Asexual Cell Division ( Mitosis)
Some Key Points What is the life cycle of a cell?
Do all cells duplicate their DNA at the same time?
How long does it take for a cell to divide?
Asexual
cell reproduction in higher eucaryotic cells is characterized as a cyclic process, in effect the life cycle of a cell. Three distinct phases are recognized in
the
Cell
Cycle:
a) interphase, b) nuclear division
(mitosis), and c) cytokinesis.
Interphase, the non-dividing phase, is the stage during which the cell spends most of its life cycle. Interphase itself is divided into 3 parts: G-1, the gap before DNA duplication, S, the synthetic phase, where the cell duplicates its DNA, and G-2, the gap after DNA duplication. During the G-1 period, the cell grows and carries out normal metabolism, organelles duplicate themselves, and the cell is preparing to replicate its DNA. During G-2 the cell is making cytoskeletal elements and is preparing for mitosis. RNA and protein synthesis continue throughout interphase, but histone mRNA and synthesis occur only during the S phase, while the cell duplicates its DNA .
The generation time, from once cell division cycle to the next, is species and environmentally dependent. Onion root tip cells (Allium cepa) divide every 12 hours, while rat liver parenchymal cells divide about one time per year. Sea urchin eggs, from a cool habitat, divide every 60 min, while those in a warm marine habitat do so every 30 min. The determination of interphase and cell cycle times in dividing cells is determined by pulse-chase experiments. Group of synchronously, or asynchronously, dividing cells is given a 1 hour pulse of radioactive thymidine, which will be incorporated into DNA as it replicates. Cells can be withdrawn at specific intervals, prepared for microscopic autoradiography, and a plot of the % of labeled cells vs. time is created. From such a plot the time intervals for S, M, and G-2 can be calculated.
The eucaryotic cell cycle, which evolved under a circadian rhythm, typically lasts between 10 and 20 hours in rapidly proliferating adult cells. The cycle may be arrested for weeks or months in quiescent cells. Prolonged arrest of this type usually occurs during the G1 phase and is sometimes referred to as G 0. Some embryonic cells (example: Drosophilia melangaster) can complete entire cycles and divide in only 11 minutes. In these special cases, G 1 and G 2 are undetectable, and mitosis alternates with DNA synthesis. The length of the S phase varies dramatically, with fruit flies embryos taking only minutes, while broad bean root cells can take 7.5 hours.
The nuclear division phase of the eucaryotic asexual cell cycle is Mitosis. Mitosis separates the duplicated DNA, as chromosomes, into two separate cell nuclei. An essential feature of mitosis is the separation of the chromatids to opposite poles of the mitotic spindle. This ensures that each of the daughter cells will receive a complete set of chromosomes. During mitosis each duplicated chromosome splits length-wise, forming two identical chromatids. Each pair of chromatids is separated into two daughter cells during mitosis. The chromatids are moved via their attachment to bunches of microtubules called the mitotic spindle. The two poles of the mitotic spindle hold centrosomes, which organize the microtubule arrays. The centrosomes in animal cells each contain a pair of cylindrical centrioles. Centrioles are cylindrical array of microtubules (MT's), made of 9 triplets of MT's, structurally identical to a basal body. Basal bodies give rise to cilia & flagella, which are composed of 9 outer MT doublets (with motor protein dynein arms) wrapped around a central core of 2 MT's.
Centrioles duplicate near the start of the S phase of DNA replication. Mitotic spindle microtubules in animal cells with centrioles are tightly organized, but seem less tightly organized in plant cells, implying an organizational role for the centriole. Yet, the significance of centrioles in cell division is not clear since they are not found in plant cells which also undergo mitosis.
Cytokinesis, the division of the cytoplasm, comes after mitosis. Animal and plant cells differ in the pattern of cytokinesis. Cytokinesis in animal cells is achieved through the constriction of the cell by a ring of contractile microfilaments consisting of actin and myosin protein. In plant cells, the cytoplasm is divided by the formation of a cell plate, which make up the new cell wall, between the two daughter cells. During late telophase, a cell plate arises from small, Golgi-derived vesicles, which join together across the equator of the mitotic spindle to form a disk-shaped structure. All the vesicles supply their membranes to help form the new plasma membrane, while the vesicle's medium contribute to forming the new cell wall. Sets of Golgi vesicles stretch the edge of the cell plate until it reaches and fuses with the side walls of the parent cell, thus physically separating the two new daughter cells. Cellulose synthesis completes the cell plate, which now becomes the primary plant cell wall.
