Biology Chapter 8 Reading Guide v Cell cycle Ø Cell reproduction Cell division the reproduction of cell through duplication of the genome and division of the cytoplasm Ø Chromosomes a threadlike gene found in the nucleus of eukaryotic cells and most visible during mitosis and meiosis also the main gene carrying structure of a prokaryotic cell consists of a very long piece of chromatin a commination of DNA and protein Ø Asexual reproduction the creation of genetically identical offspring by a single parent without the participation of sperm or egg Sea stars new individuals from fragmented pieces Ø Sexual reproduction the creation of genetically unique offspring bu the fusion of 2 haploid sex cells (gametes) forming a diploid zygote v Binary fission a means of asexual reproduction in which a parent organism often a single cell divides into two genetically identical individuals of about equal size Ø Prokaryotes v The cell cycle or the cell division cycle is the series of events that takes place in the eukaryotic cell between its formation and the moment it replicates itself. These events can be divided into two parts interphase (in between divisions phases grouping G1 phase S phase and G2 phase) during which the cell is forming and carries on with its normal metabolic functions. The mitotic phase (M mitosis) during which the cell is replicating itself. Thus cell division cycle is and essential process by which a single cell fertilized egg develops into a mature organism and the process by which hair skin and blood cells and some internal organs are formed v Interphase is a phase of the cell cycle, defined only by the absence of cell division. During interphase, the cell obtains nutrients, and duplicates (copies) its chromatids (genetic material). The genetic material or chromatids are located in the nucleus of the cell and are made of the molecule DNA. v Chromatids are connected by the centromere and have a LONG AND SHORT ARM. v Most eukaryotic cells spend most of their time in interphase. For example, human skin cells, which divide about once a day, spend roughly 22 hours in interphase. About 90 percent of cells are in interphase. Some cells, such as nerve cells, can stay in interphase for decades. There are 3 parts of interphase: G 1 (growth 1 in which the cell creates organelles and begins
metabolism), S phase (DNA synthesis in which the chromosomes of the cell are copied) and G 2 (growth 2 in which the cell grows in preparation for cell division). v Sometimes the cells exit the cell cycle (usually from G 1 phase) and enter the G 0 phase. In the G 0 phase, cells are alive and metabolically active, but do not divide. In this phase cells do not copy their DNA and do not prepare for cell division. Many cells in the human body, including those in heart muscle, eyes, and brain are in the G 0 phase. If these cells are damaged they cannot be replaced. v The G 1 phase is a period in the cell cycle during interphase, after cytokinesis (process whereby a single cell is divided into two identical daughter cells whenever the cytoplasm is divided) and before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized (made), so the cell requires both structural proteins and enzymes, resulting in great amount of protein synthesis. v The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase. Following G1, the cell enters the S stage, when DNA synthesis or replication occurs. At the beginning of the S stage, each chromosome is composed of one coiled DNA double helix molecule, which is called a chromatid. At the end of this stage, each chromosome has two identical DNA double helix molecules, and therefore is composed of two sister chromatids. During S phase, the centrosome is also duplicated. v G 2 phase is the third, final, and usually the shortest subphase during interphase within the cell cycle in which the cell undergoes a period of rapid growth to prepare for mitosis. It follows successful completion of DNA synthesis and chromosomal replication during the S phase, and occurs during a period of often four to five hours. Although chromosomes have been replicated they cannot yet be distinguished individually because
they are still in the form of loosely packed chromatin fibers. The G 2 phase continues growth of the cell and prepares the cell for mitosis (M phase) by producing all of the enzymes that the cell will need in order to divide v After the G 2 phase of interphase, the cell is ready to start dividing. The nucleus and nuclear material (chromosomes made of DNA) divide first during stage known as MITOSIS. Mitosis is also called KARYOKINESIS (karyon means nucleus) because only the nucleus is dividing. v Mitosis is the process in which a eukaryotic cell (cell containing a nucleus) separates its already duplicated chromosomes (copied during the S phase) into two sets of chromosomes so there will be two identical nuclei. It is generally followed by cytokinesis which divides the cytoplasm and cell membrane. This results in two identical cells (both have an identical set of chromosomes) with an equal distribution of organelles and other cellular components. The mitotic (M) phase and cytokinesis (C phase) together are called cell division, the division of the parent cell (original) into two daughter cells (new cells), each with the same genetic information (chromosomes) as the parent cell. Mitosis does NOT occur in prokaryotic cells that do NOT have a nucleus. In multicellular organisms, the somatic cells (body cells) undergo mitosis, while germ cells cells destined to become sperm in males or ova (eggs) in females divide by a related process called meiosis. Prokaryotic cells (bacteria), which lack a nucleus, divide by a process called binary fission.
v The process of mitosis (division of the nucleus) is divided into four stages (Prophase, Metaphase, Anaphase, and Telophase). Immediately following nuclear division (mitosis), the cell membrane must also divide (cytokinesis). Animal cells divide the cytoplasm by constricting the cell membrane in the middle to form a cleavage furrow. Plant cells form a cell plate in the center to divide the cytoplasm. At Interphase, there is only one cell, but after cytokinesis there are two identical cells. v During interphase, the genetic material is called chromatin and can NOT be clearly seen because it isn t tightly coiled. When prophase begins, the DNA molecules are progressively shortened and condensed by coiling, to form visible chromosomes. Enzymes during prophase break down the nuclear membrane and nucleolus so they are no longer visible. Spindle fibers also form in
prophase which will attach to the chromosomes. At metaphase, the spindle fibers attach themselves to the centromeres of the chromosomes and align the chromosomes at the equator (middle of the cell). Anaphase is the next stage. The spindle fibers shorten and the centromere splits separating the two sister chromatids. During telophase, the chromosomes pairs (chromatids are pulled to opposite poles of the cell. The nuclear envelope and nucleolus reform before the chromosomes uncoil. The spindle fibers disintegrate. v Growth factor a protein secreted by certain body cells that stimulates other cells to divide v Density dependent inhibition the ceasing of cell division that occurs when cells touch one another v Anchorage dependence the requirement that to divide a cell must be attached to a solid surface v Cell cycle control cynically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle Ø Default stage checkpoint in animal cells Halt of the cell cycle of these checkpoints unless overridden by specific go- ahead signals
Ø Major checkpoints cell cycle Dividing the G1 and G2 sub- phases of interphase and in the metaphase v The growth factor binds to the cells receptor protein embedded in the plasma membrane then the receptor protein passes on the signal to a relay protein in the cytoplasm to signal another relay protein that is in the nucleus which signals the checkpoint in the cell cycle to go v Biochemical reactions that cause relay proteins to take signal to the nucleus = transduction pathway v Cancer Ø Tumor an abnormal mass of rapidly growing cells that forms within otherwise normal tissue Ø Benign tumor an abnormal mass of cells that remains as its original site in the body Ø Malignant tumor an abnormal tumor tissue mass that can spread to neighboring tissue and to other pairs of the body a cancerous tumor Ø Metastasis the spread of cancer cells beyond their original site Ø Carcinoma cancer that originates in the coverings of the body such as skin or the lining of the intestinal tract Ø Sarcoma cancer o the supportive tissues such as bone cartilidge and the muscle Ø Lymphoma cancer of the tissues that form white blood cells Ø Researchers have discovered that cancer cells respond to growth factors and checkpoints in the cell cycle as follows: not need normal dignals that regulate
the cell cycle many have defective cell cycle control systems, proceed past checkpoints in absence of growth factors Ø Immortal because they can go on dividing indefinitely as long as they have a supply of nutrients normal cells divide only 20-50 times Ø Ways that cancer is treated Removed surgically High energy radiation Chemotherapy Drugs Ø Side effects of radiation Ovaries/testes causes sterility Ø Side effects of chemotherapy Nausea from effects intestinal cells hair loss effects immune cell production v Meiosis Ø Somatic cell any cell in a multicellular organism except a sperm or egg cell that develops into a sperm or egg Ø Homologous chromosome the two chromosomes that make up a matened pair staining pattern and possessed genes for the same characteristics t corresponding loci one is inherited from organisms father and the other from the mother Ø Locus the particular site where a gene is found on a chromosome homologues chromosomes have corresponding gene loci Ø Sex chromosomes a chromosome that determines whether an individual is male or female Ø Life cycle the entire sequence of stages in the life of an organism from the adults of one generation to the adults of the next Ø Diploid in an organism that reproduces sexually a cell containing two homologous sets of chromosomes one set inherited from each parent a 2n cell Ø Gametes a sex cell haploid egg or sperm the union of two gametes of opposite sex (fertile action) produces a zygote Ø Haploid in the life cycle of an organism that reproduces sexually a cell containing a single set of chromosomes, n cell Ø Fertilization the union of the nucleus of the sperm cell with the nucleus of an egg cell producing a zygote Ø Zygote the diploid fertilized egg which results from the union of sperm cell nucleus and an egg cell nucleus Ø Meiosis in a sexually reproducing organism the division of a single diploid nucleus into 4 haploid daughter nuclei Meiosis and cytokinesis produce haploid gametes from diploid cells in the reproductive organs of the parents Ø Phases Interphase Meiosis I Prophase I
Metaphase I Anaphase I Telophase I and cytokinesis Meiosis II Prophase II Metaphase II Anaphase II Telophase II and cytokinesis v Genetic variability Ø Formation random combination chromosomes in meiosis and of genes on different pairs of homologous chromosomes by the passage according to the laws of probability of one of each diploid pair of homologous chromosomes into each gamete independently of each other pair Ø Crossing over homologous chromosomes during synapis in prophase I of meiosis also exchange of segments between DNA molecules in prokaryotes Ø Genetic recombination the production by crossing over/independent assortment combination different from those in the parents more specifically mean the production by crossing over of eukaryotic/prokaryotic chromosomes with gene combinations different from original chromosomes Ø Karyotype display of which graphs of the metaphase chromosomes of a cell changed by size and centromere position may be used identify chromosomal abnormalities Ø Homologous chromosomes contribute to the genetic variability of gametes Can bear two different kinds of genetic information for the same characteristic Ø Crossing over contributing to genetic variability of genes Produces hybrid chromosomes with new combinations maternal/parental genes 4 types not 2 Ø How does independent orientation contribute to genetic variability of gametes Placement chromosomes results in different (completely) from original parental cell Daughter cell combinations make different cells based on locations at metaphase plate Ø Trisomy 21 down syndrome Ø Down syndrome a human genetic disorder resulting from the presence of an extra chromosome 21 Ø Nondisjunction an accident of meiosis/mitosis in which a pair of homologous chromosomes or pair system chromatids fail to separate at anaphase Ø Klinefelter s syndrome (males) have male sex organs normal intelligence testes abnormally small, sterile, often larger breasts and other female (secondary) characteristics, has more than 3 sex chromosomes Ø Turner syndrome lacks X chromosome
Ø Polyploidy an organism that has more than two complete sets of chromosomes as a result of an accident of cell division Ø Effects child with down syndrome Found face fold inner corner of the eye flattened nose bridge small irregular teeth short stature heart defects susceptibility respiratory infections leukemia Alzheimer s disease Ø What is the genetic combination of sex chromosomes in Klinefelter s and Turner syndrome and characteristics Klinefelter s: XXYY, XXXY, XXXXY means more likely disabilities (developmental) Characteristic: small testes, mini breasts, female characteristics Turner: X0 Characteristic: short stature webbed skin neck and shoulders sterile sex organ not fully mature Poor development of breasts and other secondary sex characteristics with normal intelligence Ø In what animal/plant species have scientists observed polyploidy Wheat Potatoes Apples Cotton Fishes Amphibians Rat