Topic 8 Mitosis & Meiosis Ch.12 & 13. The Eukaryotic Genome. The Eukaryotic Genome. The Eukaryotic Genome

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1 Topic 8 Mitosis & Meiosis Ch.12 & 13 The Eukaryotic Genome pp , Genome All of the genes in a cell. Eukaryotic cells contain their DNA in long linear pieces. In prokaryotic cells, there is a singular circular piece of DNA. Fig The Eukaryotic Genome Chromosome - One very long DNA molecule, containing thousands of genes. Gene - A discrete unit of DNA which codes for a specific protein. Chromatin Term for DNA when it is condensed (coiled around proteins called histones). The Eukaryotic Genome A duplicated chromosome consists of two sister chromatids. Sister chromatids - Contain identical copies of the DNA molecule. They make up one leg of the X shaped chromosome. Centromere Where sister chromatids are attached. Fig Fig The Eukaryotic Genome Eukaryotic organisms vary in the number of sets of chromosomes they possess. A set of chromosomes is represented by n. Haploid (n) - A cell containing only one set of chromosomes. Diploid (2n) - A cell containing two sets of chromosomes, one set inherited from each parent. Polyploid (3n, 4n, etc.) - A cell containing more than two sets of chromosomes. Fig

2 The Eukaryotic Genome In diploid cells, each parent provided a complete set of information. The chromosomes from each parent carry the exact same genes, but the forms of the genes (alleles) may vary. Homologous chromosomes Chromosomes that are the same length, have centromeres at the same location and contain the same genes. Allele Different forms of the same gene (i.e. a gene for freckles has the alleles present and absent ). Cell Reproduction in Bacteria pp Fig In prokaryotes the genome is one circular molecule of DNA (maybe some plasmids). Reproducing the cell is by binary fission. This process involves: -The DNA duplicating. -The DNA attaching to the cell membrane. -The cell growing to about twice its normal size. -Then splitting in half. Cell Reproduction in Eukaryotic Cells pp , Karyokinesis Division of the karyon (DNA in the nucleus) of a cell. Binary fission involves duplicating and separating DNA. However binary fission is not a sufficient means of separating multiple chromosomes within a nucleus. Instead, eukaryotic cells undergo mitosis or meiosis. Cell Reproduction in Eukaryotic Cells Eukaryotic cells need to carefully organize and move their chromosomes during cell division. Spindle fibers - An assemblage of microtubules that orchestrate chromosome movement during cell division. Spindle fibers attach to each chromosome at the centromere. Binary fission does not use spindles and thus cannot ensure as precise cell division as in eukaryotes. 2

3 Cell Reproduction in Eukaryotic Cells In unicellular organisms, division of one cell reproduces the entire organism. In multicellular organisms cells are also produced for reproduction, as well as for growth and repair. Cell Reproduction in Eukaryotic Cells There are two types of reproduction: 1. Asexual reproduction One parent produces genetically identical offspring. E.g. budding, fragmentation 2. Sexual reproduction Two parents give rise to offspring that have unique combinations of genes. The Cell Cycle pp The Cell Cycle The life cycle of a cell, from its origin to its division into two cells. There are 2 major phases: 1. Interphase Most of a cell s life, including: - Gap 1 (G1) phase Cell undergoes regular activity. - Synthesis (S) phase DNA is duplicated. - Gap 2 (G2) phase Cell is ready to divide but hasn t yet. 2. Mitotic (M) phase - Mitosis and cytokinesis. Fig Mitosis pp Mitosis Nuclear division in eukaryotic cells in which chromosome number in the daughter nuclei is conserved. Mitosis is broken down into five subphases: 1. Prophase Chromatin condenses so that individual chromosomes in the form of sister chromatids become visible. They are attached to each other by the centromere. Mitotic spindle begins to form. Fig

4 2. Prometaphase Nuclear envelope dissolves. Microtubules attach to the chromatids at the kinetochore in the centromere. Fig Metaphase Chromatids line up in the middle of the cell. Fig Anaphase Sister chromatids separate. Microtubules shorten and chromosomes are brought to opposite ends of the cell. Fig Telophase Two new nuclei form and cytokinesis occurs afterwards. Cytokinesis - Division of the cytoplasm. Occurs after mitosis and is not considered part of mitosis. Fig Fig Mitosis Review Parent cell Mitosis Review 1 chromosome Parent cell 2 chromatids (you would also say this is 1 chromosome) Daughter cell Daughter cell 1 chromosome 1 chromosome 4

5 Meiosis pp Mitosis results in the production of genetically identical cells. To create the variation needed for sexual reproduction, eukaryotes use meiosis. Meiosis Cell division in sexually reproducing organisms that results in cells with half the number of chromosomes of the original cell. Meiosis There are two rounds of division in meiosis: Meiosis I - Separates homologous chromosomes. Meiosis II - Separates sister chromatids. Fig Meiosis The preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. However, at the end of meiosis, haploid cells are produced. These need to fuse with other haploid cells, so they can not continue the cell cycle the same way diploid cells do. Meiosis I: Prophase I Chromosomes condense and homologous chromosomes pair up, aligned gene by gene to form tetrads (four chromatids). This pairing is called synapsis. Crossing over occurs. Meiosis I: Metaphase I Tetrads line up at equator of the cell, with one chromosome facing each pole. Meiosis I: Anaphase I The pairs of homologous chromosomes separate. One chromosome moves toward each pole. Sister chromatids remain attached at the centromere and move as one unit toward the pole. 5

6 Meiosis I: Telophase I Each half of the cell has a haploid set of chromosomes. Each chromosome still consists of two sister chromatids. Cytokinesis usually occurs simultaneously. In animal cells, a cleavage furrow forms, in plant cells, a cell plate forms. Meiosis II: Prophase II Meiosis II is very similar to mitosis. In prophase II, a spindle apparatus forms. Meiosis II: Metaphase II The sister chromatids are arranged at the equator of the cell. Because of crossing over, the two sister chromatids of each chromosome are no longer genetically identical. The kinetochores of sister chromatids attach to microtubules extending from opposite poles. Meiosis II: Anaphase II The sister chromatids separate. The sister chromatids of each chromosome now move as two newly individual chromosomes. Meiosis II: Telophase II Nuclei reform and the chromosomes begin decondensing. Cytokinesis separates the cytoplasm. At the end of meiosis, there are four daughter cells, each with a haploid set of chromosomes. Each daughter cell is genetically distinct from the others and from the mother cell. Cleavage furrow Two haploid cells form; chromosomes are still double Haploid daughter cells Sister chromatids forming separate During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes 6

7 Cytokinesis p.251 Cytokinesis in animal cells results from a pinching off that results in the formation of a cleavage furrow. Cleavage furrow deepens until the cell is pinched in two. Cytokinesis Plants have a cell wall, so it is not possible to pinch off. Instead a new cell wall is formed at the middle of the dividing cell. This new wall is called a cell plate. Fig Fig Mitosis vs. Meiosis pp Fig Property Mitosis Meiosis DNA replication During interphase During interphase Divisions One Two Synapsis and crossing over Genetic composition of daughter cells Role in animal body Does not occur Two diploid, identical to parent cell Produces cells for growth and tissue repair Occurs in prophase I Four haploid, different from parent cell and each other Produces gametes Fig Creating Variation in Meiosis pp There are many disadvantages to sexual reproduction. The single advantage is that it creates variation in offspring from the genetic recombination of two individuals. If the environment changes and all offspring are genetically identical, all offspring could die. Genetically unique offspring will increase the likelihood that some individuals, and through them the entire species, will survive. 7

8 Fig Creating Variation in Meiosis The variation seen in sexual reproduction is produced through three processes: 1. Crossing Over Crossing over begins very early in prophase I, as homologous chromosomes pair up. Homologous portions of two nonsister chromatids trade places. This contributes to genetic variation by combining DNA from two parents into a single chromosome. I.e. creating recombinant chromosomes. Fig Independent Assortment Homologous pairs of chromosomes orient randomly at metaphase I. Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs. The number of combinations possible when chromosomes assort independently into gametes is 2 n, where n is the haploid number. E.g. for humans 2 23 possible combinations. Fig

9 3. Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum. The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations. Each zygote has a unique genetic identity. Sexual Life Cycles pp , Animals (Humans) - Meiosis produces haploid gametes. Haploid sperm and haploid egg fuse and syngamy (fertilization) produces a diploid zygote. The diploid zygote then divides by mitosis to produce a multicellular diploid adult. Fig Sexual Life Cycles 2. Plants Plants have alternation of generations. A multicellular diploid sporophyte undergoes meiosis to produce haploid spores. The spores develop into a haploid gametophyte which produces gametes through mitosis. The gametes fuse to form a diploid zygote, which develops into a sporophyte through mitosis. Fig Sexual Life Cycles 3. Fungi A multicellular haploid organism may fuse with another haploid individual. Plasmogamy When the cytoplasm of two haploid individuals fuse. Later the nuclei of these two individuals can fuse and form a diploid zygote (karyogamy). The diploid zygote then undergoes meiosis to produce haploid spores. The spores grow into a haploid multicelluar adult through mitosis. 9

10 Fig Fig