Bisc 102 - Chapters 8 / 9: DNA Replication, Binary Fission, and Mitosis / Sexual Reproduction and Meiosis
Bisc 102 - Chapters 8 / 9: DNA Replication, Binary Fission, and Mitosis / Sexual Reproduction and Meiosis Bisc 102
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This 9 page Class Notes was uploaded by Alexis Neely on Monday September 26, 2016. The Class Notes belongs to Bisc 102 at University of Mississippi taught by Carla Beth Carr in Fall 2016. Since its upload, it has received 10 views. For similar materials see Inquiry Into Life Human Biology in College of Liberal Arts at University of Mississippi.
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Date Created: 09/26/16
8.1 Cells Divide, and Cells Die - Cell division produces a continuous supply of replacement cells everywhere in the body. - No living organism can reproduce without cell division, and the grown and development of a multicellular organism also require the production of new cells. Sexual Life Cycles Include Mitosis, Meiosis, and Fertilization - For a single-celled organism, asexual reproduction i s when one cell replicates its genetic material and splits into two. Except for the occasional mutation, asexual reproduction generates genetically identical offspring. - Sexual reproduction, in contrast, is the production of offspring whose genetic makeup comes from two parents. Each parent contributes a sex cell, and the fusion of these cells signals the start of the next generation (offspring genetically differ from each other and parents). - Meiosis is a specialized type of cell division that gives rise to nuclei that are genetically different from one another. In humans and many other species, these nuclei are packaged into gametes: sperm cells for males, and egg cells for females. The variation among gametes explains why siblings generally look different from one another (except identical twins). - Fertilization is the union of the sperm and the egg cell, producing a zygote (the first cell of the new offspring). Immediately after fertilization,the other type of cell division - mitotic - takes over. Mitosis divides a eukaryotic cell’s genetic information into two identical nuclei. Cell Death is Part of Life - Cells die in predictable ways, carving distinctive structures. A poptosis, also called “programmed cell death,” is a normal part of development. - During early development, both cell division and apoptosis shape new structures. (e.g. cells in the tail of a tadpole die as it becomes an adult) 8.2 DNA Replication Precedes Cell division - Before any cell divides - by binary fission, mitotically, or meiotically - it must first duplicate its entire enome, which consists of all of the cell’s genetic material. - The genome may consists of one or more chromosomes, individual molecules of DNA with their associated proteins. - An army of enzymes copies DNA just before a cell divides. Enzymes called helicases unwind and “unzip” the DNA molecule. Next, the D NA polymerase enzyme adds new DNA nucleotides that are complementary to the bases on each exposed strand. As the new DNA strands grow, hydrogen bonds form between the complementary bases, and two DNA molecules form in the place of one. 8.3 Bacteria and Archaea Divide by Binary Fission - In prokaryotes, reproduction occurs by b inary fission, an asexual process that replicates DNA and distributes it (along with other cell parts) into two daughter cells. - 1. Parent cell contains one chromosome. - 2. DNA replicates and attaches to cell membrane. - 3. Membrane growth between the two attachment points moves the DNA molecules apart as new cell wall material is deposited. - 4. The result of binary fission: two daughter cells, each identical to the original. - Cytokinesis divides the cytoplasm into two daughter cells. 8.4 Replicated Chromosomes Condense as a Eukaryotic Cell Prepares to Divide - Binary fission is relatively uncomplicated because the genetic material in prokaryotic cells typically consists of a single circular DNA molecule, so cell division is relatively simple. In a eukaryotic cell, however, distributing DNA into daughter cells is more complex because the genetic information consists of multiple chromosomes inside a nucleus. - A eukaryotic cell must have access to the information in its DNA, and it a dividing cell must package its DNA into a portable form that can easily move into the two daughter cells. - Condensed DNA is easier for the dividing cell to manage than is an unwound chromosome. - Eukaryotic chromosomes consist of chromatin, which is a collective term for all of the cell’s DNA and its associated proteins. These proteins include the many enzymes that help replicated the DNA and transcribe it to RNA. Others serve as scaffolds around which DNA - entwines, helping to pack the DNA efficiently inside the cell. - Chromatin is organized into n ucleosomes, each consisting of a stretch of DNA wrapped around eight proteins (histones). A continuous thread of DNA connects nucleosomes like beads o a string. When the cell is not dividing, chromatin is barely visible because the nucleosomes are loosely packed together. The cell can therefore access the information in the DNA to produce the proteins that it needs. DNA replication in preparation for cell division also requires that the cell’s DNA be unwound. - The chromosome’s appearance changes shortly after DNA replication.The nucleosomes gradually fold into progressively larger structures, until the chromosome takes on its familiar, compact shape. Once condensed, a chromosome has readily identifiable parts. - Two chromatids make up the replicated chromosome. Because these paired chromatids have identical DNA sequences, they are called “sister chromatids.” - The centromere is a small section of DNA and associated proteins that attaches the sister chromatids to each other. It often appears as a constriction in a replicated chromosomes. As a cell’s genetic material divides, the centromere splits, and the sister chromatids move apart. At that point, each chromatid becomes an individual chromosome. 8.5 Mitotic Division Generates Exact Cell Copies - Actively dividing skin cells illustrate the ell cycle which describes the events that occur in one complete round of cell division. - Biologists divide the cell cycle into stages. I nterphase is the interval between successive cell divisions’ protein synthesis, DNA replication, and many other events occur during interphase. Next is mitosis during which the contents of the nucleus divide. In cytokinesis, the cell splits into two daughter cells. After cytokinesis is complete, each daughter cell enters interphase, and the cell cycle begins anew. DNA Is Copied during Interphase - Interphase is divided into “gap” phases (designated G1 and G2), separated by a “synthesis” (S) phase. During G 1 phase, the cell grows, carries out its basic functions, and produces the new organelles and other components it will require if it divides. - During S phase, enzymes replicate the cell’s genetic material and repair damaged DNA. As S phase begins, each chromosome includes one DNA molecule. By the end of the S phase, each chromosome consists of two attached sister chromatids. - In an animal cell, another event that occurs during S phase is the duplication of the centrosome. Centrosomes are structures that organize the mitotic spindle, a set of microtubule proteins that coordinate the movements of the chromosomes during mitosis. Each centrosome includes proteins enclosing a pair of barrel-shaped centrioles. Most plant cells lack centrosomes’ they organize their spindle fibers throughout the cell. - In G2 phase, the cell continues to grow but also prepares to divide, producing the proteins that will help coordinate mitosis. The DNA winds more tightly around its associated proteins, and this start of chromosome condensation signals the start of mitosis. Interphase has ended. Chromosomes Divide during Mitosis - Mitosis separates the genetic material that replicated during S phase. Biologists divide mitosis into four main stages. However, the process does not actually stop as each stage ends. - During prophase, DNA coils very tightly, shortening and thickening the chromosomes. As the chromosomes condense, they become visible when stained and viewed under a microscope. For now, the chromosomes remain randomly arranged in the nucleus. - During prophase, the two centrosomes migrate toward opposite ends of the cell, and the spindle begins to form. The nucleolus - the darkened area in the nucleus - disappears. The nuclear envelope breaks into small pieces, as does the surrounding endoplasmic reticulum. The spindle fibers are now free to attach to the chromosomes. - As metaphase begins, the spindle aligns the chromosomes down the center, or equator, of the cell. This alignment ensures that each cell will receive one copy of each chromosome. - In anaphase, the centromeres split and the spindle fibers pull the sister chromatids (now chromosomes) toward opposite poles of the cell. At the same time, some microtubules in the spindle lengthen in a way that moves the poles farther apart, stretching the dividing cell. - Telophase, the final stage of mitosis, essentially reverses the events of prophase. The spindle disassembles, and the chromosomes begin to unwind. In addition, a nuclear envelope and nucleolus form at each end of the stretched-out cell. As telophase ends, the division of the genetic material is complete, and the cell contains two nuclei - but not for long. The Cytoplasm Splits in Cytokinesis - In cytokinesis, the cytoplasm and the two nuclei are distributed into the two forming daughter cells, which then physically separate. The process differs somewhat between animal and plant cells. - In an animal cell, the first sign of cytokinesis is the c leavage furrow, a slight indentation around the middle of the dividing cell. A ring of proteins beneath the cell membrane contracts like a drawstring, separating the daughter cells. - A dividing plant cell must construct a new wall that separates the two daughter cell.s The first sign of cell wall construction is the ell plate, a structure that appears at the midline of the dividing plant cell. The cell plate grows and consolidates as vesicles from the Golgi apparatus deliver cellulose, other polysaccharides, and proteins. The resulting layer of cellulose fibers embedded in surrounding material makes a strong, rigid wall that gives a plant cell its shape. 8.6 Cancer Arises When Cells Divide out of Control Chemical Signals Regulate Cell Division - Cells divide in response to a variety of chemical signals, many of which originate outside the cell. Proteins that stimulate division bind to receptors on a receiving cell’s membrane, and then a cascade of chemical reactions inside the cell initiates division. - Several internal “checkpoints” ensure that a cell does not enter one stage of the cell cycle until the previous stage is complete. - Precise timing of the many chemical signals that regulate the cell cycle is essential. Too little cell division, and an injury may go unrepaired; too much, and an abnormal growth forms. Cancer Cells Lost Control of the Cell Cycle - Sometimes a tumor, or an abnormal mass of tissue, forms when the body loses control over cell division and apoptosis. - Benign tumors are usually slow-growing and harmless, unless they become large enough to disrupt nearby tissues or organs. A tough capsule surrounding the tumor prevents it from invading nearby tissues or spreading to other parts of the body. Warts and moles are examples of benign tumors of the skin. - In contrast, malignant tumors invade adjacent tissue. Because it lacks a surrounding capsule, a malignant tumor is likely to metastasize, meaning that its cells can break away from the original mass and travel in the bloodstream or lymphatic system to colonize other areas of the body. Cancer is a class of diseases characterized by malignant cells. - Solid tumors of the breast, lung, skin and other major organs are most familiar, but cells in the blood-forming tissues of the bone marrow can also divide out of control (leukemia). - Cancer begins when a single cell accumulates genetic mutations that cause it to break through its cell cycle controls. Each cell passes loss of control to daughter cells.With enough nutrients and space, cancer cells can divide uncontrollably and eternally. As they do so, they may crush vital organs, block the body’s passageways, and divert nutrients from other body cells. Cancer Treatments Remove or Kill Abnormal Cells - Traditional cancer treatments include surgical tumor removal, drugs (chemotherapy), and radiation. Chemotherapy drugs target rapidly dividing cells, whether cancerous or not. Examples of these cells are in bone marrow, digestive tract, and hair follicles. (death of these cells account for most notorious cancer side effects, but healthy cell usually return after the treatments end). - Surgery can cure cancers that have not spread. Once cancer metastasizes, however, it becomes difficult to locate and treat all of the tumors. DNA replication errors introduce mutations in rapidly dividing cancer cells. Treatments that shrank the original tumor may have no effect on this new, changed growth. Genes and Environment Both Can Increase Cancer Risk - Proteins control both the cell cycle and apoptosis. Genes encode proteins, so genetic mutations play a key role in causing cancer. - Sometimes a person inherits mutated versions of genes from one or both parents. The parent may have had cancer, or the mutations may have arisen spontaneously in sperm or egg producing cells. - Often, people develop cancer after exposure to harmful chemicals, radiation and viruses, all of which may alter genes. Poor diet, exercise habits, sun exposure, and cigarette smoking also raise cancer risks. 9.1 Why Sex? - In asexual reproduction, into two. Some DNA may mutate during replication, but the offspring are virtually identical. Examples of asexual organisms include bacteria, archaea and single-celled eukaryotes such as the amoeba. Many plants, fungi and other multicellular organisms also reproduce asexually. - Sexual reproduction requires two parents. The male contributes sperm cells, one of which fertilizes a female’s egg cell. Each time the male produces sperm, he scrambles the genetic information that he inherited from his own parents. A similar process occurs as the female produces eggs. The resulting variation among sex cells ensures that the offspring from two parents are genetically different from one another. - The mass production of identical offspring makes sense in habitats that never change, but conditions rarely remain constant in the real world. Temperatures rise and fall, prey species disappear and new parasites emerge. Genetic variability increases the chance that at least some individuals will have a combination of traits that allows them to survive and reproduce, even if some poorly suited individuals die. Asexual reproduction typically cannot create or maintain this genetic diversity, but sexual reproduction can. 9.2 Diploid Cells Contain Two Homologous Sets of Chromosomes - A chromosome is a single molecule of DNA and its associated proteins. - A sexually reproducing organism consists of mostly d which contain two full sets of chromosomes (one from each parent). Example: each diploid human cell contains 46 chromosomes, arranged in 23 pairs. - Of the 23 chromosome pairs in a human cell, 22 pairs consist of a utosomes - chromosomes that are the same for both sexes. The remaining pair is made up of the two sex chromosomes, which determine whether an individual is female or male. Females have two X chromosomes, whereas males have one X and one Y chromosome. - The two members of most chromosome pairs are homologous to each other. In a homologous pair, the two chromosomes look alike and have the same sequences of genes. In addition, the two members of a homologous pair of chromosomes carry the same sequence of genes. - However, homologous pairs are not identical. Instead, the two homologs differ in the combination of alleles, or versions of the genes they carry. Each allele of a gene encodes a different version of the same protein. A chromosome typically carries exactly one allele of each gene, so a person inherits one allele per gene from each parent. Depending on the parents’ chromosomes, the two alleles may be identical or different. Overall, however, the members of each homologous pair of chromosomes are at least slightly different from each other. - Inheriting a set of chromosomes from each parent is like acquiring two complete sets of cookbooks, each containing slightly different recipes for the same foods. - Unlike the autosome pairs, however the X and Y chromosomes are not homologous to each other. X is much larger than Y, and its genes are completely different. Nevertheless, in males, the sex chromosomes behave as homologous chromosomes during meiosis. 9.3 Meiosis Is Essential in Sexual Reproduction - Sperm cells and egg cells are not diploid. They are h aploid cells, that is, they contain only one full set of genetic information instead of the two sets that characterize diploid cells. - These haploid cells, called g ametes, are sex cells that combine to form a new offspring. Fertilization merges the gametes from two parents, creating a new cell: the diploid zygote, which is the first cell of the new organism. The zygote has two full sets of chromosomes, one set from each parent. In most species, the zygote begins dividing shortly after fertilization. - The life of a sexually reproducing, multicellular organism requires two ways to package DNA into newly forming cells. - Mitosis divides a eukaryotic cell’s chromosomes into two identical daughter nuclei. Mitotic cell division produces the cells needs for growth, development, and tissue repair. Meiosis forms genetically variable sex cells used in reproduction, with each gamete nucleus containing half as many chromosomes as the organism’s diploid cells. - Only germ cells can undergo meiosis. In humans and other animals, these specialized diploid cells occur only in the ovaries and testes. The rest of the body’s diploid cells, called somatic cells, do not participate directly in reproduction. (e.g. muscle cells and neurons) - The human life cycle is of course most familiar to us, and many animals reproduce in essentially the same way. Gametes are the only haploid cells in our life cycle; all other cells are diploid. Sexual reproduction, however, can take many other forms as well. In some organisms, including plants, both the haploid and the diploid stages are multicellular. 9.4 In Meiosis, DNA Replicates Once, but the Nucleus Divides Twice - The interphase that comes before meiosis is similar to interphase in the mitotic cell cycle. The cell grows, DNA replicates, and the cell produces the enzymes and other proteins necessary to divide the cell. Afterward, each of the cell’s chromosomes consists of two identical sister chromatids attached at a centromere. Finally, late in interphase, chromatin begins to condense, and the cell produces the microtubule proteins that will become the spindle. The names of the meiotic phases are also similar to those in mitosis. - Meiosis includes two divisions (meiosis I and meiosis II), creating four haploid cells from one specialised diploid cell. Second, meiosis shuffles genetic information, setting the stage for each haploid nucleus to receive a unique mixture of alleles. - During prophase I (that is, prophase of meiosis I), the replicated chromosomes condense. A spindle begins to form from microtubules assembled at the centrosomes, spindle attachment points grow on each centromere, and the nuclear envelope breaks up. - In metaphase I, the spindle aligns the paired homologs down the center of the cell. Each member of a homologous pair attaches to a spindle fiber stretching to one pole. The stage is therefore set for the homologous pairs to separate in a naphase I, and the chromosomes complete their movement to opposite poles in t elophase I. Cytokinesis typically occurs after telophase I, splitting the original cell into two. - A second interphase precedes meiosis II in many species. During this time, the chromosomes unfold into very thin threads. The cell produces proteins, but the DNA does not replicate a second time. - Meiosis II strongly resembles mitosis. The process begins with prophase II, when the chromosomes again condense and become visible. In m etaphase II, the spindle aligns the chromosomes down the center of each cell. In a naphase II, the centromeres split, and the separated sister chromatids move to opposite poles. In telophase II, nuclear envelopes form around the separated sets of chromosomes. Cytokinesis then separates the nuclei into individual cells. The overall result: One diploid cell has divided into four haploid cells. 9.5 Meiosis Generates Enormous Variability Crossing Over Shuffles Alleles - Crossing over is a process in which two homologous chromosomes exchange genetic material. During prophase I, the homologs align themselves precisely, gene by gene. chromosome - 69 total, called triploidy). Most human polyploids fail to live past the very early stages of development. - Polyploidy is an important force in plant evolution. In contrast to humans, about 30% of flowering plant species tolerate polyploidy well, and many crop plants are polyploids. (e.g. durum wheat in pasta) Nondisjunction Results in Extra or Missing Chromosomes - Some gametes have just one extra or missing chromosome. The cause of the abnormality is an error called ondisjunction, which occurs when chromosomes fail to separate at either anaphase I or anaphase II. The result is a sperm or egg cell with two copies of a particular chromosome or none at all. When such a gamete fuses with another at fertilization, the resulting zygote has either 45 or 47 chromosomes instead of the normal 46. - Most embryos with incorrect chromosome numbers cease developing before birth; they account for about half of all spontaneous abortions (miscarriages) that occur in early pregnancy. - Extra genetic material causes fewer problems than missing material. Most children with the wrong number of chromosomes have an extra (trisomy). - A person with trisomy 21, the most common cause of Down syndrome, has three copies of chromosome 21. Distinctive facial features, unique pattern of hand creases, congenital heart defects. Probability of Down syndrome child increases with age. Most common autosomal trisomy is trisomy 21. - A gamete that contains two X or Y chromosomes instead of one can be produced. A zygote with too many sex chromosomes is then produced (XXX, XXY, XYY). A gamete may also lack a sex chromosome altogether. If one gamete contains an X chromosomes and the other gamete has neither X nor Y, the resulting zygote is XO. Interestingly, medical researchers have never reported a person with one Y and no X chromosome. When a zygote lacks an X chromosome, so much genetic material is missing that it probably cannot sustain more than a few cell divisions.
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