General Biology I Lecture Notes PDF

General Biology I BIO 101 LECTURE 03 GENERAL REPRODUCTION IN LIVING ORGANISMS Outlines Introduction General Reproduction Asexual Reproduction. Sexual Reproduction. 0bjectives By the end of this lecture, you will be able to: Give the definition of Repro...

General Biology I BIO 101 LECTURE 03 GENERAL REPRODUCTION IN LIVING ORGANISMS Outlines Introduction General Reproduction Asexual Reproduction. Sexual Reproduction. 0bjectives By the end of this lecture, you will be able to: Give the definition of Reproduction. Explain the two types of reproduction in living things. Describe the process of vegetative reproduction in plant Discuss the differences between Mitosis and Meiosis. Introduction In a general sense reproduction is one of the most important concepts in biology. It can be define as a means of making a copy, a likeness of the organisms to the their parents, and thereby providing for the continued existence of species. Although reproduction is often considered solely in terms of increase in the production of offspring in animals and plants, the more general meaning has far greater significance to living organisms. To appreciate this fact, the origin of life and the evolution of organisms must be considered as a means of increasing the population growth. At the lowest level, therefore, reproduction is chemical replication. As evolution level reproduction is progressed, cells of successively higher levels of complexity must have arisen, and it was absolutely essential that they had the ability to make likenesses of themselves. Introduction In the unicellular level organisms have the ability of one cell to reproduce itself means the reproduction of a new individual; While in multicellular level organisms grows and regeneration. Multicellular organisms also reproduce in the strict sense of the term where they make copies of themselves in the form of offspring but they do so in a variety of ways, many involving complex organs and elaborate hormonal mechanisms. Reproduction: is the process by which living organisms produce offspring of their own kind. Reproduction occurs either asexually (involves only one parent) or sexually (involves a combination of two parents) through cell division processes called mitosis and meiosis respectively. Growth and repair of old/dead cells in living organisms occur as a result of mitosis General Reproduction According to the cell theory, all living organisms are composed of one or more cells which originate from pre-existing cells by the process of cell division. Cells divide mainly for growth, repair of old / dead cells and reproduction, which are very important for the survival of organisms. The chemical constituents of cytoplasm (that part of the cell outside the nucleus), are not resynthesized from DNA every time a cell divides. This is because each of the two daughter cells formed during cell division usually inherits about half of the cellular material from the mother cell and is important because the presence of essential enzymes enables DNA to replicate even before it has made the enzymes necessary to do so. General Reproduction Cells of higher organisms contain complex structures, and each time a cell divides the structures must be duplicated. The method of duplication varies for each structure, and in some cases the mechanism is still uncertain. One striking and important phenomenon is the formation of a new membrane. Cell membranes although they are very thin and appear to have a simple form and structure, contain many enzymes and are sites of great metabolic activity. This applies not only to the membrane that surrounds the cell but to all the membranes within the cell. New membranes, which seem to form rapidly, are indistinguishable from old ones. Asexual Reproduction. Mitosis is the most common mode of binary fission, in which the division of a cell into two separate and similar parts take place. In bacteria (prokaryotes) the chromosome replicates and then divides in two, after which a cell wall forms across the elongated parent cell. In higher organisms (eukaryotes) there is first an elaborate duplication and then a separation of the chromosomes (mitosis), after which the cytoplasm divides in two. In the hard-walled cells of higher plants, a median plate forms and divides the mother cell into two compartments; in animal cells, which do not have a hard wall, a delicate membrane pinches the cell in two, much like the separation of two liquid drops. Budding yeast cells provide an interesting exception. In these fungi the cell wall forms a bubble that becomes engorged with cytoplasm until it is ultimately the size of the original cell. The nucleus then divides, one of the daughter nuclei passes into the bud, and ultimately the two cells separate. The process of Mitosis. Interphase: This is the preparatory stage that last from the end of one cell division to the beginning of the next, taking up about 90% of time in the cell cycle. During interphase, the cell grows in size and more nucleic materials and cellular organelles are produced. This stage is divided into 3 sub-stages: G1 (Gap 1) – this is the period where the cell grows and prepare to replicate its DNA; no division takes place in this phase. S (Synthesis phase) – this is the period where the chromosomes (46 in human somatic cells) inside the nucleus replicate, this results in two identical copies of chromosomes (92), called sister chromatids. The two sister chromatids are attached to each other at a point called the centromere. At this stage the sister chromatids remain largely invisible to microscope. The process of Mitosis. G2 (Gap 2) – at this period organelles and other material required for cell division are formed and/or replicated, e.g., the centrioles in animal cells replicate themselves, to form 2 pairs. Mitosis: This is the part of the cell cycle when the cell nucleus is replicated and divided into two nuclei containing genetically identical material. There are five (5) main phases in mitosis: Prophase – in this phase the sister chromatids become visible. The nuclear envelope breaks up, exposing the chromosomes. The centrioles slowly migrate to opposite poles of the cell. As they move apart, a network of protein fibers known as spindle fibers made up of microtubules is formed, which then attach to structures called Kinetochore, on the centromere of the sister chromatids for subsequent motion towards the center (equator). The process of Mitosis. Prometaphase and Metaphase – in prometaphase the chromosomes gradually migrate to the center, and in metaphase the chromosomes are lined up along the equatorial plate - an imaginary line in the center of the cell. The chromosomes move here with the help of the spindle fibers from the centrioles. Anaphase – In this phase the centromere of each chromosome are pulled by the spindle fibers, causing the chromosomes to separate, creating two daughter (complete set of 46 chromosomes each) chromosomes. One complete set of chromosomes (46) is pulled to one side of the cell, while the other identical set (46) is pulled to the opposite pole. Telophase – begins once the separated chromosomes reach the opposite poles of the cell. The network of spindle fibers disappears; around each daughter chromosome, a nuclear envelope begins to form and the chromosomes begin to decondense. At this point, the actual process of mitosis (nuclear division - karyokinesis) is complete. The process of Mitosis. The process of Mitosis. Cytokinesis: is the final stage of the cell cycle; it can occur anywhere from Anaphase into Telophase once the chromosomes have separated. The cytoplasm of the cell begins to split. It divides roughly in half. These two halves are identical copies of one another. They are now fully functioning cells; each is known as a daughter cell. The cell cycle for somatic cells is now complete and each daughter cell begins the process again. Mitosis and cytokinesis make up 10 % of the time in the cell cycle. Cytokinesis proceeds differently in animal and plant cells: In animal cells, the cytoplasm divides when a groove called the cleavage furrow forms through the middle of the parent cell. The cleavage furrow deepens until the parent cell pinches into two new identical cells. In plant cells the material for a new cell wall called the cell plate divides the cell into two new identical cells. Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death Types of asexual reproduction: Binary fission: the simplest form of asexual reproduction; the parent cell divides into two equal daughter cells – no parent is left; occur in unicellular organisms such as bacteria, algae, ameba, paramecium. Budding: Spore formation Vegetative reproduction: Is a form of asexual reproduction, where plants produce offspring from roots, stems and leaves (vegetative structures of plants), rather than sexually through seeds. Vegetative reproduction is divided into natural and artificial vegetative reproduction. Natural vegetative reproduction in plants takes place by means of bulbs, corms, tubers, runners and rhizomes. These are all modified stems, capable of reproducing identical offspring to the parent plant. Examples of such plants include: onions, tulips, lilies, gladioli, potatoes, strawberries, ginger, etc. Cutting Layering Grafting Regeneration The ability of an organism to regrow lost body parts, examples includes organisms such as starfish, earthworms, hydras and planarian. As organisms become more complex the power of regeneration disappears. Organisms that can regenerate usually do not reproduce in this manner. Regeneration Sexual Reproduction Meiosis and sexual reproduction: Meiosis is the process by which sex cells (gametes) are formed: originally each chromosome of the cell is in a pair (diploid); during meiosis these diploid pairs of chromosomes are separated so that each sex cell has only one of each pair of chromosomes (haploid). During the two successive meiotic divisions involved in the production of eggs, a primordial diploid egg cell is converted into a haploid egg and three small haploid polar bodies (minute cells). In this instance the egg receives far more cytoplasm than the polar bodies. Most multicellular organisms reproduce sexually by means of a cellular division process called meiosis - the division of a diploid nucleus to form four haploid genetically different daughter nuclei. Meiosis functions to ensure the continuity & survival of organisms by making sexual reproduction possible; increases the genetic variation of the population; and the formation of gametes in animals. Sexual Reproduction In meiosis each cell divides twice, however the chromosomes replicate only once, this process is divided into the following stages: 1. Interphase: Just as in mitosis of somatic cells, during interphase of germ cells, the cell grows; diploid chromosomes inside the nucleus replicate, bringing the total number of individual chromosomes (chromatid) to 92. 2. Meiosis I: Is the first division of the reduction division of meiosis. There are 4 sub-phases in meiosis I: Prophase I – during this phase each homologous chromosome pair up, and are fastened together by their centromeres. This pairing process is known as synapsis, and each group of four chromatids is called a tetrad. Synapsis is when crossing over occurs. During crossing over, a DNA segment is exchanged between two non-sister chromatids of a homologous pair of chromosomes. Metaphase I – at this phase the homologous chromosome pairs are lined up next to each other, along the equatorial plate; spindle microtubules (fibers) fasten at the centromeres. Sexual Reproduction Anaphase I – at this phase the homologous chromosomes separate due to the spindle fibers pulling them apart, and move to opposite ends (poles) of the cell. This process of separation is called disjunction. There are half as many chromosomes as the original cell (92), however – each set is double stranded (46 in each cell). Telophase I – ends the first meiotic division; cytoplasm divides (cytokinesis) forming two daughter cells each with half the number of chromosomes in the parent cell but in replicated form (sister chromatids). 3. Meiosis II: Is the second division of meiosis. It occurs in both of the newly formed daughter cells simultaneously and is similar to mitosis, except that 4 daughter cells rather than 2 are produced with haploid number of chromosomes. It consists of 4 sub-phases: Prophase II – in this second meiotic phase each daughter cell form spindles; chromosomes move toward the middle. Metaphase II – in this second meiotic phase chromosomes become fastened to spindles at the centromeres and line up at the equator. Anaphase II – in this phase the centromeres divide and the two chromatids separate, resulting in single stranded chromosomes which move toward the poles. Sexual Reproduction Telophase II – during Telophase II, both daughter cells divide further creating 4 daughter cells. Each of these 4 daughter cells contains 23 chromosomes, making them haploid, and none of the 4 is exactly alike (due to crossing over and independent assortment). Then the nuclear membrane reforms again. Differences between Mitosis and Meiosis Differences between Mitosis and Meiosis 1.Cell Division Mitosis: A somatic cell divides once. Cytokinesis (the division of the cytoplasm) occurs at the end of Telophase. Meiosis: A reproductive cell divides twice. Cytokinesis happens at the end of Telophase I and telophase II. 2. Daughter Cell Number Mitosis: Two daughter cells are produced. Each cell is diploid containing the same number of chromosomes. Meiosis: Four daughter cells are produced. Each cell is haploid containing one-half the number of chromosomes as the original cell. Differences between Mitosis and Meiosis 3.Genetic Composition Mitosis: The resulting daughter cells in mitosis are genetic clones (they are genetically identical). No recombination or crossing over occur. Meiosis: The resulting daughter cells contain different combinations of genes. Genetic recombination occurs as a result of the random segregation of homologous chromosomes into different cells and by the process of crossing over (transfer of genes between homologous chromosomes). 4. Length of Prophase Mitosis: During the first mitotic stage, known as prophase, chromatin condenses into discrete chromosomes, the nuclear envelope breaks down, and spindle fibers form at opposite poles of the cell. A cell spends less time in prophase of mitosis than a cell in prophase I of meiosis. Meiosis: Prophase I consist of five stages and lasts longer than prophase of mitosis. The five stages of meiotic prophase I are Leptotene, zygotene, pachytene, diplotene, and diakinesis. These five stages do not occur in mitosis. Genetic recombination and crossing over take place during prophase I. Differences between Mitosis and Meiosis 5. Tetrad Formation Mitosis: Tetrad formation does not occur. Meiosis: In prophase I, pairs of homologous chromosomes line up closely together forming what is called a tetrad. A tetrad consists of four chromatids (two sets of sister chromatids). 6. Chromosome Alignment in Metaphase Mitosis: Sister Chromatids (duplicated chromosome comprised of two identical chromosomes connected at the centromere region) align at the metaphase plate (a plane that is equally distant from the two cell poles). Meiosis: Tetrads (homologous chromosome pairs) align at the metaphase plate in metaphase I. 7. Chromosome Separation Mitosis: During anaphase, sister chromatids separate and begin migrating centromere first toward opposite poles of the cell. A separated sister chromatids becomes known as daughter chromosome and is considered a full chromosome. Meiosis: Homologous chromosomes migrate toward opposite poles of the cell during anaphase I. Sister chromatids do not separate in anaphase Questions 1-Define the term reproduction in unicellular and multicellular organisms. 2-Mension the two types of reproduction in living things with example. 3-Write short note on the following Vegetative reproduction Cutting Grafting Budding Layering 3-Explain the process of Mitosis in cell division 4-Using an illustration describe the process of Meiosis1 and Meiosis 2 5-List and Explain the differences between Mitosis and Meiosis 6- With a well label diagram explain the process of Regeneration.