 46 double-stranded chromosomes.

Meiosis features the same four stages that are associated with mitosis: prophase, metaphase, anaphase and telophase.  However, in meiosis during prophase, synapsis occurs.  The two pairs of homologous chromosomes come together and form a tetrad.  When synapsis occurs, pieces of DNA can be exchanged.  This exchange of genetic material is called crossing-over.  This results in genetic recombination.

In mitosis, every double-stranded chromosome lines up on the spindle, and then in anaphase the centromere separates the two single-stranded chromosomes.  In meiosis, however, the two pairs of homologous chromosomes – a tetrad – line up together on the spindle.  Then in anaphase, the centromere does not break up.  The two identical pairs of double-stranded chromosomes separate, each with its centromere intact.

At telophase, two daughter cells are formed.  Both cells are now called second spermatocytes, each containing 23 double-stranded chromosomes and is called a haploid cell.  Each daughter now contains duplicate chromosomes still joined by a centromere.

Each of the two daughter cells goes through a second division that is just like mitosis.  There is a prophase, metaphase, anaphase and telophase.  In each cell, a spindle forms and the 23 double-stranded chromosomes joined by a centromere line up and then separate at the centromeres.  The result is four daughter cells, each one with 23 single-stranded chromosomes.  At this stage, they are called spermatids.

So actually, there are two cell divisions.  The first division is called the first meiotic division.  It is also called the reduction division since the two daughter chromosomes end up with 23 double-stranded chromosomes instead of 46 single-stranded chromosomes.

The second division is very much like mitosis.  In each daughter cell, a spindle forms and the 23 centromeres separate.  Then the centromeres divide and each daughter cell in turn gives rise to two new daughter cells, each one having 23 single-stranded chromosomes.

The testis is where meiosis takes place in male.  The testis has a lot of little tubes called seminiferous tubules in it.  These seminiferous tubules get together to form one big tube called the vas deferens.  The wall inside of the seminiferous tubule is made of cells, and the cells are called spermatogonia.  Spermatogonia, like most cells, are diploid but they undergo meiosis and produce haploid cells, which are sperm cells or spermatozoa.

Once spermatozoa are produced, they move through the seminiferous tubules to a structure called the epididymis.  When spermatozoa leave the epididymis, they are mature.  Then they pass into the vas deferens.  The vas deferens leads to the urethra.  Fluid is added by two accessory glands, the seminal vesicles and the prostate.  The urethra is the way out, and it runs the length of the penis.

Ovaries are the site of meiosis in females.  Females have two ovaries.  When they undergo meiosis, they produce egg cells.  One egg cell is called an ovum, and the plural for ovum is ova.  The formation of ova is called oogenesis.  So the cells inside the female ovary undergo meiosis and produce ova.

In the male, the cell that undergoes meiosis is called a spermatogonium.  In the female, it is called an oogonium.  The oogonium, like most other cells, is diploid but it undergoes meiosis and produces a haploid ovum.  The one major difference is that oogenesis produces only one ovum, not four.  In spermatogenesis, one spermatogonium undergoes meiosis and produces four spermatozoa.  In oogenesis, one primary oocyte produces only one ovum.

The reason for this is that the primary oocyte undergoes the first meiotic division just like the spermatogonia.  It produces two daughter cells called secondary oocytes, and each one has 23 double-stranded chromosomes.  Each daughter cell goes on into the second meiotic division and produces two new haploid daughter cells, but three of those cells get only a tiny amount of cytoplasm and degenerate.  So we are left with only one haploid daughter cell, and that is called an ovum.

The three tiny cells that degenerate during oogenesis are called polar bodies.  Oogenesis produces only one ovum because it wants to conserve as much cytoplasm as possible.

There is one more difference between oogenesis and spermatogenesis.  At birth, all the oogonia are present and arrested in prophase of the first meiotic division.  At the time of ovulation, the ovum is sent out of the ovary into the fallopian tube, which connects the ovary to the uterus.  If at the time the ovum is released there happens to be a spermatozoan in the fallopian tube, fertilization occurs.  Fertilization means that the sperm cell penetrates the ovum.

The sperm has enzymes in a portion of its head called the acrosome which enable the sperm to penetrate the layers of cells that cover the ovum.  The outer layer is called the corona radiata, and the inner layer is called the zones pellucida.  The result is a zygote, which develops into an embryo.