Fluent Essays

Short lab assignment, DO NOT SUBMIT FORUM https://docs.google.com/forms/d/e/1FAIpQLSdpJ11vBM-XJf_nkPpwocIslipYD7LIkBD3Emh9QdoDrRQmeA/viewform Cell Division Cell Division in Prokaryotes Binary Fission Definition Bacterial cells divide by a method of asexual reproduction known as binary fission. Fission means splitting. So in the process the genetic material is replicated, the cell grows larger and then splits into two. Genetic Material of the Bacterial Cell The genetic information of a bacterial cell exists as a single, circular, double-stranded DNA molecule. Bacterial cells are prokaryotic cells; they lack a nucleus. The DNA of the bacterial cell is not surrounded by and enclosed within a nuclear membrane. It lies free within the protoplasm of the bacterial cell. Although the bacterial cell lacks a nucleus, the area of the cell protoplasm where the nucleus is found is called the nucleoid. Prior to the Division of the Cell the DNA must be Doubled Prior to the division of the bacterial cell, the DNA must be replicated, producing two copies that can be equally distributed to each of the two daughter cells. Replication of the DNA at a specific site on the DNA molecule called the origin of replication. The replication enzymes copy the DNA of both strands, moving around the circular DNA in both directions simultaneously until a specific site of termination is reached. When these enzymes have proceeded all the way around the circle of DNA, the cell possesses two copies of the genome. These “daughter” genomes are attached side-by-side to the plasma membrane. Elongation of the Cell and Segregation of DNA to Opposite Ends of the Cell As the DNA replicates, the cell elongates. The two circular molecules of DNA now separate and move apart toward opposite ends of the cell. Fission of the Cell into two Daughter Cells After the DNA molecules have been segregated to opposite ends of the cell, the bacterial cell will divide to form two daughter cells. Then a group of proteins that will operate together to separate the cell into two assemble at the site of separation. A key component of this group of division machinery proteins is the protein FtsZ. FtsZ proteins begin the separation process by forming a ring in the middle of the cell. Other components of the division apparatus then join the FtsZ ring, forming new plasma membrane that separates the cytoplasm into the two cells. This is followed by the formation of cell wall material in the separation zones. The result of the process of binary fission is two cells, each with its own circular, double stranded, DNA molecule. The cell will now begin to split into two cells by a process called septation. This occurs as a septum forms in the middle of the cell. A protein called FtsZ forms a ring in the middle of the cell. As this process proceeds, the cell lays down new plasma membrane and cell wall materials in the zone between the attachment sites of the two daughter genomes. A new plasma membrane grows between the genomes; eventually, it reaches all the way into the center of the cell, dividing it in two. Because the membrane forms between the two genomes, each new cell is assured of retaining one of the genomes. Finally, a new cell wall forms around the new membrane. Cell Division in Eukaryotes Chromosomes In Eukaryotic cells the DNA is contained within chromosomes that differ greatly in their structure and complexity from the single circular molecules of DNA found in bacteria. Eukaryotic cells contain a number of chromosomes in which the DNA is arranged linearly, forming a complex in which it is associated with positively-charged proteins called histone and wound into tightly condensed coils. Discovery of Chromosomes “Chromosomes were first observed by the German embryologist Walther Fleming in 1882, while he was examining the rapidly dividing cells of salamander larvae. When Fleming looked at the cells through what would now be a rather primitive light microscope, he saw minute threads within their nuclei that appeared to be dividing lengthwise. Fleming called their division mitosis, based on the Greek word mitos, meaning “thread” (Raven and Johnson, 2002, p. 209).” A chromosome is a rod-shaped body found in the nuclear area of the cell during division. Note that chromosomes become visible only when the cell begins to divide. The chromosomes contain the genes. Each eukaryotic chromosome is believed to contain a single linear molecule of DNA. “The chromosome is the vehicle by which hereditary information is physically transmitted from one generation to the next (Raven and Johnson, 2002, glossary p. G-3).” The Structure of Chromosomes Composition of Chromatin Chromosomes are composed of chromatin, which is composed of DNA and proteins. The DNA of a chromosome is one very long, double-stranded fiber that extends unbroken through the entire length of the chromosome. A typical human chromosome contains about 140 million (1.4 x 108) nucleotides in its DNA. Highly condensed portions of the chromatin are called heterochromatin. Some of these portions remain permanently condensed, so that their DNA is never expressed. The remainder of the chromosome, called euchromatin, is condensed only during cell division, when compact packaging facilitates the movement of the chromosomes. At all other times, euchromatin is present in an open configuration, and its genes can be expressed. Chromosome Coiling The DNA within the chromosomes resembles a string of beads. Every 200 nucleotides, the DNA duplex is coiled around a core of eight histone proteins, forming a complex known as a nucleosome. Unlike most proteins, which have an overall negative charge, histones are positively charged, due to an abundance of the basic amino acids arginine and lysine. They are thus strongly attracted to the negatively charged phosphate groups of the DNA. The histone cores promote and guide the coiling of the DNA. The DNA wrapped in nucleosomes is further coiled into an even more compact structure called the solenoid. Chromosome Karyotypes “Chromosomes may differ widely in appearance. They vary in size, staining properties, the location of the centromere (a constriction found on all chromosomes), the relative length of the two arms on either side of the centromere, and the positions of constricted regions along the arms. The particular array of chromosomes that an individual possesses is called its karyotype. Karyotypes show marked differences among species and sometimes even among individuals of the same species.” “To examine a human karyotype, investigators collect a cell sample from blood, amniotic fluid, or other tissue and add chemicals that induce the cells in the sample to divide. Later, they add other chemicals to stop cell division at a stage when the chromosomes are most condensed and thus most easily distinguished from one another. The cells are then broken open and their contents, including the chromosomes, spread out and stained. To facilitate the examination of the karyotype, the chromosomes are usually photographed, and the outlines of the chromosomes are cut out of the photograph and arranged in order (Raven and Johnson, 2002, p. 211).” How Many Chromosomes Are in a Cell? “With the exception of the gametes (eggs or sperm) and a few specialized tissues, every cell in a human body is diploid (2n). This means that the cell contains two nearly identical copies of each of the 23 types of chromosomes, for a total of 46 chromosomes. The haploid (1n) gametes contain only one copy of each of the 23 chromosome types, while certain tissues have unusual numbers of chromosomes – many liver cells, for example, have two nuclei, while mature red blood cells have no nuclei at all. The two copies of each chromosome in body cells are called homologous chromosomes, or homologues (Greek homologia, “agreement”). Before cell division, each homologue replicates, producing two identical sister chromatids joined at the centromere, a condensed area found on all eukaryotic chromosomes. Hence, as cell division begins, a human body cell contains a total of 46 replicated chromosomes, each composed of two sister chromatids joined by one centromere. The cell thus contains 46 centromeres and 92 chromatids (2 sister chromatids for each of 2 homologues for each of 23 chromosomes). The cell is said to contain 46 chromosomes rather than 92 because, by convention, the number of chromosomes is obtained by counting centromeres (Raven and Johnson, 2002, p. 211).” Chromosome Number “Since their initial discovery, chromosomes have been found in the cells of all eukaryotes examined. Their number may vary enormously from one species to another. A few kinds of organisms – such as the Australian ant Myrmecia, the plant Halopappus gracilis, a relative of the sunflower that grows in North American deserts; and the fungus Penicillium – have only 1 pair of chromosomes, while some ferns have more than 500 pairs. Most eukaryotes have between 10 and 50 chromosomes in their body cells.” “Human cells each have 46 chromosomes, consisting of 23 nearly identical pairs. Each of these 46 chromosomes contains hundreds or thousands of genes that play important roles in determining how a person’s body develops and functions. For this reason, possession of all the chromosomes is essential to survival. Humans missing even one chromosome, a condition called monosomy, do not survive embryonic development in most cases. Nor does the human embryo develop properly with an extra copy of any one chromosome, a condition called trisomy. For all but a few of the smallest chromosomes trisomy is fatal, and even in those few cases, serious problems result. Individuals with an extra copy of the very small chromosome 21, for example, develop more slowly than normal and are mentally retarded, a condition called Down syndrome (Raven and Johnson, 2002, p. 209).” Phases of the Cell Cycle “Cell cycle: The repeating sequence of growth and division through which cells pass each generation (Raven and Johnson, 2002, glossary G-3).” “This division process is diagrammed as a cell cycle, consisting of five phases.” The Five Phases (Raven and Johnson, 2002, p. 212) “G1 is the primary growth phase of the cell. For many organisms, this encompasses the major portion of the cell’s life span.” “S is the phase in which the cell synthesizes a replica of the genome.” “G2 is the second growth phase, which preparations are made for genomic separation. During this phase, mitochondria and other organelles replicate, chromosomes condense, and microtubules begin to assemble at a spindle.” “G1, S, and G2 together constitute interphase, the portion of the cell cycle between cell divisions.” “M is the phase of the cell cycle in which the microtubular apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart. Called mitosis, this process is the essential step in the separation of the two daughter genomes. We will discuss mitosis as it occurs in animals and plant, where the process does not vary much (it is somewhat different among fungi and some protists). Although mitosis is a continuous process, it is traditionally subdivided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.” “C is the phase of the cell cycle when the cytoplasm divides, creating two daughter cells. This phase is called cytokinesis. In animal cells, the microtubule spindle helps position a contracting ring of actin that constricts like a drawstring to pinch the cell in two. In cells with a cell wall, such as plant cells, a plate forms between the dividing cells.” Duration of the Cell Cycle “The time it takes to complete a cell cycle varies greatly among organisms. Cells in growing embryos can complete their cell cycle in less than 20 minutes; the shortest known animal nuclear division cycles occur in fruit fly embryos (8 minutes). Cells such as these simply divide their nuclei as quickly as they can replicate their DNA, without cell growth. Half of the cycle is taken up by S, half by M, and essentially none by G1 or G2. Because mature cells require time to grow, most of their cycles are much longer than those of embryonic tissue. Typically, a dividing mammalian cell completes its cell cycle in about 24 hours, but some cells, like certain cells in the human liver, have cell cycles lasting more than a year. During the cycle, growth occurs throughout the G1 and G2 phases (referred to as “gap” phases, as they separate S from M), as well as during the D Phase. The M phase takes only about an hour, a small fraction of the entire cycle.” “Most of the variation in the length of the cell cycle from one organism or tissue to the next occurs in the G1 phase. Cells often pause in G1 before DNA replication and enter a resting state called G0 phase; they may remain in this phase for days to years before resuming cell division. At any given time, most of the cells in an animal’s body are in G0 phase. Some, such as muscle and nerve cells, can resume G1 phase in response to factors released during injury.” Interphase: Preparing for Mitosis “The events that occur during interphase, made up of the G1, S, and G2 phases, are very important for the successful completion of mitosis. During G1, cells undergo the major portion of their growth. During the S phase, each chromosome replicates to produce two sister chromatids, which remain attached to each other at the centromere. The centromere is a point of constriction on the chromosome, containing a specific DNA sequence called a kinetochore to which is bound a disk of protein. This disk functions as an attachment site for fibers that assist in cell division. Each chromosome’s centromere is located at a characteristic site.” “The cell grows throughout interphase. The G1 and G2 segments of interphase are periods of active growth, when proteins are synthesized and cell organelles produced. The cell’s DNA replicates only during the S phase of the cell cycle.” “After the chromosomes have replicated in S phase, they remain fully extended and uncoiled. This makes them invisible under the light microscope. In G2 phase, they begin the long process of condensation, coiling ever more tightly. Special motor proteins are involved in the rapid final condensation of the chromosomes that occurs early in mitosis. Also during G2 phase, the cells begin to assemble the machinery they will later use to move the chromosomes to opposite poles of the cell. In animal cells, a pair of microtubule-organizing centers called centrioles replicate. All eukaryotic cells undertake an extensive synthesis of tubulin, the protein of which microtubules are formed.” Mitosis in the Animal Cell Each of us developed from a single cell. Man consists of approximately 1014 (100 trillion) cells. Not only must these be formed and differentiated as the body matures, they must also, in many instances be constantly replaced as they pass through their life cycle and die. Mitosis – a method of cell division in which there is an equal distribution of chromosomal material to each of the daughter cells that result from the division. Generally, mitosis includes the self-duplication of DNA and the equal distribution of this nuclear substance into two daughter nuclei. The sequence of events during mitosis is divided into the following stages or phases: 1) interphase 2) prophase 3) prometaphase 4) metaphase 5) anaphase 6) telophase. Interphase is not a dividing stage. It is the stage in-between divisions. Strictly speaking, it is not part of mitosis. These stages are not identified by abrupt changes. (Mitosis is continuous) However, certain significant events occur that will make it possible for you to distinguish each stage. As is true of almost all other events in the cell, reproduction also begins in the nucleus itself. The first step is replication (duplication) of all genes and of all chromosomes. The next step is division of the two sets of genes between two separate nuclei. And the final step is splitting of the cell itself to form two new daughter cells, a process called mitosis. The complete life cycle of a cell that is not inhibited in some way is about 10 to 30 hours from reproduction to reproduction, and the period of mitosis lasts for approximately one-half hour. The period between mitoses is called interphase. However, in the body there are almost always inhibitory controls that slow or stop the uninhibited life cycle of the cell and give cells life cycle periods that vary from as little as 10 hours for stimulated bone marrow cells to an entire lifetime of the human body for nerve cells. Interphase In this stage the nucleus is large. Nucleic acids and proteins are synthesized as a preparatory (prior to) step to division. Chromosomes are in the form of chromatin. This is a network of nucleic acids and proteins. Under the light microscope, it appears as granules spread throughout the nucleus. Chromatin is the material that makes up chromosomes. The Chromosomes are not individually distinguishable in interphase. Nucleoli are visible. DNA is replicated. Replication of the Genes The genes are reproduced several hours before mitosis takes place, and the duration of gene replication is only about one hour. When replication begins, it occurs for all genes at the same time and not for only part of them. Furthermore, the genes are duplicated only once. The net result is two exact duplicates of all genes, which respectively become the genes in the two new daughter cells that will be formed at mitosis. Following replication of the genes, the nucleus continues to function normally for several hours before mitosis begins abruptly. Chemical and Physical Events: The genes are duplicated in the following way: First, the two strands of the DNA helix of the gene pull apart. Second, each of these strands combines with deoxyribose nucleotides. Each of the bases on each strand of DNA in the chain attracts a nucleotide containing the appropriate complementary base. In this way, the appropriate nucleotides are lined up side by side. Third, appropriate enzyme mechanisms then provide energy and cause polymerization of the nucleotides to form a new DNA strand. Prophase Early Prophase Replication of the Centrioles The first event of mitosis takes place in the cytoplasm occurring in the small structures called centrioles. Two pairs of centrioles lie close to each other near one pole of the nucleus. Each centriole is a small cylindrical body about 0.4 (long and about 0.15 (in diameter, consisting mainly of nine parallel, tubular –like structures arranged around the inner wall of the cylinder. The two centrioles of each pair lie at right angles to each other. A pair of centrioles forms a structure called a centrosome. Formation of Spindle Apparatus In so far as is known, the two pairs of centrioles remain dormant during interphase until shortly before mitosis is to take place. At that time, the two pairs begin to move apart from each other. This is caused by protein microtubules growing between the respective pairs and actually pushing them apart. At the same time, microtubules grow radially away from each of the pairs. Some of these penetrate the nucleus. The set of microtubules connecting the two centriole pairs is called the spindle, and the entire set of microtubules plus the two centrioles is called the mitotic apparatus. While the spindle is forming, the chromatin material of the nucleus (the DNA) becomes condensed into well-defined but disoriented chromosomes. They become visible as long thin threads (spireme threads). The chromosome threads split lengthwise to form two identical halves known as chromatids. Chromatid – one of the identical halves that makes up a chromosome. The nucleolus and nuclear membrane begin to disappear. Late Prophase The chromatids shorten and thicken to form the chromosomes. The nucleolus and nuclear membrane disappear. Some of the microtubules from the forming mitotic apparatus become attached to the chromosomes. This attachment always occurs at the point on each chromosome, at a small condensed portion called the centromere. Prometaphase During prometaphase, the condensed chromosomes become attached to the spindle by their kinetochores. Each chromosome possesses two kinetochores, one attached to the centromere region of each sister chromatid. Some of the spindle fibers from each pole attach to the kinetochore of a chromatin. These are known as kinetochore microtubules. The kinetochore microtubules attach such that the kinetochores of sister chromatids are attached to kinetochore microtubules from opposite poles. Other spindle fibers, known as nonkinetochore microtubules extend from each pole and interact with other nonkinetochore microtubules coming from the opposite pole. Metaphase Early Metaphase During metaphase the centriole pairs are pushed far apart by the growing spindle, and the chromosomes are thereby pulled tightly by the attached microtubules to the very center of the cell, lining up in the equatorial plane of the mitotic spindle. Late Metaphase Separation of chromosomes is completed by splitting of the centromere. Anaphase – Early, Mid, and Late In anaphase, the chromatids move apart from each other. By convention, when the chromatids begin to move apart from one another, they are called chromosomes again. A microtubule connecting with one pair of centrioles pulls one chromatid and a microtubule connecting with the other centriole pair pulls the opposite chromatid. This movement results in the separation of chromatids so that one member of each pair will arrive at one or the other cell poles. Termination of anaphase movement occurs when the chromosomes form a densely packed group at the two poles. Telophase Early Telophase Chromosomes aggregate at poles. Late Telophase 1) Chromosomes reorganize to form the chromatin net. 2) The nucleolus and nuclear membrane reappear 3) The Spindle fibers disappear Each of the two pairs of centrioles is replicated during telophase, the mechanism of which is not understood. These new pairs of centrioles remain dormant through the next interphase until a mitotic apparatus is required for the next cell division. Mitosis in the Plant Cell Compared to Mitosis in the Animal Cell Plant Cells Animal Cells Lack centrioles and centrosomes Centrioles and centrosomes are present Lack astral rays Astral rays are present Division of the cytoplasm (cytokinesis) takes place by cell-plate formation Division of the cytoplasm (cytokinesis) takes place by constriction of cytoplasm (cleavage furrow) In the animal cell, small cylindrical structures known as centrioles organize the spindle fibers, thin threads of protein that form the spindle apparatus. The spindle fibers pull the chromosomes apart toward opposite poles of the cell during anaphase. In animal cells, the mitotic apparatus consists of three components: the asters, which form about each centrosome, the gelatinous spindle, and the traction fibers, which connect the centromeres of the various chromosomes to either centrosome. Plant cells lack centrioles (and centrosomes) but still produce a spindle apparatus. Although plant cells lack centrioles, they have a microtubule-organizing center instead. It does the same job as the centrioles, but for some reason, is not as visible in electron micrographs. In animal cells, special spindle fibers form around the centrioles that are located at each pole of the cell. They are known as astral rays. They are not found in plant cells. In telophase of mitosis, the cell divides into daughter cells. In plant cells the cytoplasm is divided by formation of a cell plate. The cell plate will eventually form cell walls between the daughter cells. In animal cells, the cytoplasm is divided into the two daughter cells by constriction of the cytoplasm. The cytoplasm pinches inward in the middle, eventually separating the two cells. The constriction is also known as a cleavage furrow. Meiosis Meiosis is the process by which the chromosome number of a reproductive cell becomes reduced to half the diploid (2n) or somatic number; results in the formation of gametes in animals, or of spores in plants. Haploid and Diploid Haploid – a single set of chromosomes. Diploid - a double set of chromosomes. Cells that have more than one double set of chromosomes are known as polyploid. The haploid number is designated n and the diploid number as 2n. In humans, for example, n = 23 and therefore 2n = 46. When a sperm fertilized an egg, the two haploid nuclei fuse, n + n = 2n, and the diploid number is restored. The diploid cell produced by the fusion of two gametes is known as a zygote. In every diploid cell, each chromosome has a partner. These pairs of chromosomes are known as homologous pairs, or homologues. The two resemble each other in size and shape and also in the type of hereditary information each contains. One homologue is derived from one parent, and its partner is derived from the other parent. In the special kind of nuclear division called meiosis, the diploid number of chromosomes is reduced to the haploid number, thus counterbalancing the effects of fertilization. Moreover, meiosis is in itself a source of new chromosome combinations. The Phases of Meiosis Meiosis consists of two successive nuclear divisions, conventionally designated meiosis I and meiosis II. In meiosis I, the homologues separate; in meiosis II, the chromatids separate. In the following discussion we shall describe meiosis in a plant cell in which the diploid number is 8 (n = 4). Four of the eight chromosomes were inherited from one parent and four from the other parent. Each chromosome has a homologue or homologous chromosome, which is of similar size and shape. One of the members of this homologous pair of chromosomes came from one parent; the other member came from the other parent. Prophase I During interphase, the chromosomes are replicated, so that by the beginning of meiosis I each chromosome consists of two identical sister chromatids held together at the centromere. Insert Diagram of Homologous Chromosomes In the first prophase of meiosis, prophase I, the chromatin condenses and the nuclear envelope begins to dissolve. During this first prophase, the homologous chromosomes come together in pairs. The pairing of homologous chromosomes is known as synapsis. Since each chromosome consists of two identical chromatids, the pairing of the homologous chromosomes actually involves four chromatids; this complex of paired homologous chromosomes is known as a tetrad. Crossing Over Crossing over, the interchange of segments of one chromosome with corresponding segments from its homologue, takes place at this time. Insert Diagram of Crossing Over Crossing over is an important source of variation in the hereditary material. Toward the end of prophase I, the spindle apparatus is formed, the nucleolus disappears, and the nuclear envelope breaks down. Metaphase I In metaphase I, the homologous pairs (four in this example) line up along the equatorial plane of the cell. Each pair consists of four chromatids (two homologous chromosomes). The spindle has formed, and spindle fibers attach to each homologue at its kinetochore (part of centromere). If this were an animal cell, centrioles and asters would be present. Anaphase I At anaphase I, the homologues, each consisting of two chromatids, separate, as if pulled apart by the spindle fibers attached to their kinetochores. However, the two sister chromatids of each chromosome do not separate as they did in mitosis. Telophase I By the end of the first meiotic division, telophase I, the homologues have moved to the poles. Each group now contains only half the number of chromosomes as the original nucleus. Moreover, these chromosomes may be different from any one that was present in the original cell because of exchanges taking place during crossing over. Depending on the species, new nuclear envelopes may or may not form, and cytokinesis may or may not take place. Meiosis II Meiosis II resembles mitosis except that it is not preceded by replication of the chromosomal material. At the beginning of the second meiotic division, the chromosomes, which may have dispersed somewhat, condense fully again. There are four in each nucleus (the haploid number), and they are still in the form of two chromatids held together at the centromere. Prophase II During prophase II, the nuclear envelopes, if present, dissolve, and new spindle fibers begin to appear. Metaphase II During metaphase II, the four chromatid pairs in each nucleus line up on the equatorial plane. Anaphase II At anaphase II, as in mitosis, the sister chromatids separate, and each resulting chromosome moves toward one of the poles. Telophase II During telophase II, the spindles disappear and a nuclear envelope forms around each set of chromosomes. There are now four nuclei in all, each containing the haploid number of chromosomes. Cell division (cytokinesis) proceeds as it does following mitosis. Cell walls form, dividing the cytoplasm, and the cells begin to differentiate into spores. So, beginning with one cell containing eight chromosomes (four homologous pairs), we end with four cells, each with four chromosomes (no homologous pairs). The haploid cells …