Cell Division
Cell division is the process by which a parent cell divides into two or more daughter cells. It is essential for growth, repair, and reproduction in living organisms. The two main types of cell division are mitosis, which produces two identical daughter cells, and meiosis, which produces four genetically diverse daughter cells.
7 Key excerpts on "Cell Division"
eBook - ePub- Abdul Hamid Rana(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
...CHAPTER 1 C ell Division I n a cell population that is constantly being renewed, individual cells divide periodically. A typical somatic Cell Division, mitosis, consists of an equal division of nuclear material, so that the two newly formed daughter cells receive exactly the number and kind of chromosomes that the parent cell had. This separation of nuclear material is then followed by division of the cytoplasm. Before a cell can undergo division, it must increase its mass and contents, and double the mass of its DNA. All of this occurs during the growth period known as interphase. Following this is the Μ phase, during which nuclear division (mitosis) and cytoplasmic division (cytokinesis) take place. Duplicated DNA must be divided precisely between daughter cells. Although the cell cycle is continuous, for simplicity and clarity, both interphase and Μ phase are subdivided into stages. The interphase is composed of the G 1, S and G 2 periods. I. Interphase A. G 1 (Gap 1) Period After completion of Cell Division, the daughter cells enter the preduplication period, Gi (gapi). During this period there is synthesis of RNA and proteins, and total cell mass is increased. After this, the cell is held at a restriction point. Any cell that passes this point will complete the rest of the stages of the cycle. A trigger or unstable protein (S phase activator) has been proposed. An accumulation of a threshold amount of this protein helps the cell to exceed the restriction point. The quiescent cells that do not accumulate this protein are arrested at the restriction point and are considered to be in the G 0 period of interphase. This may be one of the mechanisms by which tissue growth is controlled. Crowding (contact inhibition) and starvation may also inhibit Cell Division. In many tissues, cells divide only when new cells are needed. Neoplastic cells appear to have lost these growth controls. Figure 1.1 Chromosomes (see Color Plate 1.1)...
eBook - ePub- Michelle Peckham(Author)
- 2011(Publication Date)
- Wiley-Blackwell(Publisher)
...6 Cell Division In the cell cycle, cells spend most of their time in interphase (phase between each mitosis). Interphase is divided up into three phases: G 1 (growth 1): growth phase 1; S (synthesis): DNA replication; G 2 (growth 2): growth phase 2. Following G 2, the cells can then enter mitosis. Some cells enter G 0 after mitosis: a resting/quiescent/senescent stage, in which cells have stopped dividing. Many cells in the body are terminally differentiated, and do not divide, an example being skeletal muscle. Therefore, you will not commonly find examples of dividing cells in tissue sections, but they can be seen occasionally, depending on the tissue. Mitosis Each cell contains two pairs of chromosomes, one of which is paternally, and one maternally derived. Cell Division occurs about once every 24–48 hours in cells that have not yet terminally differentiated. Cell Division only takes about 30–60 minutes. Dividing cells can sometimes be observed in tissue sections and are often called ‘mitotic figures’. The different phases of Cell Division can be identified in tissue sections (Fig. 6). Prophase In prophase (Fig. 6, stage 1), the centrosome duplicates and the two resultant centrosomes move apart to form the poles of the mitotic spindle. The replicated chromosomes condense, and asso-ciate (sister chromatids). They are held together along their length. Pairs of paternal and maternal chromosomes remain separate. Prometaphase In prometaphase (Fig. 6, stage 2), the nuclear membrane breaks down, and the spindle is formed. There are three main types of microtubules. Astral microtubules: These grow out from the poles to towards the plasma membrane anchoring the spindle in the center of the cell. Kinetochore microtubules: These grow out from the poles and attach to the kinetochores of the chromosomes. Spindle microtubules: These can attach to the arms of chromosomes. Chromosome movement is highly dynamic during this stage. Metaphase In metaphase (Fig...
eBook - ePubAdvanced Molecular Biology
A Concise Reference
- Richard Twyman(Author)
- 2018(Publication Date)
- Garland Science(Publisher)
...Chapter 2 The Cell Cycle Fundamental concepts and definitions The cell cycle is the sequence of events between successive Cell Divisions. Many different processes must be coordinated during the cell cycle, some of which occur continuously (e.g. cell growth) and some discontinuously, as events or landmarks (e.g. Cell Division). Cell Division must be coordinated with growth and DNA replication so that cell size and DNA content remain constant. The cell cycle comprises a nuclear or chromosomal cycle (DNA replication and partition) and a cytoplasmic or Cell Division cycle (doubling and division of cytoplasmic components, which in eukaryotes includes the organelles). The DNA is considered separately from other cell contents because it is usually present in only one or two copies per vegetative cell, and its replication and segregation must therefore be precisely controlled. Most of the remainder of the cell contents are synthesized continuously and in sufficient quantity to be distributed equally into the daughter cells when the parental cell is big enough to divide. An exception is the centro-some, an organelle that is pivotal in the process of chromosome segregation itself, which is duplicated prior to mitosis and segregated into the daughter cells with the chromosomes (the centrosome cycle). In eukaryotes, the two major events of the chromosomal cycle, replication and mitosis, are controlled so that they can never occur simultaneously. Conversely, in bacteria the analogous processes, replication and partition, are coordinated so that partially replicated chromosomes can segregate during rapid growth...
eBook - ePubUnderstanding Cancer
An Introduction to the Biology, Medicine, and Societal Implications of this Disease
- J. Richard McIntosh(Author)
- 2019(Publication Date)
- Garland Science(Publisher)
...Both cell cycle checkpoints and the ability to initiate apoptosis are commonly lost in cancer cells. These deficiencies contribute to their genetic instability. ESSENTIAL CONCEPTS • Cells reproduce by cycles of growth and division. Interphase is the time between Cell Divisions. During interphase, cells synthesize all the materials necessary for two cells. Thus, the parent cell prepares a dowry of materials for its daughter cells, enabling them to grow and divide again. • During interphase a cell replicates its DNA by going through a round of DNA synthesis (S phase). After Cell Division but before S phase, there is a gap in time called G1. After S phase and before chromosome segregation, there is a second gap called G2. Human cells commit to either differentiate or replicate during G1 phase. • The mitotic spindle is a machine made from microtubules and associated proteins that organizes and segregates both the chromosomes and the centrosomes to opposite ends of the parent cell. The parent cell then builds a ring of microfilaments around its center and pulls the ring tight to divide itself into two. • Mitosis is quite accurate but not perfect. Mistakes in mitosis can lead the daughter cells to contain the wrong number of chromosomes. This condition reflects an instability in genome management that is characteristic of cancer cells. • Many of the cells in an adult have withdrawn from the cell cycle. They have turned on the expression of genes that allow them to differentiate, so they can perform a particular function for the good of the body as a whole. • Cell differentiation depends on the regulated expression of specific genes. Specificity of gene expression is achieved by transcription factors that guide the action of RNA polymerase to the right places on DNA...
eBook - ePubBiology
A Self-Teaching Guide
- Steven D. Garber(Author)
- 2020(Publication Date)
- Jossey-Bass(Publisher)
...These steps include the division of the nucleus, known as karyokinesis, as well as the division of the rest of the cell, which is called cytokinesis. Before the cell begins to divide, the genetic information located in the nucleus in the form of chromosomes must be duplicated. The chromosomes are the structures in the cells that contain the greens. All the instructions concerning the life processes of a cell emanate from the chromosomes. At the ends of the chromosomes are regions of repetitive DNA called telomeres that protect the chromosomes from fusing with neighboring chromosomes, and the telomeres protect the chromosomes from deteriorating. Many people who live over 100 years have longer telomeres than people who don't live as long. Figure 3.1 (a) Generalized cyanobacterium (or blue-green bacterium or blue-green algae) undergoing Cell Division; and (b) electron photomicrograph of an actual cyanobacterium dividing in two. It is only when the two sets of chromosomes are segregated in separate parts of the cell that the cytoplasm and other requisite materials may divide and the parent cell can complete its division. Interphase Compared to the rest of the cell's life, Cell Division is a brief and distinct stage in the cell's life history. Interphase, although the longest and most physiologically dynamic part of the cell's life history, is not considered part of Cell Division. Rather, this is the stage during which the cell is growing, metabolizing, and maintaining itself. During interphase, the nucleus exists as a distinct organelle, bound by the nuclear membrane. Inside the nucleus are long, thin, unwound strands of chromosomes. While unwound throughout interphase, the chromosomes influence the activities of the cell...
eBook - ePub- Archana Sharma(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
...The result is the formation of two daughter nuclei. These daughter nuclei are ready for metabolic phase. Nuclear division is followed by the division of cytoplasm or cytokinesis, leading to formation of two daughter cells. Equational separation in normal mitosis is responsible for every cell containing the same genetic component. This type of division ensures equal number of chromosomes in all cells of the soma or body. Another type of division characterizing the germinal line is meiosis, where the chromosome number undergoes reduction. This division occurs in male and female organs of flowers as well as in testis and ovarian systems of animals. Meiosis entails two divisions of the nucleus, one of which is equational and the other is reductional. It characteristically results in the separation of homologous chromosomes and halving of the chromosome number. Of the two divisions in meiosis, in the first division, prophase is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In leptotene, chromosomes are long, thin, and optically single threads, showing little of the coiling characteristic of somatic prophase. The threads contain a series of chromatic beads called chromo-meres. In zygotene, the homologous chromosomes begin to pair, usually at the ends, near the kinetochore or both. A general pairing is followed by closer chromomere to chromomere association or synapsis which may be complete or not, depending on the species and on conditions within the organisms. During pachytene, interchange of segments takes place between the paternal and maternal sets of chromosomes through a process known as synapsis or crossing-over. The chiasmata, which is the visible sign of crossing-over, involves only two out of four strands at any locus. Breakage and reunion of segments are achieved during this phase, and the two chromosomes originating thereby contain interchanged segments (Fig. 2.1)...
eBook - ePub- Thomas D. Pollard, William C. Earnshaw, Jennifer Lippincott-Schwartz, Graham Johnson(Authors)
- 2016(Publication Date)
- Elsevier(Publisher)
...As cells exit mitosis, the nuclear envelope reassembles on the surface of the chromosomes to reform the daughter nuclei. Then the process of cytokinesis cleaves the daughter cells. A key feature of the cell cycle is a series of built-in quality controls, called checkpoints (Fig. 1.9), which ensure that each stage of the cycle is completed successfully before the process continues to the next step. These checkpoints also detect damage to cellular constituents and block cell-cycle progression so that the damage may be repaired. Misregulation of checkpoints and other cell-cycle controls predisposes to cancer. Remarkably, the entire cycle of DNA replication, chromosomal condensation, nuclear envelope breakdown, and reformation, including the modulation of these events by checkpoints, can be carried out in cell-free extracts in a test tube. Welcome to the Rest of the Book This overview should prepare the reader to embark on the following chapters, which explain our current understanding of the molecular basis of life at the cellular level. This journey starts with the evolution of the cell and introduction to the molecules of life. The following sections cover membrane structure and function, chromosomes and the nucleus, gene expression and protein synthesis, organelles and membrane traffic, signaling mechanisms, cellular adhesion and the extracellular matrix, cytoskeleton and cellular motility, and the cell cycle. Enjoy the adventure of exploring all of these topics. As you read, appreciate that cell biology is a living field that is constantly growing and identifying new horizons. The book will prepare you to understand these new insights as they unfold in the future. Chapter 2 Evolution of Life on Earth N o one is certain how life began, but the common ancestor of all living things populated the earth more than 3 billion years ago, not long (geologically speaking) after the planet formed 4.5 billion years ago (Fig. 2.1)...
