14.1: Introduction
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Introduction to Mitosis
In eukaryotic cells, the time and phases from the beginning of one cell division until the beginning of the next cell division is called the cell cycle. The first phase of the cell cycle is interphase. Interphase is the time during which the cell performs its normal functions and prepares for cell division. Cells spend most of their time in this phase. During interphase, chromosomes are not visible because they are decondensed (present only as a tangled mass of thin threads of DNA with associated proteins, called chromatin). The nuclear membrane is present, and visible, as is the nucleolus.
Interphase includes two gap phases, G1 and G2, where the cell increases in size and synthesizes new organelles, enzymes, and other proteins that are needed for cell division. In between the two gap phases, the DNA replicates in preparation for cell division. This stage is called the S phase. At the beginning of S phase, chromosomes are single and unreplicated. By the end of S phase, each chromosome has made an exact copy of itself and consists of two sister chromatids. At this point in the cell cycle the sister chromatids are held together tightly at the centromere. While the two sister chromatids are physically joined together they are still considered one replicated chromosome. When the sister chromatids physically separate, later during the cell cycle, they are then considered to be individual chromosomes.
In animal cells, interphase is also when the centrosome (consisting of two centrioles) is replicated. Spindle fibers form from and radiate outward from the centrosomes to attach to and move chromosomes during cell division.
Interphase is followed by mitosis (in the somatic cells) or meiosis (in reproductive cells), which is when replicated chromosomes and cytoplasm separate, during the process of karyokinesis and cytokinesis respectively.
Chromosomes that are the same length, have the same centromere location and the same gene sequences and positions are called homologous chromosomes. Human somatic cells contain pairs of homologous chromosomes. Each homologous pair consists of one maternal chromosome and one paternal chromosome. Cells that contain two copies of each chromosome are called diploid (2n, where n is the number of different chromosomes in a single set). Human sex cells (eggs and sperm) contain only one copy of each chromosome. Cells with only one copy of each chromosome are haploid (n). When the haploid sperm (n) and egg (n) combine during fertilization this forms a diploid zygote (2n).
MITOSIS
Mitosis is nuclear division that results in two cells containing the same number of chromosomes as the parent cell. Most human cells (skin, muscle, bone, etc.) divide by mitosis. This process is necessary for the normal growth and development of a multicellular eukaryotic organism from a zygote (fertilized egg), as well as growth and the repair and replacement of cells and tissues. At the end of mitosis, two daughter cells are formed that are identical to the original (parent) cell. Mitosis is also a form of asexual reproduction in unicellular eukaryotes.
Mitosis is a complex and highly regulated process. It occurs in the following 4 separate phases: prophase, metaphase, anaphase, and telophase. Telophase is quickly followed by cytokinesis.
Prophase: Cells prepare for division by coiling and condensing their chromatin into chromosomes. By late prophase, individual chromosomes can be seen, each consisting of two sister chromatids joined at a centromere. Spindle fibers begin to form from the centrosomes, which have begun to migrate to opposite “poles” of the cell. The nucleoli and the nuclear membrane degrade.
Metaphase: Spindle fibers (called kinetochore microtubules or kinetochore spindle fibers) that emanate from the centromeres attach to the kinetochore (a proteinaceous area) on the sister chromatids. The fibers pull and otherwise manipulate the chromosomes to align them on the plane that passes through the center of the cell (metaphase plate). This “plate” is not an actual structure; it merely signifies the location of replicated chromosomes prior to their impending separation.
Other non-kinetochore spindle fibers or tubules (aka polar microtubules), emanating from the two centrosomes, elongate and eventually overlap with each other near the metaphase plate.
Anaphase: The centromeres divide, with the help of separase enzymes, and separate the sister chromatids. This happens simultaneously in all the chromosomes. The kinetochore spindles shorten and pull each chromatid to which they are attached toward the pole (and centrosome) from which they originate. This equally distributes exactly half the chromosomal material to each side of the cell. In late anaphase, the non-kinetochore spindles begin to elongate, lengthening the cell.
Telophase: The non-kinetochore microtubules continue to elongate, further elongating the cell in preparation for cytokinesis (splitting of the cytoplasm). The chromosomes reach their respective poles. The kinetochores disappear. The two nuclear membranes (one in each half of the cell) begin to form around the chromosomes. Nucleoli reappear and the chromosomes in each soon-to-be new cell begin to decondense back into chromatin. Mitosis is complete at the end of this stage.
Cytokinesis (splitting of the cytoplasm):
In animal cells and all other eukaryotes without a cell wall, cytokinesis is achieved by means of a constricting “belt” of protein fibers that slide past each other near the equator of the cell. As this occurs, the diameter of the belt decreases, pinching the cell to form a cleavage furrow around the cell’s circumference. As constriction proceeds, the furrow deepens until it eventually slices its way into the center of the cell. At this point, the cell is divided into two.
Plant cell walls are far too rigid to be split apart by contracting proteins. Instead, these cells assemble membrane proteins (in vesicles that bud off the Golgi apparatus) in their interior at right angles to the spindle apparatus. This expanding membrane partition, called a cell plate, continues to grow outward until it reaches the interior surface of the plasma membrane and fuses with it. This divides the cell in two.
References
Belwood, Jacqueline; Rogers, Brandy; and Christian, Jason, Foundations of Biology Lab Manual (Georgia Highlands College). “Lab 10: Mitosis & Meiosis,” (2019). Biological Sciences Open Textbooks. 18. CC-BY