Mitosis and Meiosis for AP Biology
Practice problems for these concepts can be found at : Cell Division Review Questions for AP Biology
During mitosis, the fourth stage of the cell cycle, the cell actually takes the second copy of DNA made during the S phase and divides it equally between two cells. Single-cell eukaryotes undergo mitosis for the purpose of asexual reproduction. More complex multicellular eukaryotes use mitosis for other processes as well, such as growth and repair.
Mitosis consists of four major stages: prophase, metaphase, anaphase, and telophase. These stages are immediately followed by cytokinesis—the physical separation of the newly formed daughter cells. During interphase, chromosomes are invisible. The chromatin—the raw material that gives rise to the chromosomes—is long and thin during this phase. When the chromatin condenses to the point where the chromosome becomes visible by microscope, the cell is said to have begun mitosis. The AP Biology exam is not going to ask you detailed questions about the different stages of mitosis; just have a general understanding of what happens during each step.
Prophase. Nucleus and nucleolus disappear; chromosomes appear as two identical, connected sister chromatids; mitotic spindle (made of microtubules) begins to form; centrioles move to opposite poles of the cell (plant cells do not have centrioles).
Metaphase. For metaphase, think middle. The sister chromatids line up along the middle of the cell, ready to split apart.
Anaphase. For anaphase, think apart. The split sister chromatids move via the microtubules to the opposing poles of the cell. The chromosomes are pulled to opposite poles by the spindle apparatus. After anaphase, each pole of the cell has a complete set of chromosomes.
Telophase. The nuclei for the newly split cells form; the nucleoli reappear, and the chromatin uncoils.
Cytokinesis. Newly formed daughter cells split apart. Animal cells are split by the formation of a cleavage furrow, plant cells by the formation of a cell plate.
Figure 9.3 is a pictorial representation of the stages of mitosis.
Here are the definitions for words you may need to know:
Cell plate: plant cell structure, constructed in the Golgi apparatus, composed of vesicles that fuse together along the middle of the cell, completing the separation process.
Cleavage furrow: groove formed (in animal cells) between the two daughter cells that pinches together to complete the separation of the two cells after mitosis.
Cytokinesis: the actual splitting of the newly formed daughter cells that completes each trip around the cell cycle—some consider it part of mitosis; others regard it as the step immediately following mitosis.
Mitotic spindle: apparatus constructed from microtubules that assists the cell in the physical separation of the chromosomes during mitosis.
Now that I have armed you with the knowledge of the distinction between haploid and diploid, it is time to dive into the topic of meiosis, which occurs during the process of sexual reproduction. A cell destined to undergo meiosis goes through the cell cycle, synthesizing a second copy of DNA just like mitotic cells. But after G2, the cell instead enters meiosis, which consists of two cell divisions, not one. The second cell division exists because the gametes to be formed from meiosis must be haploid. This is because they are going to join with another haploid gamete at conception to produce the diploid zygote. Meiosis is like a two-part made-for-TV miniseries. It has two acts: meiosis I and meiosis II. Each of these two acts is divided into four steps, reminiscent of mitosis: prophase, metaphase, anaphase, and telophase.
Homologous chromosomes resemble one another in shape, size, function, and the genetic information they contain. In humans, the 46 chromosomes are divided into 23 homologous pairs. One member of each pair comes from an individual's mother, and the other member comes from the father. Meiosis I is the separation of the homologous pairs into two separate cells. Meiosis II is the separation of the duplicated sister chromatids into chromosomes. As a result, a single meiotic cycle produces four cells from a single cell. The cells produced during meiosis in the human life cycle are called gametes.
Again, the AP Biology exam is not going to test your mastery of the minute details of the meiotic process. However, a general understanding of the various steps is important:
Prophase I. Each chromosome pairs with its homolog. Crossover (synapsis) occurs in this phase. The nuclear envelope breaks apart, and spindle apparatus begins to form.
Metaphase I. Chromosomes align along the metaphase plate matched with their homologous partner. This stage ends with the separation of the homologous pairs.
Anaphase I. Separated homologous pairs move to opposite poles of the cell.
Telophase I. Nuclear membrane reforms; the process of division begins.
Cytokinesis. After the daughter cells split, the two newly formed cells are haploid (n).
As discussed earlier, meiosis consists of a single synthesis period during which the DNA is replicated, followed by two acts of cell division. With the completion of the first cell division, meiosis I, the cells are haploid because they no longer consist of two full sets of chromosomes. Each cell has one of the duplicated chromatid pairs from each homologous pair. The cell then enters meiosis II.
Prophase II. The nuclear envelope breaks apart, and spindle apparatus begins to form.
Metaphase II. Sister chromatids line up along the equator of the cell.
Anaphase II. Sister chromatids split apart and are called chromosomes as they are pulled to the poles.
Telophase II. The nuclei and the nucleoli for the newly split cells return.
Cytokinesis. Newly formed daughter cells physically divide.
Figure 9.4 is a pictorial representation of the stages of meiosis I and II.
In humans, the process of gamete formation is different in women and men. In men, spermatogenesis leads to the production of four haploid sperm during each meiotic cycle. In women, the process is called oogenesis. It is a trickier process than spermatogenesis, and each complete meiotic cycle leads to the production of a single ovum, or egg. After meiosis I in females, one cell receives half the genetic information and the majority of the cytoplasm of the parent cell. The other cell, the polar body, simply receives half of the genetic information and is cast away. During meiosis II, the remaining cell divides a second time, and forms a polar body that is cast away, and a single haploid ovum that contains half the genetic information and nearly all the cytoplasm of the original parent cell. The excess cytoplasm is required for proper growth of the embryo after fertilization. Thus, the process of oogenesis produces two polar bodies and a single haploid ovum.
To review, why is it important to produce haploid gametes during meiosis? During fertilization, a sperm (n) will meet up with an egg (n), to produce a diploid zygote (2n). If either the sperm or the egg were diploid, then the offspring produced during sexual reproduction would contain more chromosomes than the parent organism. Meiosis circumvents this problem by producing gametes that are haploid and consist of one copy of each type of chromosome. During fertilization between two gametes, each copy will match up with another copy of each type of chromosome to form the diploid zygote.
In meiosis during prophase I, the homologous pairs join together. This matching of chromosomes into homologous pairs does not occur in mitosis. In mitosis, the 46 chromosomes simply align along the metaphase plate alone.
An event of major importance that occurs during meiosis that does not occur during mitosis is known as crossover (also known crossing over) (Figure 9.5). When the homologous pairs match up during prophase I of meiosis, complementary pieces from the two homologous chromosomes wrap around each other and are exchanged between the chromosomes. Imagine that chromosome A is the homologous partner for chromosome B. When they pair up during prophase I, a piece of chromosome A containing a certain stretch of genes can be exchanged for the piece of chromosome B containing the same genetic information. This is one of the mechanisms that allows offspring to differ from their parents. Remember that crossing over occurs between the homologous chromosome pairs, not the sister chromatids.
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