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1/14/2010 Cell Division Video Chromosomes & Cellular Reproduction Ch 6 & 7.1 pg. 118-132 & 144-151 \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\Notes\Video Clips\Introduction_to_Cell_Division.asx Biology Mrs. Stolipher Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Formation of New Cells by Cell Division • Cell division, also called cell reproduction, occurs in humans and other organisms at different times in their life. • The formation of gametes involves yet a special type of cell division. Gametes are an organism’s reproductive cells, such as sperm or egg cells. • When a cell divides, the DNA is first copied and then distributed. Prokaryotic Cell Reproduction • Prokaryotes reproduce by a type of cell division called binary fission. • Binary fission is a form of asexual reproduction that produces identical offspring. • In asexual reproduction, a single parent passes exact copies of all of its DNA to its offspring. • Binary fission occurs in two stages: first, the DNA is copied (so that each new cell will have a copy of the genetic information), and then the cell divides. • Eventually the dividing prokaryote is pinched into two independent cells. Binary Fission Eukaryotic Cell Reproduction • A gene is a segment of DNA that codes for a protein or RNA molecule. • When genes are being used, the DNA is stretched out so that the information it contains can be used to direct the synthesis of proteins. • As a eukaryotic cell prepares to divide, the DNA and the proteins associated with the DNA coil into a structure called a chromosome. • The two exact copies of DNA that make up each chromosome are called chromatids. • The two chromatids of a chromosome are attached at a point called a centromere. • The chromatids, which become separated during cell division and placed into each new cell, ensure that each new cell will have the same genetic information as the original cell. 1 1/14/2010 Gene Chromosome Structure \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec01\vc02\hx406_01_v02fs.htm Parts of a Chromosome \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec01\vc04\hx406_01_v04fs.htm How Chromosome Number and Structure Affect Development Sets of Chromosomes Comparing Cell Division in Prokaryotes and Eukaryotes \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec01\vc01 Homologous Chromosomes \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec01\vc05\hx406_01_v05fs.htm • Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content. • Each homologue in a pair of homologous chromosomes comes from one of the two parents. • The 46 chromosomes in human somatic cells are actually two sets of 23 chromosomes. 2 1/14/2010 Sets of Chromosomes • When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid. Comparing Haploid and Diploid Cells \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec01\vc06 • When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid. • The fusion of two haploid gametes—a process called fertilization—forms a diploid zygote. A zygote is a fertilized egg cell. Chromosome Number of Various Organisms Sex Chromosomes • Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual. • The sex chromosomes, one of the 23 pairs of chromosomes in humans, contain genes that will determine the sex of the individual. • In humans and many other organisms, the two sex chromosomes are referred to as the X and Y chromosomes. Change in Chromosome Number Karyotype • Humans who are missing even one of the 46 chromosomes do not survive. • Humans with more than two copies of a chromosome, a condition called trisomy, will not develop properly. • Abnormalities in chromosome number can be detected by analyzing a karyotype, a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size. 3 1/14/2010 Chromosome Number Change in Chromosome Structure \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec01\vc09\hx406_01_v09fs.htm • Changes in an organism’s chromosome structure are called mutations. • Breakage of a chromosome can lead to four types of mutations: 1. deletion mutation 2. duplication mutation 3. inversion mutation 4. translocation mutation Types of Chromosome Mutations \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec01\vc11\hx406_01_v11fs.htm The Life of a Eukaryotic Cell The Cell Cycle • The cell cycle is a repeating sequence of cellular growth and division during the life of an organism. • A cell spends 90 percent of its time in the first three phases of the cycle, which are collectively called interphase. • The five phases of the cell cycle are: Control of the Cell Cycle 1. First growth (G1 = Gap 1) phase During the G1 phase, a cell grows rapidly and carries out its routine functions. • The cell cycle has key checkpoints (inspection points) at which feedback signals from the cell can trigger the next phase of the cell cycle (green light). 2. Synthesis (S) phase A cell’s DNA is copied during this phase. • Other feedback signals can delay the next phase to allow for completion of the current phase (yellow or red light). 3. Second growth (G2 = Gap 2) phase In the G2 phase, preparations are made for the nucleus to divide. • Control occurs at three principal checkpoints: 1. Cell growth (G1) checkpoint This checkpoint makes the decision of whether the cell will divide. 4. Mitosis The process during cell division in which the nucleus of a cell is divided into two nuclei is called mitosis. 2. DNA synthesis (G2) checkpoint DNA replication is checked at this point by DNA repair enzymes. 5. Cytokinesis The process during cell division in which the cytoplasm divides is called cytokinesis. 3. Mitosis checkpoint This checkpoint triggers the exit from mitosis. 4 1/14/2010 When Control Is Lost: Cancer Control of the Cell Cycle \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec02\vc05 • Certain genes contain the information necessary to make the proteins that regulate cell growth and division. • If one of these genes is mutated, the protein may not function, and regulation of cell growth and division can be disrupted. • Cancer, the uncontrolled growth of cells, may result. \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\Notes\Video Clips\Cancer_Cells.asx Forming the Spindle Chromatid Separation in Mitosis • During mitosis, the chromatids on each chromosome are physically moved to opposite sides of the dividing cell with the help of the spindle. • When a cell enters the mitotic phase, the centriole pairs start to separate, moving toward opposite poles of the cell. • As the centrioles move apart, the spindle begins to form. • Spindles are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division. Separation of Chromatids by Attaching Spindle Fibers • The chromatids are moved to each pole of the cell in a manner similar to bringing in a fish with a fishing rod and reel. • When the microtubule “fishing line” is “reeled in,” the chromatids are dragged to opposite poles. • As soon as the chromatids separate from each other they are called chromosomes. Interphase • Includes G1, S, & G2 phases of cell cycle • Rest & growth stage where cell doubles in size, duplicates all organelles & DNA • Happens before mitosis 5 1/14/2010 Prophase Mitosis • • • Is a period of nuclear division Produces 2 identical daughter cells each containing a complete set of chromosomes Is made up of 4 phases 1. 2. 3. 4. Prophase Metaphase Anaphase telophase • Is the 1st & longest phase of mitosis • Individual chromosomes become visible • Nuclear membrane & nucleolus disappear • Centrioles appear & migrate to opposite poles • Spindle forms Video Clip \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\Notes\Video Clips\Prophase__The_First_Stage_of_Mitosis.asx Anaphase Metaphase • Centromere splits • Spindle fibers shorten • Sister chromatids are pulled to opposite poles (sides) • Chromosomes attach to spindle fibers at their centromeres • Chromosomes line up at the equator (center of cell) Anaphase Metaphase Video Clip Video Clip \\plankton\userdata\Amy.Stolipher\ My Documents\Adv. Bio\Notes\Video Clips\Anaphase__The_Third_Stage_of _Mitosis.asx \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\Notes\Video Clips\Metaphase__The_Second_Stage_of_ Mitosis.asx Telophase • Is the reverse of prophase • Chromosomes uncoil & become less visible • Spindle disappears • Nuclear membrane reforms • Nucleolus reappears Telophase Cell plate Mitosis \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec03\vc01 Video Clip \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\Notes\Video Clips\Telophase__The_Fourth_Stage_of_Mitosis.asx 6 1/14/2010 Cytokinesis • As mitosis ends, cytokinesis begins. Comparing Cell Division in Plants and Animals In Animals In Plants • the cytoplasm of the cell is divided in half and the cell membrane grows to enclose each cell, forming two separate cells as a result. • The end result of mitosis and cytokinesis is two genetically identical cells where only one cell existed before. \\plankton\userdata\Amy.Stolipher\My Documents\Adv. Bio\HOLT\Visual Concepts\student\ch06\sec03\vc02\hx406_03_v02fs.htm Results of Mitosis • produces 2 new daughter cells that are identical in structure & function to the original parent cell • In unicellular organisms = 2 new organisms • In multicellular organisms = groups of cells that work together as a tissue Chapter 7 Diploid vs. Haploid • Human body (somatic) cells have 46 chromosomes – 23 pairs • 1 chromosome in each pair came from Dad & the other one came from Mom • This type of cell is called diploid – It has 2 of each kind of chromosome – Abbreviated 2n •Gametes (egg & sperm) only have one of each kind of chromosome – in humans 23 chromosomes - this type of cell is called haploid Mitosis Video Quiz • Take out a sheet of paper & number 1-5 Formation of Haploid Cells • Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells, such as gametes or spores. • Meiosis involves two divisions of the nucleus— meiosis I and meiosis II. • Before meiosis begins, the DNA in the original cell is replicated. Thus, meiosis starts with homologous chromosomes. - abbreviated n 7 1/14/2010 8 Stages of Meiosis Meiosis I Prophase I The nuclear envelope breaks down. Homologous chromosomes pair. Crossing-over occurs when portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome. Metaphase I The pairs of homologous chromosomes are moved by the spindle to the equator of the cell. Anaphase I The chromosomes of each pair are pulled to opposite poles of the cell by the spindle fibers. Telophase I Individual chromosomes gather at each of the poles. In most organisms, the cytokinesis occurs. MEIOSIS I: Homologous chromosomes separate Meiosis II INTERPHASE Prophase II A new spindle forms around the chromosomes. Metaphase II The chromosomes line up along the equator and are attached at their centromeres to spindle fibers. Anaphase II The centromeres divide, and the chromatids (now called chromosomes) move to opposite poles of the cell. Telophase II A nuclear envelope forms around each set of chromosomes, PROPHASE I Centrosomes (with centriole pairs) Nuclear envelope METAPHASE I Microtubules Sites of crossing over attached to Spindle kinetochore Chromatin Sister chromatids Tetrad ANAPHASE I Metaphase plate Centromere (with kinetochore) Sister chromatids remain attached Homologous chromosomes separate and the cell undergoes cytokinesis. Meiosis Video Clips MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Prophase I Video Clips\Prophase_One.asx Video Clips\Metaphase_One.asx Metaphase I Cleavage furrow Anaphase I Telophase I Sister chromatids separate Haploid daughter cells forming Video Clips\Anaphase_One.asx Video Clips\Telophase_One.asx Prophase II Video Clips\Prophase_Two.asx Metaphase II Anaphase II Video Clips\Metaphase_Two.asx Video Clips\Anaphase_Two.asx Telophase II Meiosis Video Video Clips\Telophase_Two.asx Comparing Meiosis and Mitosis MITOSIS MEIOSIS PARENT CELL (before chromosome replication) PROPHASE Video Clips\Meiosis.asx ..\HOLT\Visual Duplicated chromosome (two sister chromatids) METAPHASE Site of crossing over MEIOSIS I PROPHASE I Concepts\student\ch07\sec01\vc05\hx407_01_v05fs.htm Tetrad formed by synapsis of Chromosome Chromosome homologous replication replication chromosomes 2n = 4 ..\HOLT\Visual Chromosomes Tetrads Concepts\student\ch07\sec01\vc06\hx407_01_v06fs.htm align at the align at the ANAPHASE TELOPHASE 2n Daughter cells of mitosis metaphase plate metaphase plate Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together 2n No further chromosomal replication; sister chromatids separate during anaphase II METAPHASE I ANAPHASE I TELOPHASE I Haploid n=2 Daughter cells of meiosis I MEIOSIS II n n n n Daughter cells of meiosis II 8 1/14/2010 Independent Assortment Meiosis and Genetic Variation The random distribution of homologous chromosomes during meiosis is called independent assortment. • Meiosis is an important process that allows for the rapid generation of new genetic combinations. • Three mechanisms make key contributions to this genetic variation: 1. independent assortment 2. crossing-over 3. random fertilization ..\HOLT\Visual Concepts\student\ch07\sec01\vc02\hx407_01_v02fs.htm Crossing-Over and Random Fertilization • Meiosis and the joining of gametes are essential to evolution. No genetic process generates variation more quickly. • The DNA exchange that occurs during crossing-over adds even more recombination to the independent assortment of chromosomes that occurs later in meiosis. • The pace of evolution is sped up by genetic recombination. The combination of genes from two organisms results in a third type, not identical to either parent. • Thus, the number of genetic combinations that can occur among gametes is practically unlimited. • Furthermore, the zygote that forms a new individual is created by the random joining of two gametes. Importance of Genetic Variation ..\HOLT\Visual Concepts\student\ch07\sec01\vc01\hx407 _01_v01fs.htm 9