Cellular Reproduction, Cellular Respiration, and Photosynthesis
Mitosis
Mitosis is a form of cell division where two identical nuclei are produced from one nucleus. DNA contains the genetic information of the cell. It is stored in the nucleus. During mitosis, DNA in the nucleus must be copied, or replicated. Recall that this happens during the S phase of the cell cycle. During the M phase, this copied DNA is divided into two complete sets, one of which goes to a daughter cell.
When DNA replicates, it condenses to form chromosomes that resemble an X. The DNA forms chromosomes by wrapping around proteins called histones. As shown below, it takes two identical sister chromatids to form a chromosome. A centromere holds the sister chromatids together.
Four phases take place during mitosis to form two identical daughter cells:
- Prophase: The nuclear membrane disappears, and other organelles move out of the way. The spindle, made of microtubules, begins to form. During prometaphase, the microtubules begin to attach to the centromeres at the center of the chromosome.
2. Metaphase: Spindle fibers line the chromosomes at the center of the cell. This is because they are pulled equally by the spindle fibers, which are attached to the opposite poles of the cell.
3. Anaphase: The chromosomes are pulled to the opposite poles of the cell.
4. Telophase: The chromosomes de-condense, the nuclear membrane reappears, and other parts of the cell return to their usual places in the cell.
The cell divides into two daughter cells by way of cytokinesis.
Test Tip
There is a popular mnemonic to help remember the order of the phases for mitosis:
[Please] Pee on the MAT
The “please” refers to prophase, while “pee” refers to prometaphase. MAT stands for metaphase, anaphase, and telophase, respectively.
Meiosis
Meiosis, sexual cell division in eukaryotes, involves two phases of mitosis that take place one after the other but without a second replication of DNA. This provides the reduction in chromosome number from 2n to n needed for fertilization to restore the normal 2n state.
Diploid multicellular organisms use meiosis, which reduces the number of chromosomes by half. Then, when two haploid (n) sex cells (sperm, egg) unite, the normal number of chromosomes is restored. Diploid organisms, such as humans and oak trees, have two copies of every chromosome per cell (2n), as opposed to n, when one copy of every chromosome is present per cell.
Keep In Mind
During prophase I of meiosis, crossing over occurs to increase genetic diversity. Corresponding chromosomes from the mother and the father of the organism undergoing meiosis are physically bound, and X-shaped structures called chiasmata form. These are where corresponding DNA from the different parental chromosomes are exchanged, resulting in increased diversity.
The process of meiosis is divided into two rounds of cell division: meiosis I and meiosis II.
Cytokinesis looks different in plant and animal cells. Plant cells build a new wall, or cell plate, between the two cells, while animal cells split by slowly pinching the membrane toward the center of the cell as the cell divides. Microtubules are more important for cytokinesis in plant cells, while the actin cytoskeleton performs the pinching-off operation during animal cytokinesis.
The phases that occur in mitosis (prophase, metaphase, anaphase, and telophase) also occur during each round of meiosis. Also, cytokinesis occurs after telophase during each round of cell division. However, DNA replication does not happen when meiosis I proceeds to meiosis II. The result of meiosis is one diploid cell that divides into four haploid cells, as shown in the following images.
Key Point
Meiosis and mitosis both require cytokinesis to physically separate a cell into daughter cells. Also, the sequence of events that occur in mitosis are the same in meiosis. However, there are two primary differences between the types of cell division: (1) meiosis has two rounds of cell division, and (2) daughter cells are genetically identical to the parent cell in mitosis but are not genetically identical in meiosis.
Subscribe to the online course to gain access to the full lesson content.
If your not ready for a subscription yet, be sure to check out our free practice tests and sample lesson at this link