BSCI222 Genetics Chapter 2
BSCI222 Genetics Chapter 2 BSCI222
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This 9 page Class Notes was uploaded by Colin Fields on Thursday September 1, 2016. The Class Notes belongs to BSCI222 at University of Maryland taught by Dr. Paczolt in Fall 2016. Since its upload, it has received 59 views. For similar materials see Genetics in Biology at University of Maryland.
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Genetics Chapter 2 2.1 Prokaryotic and Eukaryotic Cells Differ in a Number of Genetic Characteristics Trait Prokaryotic Eukaryotic Nucleus Absent Present Cell Diameter 1 to 10 micrometers 10 to 100 micrometers Gnome Usually one circular Multiple linear DNA molecule DNA molecules DNA Small amount Large amount Archea have some Complexed with histones histones Organelles Usually absent Present Prokaryote: unicellular organism with a relatively simple cell structure Eukaryote: both multi and unicellular organisms with compartmentalized cell structures by way of intracellular membranes Eubacteria: a type prokaryote without histones, distinct from archea Archea: a type of prokaryote that can have some histones and is more closely related to Eukaryotes than Eubacteria Nucleus: nuclear membrane that segregates DNA from cytoplasm + that DNA Histone: proteins that DNA wraps around to condense Limits access of proteins and cofactors to DNA to regular gene expression Chromatin: tightly packed DNA-histone complex, makes up chromosomes Plasmid: a few bacteria have this, an additional chromosomes that contain important genes Virus: an outer protein coat surrounding nucleic acid Neither alive nor dead Rely on hosts to replicate More closely related to hosts than other viruses Suggested to have evolved from hosts not other viruses 2.2 Cell Reproduction Requires the Copying of the Genetic Material, Separation of the Copies, and Cell Division Prokaryotic Cell Reproduction 1. Binary Fission a. Replication starts at origin of replication b. Origins move to opposite sides of cell c. Proteins in some bacteria anchor origins to plasma membrane d. New cell wall forms between chromosomes 2. Produces identical daughter cells 3. Does not produce diversity Eukaryotic Cell Reproduction 1. Eukaryotic Chromosomes a. No real relationship between number of chromosomes per cell and organism complexity b. Usually two sets of chromosome, one form each parent c. Homologous pair: two chromosomes which are alike in size and structure which carries genetic information for the same set of hereditary characteristics d. Allele: a copy of a specific gene e. Diploid: cells that carry two sets of genetic information f. Haploid: reproductive and a few other cells that carry one set of genetic information g. Polyploid: cells that carry more than 2 sets of genetic information 2. Chromosome Structure a. DNA molecules are coiled tightly around histone proteins to be compacted into the nucleus as rod shaped chromosomes b. Essential elements i. Centromere: attachment point for spindle microtubules. Spindles move chromosomes during cell division ii. Telomere: Repeated sequences at the ends of chromosomes that protect the core genetic material from degradation iii. Origins of replication: Where replication of DNA starts c. The centromere appears as a constricted region of the chromosome. Kinetochores assemble on the centromere and are where spindle microtubules attach. d. Shape Classification i. Metacentric – centromere is roughly in the center of the chromosome ii. Telocentric – centromere is on one end of chromosome iii. Submetacentric – centromere is between chromosome center and telomere iv. Acrocentric – center is near the telomere e. Sister Chromatids: chromosome + the copy of itself made during DNA synthesis. Attached at centromere The Cell Cycle and Mitosis Cell Cycle: The stages through which a cell passes from one division to the next Interphase: period between cell division in which the cell grows, develops, and functions M (mitotic) phase: period of active cell division including mitosis and cytokinesis 1. Interphase a. Three subphases, G1, S, and G2 b. G1 i. The cell grows ii. Proteins necessary for cell division are synthesized iii. Usually lasts several hours iv. G1/S checkpoint is reached and holds the cell in G1 until enzymes necessary for DNA replication are all made v. Once G1/S check is passed the cell is committed to divide c. G0 i. Cell can inter this phase before the G1/S checkpoint ii. Nondividing, stable state, little cell growth iii. Can reenter into the active cell cycle from this state d. S i. Chromosomes duplicate e. G2 i. Several biochemical events necessary fro cellular division take place ii. G2/M checkpoint is when cell transitions into miotic cycle iii. Checkpoint is passed only if DNA is completely replicated and undamaged f. During interphase the chromosomes are in a relaxed state and cannot be visualized with a microscope 2. M Phase a. This is where cell division happens b. 6 states, prophase, prometaphase, metaphase, anaphase, telophase, and cyotkinesis c. Prophase i. Sister chromatids become visible ii. Mitotic spindle, an array of microtubules, forms 1. In animals, grows from a pair of centrosomes that migrate to opposite sides of cells 2. In plants, there are not necessarily centrosomes or centrioles d. Prometaphase i. Marked by the disintegration of the nuclear membrane ii. Spindle microtubules (tubulin) enter nuclear region and stabilizes by attaching to a kinetochore iii. Bi-orientation: arrangement where each sister chromatid is connected to a separate centrosome by spindle fibers e. Metaphase i. Chromosomes arrange in the center of the cell in a coplanar manner (plane called metaphase plate) ii. Spindle-assembly checkpoint assures proper alignment 1. This is done by tension checks using sister chromatids f. Anaphase i. Cohesion proteins holding sister chromatids together are broken down by separase 1. This marks the beginning of anaphase ii. Chromosome movement is caused by disassembly of tubulin at both ends 1. Molecular motor proteins disassemble from the spindle and pull the chromosomes to the spindle pole iii. Sister chromatids separate into daughter chromasomes g. Telophase i. Marked by arrival of chromosomes at spindle poles ii. Nuclear membranes reform iii. Chromosomes relax and lengthen h. Cytokinesis i. Cytoplasm divides Genetic Consequences of the Cell Cycle 1. Identical daughter cells 2. Same number of chromosomes as regular somatic cells 3. Cytoplasmic components are roughly evenly divided between daughter cell though no mechanism ensures this 2.3 Sexual Reproduction Produces Genetic Variation Through the Process of Meiosis 1. Meiosis: produces gametes which have half the number of chromosomes as somatic cells 2. Fertilization: union of two haploid gametes Meiosis 1. Has the interphase stages of mitosis 2. Meiosis I and Meiosis II 3. Meiosis I a. Prophase I i. Leptotene 1. Chromosomes contract ii. Zygotene 1. Homologous chromosomes pair up 2. Synapsis: very close pairing association of homologous chromosomes a. Synpased chromosomes consist of 4 chromatids called a bivalent or tetrad iii. Pachytene 1. Chromosomes shorten and thicken 2. 3 part synaptomental complex develops between homologous chromosomes a. Involved with proper separation of chromosomes iv. Crossing over takes place v. Diplotene 1. Centromeres of paired chromosomes move apart 2. Homologues remain attached at chiasmata caused by crossover vi. Diakinesis 1. Nucleolus and nuclear membrane breaks down 2. Spindle fibers form b. Metaphase I i. Homologous chromosomes align on the metaphase plate c. Anaphase I i. Homologous chromosomes separate and move to opposite poles d. Telophase I i. Chromosomes arrive at spindle poles ii. Cytoplasm divides 4. Interkinesis a. Between Meiosis I and II b. Nuclear membrane reforms c. Spindle breaks down d. Chromosomes relax 5. Meiosis II a. Prophase II i. Only with cells that did the 3 parts of Interkinesis otherwise they skip to metaphase II ii. Events of Interkinesis reverse b. Metaphase II i. Chromosomes line up on metaphase plate c. Anaphase II i. Kinetochores of sister chromosomes separate ii. Chromatids move to opposite poles of cell d. Telophase II i. Chromosomes arrive at poles ii. Nuclear envelope reforms iii. Cytoplasm divides Sources of Genetic Variation in Meiosis 1. Each parent cell produces 4 daughter cells 2. Chromosome number is reduced by half 3. Genetically different daughter cells 4. Crossing Over a. Prophase I b. Exchange of genetic material between chromatids from different homologous chromosomes c. Interchromosomal recombination: creation of new combinations of alleles on a chromatid 5. Random Separation of Homologous Chromosomes a. Anaphase I b. Homologous chromosomes align and separate in a random manner n c. Can produce 2 combinations of chromosomes where n is the number of homologous pairs d. Independent assortment The Separation of Sister Chromatids and Homologous Chromosomes 1. Cohesion: proteins that hold sister chromatids together 2. Separase: enzyme that breaks down cohesion 3. During Meiosis I the protein shugoshin protects cohesion at centromere preventing separation of sister chromatids in Anaphase I Meiosis in the Life Cycle of Animals and Plants 1. Meiosis in Animals a. Spermatogenesis: production of gametes in males that occurs in the testes b. Spermatogonia: cells produced by mitotic division of primordial germ cells i. Can divide mitotically repeatedly to produce more spermatoginia ii. Can initiate meiosis c. Primary spermatocyte: spermatogonium that has entered prophase I of meiosis d. Secondary spermatocytes: two haploid daughter cells produced by a primary spermatocyte during meiosis I e. Spermatids: two haploid cells produced from a secondary spermatocyte f. Oogenesis: female production of gametes g. Oogonia: female version of spermatogonia h. Primary oocytes: female version of primary spermatocyte i. Secondary oocyte: most of the cytoplasm in meiosis I is given to one of the haploid daughter cells j. First Polar Body: haploid daughter cell from meiosis I that has very little cytoplasm and other components k. Ovum: meiosis II also produces a polar body and the final, larger, cell becomes the ovum, the mature female gamete l. Spermatogenesis happens all throughout a male’s reproductive life whereas oogenesis is halted after a usually short period of time 2. Meiosis in Plants a. Two distinct structures (generations) i. Multicellular diploid sporophyte 1. meiosis ii. Multicellular haploid gametophyte 1. mitosis b. Generations alternate c. Produces spores from meiosis which then produce gametes through mitosis d. Sporophyte is the obvious vegetative part of the plant that contains a few gametophytes e. Stamen: male part of the flower that contains diploid reproductive cells called microsporocytes i. Microsporocytes: undergo meiosis to produce 4 haploid microspores ii. Microspores divide mitotically producing an immature pollen grain consisting of two haploid nuclei 1. Tube nucleus directs the growth of a pollen tube 2. Generative nucleus divides mitotically to produce two sperm cells iii. Pollen grain is the male gametophyte f. Ovary: female part of the flower that contains diploid cells called megasporocytes i. Megasporocytes: undergoes meiosis to produce 4 haploid megaspores of which only one survives 1. Surviving megaspore divides mitotically 3 times producing 8 haploid nuclei that comprise the female gametophyte aka embryo sac g. When the plant flowers, the stamen open and release pollen grains h. Pollen land on the stigma, atop the style, whose base has the ovary i. Pollen grains that germinate grow a tube down into the ovary j. Fertilization produces a diploid zygote k. Other sperm fuses with 2 nuclei producing a triploid endosperm that stores food for the embryonic plant