The Cell Theory: Cell Division, Organelles, and Chromosomes - Assignment Example

? 15 December Cell Theory According to the cell theory, "cell is the basic unit of life". Although cells are alive by themselves, they are usually a part of a larger living organism. Bacteria the smallest living organisms on earth and are made of a single cell (unicellular, where 'uni' means 'one'), while larger organisms like tigers and whales are made of many cells (multicellular, where 'multi' means 'many'). The cell theory encompasses three basic theories. Firstly, "all living things are made of cells". Secondly, a single cell is the smallest unit of life as it is the smallest living thing that performs all life functions. Finally, all cells originate from pre-existing cells. The process of formation of new cells from older cells is called cell division. Each cell contains information in the form of nucleic acids, which is passed on from parent cell to daughter cell during cell division. Deoxyribonucleic acid (DNA) is the nucleic acid that is responsible for transferring information during cell division. All cells carry out metabolism, which is the process by which they consume food and process it to derive energy for performing their functions. Cells were discovered by Robert Hooke in 1663. This discovery was made when he observed a section of cork cambium under a primitive microscope. He observed that the cork tissue was made of compartments that reminded him of small rooms which are usually inhabited by monks. In Latin, such rooms were called 'cellula'. Hooke therefore named the compartments he observed under the microscope as 'cells'. The cells that Hooke observed were actually dead and did not have cell organelles. Hooke was, however, unaware of this fact. Anton van Leeuewenhoek was the first to observe live cells under a self-made microscope. He observed live moving organisms, which he later called 'animalcules' that means 'little animals'. He drew diagrams of these animalcules in his letters to the Royal Society of London to inform them of his discovery. Cell Organelles A cell organelle is the part of a cell that has a specific function. Organelles are usually located inside the cytoplasm of a living cell, and are typically enclosed in a membrane called plasma membrane. They are called "organelles" because their importance to a cell is analogous to the importance of organs of to a body. Both prokaryotes and eukaryotes have cell organelles; however, there are some important differences between the two. The organelles in prokaryotic cells are not organized or enclosed in plasma membranes. Major cell organelles include chloroplasts, endoplasmic reticulum, golgi apparatus, mitochondria, nucleus and vacuoles. Chloroplasts have double membranes as well as their own DNA, and perform photosynthesis. They are present in plants and some algae. Endoplasmic reticulum (ER) has a single membrane and is responsible for transport as well as synthesis of proteins. It is of two types – rough ER, whose surface appears to be rough because of the presence of ribosomes, and smooth ER, whose surface appears to be smooth. Golgi apparatus is also singe-membraned and is responsible for protein sorting and modification. The mitochondrion is also called as the powerhouse of the cell because it is responsible for energy production, and like chloroplasts, it also has a double membrane and its own DNA. The nucleus has both Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) and is responsible for maintenance and proper distribution of DNA during cell division. Vacuoles function as storage reservoirs of the cell. Minor organelles of the cell include autophagosomes that collect material from the cytoplasm for degradation, centrioles that enable cell division, ribosomes that help in the production of proteins, lysosomes that are responsible for processing or breakdown of large molecules into simpler ones, nucleolus that produces ribosomes, and vesicles that function as material transporters in the cell. Many other organelles exist based on the type of organism. Cell Division The process that results in the division of one cell (called parent cell), into daughter cells is called cell division. When a cell undergoes division, all of the components inside it divide simultaneously. Cell division in eukaryotes is of two types – mitosis and meosis. In case of most prokaryotes, cell division occurs via binary fission. Cells are the basic units of life and are the building blocks of the body. Organisms grow and reproduce as a result of cell division. Mitosis is the process by which a somatic (vegetative) cell divides into two, and meiosis is the process by which one sex cell divides into four daughter cells to produce gametes. While mitosis helps in growth and repair, meiosis helps in the production of gametes that help in sexual reproduction. Cytokinesis is the process that results in a physical separation of the daughter cells. Binary fission is the process in which a single unicellular organism such as a bacterium divides into two. Bacteria and other simple unicellular organisms have a single DNA molecule as the chromosome. During binary fission, the DNA molecule replicates into two and each daughter cell receives one molecule. Binary fission is the simplest type of cell division. Mitosis occurs in somatic cells that usually have more than one chromosome. It results in the production of two daughter cells. Firstly, all the chromosomes are duplicated and then each copy of the chromosome is segregated into each of the two daughter cells. A spindle apparatus makes sure that the chromosomes are distributed equally between the two daughter cells. At the end of mitosis, each daughter cell has the same number of chromosomes as the parent cell. Meiosis occurs in sex cells called gametes and results in the production of four daughter cells from a single parent cell. The process of meiosis involves an initial cell division that results in two daughter cells that have half the number of chromosomes of the parent cell. These two daughter cells divide further by a process similar to mitosis, resulting in two more daughter cells, finally giving four daughter cells from a single parent cell. Chromosomes Chromosomes are DNA molecules that occur in the cell nucleus in case of eukaryotes and in the cytoplasm in case of prokaryotes. Chromosomes also contain proteins that give them a specific structure. They are responsible for carrying the genetic information of an organism and are made of many genes. All chromosomes occur in pairs, one set is from the father and one from the mother. They are present in every cell of an organism. When cell division occurs, chromosomes divide too. Each chromosome duplicates first and the resulting daughter chromosomes are distributed equally between the daughter cells. Chromosomes are called as chromatids when they are in the doubled state during cell division. The number of chromosomes for each organism is fixed and the number varies from one species to the other. Humans have 23 pairs of chromosomes, the total chromosome number being 46. Each of the 23 pairs of chromosomes has one set from the father and the other set from the mother and is called the diploid genome. The chromosomes include one pair of sex chromosomes. Each pair of sex chromosomes has an X-chromosome derived from the mother's egg and an X or Y-chromosome derived from the father's sperm. Sex cells divide through meiosis and the resultant daughter cells have a haploid genome (a single set). If an organism lacks the usual number of chromosomes or if there is an abnormality in any one or more chromosomes, the organism may either die or will suffer from abnormalities or disorders. For instance, Down's syndrome is a genetic disorder that occurs when an organism has an extra chromosome (21st chromosome). The presence of an extra X chromosome in a male results in Klinefelter's syndrome. Turner syndrome results when an individual has only one sex chromosome (X chromosome). Abnormalities in all chromosomes except the sex chromosomes are called autosomal. Chromosomal abnormalities can be detected by karyotyping, in which all the chromosomes are arranged in a format called karyogram/idiogram. The Cell Cycle The cell cycle refers to the cycle of cell division, which involves a series of successive events that begin with duplication of cell organelles and end in cell division. Cell division results in the production of two exactly alike daughter cells from a single parent cell. Cell cycle has five distinct stages, namely G0, G1, S, G2, and M. The M phase is the mitotic stage that is further divided into prophase, metaphase, anaphase, and telophase. The G1, S, and G2 phases are collectively called the interphase. The cell cycle culminates in cytokinesis, in which the cytoplasm of the parent cell physically divides into two daughter cells. The G0 ­phase is the resting phase of the cell. This stage is followed by interphase that includes G1, S, and G2 phases. This is the preparatory phase in which the cell prepares to divide. The G1 phase is a gap phase, also known as growth phase. During this phase, the cell carries out biosynthetic activities at a high rate. Enzymes and other molecules that will be required later in the S phase are synthesized in this phase. These enzymes are especially those that are need for DNA replication. The next phase, S, begins with the commencement of DNA synthesis. During this phase, the DNA of the cell doubles, and protein synthesis is slow. This phase is followed by the G2 phase. This is the second gap phase and continues from the S phase until the cell begins mitosis in the M phase. This phase is characterized by increased biosynthesis of material, such as microtubules, that will be required in the next phase. A cell cannot proceed from the G2 to M phase if protein synthesis is inhibited in this phase. The G2 phase is followed by the M phase, which is the mitotic phase of the cell. The M phase is characterized by mitosis and nuclear division (karyokinesis), followed by cytokinesis. Each phase in the cell cycle proceeds only if the previous phase is proper and complete. After one cell division is complete, the cell commences a new cell cycle beginning with the interphase. A cell remains morphologically indistinguishable throughout the interphase. Mitosis Mitosis is the process in which a cell divides itself into two identical daughter cells. Before the occurrence of mitosis, a cell undergoes other phases of the cycle that prepare it to undergo division. These phases are collectively called the interphase. During the interphase, DNA is duplicated and wound up to form thick chromosomes that are individually made of two chromatids joined at the center by a centromere. Mitosis is divided into prophase, metaphase, anaphase, and telophase, and culminates in cytokinesis. During prophase, chromosomes move to the opposite sides of the nucleus and spindle fibres (microtubules) begin to form. The envelope surrounding the nucleus begins to disintegrate. This phase is followed by the metaphase in which chromosomes line up at the center of the cell, also known as equator or metaphase plate. This occurs because the spindle fibers that are attached to the centromeres of the chromosomes pull the chromosomes to bring them into place. The next phase is the anaphase during which the two chromatids of each chromosome separate and each chromatid is pulled to the opposite poles of the cell by spindle fibers. The cell stretches out at the poles. This phase is followed by the telophase, which is the last phase of mitosis. During this phase, a complete set of chromosomes has reaches each end of the cell. As the spindle fibers begin to disintegrate, a nuclear membrane begins to form, encircling each chromosome set. The chromosomes begin to uncoil, resulting in thin strands of DNA, called chromatin. A dark nucleolus also begins to appear within the nucleus. At this stage, only the nucleus has divided and this is called karyokinesis. Once the nucleus has divided and the two sets of duplicated chromosomes have separated, the cell begins cytokinesis. The cytoplasm of the cell divides and the cell wall pinches in, physically dividing the cell into two identical daughter cells. After mitosis, the cells begin a new cell cycle, starting with the interphase. Cytokinesis Cytokinesis is the physical division of the cell after mitosis and meiosis into two daughter cells. During this process, the cell's cytoplasm splits into two equal halves. It occurs just after the beginning of anaphase and continues until the telophase stage in both mitosis and meiosis. It ends once the cell has completely separated into two. However, it is not a part of mitosis. Cytokinesis is an important part of cell division. During the process of cytokinesis, a cleavage furrow appears at the center of the cell in which two chromosomal sets have moved to the opposite poles. The cleavage furrow appears because a ring of proteins is formed at the center and the cell wall is pinched in. As the cell wall pinches in, the cytoplasm divides. Once the cell wall is completely pinched off, the two daughter cells close off completely and are physically separated from one another. Once cytokinesis is complete, the cell has completed its division and the two resultant cells go back into interphase, thereby repeating the cell cycle. If cytokinesis occurs when a cell has not undergone mitosis, the resulting daughter cells would not be alike and may not even function normally. Therefore, if mitosis does not occur, a cell will not undergo nuclear division (karyokinesis) and so, one cell would have the nucleus and the other would not. Hence, the various stages of the cell cycle and cell division need to progress perfectly, in order to obtain two normal, functional and identical daughter cells. Cytokinesis in plants is different from that observed in animals. Plants have rigid cell walls and so, their cells cannot move apart. Instead of splitting the cell into two halves during cytokinesis, a cell plate is formed at the center of the dividing cell. This plate begins appearing during the latter part of anaphase and during the telophase. Once the cell cytoplasm and organelles have divided equally between the two daughter cells, the cell plate becomes rigid and closes off the two cells resulting in separation. Works Cited Wikipedia.org, 2011. Web. 15 Dec. 2011. Read More