Meiosis
Many organisms pass their genes to their offspring through sexual reproduction. This begins when two gametes unite to form an embryo that is genetically unique from the parent organisms. The embryo then grows into an adult who in turn passes their genetic information on to their offspring.
Gametes are formed through a process called meiosis. The cells that undergo meiosis to produce the gametes are called germ-line cells.
In diploid organisms, germ-line cells have two copies of each chromosome. Germ-line cells undergo meiosis to produce haploid gametes which have only one copy of each chromosome. These haploid gametes fuse to form a diploid embryo that grows into the adult.
Meiosis is just one step in the life cycle of a germ-line cell. Similar to mitosis, the cells also pass through the interphase, G1, S, and G2 stages before they enter meiosis. The DNA inside a germ-line cell is duplicated before meiosis begins during the S phase. The duplicated germ-line chromosomes are called sister chromatids. These chromatids remain attached to each other until the second cell division event in meiosis.
There are two cell division events during meiosis. The first division, meiosis I, results in two unique daughter cells that have half the amount of DNA as the parent germ-line cell. The second division, meiosis II, results in four unique haploid cells that only have one copy of each chromosome. These haploid cells are the gametes that could go on to produce an offspring through sexual reproduction.
Let’s look more closely at each of the division events. Meiosis begins with prophase I. In this stage, the DNA condenses to form chromosomes. Here we see the duplicated sister chromatids joined together at the centromere. They stay fused at the centromere throughout Meiosis I. Next, each pair of homologous chromosomes undergoes synapsis to form a complex involving two pairs of sister chromatids. Chromosomal material is exchanged between the two pairs of sister chromatids. This event is called recombination or more commonly, crossing over. After crossing over, the sister chromatids for each chromosome are no longer identical to one another. This is one of the reasons why no two siblings (aside from twins) are genetically identical.
There are several more key steps in prophase I. The nuclear membrane begins to break down. Then the two centrosomes migrate to opposite ends of the cell and microtubules appear. The microtubules then attach to the chromosomes.
The next phase of meiosis I is called metaphase I. Here the synapsed chromosomes align at the equator of the cell. The chromosomes align randomly which results in different combinations each time meiosis occurs.
The next phase is anaphase I. During this phase, homologous chromosomes separate and migrate to the two poles of the cell. Importantly, the sister chromatids remain attached at their centromeres.
The final steps of meiosis I are telophase I and cytokinesis. Here the cell divides into two daughter cells.
Each of these two cells now undergo meiosis II. Meiosis II is similar to mitosis.
The first stage of meiosis II is prophase II. Again, chromosomes condense, the nuclear envelop breaks down, and the spindle apparatus forms. The major difference between prophase II and prophase I is the fact that the daughter cells have only one copy of each homologous chromosome. So, in prophase II, there is no synapsis of homologous chromosomes or crossing over.
In metaphase II, the chromosomes align at the equator of the cell. Again, the alignment is random. Since the sister chromatids are no longer identical, there will be many different possible ways for these chromosomes to align.
In anaphase II, the sister chromatids are pulled apart as the microtubules shorten. Also, the ends of the cell are pushed further apart as microtubules elongate.
In telophase II, the nuclear membrane reforms, and the cytoplasm is divided into the two haploid daughter cells. This division is called cytokinesis. Since meiosis II began with two cells, and each of those cells were split into two cells, we now have 4 unique haploid cells at the end of meiosis. These cells are gametes.
Two gametes, one from the father and one from the mother, may fuse to produce a diploid embryo. The resulting embryo then grows through many cycles of mitosis.
Mitosis
Cell division is required for an organism to grow, mature, and maintain tissues. During the mitotic phase, a cell will undergo mitosis to form two new nuclei and then divide to form two new individual cells during cytokinesis.
Mitosis is the process of dividing the duplicated DNA of a cell into two new nuclei. Mitosis is split into distinct stages. The first stage is prophase; the DNA condenses, organizes, and the classic chromosome structure appears. Next comes prometaphase where microtubules attach to the chromosomes. This step is followed by metaphase where the chromosomes align. Metaphase is followed by anaphase where the chromosomes separate. Finally, during telophase nuclear membranes reappear around the two sets of chromosomes. Mitosis is now complete. After mitosis two new cells are formed by a process called cytokinesis.
Mitosis is only one part of what is called the cell cycle. For many eukaryotic cells, a cell is duplicated every 24 hours. Most of the life of a cell is spent in interphase. Interphase consists of three stages called G1, S, and G2.
G1 (or Gap 1) is the first growth stage of interphase. In G1, the cell grows to nearly its full size and performs many of its specific biochemical functions that aid the organism.
Next is the S (or synthesis) phase. This is an important stage, because it is during the S phase that DNA in the nucleus is replicated.
The cell next enters another growth stage called G2 (or Gap 2). It is during G2 that the cell finishes growing. Once the cell has duplicated DNA in the nucleus, and two centrosomes have appeared in the cytoplasm, mitosis can begin. For a typical eukaryotic cell this will last about 80 minutes.
During the first stage of mitosis, called prophase, we first see the classic chromosome structure. This occurs through a condensation process. At the same time, protein strands called microtubules appear from the centrosomes in animals. Finally, a structure found within the nucleus, the nucleolus, disappears.
Next, prometaphase begins when the nuclear membrane is broken down. At the same time, microtubule strands, or spindle fibers, are growing from the centrosomes. These strands attach to a protein structure called the kinetochore. One kinetochore is attached to the centromere of each sister chromatid.
Next comes metaphase. During this stage the sister chromatids align along the center of the cell so that both chromatids face toward opposite poles of the cell.
Now the sister chromatids are ready to be separated. This occurs during anaphase through a shortening of the microtubules attached to the kinetochores. Additionally, the poles of the cell move farther apart and cause increased separation of sister chromatids. At the end of anaphase, the sister chromatids have moved to the two ends of the cell.
Telophase is the final stage of mitosis. It is here the components of the new cells begin to appear. At this point the spindle fibers are broken up. A new nuclear membrane surrounds the chromosomes at the end of each cell. And the chromosomes uncoil and return to an uncondensed state. Mitosis is now complete. The formation of two cells is all that remains.
Following mitosis, the cell undergoes a process called cytokinesis. First the cell is compressed by a contractile ring that divides the cell in nearly equal halves. By now the organelles in the cell have been replicated, and are now divided between the two halves of the cell. This includes mitochondria, golgi bodies, and the rough ER. Plant cells also have chloroplasts. Once split, the two new cells are now fully in the G1 stage of interphase and ready again to begin their growth.
Let’s watch the process one more time. Mitosis begins with prophase. Notice the DNA condensing into chromosomes during this stage. Microtubules appear during prometaphase, and the nuclear membrane breaks down. Metaphase occurs when the chromosomes are aligned at the center of the cell. During anaphase the chromosomes are moving apart. The telophase stage is marked by the appearance of new nuclear membranes. This is the end of mitosis.
Finally, the splitting of the cell occurs during cytokinesis. The two new cells are now ready to grow and perform their specialized functions.