Module 3. Cell Division PDF

Cellular reproduction Outline 1 The Cell Cycle 2 Mitosis and Cytokinesis 3 The Cell Cycle and Cancer 4 Meiosis 2  Cell Cycle ü All cells are derived from pre- existing cells ü New cells are produced for growth and to replace damaged or old cells ü...

Cellular reproduction Outline 1 The Cell Cycle 2 Mitosis and Cytokinesis 3 The Cell Cycle and Cancer 4 Meiosis 2  Cell Cycle ü All cells are derived from pre- existing cells ü New cells are produced for growth and to replace damaged or old cells ü Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals) 3 Cell Cycle The cell cycle is an orderly set of stages from the first division to the time the resulting daughter cells divide Just prior to the next division: ◦ The cell grows larger ◦ The number of organelles doubles ◦ The DNA is replicated The three major stages of the cell cycle: ◦ Interphase (includes several stages) ◦ Mitotic Stage ◦ Cytokinesis 4 Keeping Cells Identical The instructions for making cell parts are encoded in the DNA, so each new cell must get a complete set of the DNA molecules 5  DNA Replication Original DNA üDNA must be copied strand or replicated before cell division Two new, identical DNA üEach new cell strands will then have an identical copy of the DNA 6  Types of Cell Reproduction ü Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells ü Mitosis & binary fission are examples of asexual reproduction ü Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells ü Meiosis is an example 7  Prokaryotic Chromosome üThe DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane 8 Cell Division in Prokaryotes Parent cell ü Prokaryotes such as bacteria divide into 2 identical cells by the process of binary fission Chromosome doubles ü Single chromosome makes a copy of itself ü Cell wall forms between the Cell splits chromosomes dividing the cell 2 identical daughter cells 17  The Cell Cycle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Interphase S G1 checkpoint (growth and DNA Cell cycle main checkpoint. G1 replication) If DNA is damaged, G2 checkpoint G2 apoptosis G1 (growth and final Mitosis checkpoint. will occur. Otherwise, the preparations for Mitosis will occur (growth) cell G0 M division) G2 if DNA has is committed to divide when replicated properly. growth signals are present is es Apoptosis will e ase se n e as i se and nutrients are available. ok as h occur if the DNA is ha h t op Anapha Cy op Metaph lop r pr damaged and P te T e cannot be repaired. La M M checkpoint Spindle assembly checkpoint. Mitosis will not continue if chromosomes are not properly aligned. 10 The Cell Cycle Interphase ◦ Most of the cell cycle is spent in interphase ◦ Cell performs its usual functions ◦ Time spent in interphase varies by cell type ◦ Nerve and muscle cells do not complete the cell cycle (remain in the G0 stage) 11 The Cell Cycle Interphase consists of: G1, S, and G2 phases ◦ G1 Phase: ◦ Recovery from previous division ◦ Cell doubles its organelles ◦ Cell grows in size ◦ Cell accumulates raw materials for DNA synthesis ◦ S Phase: ◦ DNA replication ◦ Proteins associated with DNA are synthesized ◦ Chromosomes enter with 1 chromatid each ◦ Chromosomes leave with 2 identical chromatids (sister chromatids) each ◦ G2 Phase: ◦ Between DNA replication and onset of mitosis ◦ Cell synthesizes proteins necessary for division 12 What’s Happening in Interphase? What the cell looks like Animal Cell What’s occurring 13  Eukaryotic Chromosomes ü All eukaryotic cells store genetic information in chromosomes üMost eukaryotes have between 10 and 50 chromosomes in their body cells üHuman body cells have 46 chromosomes or 23 identical pairs 14 Eukaryotic Chromosomes DNA is in very long threads ◦Chromosomes ◦Stretched out and intertwined between divisions ◦DNA is associated with histones (proteins) ◦DNA and histone proteins are collectively called chromatin Before mitosis begins: ◦ Chromatin condenses (coils) into distinctly visible chromosomes ◦ Each species has a characteristic chromosome number 15 Diploid Chromosome Numbers of Some Eukaryotes 16 Eukaryotic Chromosomes At the end of S phase: ◦ Each chromosome internally duplicated ◦ Consists of two identical DNA chains ◦ Sister chromatids (two strands of genetically identical chromosomes) ◦ Attached together at a single point (called centromere) During mitosis: ◦ Centromeres holding sister chromatids together separate ◦ Sister chromatids separate ◦ Each becomes a daughter chromosome ◦ Sisters of each type are distributed to opposite daughter nuclei 17 Chromosomes in Dividing Cells üDuplicated chromosomes are called chromatids & are held together by the centromere Called Sister Chromatids 11 Duplicated Chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sister chromatids centromere kinetochore one chromatid a. 9,850 b. © Andrew Syred/Photo Researchers, Inc. 19  Karyotype üA picture of the chromosomes from a human cell arranged in pairs by size üFirst 22 pairs are called autosomes üLast pair are the sex chromosomes üXX female or XY male 20 Henrietta Lack’s Karyotype The Cell Cycle The cell cycle is controlled by internal and external signals A signal is a molecule that either stimulates or inhibits a metabolic event. ◦Internal signals ◦ Family of proteins called cyclins (CDK) that increase and decrease as the cell cycle continues ◦ Without cyclins, the cell cycle stops at G1, M or G2 (checkpoints) ◦ Allows time for any damage to be repaired 22 The Cell Cycle Apoptosis is programmed cell death It involves a sequence of cellular events: ◦ fragmenting of the nucleus, ◦ blistering of the plasma membrane ◦ engulfing of cell fragments. Apoptosis is caused by enzymes called caspases. Mitosis and apoptosis are opposing forces ◦ Mitosis increases cell number ◦ Apoptosis decreases cell number 23  Mitosis ü Division of the nucleus ü Also called karyokinesis ü Only occurs in eukaryotes ü Has four stages ü Doesn’t occur in some cells such as brain cells, nerve cells, and muscle cells. 24 Four Mitotic Stages üProphase üMetaphase üAnaphase üTelophase 25  Early Prophase ü Chromatin in nucleus condenses to form visible chromosomes ü Mitotic spindle forms from fibers in cytoskeleton or centrioles (animal) Nucleolus Cytoplasm Nuclear Membrane Chromosomes 26 Late Prophase ü Nuclear membrane & nucleolus are broken down ü Chromosomes continue condensing & are clearly visible ü Spindle fibers called kinetochores attach to the centromere of each chromosome ü Spindle finishes forming between the poles of the cell 27  Spindle Fiber attached to Chromosome Kinetochore Fiber Chromosome 28  Spindle Fibers ü The mitotic spindle form from the microtubules in plants and centrioles in animal cells ü Polar fibers extend from one pole of the cell to the opposite pole ü Kinetochore fibers extend from the pole to the centromere of the chromosome to which they attach ü Asters are short fibers radiating from centrioles 29 Sketch The Spindle 30  Metaphase ü Chromosomes, attached to the kinetochore fibers, move to the center of the cell ü Chromosomes are now lined up at the equator Equator of Cell Pole of the Cell 31 Metaphase Asters at the poles Spindle Chromosomes Fibers lined at the Equator 32  Review of Metaphase What the cell looks like What’s occurring 33  Anaphase üOccurs rapidly üSister chromatids are pulled apart to opposite poles of the cell by kinetochore fibers 34 Anaphase Sister Chromatids being separated 35  Anaphase Review What the cell looks like What’s occurring 36 Telophase üSister chromatids at opposite poles üSpindle disassembles üNuclear envelope forms around each set of sister chromatids üNucleolus reappears üCYTOKINESIS occurs üChromosomes reappear as chromatin 37 Comparison of Anaphase & Telophase 38 Cytokinesis ü Means division of the cytoplasm üDivision of cell into two, identical halves called daughter cells üIn plant cells, cell plate forms at the equator to divide cell üIn animal cells, cleavage furrow forms to split cell 39 Cytokinesis in Animal Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. X 4,000 Cleavage furrow Contractile ring X 4,000 40 (top): © Thomas Deerinck/Visuals Unlimited; (bottom): © SPL/Getty RF Cleavage furrow in Cell plate in animal cell animal cell Daughter Cells of Mitosis üHave the same number of chromosomes as each other and as the parent cell from which they were formed üIdentical to each other, but smaller than parent cell üMust grow in size to become mature cells (G1 of Interphase) 42 The diploid (2n) number includes two sets of chromosomes of each type ◦ Humans have 23 different types of chromosomes ◦ Each type is represented twice in each body cell (diploid) ◦ Only sperm and eggs have one of each type ·termed haploid (n) ◦ The haploid (n) number for humans is 23 ◦ Two representatives of each chromosome type ◦ Makes a total of 2n = 46 in each nucleus ◦ One set of 23 from individual’s father (paternal) ◦ Other set of 23 from individual’s mother (maternal) 43 Comparison of Phases of Mitosis in Animal and Plant Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrosome has centrioles Animal Cell at Interphase 20 µm aster nuclear envelope fragments Early Prophase Centrosomes have duplicated. MITOSIS chromatin condenses nucleolus Chromatin is condensing into disappears chromosomes, and the nuclear Early Prophase envelope is fragmenting. centrosome lacks centrioles Plant Cell at Interphase 25 µm Animal cell(Early prophase): © Ed Reschke; Plant cell(Early prophase): © Ed Reschke  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrosome has centrioles Animal Cell at Interphase 20 µm duplicated 20 µm aster chromosome nuclear envelope Prophase centromere fragments Nucleolus has disappeared, and duplicated chromosomes are MITOSIS chromatin condenses nucleolus visible. Centrosomes begin moving disappears spindle fibers forming apart, and spindle is in process Early Prophase Centrosomes have duplicated. Prophase of forming. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. centrosome lacks centrioles Plant Cell at Interphase 25 µm 6.2 µm cell wall chromosomes Animal cell(Early prophase, Prophase): © Ed Reschke; Plant cell(Early prophase): © Ed Reschke; Plant cell(Prophase): © R. Calentine/Visuals Unlimited  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrosome has centrioles Animal Cell at Interphase 20 µm duplicated 20 µm spindle 9 µm aster chromosome pole nuclear envelope kinetochore Prometaphase centromere fragments The kinetochore of each MITOSIS chromatid is attached to a chromatin condenses nucleolus kinetochore spindle fiber. Polar disappears kinetochore spindle fibers forming spindle fiber spindle fibers stretch from each polar spindle fiber Early Prophase Centrosomes have duplicated. Prophase Nucleolus has disappeared, and Prometaphase spindle pole and overlap. Chromatin is condensing into duplicated chromosomes are visible. chromosomes, and the nuclear Centrosomes begin moving apart, envelope is fragmenting. and spindle is in process of forming. centrosome lacks centrioles Plant Cell at Interphase 25 µm 6.2 µm spindle pole lacks 20 µm cell wall chromosomes centrioles and aster Animal cell(Early prophase, Prophase, Metaphase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase): © R. Calentine/Visuals Unlimited  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrosome has centrioles Metaphase Animal Cell at Interphase aster 20 µm duplicated 20 µm spindle 9 µm chromosomes at 20 µm Centromeres of duplicated chromosome pole metaphase plate nuclear envelope centromere kinetochore chromosomes are aligned at the fragments metaphase plate (center of fully MITOSIS chromatin formed spindle). Kinetochore condenses kinetochore nucleolus disappears spindle kinetochore spindle fiber spindle fibers attached to the spindle fiber fibers forming Early Prophase Prophase polar spindle fiber Prometaphase Metaphase sister chromatids come from Centrosomes have duplicated. Nucleolus has disappeared, and The kinetochore of each chromatid is Chromatin is condensing into chromosomes, and the nuclear duplicated chromosomes are visible. Centrosomes begin moving apart, attached to a kinetochore spindle fiber. Polar spindle fibers stretch from each opposite spindle poles. envelope is fragmenting. and spindle is in process of forming. spindle pole and overlap. centrosome lacks centrioles Plant Cell at Interphase 25 µm 6.2 µm spindle pole lacks 20 µm 6.2 µm cell wall chromosomes spindle fibers centrioles and aster Animal cell(Early prophase, Prophase, Metaphase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase): © R. Calentine/Visuals Unlimited  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrosome has centrioles Animal Cell 20 µm 20 µm 9 µm 20 µm 20 µm Anaphase at Interphase aster duplicated spindle chromosomes at daughter chromosome chromosome pole metaphase plate nuclear kinetochore Sister chromatids part and become envelope fragments centromere daughter chromosomes that move toward the spindle poles. In this MITOSIS chromatin condenses way, each pole receives the same nucleolus disappears kinetochore spindle spindle fiber kinetochore number and kinds of chromosomes fibers forming spindle fiber polar spindle fiber Early Prophase Prophase Prometaphase Metaphase Anaphase as the parent cell. Centrosomes have duplicated. Nucleolus has disappeared, and The kinetochore of each chromatid is Centromeres of duplicated chromosomes Chromatin is condensing into duplicated chromosomes are visible. attached to a kinetochore spindle fiber. are aligned at the metaphase plate (center chromosomes, and the nuclear Centrosomes begin moving apart, Polar spindle fibers stretch from each of fully formed spindle). Kinetochore spindle envelope is fragmenting. and spindle is in process of forming. spindle pole and overlap. fibers attached to the sister chromatids come from opposite spindle poles. centrosome lacks centrioles Plant Cell at Interphase 25 µm 6.2 µm spindle pole lacks 20 µm 6.2 µm 6.2 µm cell wall chromosomes spindle fibers centrioles and aster Animal cell(Early prophase, Prophase, Metaphase, Anaphase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase, Anaphase): © R. Calentine/Visuals Unlimited  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrosome has centrioles Animal Cell 20 µm duplicated 20 µm spindle 9 µm chromosomes at 20 µm 20 µm 16 µm at Interphase aster daughter chromosome cleavage furrow chromosome pole metaphase plate nuclear kinetochore Telophase envelope centromere fragments nucleolus Daughter cells are forming MITOSIS chromatin as nuclear envelopes and condenses nucleolus disappears spindle kinetochore nucleoli reappear. spindle fiber kinetochore fibers forming spindle fiber Early Prophase Prophase polar spindle fiber Prometaphase Metaphase Anaphase Telophase Chromosomes will Centrosomes have duplicated. Nucleolus has disappeared, and The kinetochore of each chromatid is Centromeres of duplicated chromosomes Sister chromatids part and become daughter Chromatin is condensing into chromosomes, and the nuclear duplicated chromosomes are visible. attached to a kinetochore spindle fiber. are aligned at the metaphase plate (center chromosomes that move toward the spindle Centrosomes begin moving apart, Polar spindle fibers stretch from each of fully formed spindle). Kinetochore spindle poles. In this way, each pole receives the same become indistinct chromatin. envelope is fragmenting. and spindle is in process of forming. spindle pole and overlap. fibers attached to the sister chromatids number and kinds of chromosomes as the parent cell. come from opposite spindle poles. centrosome lacks centrioles Plant Cell at Interphase 25 µm 6.2 µm spindle pole lacks 20 µm 6.2 µm 6.2 µm 6.6 µm cell wall chromosomes spindle fibers cell plate centrioles and aster Animal cell(Early prophase, Prophase, Metaphase, Anaphase, Telophase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase, Anaphase): © R. Calentine/Visuals Unlimited; Plant cell(Telophase): © Jack M. Bostrack/Visuals Unlimited; Comparison of Phases of Mitosis in Animal and Plant Cells Functions of mitosis: ◦ Permits growth and repair. ◦ In flowering plants, meristematic tissue retains the ability to divide throughout the life of the plant ◦ In mammals, mitosis is necessary when: ◦ A fertilized egg becomes an embryo ◦ An embryo becomes a fetus ◦ A cut heals or a broken bone mends 50 The Cell Cycle and Cancer Abnormal growth of cells is called a tumor ◦ Benign tumors are not cancerous ◦ Encapsulated ◦ Do not invade neighboring tissue or spread ◦ Malignant tumors are cancerous ◦ Not encapsulated ◦ Readily invade neighboring tissues ◦ May also detach and lodge in distant places (metastasis) ◦ Results from mutation of genes regulating the cell cycle Development of cancer ◦ Tends to be gradual ◦ May take years before a cell is obviously cancerous 51 The Cell Cycle and Cancer Characteristics of Cancer Cells ◦ Lack differentiation ◦ Are non-specialized ◦ Are immortal (can enter cell cycle repeatedly) ◦ Have abnormal nuclei ◦ May be enlarged ◦ May have abnormal number of chromosomes ◦ Often have extra copies of genes ◦ Do not undergo apoptosis ◦ Normally, cells with damaged DNA undergo apoptosis ◦ The immune system can also recognize abnormal cells and trigger apoptosis ◦ Cancer cells are abnormal but fail to undergo apoptosis 52  Uncontrolled Mitosis ü If mitosis is not controlled, unlimited cell division occurs causing cancerous tumors ü Oncogenes are special proteins that increase the chance that a normal cell develops into a tumor cell 53 The Cell Cycle and Cancer Origin of Cancer ◦ Chromosomes normally have special material at each end called telomeres ◦ These get shorter each cell division ◦ When they get very short, the cell will no longer divide ◦ Telomerase is an enzyme that maintains the length of telomeres ◦ Mutations in telomerase gene: ◦ Cause telomeres to continue to lengthen, which ◦ Allows cancer cells to continually divide 54 Meiosis and sexual reproduction Halving the Chromosome Number Meiosis ◦ Special type of cell division ◦ Used only for sexual reproduction ◦ Halves the chromosome number prior to fertilization ◦ Parents are diploid (2n) ◦ Meiosis produces haploid (n) gametes ◦ Gametes fuse in fertilization to form a diploid (2n) zygote ◦ The zygote becomes the next diploid (2n) generation 56 Halving the Chromosome Number In diploid body cells, chromosomes occur in pairs Humans have 23 different types of chromosomes Diploid (2n) cells have two chromosomes of each type Chromosomes of the same type are said to be homologous chromosomes (homologues) ◦ They have the same length ◦ Their centromeres are positioned in the same place ◦ One came from the father (the paternal homolog) the other from the mother (the maternal homolog) ◦ When stained, they show similar banding patterns 57 Homologous Chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Sister chromatids nonsister duplication chromatids duplication kinetochore centromere chromosome homologous pair chromosome paternal chromosome maternal chromosome 58 b. a: © L. Willatt/Photo Researchers, Inc. Halving the Chromosome Number Homologous chromosomes have genes controlling the same trait at the same position ◦ Each gene occurs in duplicate ◦ A maternal copy from the mother ◦ A paternal copy from the father Many genes exist in several variant forms in a large population Homologous copies of a gene may encode identical or different genetic information The variants that exist for a gene are called alleles An individual may have: ◦ Identical alleles for a specific gene on both homologs (homozygous for the trait), or ◦ A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait) 59 Halving the Chromosome Number Overview of Meiosis ◦ Meiosis I ◦ Chromosomes are replicated prior to meiosis I ◦ Each chromosome consists of two identical sister chromatids ◦ Homologous chromosomes pair up – synapsis ◦ Homologous pairs align themselves against each other side by side at the metaphase plate ◦ The two members of a homologous pair separate ◦ Each daughter cell receives one duplicated chromosome from each pair ◦ Meiosis II ◦ DNA is not replicated between meiosis I and meiosis II ◦ Sister chromatids separate and move to opposite poles ◦ The four daughter cells contain one daughter chromosome from each pair ◦ Each daughter chromosome consists of a single chromatid ◦ The daughter cells are haploid 60 Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2n = 4 61 Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrioles nucleolus centromere 2n = 4 62 Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrioles nucleolus centromere chromosome duplication 2n = 4 2n = 4 MEIOSIS I Homologous pairs synapse and then separate. 63 Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sister chromatids centrioles nucleolus synapsis centromere chromosome duplication 2n = 4 2n = 4 MEIOSIS I Homologous pairs synapse and then separate. 64  Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First division centrioles nucleolus sister chromatids centromere synapsis chromosome duplication 2n = 4 2n = 4 n=2 MEIOSIS I Homologous pairs synapse and then separate. 65  Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First division Second division centrioles nucleolus sister chromatids centromere synapsis chromosome duplication 2n = 4 2n = 4 n=2 MEIOSIS I Homologous pairs synapse and then separate. 66  Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First division Second division centrioles nucleolus sister chromatids centromere synapsis chromosome duplication 2n = 4 2n = 4 n=2 n=2 MEIOSISI MEIOSISII Homologous pairs Sister chromatids separate, synapse and then separate. becoming daughter chromosomes. 67  Overview of Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. First division Second division Four haploid daughter cells centrioles nucleolus sister chromatids centromere synapsis chromosome duplication 2n = 4 2n = 4 n=2 n=2 MEIOSISI MEIOSISII Homologous pairs Sister chromatids separate, synapse and then separate. becoming daughter chromosomes. 68 10.2 Genetic Variation Meiosis brings about genetic variation in two key ways: ◦ Crossing-over between homologous chromosomes, and ◦ Independent assortment of homologous chromosomes Crossing Over: ◦ Exchange of genetic material between non-sister chromatids during meiosis I ◦ At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues ◦ Holds homologues together ◦ Aligns DNA of non-sister chromatids ◦ Allows crossing-over to occur ◦ Then homologues separate and are distributed to different daughter cells 69  Crossing Over Occurs During Meiosis I Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. nucleoprotein lattice sister chromatids sister chromatids of a chromosome of its homologue A A a a A a B B b b B b chiasmata of nonsister chromatids c C C c 1 and 3 C c D D d d D d 12 34 1 2 3 4 1 2 3 4 Bivalent Crossing-over Daughter forms has occurred chromosomes a. b. c. d. a: Courtesy Dr. D. Von Wettstein 72  Genetic Variation Independent assortment ◦When homologous chromosome pairs align at the metaphase plate: ◦ They separate in a random manner ◦ The maternal or paternal homologue may be oriented toward either pole of mother cell ◦Causes random mixing of blocks of alleles into gametes 73  Independent Assortment Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Combination Combination 5 7 Combination 3 Combination 1 Combination 2 Combination 6 Combination 8 Combination 4 74 Genetic Variation Fertilization – union of male and female gametes ◦ Chromosomes donated by the parents are combined ◦ In humans, (223)2 = 70,368,744,000,000 chromosomally different zygotes are possible If crossing-over occurs only once ◦(423)2, or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible Crossing-over may occur several times in each chromosome 75 Genetic Variation Significance of genetic variation: ◦ Asexual reproduction produces genetically identical clones ◦ Sexual reproduction causes genetic recombinations among members of a population ◦ Asexual reproduction is advantageous when the environment is stable ◦ However, if the environment changes, genetic variability introduced by sexual reproduction may be advantageous ◦ Some offspring may have a better chance of survival 76 The Phases of Meiosis Meiosis I: ◦ Prophase I ◦ A spindle forms ◦ The nuclear envelope fragments ◦ The nucleolus disappears ◦ Each chromosome is duplicated (consists of two identical sister chromatids) ◦ Homologous chromosomes pair up and physically align themselves against each other side by side (synapsis) ◦ Synapsed homologs are referred to as a bivalent (two homologues) or a tetrad (four chromatids) ◦ Metaphase I ◦ Homologous pairs are arranged at the metaphase plate ◦ Bivalents are aligned independently of one another 77  The Phases of Meiosis Meiosis I ◦ Anaphase I ◦ Homologous chromosomes of each bivalent separate from one another ◦ Homologues move towards opposite poles ◦ Sister chromatids do not separate ◦ Each is still a duplicated chromosome with two chromatids ◦ Telophase I ◦ Daughter cells have one duplicated chromosome (n) from each homologous pair 78  The Phases of Meiosis Interkinesis ◦ Two haploid (n) daughter cells, each with one duplicated chromosome of each type ◦ Interkinesis is similar to mitotic interphase except ◦ It is usually shorter ◦ DNA replication does not occur 79 Meiosis I in Animal Cells Meiosis I in Animal Cells Meiosis I in Animal Cells Meiosis I in Animal Cells  Spermatogenesis and Oogenesis in Mammals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SPERMATOGENESIS OOGENESIS primary primary spermatocyte oocyte 2n 2n Meiosis I Meiosis I first polarbody secondary spermatocytes n secondary n oocyte Meiosis II n Meiosis II spermatids Meiosis II is completed n second after entry of sperm polarbody (fertilization) n Metamorphosis egg Fertilization and maturation n sperm Sperm nucleus n n fusion of sperm nucleus and zygote agg nucleus 2n Meiosis Compared to Mitosis Meiosis Mitosis ◦ Requires one nuclear division ◦ Requires two nuclear divisions ◦ Chromosomes do not synapse nor ◦ Chromosomes synapse and cross cross over over ◦ Centromeres dissolve in mitotic ◦ Centromeres survive Anaphase I anaphase ◦ Halves chromosome number ◦ Preserves chromosome number ◦ Produces four daughter nuclei ◦ Produces two daughter nuclei ◦ Produces daughter cells genetically ◦ Produces daughter cells genetically different from parent and each identical to parent and to each other other ◦ Used only for sexual reproduction ◦ Used for asexual reproduction and growth 85 Meiosis Compared to Mitosis 86 Meiosis Compared to Mitosis 87 Changes in Chromosome Number and Structure Euploidy is the correct number of chromosomes in a species. Aneuploidy is a change in the chromosome number ◦ Results from nondisjunction ◦ Monosomy - only one of a particular type of chromosome, ◦ Trisomy - three of a particular type of chromosome 88  Nondisjunction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pair of pair of homologous homologous chromosomes chromosomes 89  Nondisjunction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pair of pair of homologous homologous chromosomes chromosomes nondisjunction Meiosis I normal 90  Nondisjunction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pair of pair of homologous homologous chromosomes chromosomes nondisjunction Meiosis I normal Meiosis II normal nondisjunction nondisjunction 91  Nondisjunction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pair of pair of homologous homologous chromosomes chromosomes nondisjunction Meiosis I normal Meiosis II normal nondisjunction Fertilization 92  Nondisjunction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pair of pair of homologous homologous chromosomes chromosomes nondisjunction Meiosis I normal Meiosis II normal nondisjunction Fertilization Zygote 93  Nondisjunction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pair of pair of homologous homologous chromosomes chromosomes nondisjunction Meiosis I normal Meiosis II normal nondisjunction Fertilization Zygote 2n + 1 2n + 1 2n - 1 2n - 1 2n 2n 2n + 1 2n - 1 a. b. 94 Changes in Chromosome Number and Structure Trisomy occurs when an individual has three of a particular type of chromosome The most common autosomal trisomy seen among humans is Trisomy 21 ◦ Also called Down syndrome ◦ Recognized by these characteristics: ◦ short stature ◦ eyelid fold ◦ flat face ◦ stubby fingers ◦ wide gap between first and second toes 95 Trisomy 21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. extra chromosome 21 a. b. a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers 96 Changes in Chromosome Number and Structure Changes in sex chromosome number: ◦ Results from inheriting too many or too few X or Y chromosomes ◦ Nondisjunction during oogenesis or spermatogenesis ◦ Turner syndrome (XO) ◦ Female with a single X chromosome ◦ Short, with broad chest and widely spaced nipples ◦ Can be of normal intelligence and function with hormone therapy 97 Changes in Chromosome Number and Structure Changes in sex chromosome number: ◦ Klinefelter syndrome (XXY) ◦ Male with underdeveloped testes and prostate; some breast overdevelopment ◦ Long arms and legs; large hands ◦ Near normal intelligence unless XXXY, XXXXY, etc. ◦ No matter how many X chromosomes are present, the presence of a chromosome Y renders the individual male 98 Changes in Sex Chromosome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Turner syndrome missing b. Klinefelter syndrome extra chromosome X a(top): Courtesy UNC Medical Illustration and Photograph; b(top): Courtesy Stefan D. Schwarz, chromosome X 99 http://klinefeltersyndrome.org; a, b(bottom): © CNRI/SPL/Photo Researchers, Inc Changes in Chromosome Number and Structure Changes in chromosome structure include: ◦ Deletion ◦ One or both ends of a chromosome breaks off ◦ Two simultaneous breaks lead to loss of an internal segment ◦ Duplication ◦ Presence of a chromosomal segment more than once in the same chromosome ◦ Translocation ◦ A segment from one chromosome moves to a non-homologous chromosome ◦ Follows breakage of two non-homologous chromosomes and improper re-assembly 100 Changes in Chromosome Number and Structure ◦ Changes in chromosome structure include: ◦ Inversion ◦ Occurs as a result of two breaks in a chromosome ◦ The internal segment is reversed before re-insertion ◦ Genes occur in reverse order in the inverted segment 101 Types of Chromosomal Mutation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a a a b b b b c c c c + a d d d d e e e e f d f f e g g g f g a. Deletion b. Duplication a a a a b b b l b l c m c m c d d n d n d c e o e o e e f p f p f f g q q g g g h r r h c. Inversion d. Translocation 102 Deletion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a a b b + h deletion lost c c d d e e f f g g h a. b. b: Courtesy The Williams Syndrome Association 103