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Limits of Cell Growth: The larger the cell, the more difficult it is for materials to enter and leave it. Rates of Cell Growth: E. coli can double its volume in = 30 minutes. In one day, a single cell would grow into a 14 kg mass of bacteria. In 3 days = mass of the earth Controls on Cell Growth: cells of the heart and nervous system rarely grow. Cells of the skin and digestive tract grow and divide rapidly throughout Life; old, worn out, and damaged cells. Cells will grow until they come into contact with each other. Uncontrolled Cell Growth: Cancer cells grow out of control until they have used up all of their nutrients Chromosomes: Made up of chromatin (DNA and protein) Total length of DNA is = 10,000 x length of the chromosome. In 1973, Don and Ada Olins and Christopher Woodcock discovered the chromosome’s DNA was coiled around special proteins called histones. DNA and histone molecules form beadlike structures called nucleosomes. Nucleosomes form a thick fiber, shortened by a system of loops and coils. Composed of 2 sister chromatids and a centromere. A form of asexual reproduction and cell division Used by all prokaryotes and some protozoa The cell divides into two parts that each have the potential to grow to the size of the original cell. Skin cancer - the abnormal growth of skin cells - most often develops on skin exposed to the sun. Cell that reproduce by asexual reproduction reproduce constantly. The cell cycle is a series of events that occur in a cell to allow it to replicate, divide, and reproduce. Animated Mitosis Cycle http://www.cellsalive.com/mitosis.htm • Interphase • Prophase • Metaphase • Anaphase • Telophase & Cytokinesis Chromosomes are copied (# doubles) • Chromosomes appear as threadlike coils (chromatin) at the start, but each chromosome and its copy(sister chromosome) change to sister chromatids at end of this phase • Nucleus CELL MEMBRANE Cytoplasm • • • • • Consists of G1, S, and G2 Cells do not move through the cycle at the same rate G1 (Gap 1) – cell grows and develops; increase in supply of proteins, and synthesis of some organelles S (Synthesis) – DNA replication occurs G2 (Gap 2) – Synthesis of more organelles and the cell prepares for mitosis Animal Cell Plant Cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Division of the nucleus Occurs in somatic or body cells Can last from a few minutes to several days • • • • • Mitosis begins (cell begins to divide) Chromosomes become visible as they shorten and thicken Centrioles (or poles) appear and begin to move to opposite end of the cell. Nucleolus and Nuclear envelope disintegrate. Spindle fibers form between the poles. Centrioles Sister chromatids Spindle fibers Animal Cell Plant Cell Spindle fibers Centrioles Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Chromatids (or pairs of chromosomes) attach to the spindle fibers and line up along the equator or middle of the cell. • Centrioles have reached the opposite poles. • Centromere of each chromosome is attached to a spindle fiber. • Centrioles Spindle fibers Animal Cell Plant Cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm • • Centromeres split. Chromatids (or pairs of chromosomes) separate and begin to move to opposite ends of the cell and are now individual chromosomes. Centrioles Spindle fibers Animal Cell Plant Cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm • • • • • Two new nuclei form. Chromosomes reach opposite poles and appear as chromatin (less distinct). Nuclear membrane begins to reform around chromosomes. Nucleolus reappears. Mitosis ends. Nuclei Chromatin Nuclei Animal Cell Plant Cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm • Cell membrane moves inward to create two daughtercells – each with its own nucleus with identical chromosomes. In Animal Cells: Microfilaments constrict (or pinch) the cytoplasm allowing cytokinesis to occur A cleavage furrow forms allowing the cell to become two new cells In Plant Cells: A cell plate forms between two daughter nuclei and then cell walls form on either side of the cell plate. Animal Mitosis -- Review Interphase Prophase Metaphase Anaphase Telophase Interphase Plant Mitosis -- Review Interphase Prophase Metaphase Anaphase Telophase Interphase 28 http://www.cellsalive.com/mitosis.htm Meiosis Organisms that reproduce Sexually are made up of two different types of cells. 1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. 2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes. n = number of chromosomes in the set… so….2n means 2 chromosomes in the set…. Polyploid cells have more than two chromosomes per set… example: 3n (3 chromosomes per set) Gametes • The Male Gamete is the Sperm and is produced in the male gonad the Testes. • The Female Gamete is the Ovum (ova = pl.) and is produced in the female gonad the Ovaries. During Ovulation the ovum is released from the ovary and transported to an area where fertilization, the joining of the sperm and ovum, can occur…… fertilization, in Humans, occurs in the Fallopian tube. Fertilization results in the formation of the Zygote. (fertilized egg) Sperm + Ovum (egg) fertilization Zygote Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote Chromosomes • If an organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA). • Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for humans is 46 (46 chromosomes per cell). Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus (position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. 22 pairs of autosomes 1 pair of sex chromosomes Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”) eye color locus eye color locus hair color locus hair color locus Paternal Maternal Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues. Homologue Homologue Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male Sex Chromosomes “Sex Chromosomes” …….the 23rd set This person has 2 “X” chromosomes… and is a female. 23 Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are produced. During Meiosis diploid cells are reduced to haploid cells Diploid (2n) Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die. Meiosis Meiosis is Two cell divisions (called meiosis I and meiosis II) with only one duplication of chromosomes. Meiosis in males is called spermatogenesis and produces sperm. Meiosis in females is called oogenesis and produces ova. Spermatogenesis Secondary Spermatocyte n=23 human sex cell 2n=46 sperm n=23 Primary Spermatocyte n=23 Secondary Spermatocyte haploid (n) n=23 diploid (2n) n=23 4 sperm cells are produced from each primary spermatocyte. meiosis I n=23 meiosis II Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte. Interphase I • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. • Centriole pairs also replicate. Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I Prophase I • Longest and most complex phase. • 90% of the meiotic process is spent in Prophase I • Chromosomes condense. • Synapsis occurs: homologous chromosomes come together to form a tetrad. • Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids). Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids During Prophase I “Crossing Over” occurs. Crossing Over is one of the Two major occurrences of Meiosis (The other is Non-disjunction) • During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over. Crossing Over creates variation (diversity) in the offspring’s traits. nonsister chromatids chiasmata: site of crossing over Tetrad variation Question: • A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: • 10 chromosomes (haploid) Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: • 10 chromosomes Prophase I spindle fiber aster fibers centrioles Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations Metaphase I OR metaphase plate metaphase plate Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres. Anaphase I Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed. Telophase I Meiosis II • No interphase II (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis Prophase II • same as prophase in mitosis Metaphase II • same as metaphase in mitosis metaphase plate metaphase plate Anaphase II • same as anaphase in mitosis • sister chromatids separate Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg Telophase II Non-disjunction Non-disjunction is one of the Two major occurrences of Meiosis (The other is Crossing Over) • Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. • Non-disjunction results with the production of zygotes with abnormal chromosome numbers…… remember…. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring. Non-disjunctions usually occur in one of two fashions. • The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an organism has Monosomy 23 it has only one chromosome in the 23rd set. Common Non-disjunction Disorders • • • • Down’s Syndrome – Trisomy 21 Turner’s Syndrome – Monosomy 23 (X) Kleinfelter’s Syndrome – Trisomy 23 (XXY) Edward’s Syndrome – Trisomy 18 Amniocentesis • An Amniocentesis is a procedure a pregnant woman can have in order to detect some genetics disorders…..such as non-disjunction. Amniocentesis Amniotic fluid withdrawn Karyotype (picture of an individual’s chromosomes) One of the ways to analyze the amniocentesis is to make a Karyotype What genetic disorder does this karyotype show? Trisomy 21….Down’s Syndrome MITOSIS 1 cell division: 2 daughter cells 2n MEIOSIS 2 cell divisions: 4 product of meiosis 2n Somatic cells 2n Cells in sexual cycle n n n n 2n n=chromosome number Mitosis : two genetically identical diploid cells from one diploid parent cell Diploid organism: two sets of chromosomes (one from each parent); 2n Mitosis Conservative process: Daughter cells genotypes identical to parental genotypes Final Result of Meiosis : Four genetically different haploid cells; from one diploid parent cell Haploid organism: one set of chromosomes; n Meiosis Promotes variation among the product of meiosis (crossing over) Alike Both have the stages PMAT Both go through Cytokinesis Both divide and make daughter cells Both are a form of reproduction Both take place inside the cell Chromosomes duplicate and DNA replication occurs in each process Different Mitosis produces two identical daughter cell, meiosis produces four genetically different cells Crossing over and tetrad formation (synapsis) occur in prophase I of Meiosis The centromeres split in Anaphase II Mitosis has one division, Meiosis has two Mitosis occurs in somatic or body cells Meiosis occurs in reproductive organs; producing gametes Mitosis is asexual reproduction Meiosis ensures sexual reproduction