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Cell Cycle Mitosis and Meiosis

Cell Cycle Mitosis and Meiosis. How is heritable information passed to the next generation?. In Eukaryotes Heritable information is passed to the next generation via two processes the Cell cycle and mitosis or meiosis and fertilization REVIEW:

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Cell Cycle Mitosis and Meiosis

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  1. Cell CycleMitosis and Meiosis

  2. How is heritable information passed to the next generation? • In Eukaryotes Heritable information is passed to the next generation via two processes • the Cell cycle and mitosis • or • meiosis and fertilization REVIEW: Bacteria Reproduce using a simple process called binary fission genetic material is replicated, the cell moves the copies to opposite poles of the cell and the cytoplasm is divided in half.

  3. What is the “Heritable Information” • Heritable information: Genes are essentially instructions encoded by 4 nucleotide letters (ATGC), DNA. • The order of the As, Ts, Gs and Cs codes for what the cell should make. • What is a gene? A piece of DNA that contains instructions to make a specific product (such as RNA or protein) • Genes are organized and attached together into long structures called chromosomes. • Prokaryotes usually have 1 circular chromosome • Eukaryotes have multiple linear chromosomes • Chromosomes also contain regulatory DNA that controls when and how much of a product is made.

  4. What is the Genome? • The Genome includes all the chromosomes, genes, and regulatory DNA. • The genome represents all the possible responses to the environment the cell is capable of • Cells can only respond to the extent that their genome has instructions to do so. • If the genome is corrupted through mutation the cell may respond differently to the environment (or not respond at all)

  5. Eukaryotes are Diploid (two of each chromosome) • The two chromosomes are called a homologous pair of chromosomes (homologous pair, homologous chromosomes) • Homologous chromosome have the same genes in the same location • Genes on homologous chromosomes can be different versions (allele) • Example: Gene for eye color, blue version, brown version.

  6. Human Karyotype: The condensed chromosomes • Humans have • 23 homologous pairs of chromosomes • 22 autosomes • 1 pair of sex chromosomes • total of 46 chromosomes.

  7. Having two versions of each chromosome is advantageous • Second version can serve as a back up copy in case one version is defective • Having two versions provides a wider range of functions • Alternative versions of the same gene causes hybrid vigor • individual has more than one way to respond by having different versions of the same gene ( = allele) • Greater chance of surviving a changing environment

  8. 0.5 µm Chromosome Structure Genes Centromere: narrow region where replicated, IDENTICAL SISTER CHROMATIDS are attached together Sister chromatids

  9. The cell cycle is a complex set of stages that must be highly regulated • The cell cycle is controlled by checkpoints that ensure that the genetic material is undamaged • The fate of the cell is controlled by internal and external signals to ensure that a cell divides only when needed: 20 µm 100 µm 200 µm (a) Reproduction (b) Growth and development (c) Tissue renewal

  10. Two Distinct phases of the cell cycle can be observed: • Interphase • Growth • Synthesis of DNA • Preparation for Mitosis • Mitosis • A series of coordinated steps that passes a complete copy of the genome to each daughter cell • Division of the replicated genome is followed by Cytokinesis, division of the cytoplasm and cell membrane • Results in two genetically identical daughter cells.

  11. Stages of the cell cycle INTERPHASE S (DNA synthesis) • Interphase: • Gap 1: Growth, must receive a signal to divide and pass the 1stcheckpoint • Synthesis: DNA is replicated • Gap 2: Preparation for Mitosis, pass the 2ndcheckpoint • Mitosis: • Mitosis: Divide the genome into two identical sets • Pass the 3rdcheckpoint. • Cytokinesis: Divide the cytoplasm. • Cells that do not need to divide enter a phase called G0 (G naught) in which they will carry out their intended function. G1 Cytokinesis G2 Mitosis G0 MITOTIC (M) PHASE

  12. External signals can cause the cell to progress in the cell cycle or activate the apoptosis pathway. Scalpels • Example: Platelet Derived growth factor (PDGF) is released by platelets due to tissue injury. • PDGF signals to fibroblasts to exit G0 and enter the cell cycle to divide and make more fibroblasts Petri plate Without PDGF cells fail to divide With PDGF cells proliferate Cultured fibroblasts 10 µm

  13. Other external signals: Normal cells exhibit anchorage and density dependence. Anchorage dependence Many cells will not enter the cell cycle unless they are attached to a substrate Density-dependent inhibition Most cells will enter G0 when they are touching other cells on all sides Density-dependent inhibition Cancer cells are neither anchorage dependent nor density inhibited. 25 µm 25 µm (a) Normal mammalian cells (b) Cancer cells

  14. Tumor Necrosis Factor activates proteins inside of cells that lead to apoptosis • A special class of T cells called T Cytotoxic cells release Tumor Necrosis Factor and other signaling molecules that bind to receptors and trigger apoptosis. PLEASE DO NOT MEMORIZE THIS: this is only serving as an example of external signaling

  15. Cyclins and Cyclin-dependent kinases (CDKs) control the cell cycle. • Each phase has a pair of Cylins and CDKs that cause progression to the next phase of the cycle. • How do Cyclins and CDKs move the cell through the cell cycle? • CDKs are present in constant amounts in the cytoplasm but are inactive without Cyclin partner. • When the specific Cyclin is produced it forms a Cyclin-CDK complex. • Cyclin-CDK complex is an active Kinase and can phosphorylate target proteins • Cyclins are degraded at the end of a phase, inactivating the CDK

  16. Example of Cyclin-CDK Regulation of phases of the cell cycle. 1. M phase CDK is present throughout the cell cycle but is inactive. 2. M phase cyclin is produced during the G1 and S phases and accumulates. 3. M Cyclin and CDK form a complex that drives the cell into the mitosis phase 4. M- Cyclin is degraded at the end of Mitosis, signaling the end of mitosis phase G1 S 2 1 Cdk Cyclin accumulation M G2 Degraded cyclin G2 checkpoint Cdk 4 Cyclin is degraded Cyclin MPF 3 (b) Molecular mechanisms that help regulate the cell cycle

  17. Mitosis passes a complete genome to each daughter cell • Mitosis occurs after DNA replication (and only if it was successful) • Mitosis is followed by Cytokinesis which produces genetically identical daughter cells • Mitosis is a continuous process with observable structural features which can be summarized as: • Replication • Alignment • Separation • Microtubules are involved in pulling sister chromatids apart. • Coordinated steps ensure that each daughter cell receives a complete set of the chromosomes.

  18. Mitosis: Replicate, Align, Separate • Chromosomes are replicated during S phase • During mitosis chromosomes condense and sister chromosomes are attached together at the centromere • Microtubules align sister chromatids during metaphase to ensure that they will be separated • When all sister chromatids are aligned they are pulled apart

  19. Meiosis and Sexual Reproduction Produces Genetic Variation! • Meiosis is a reduction division that segregates homologous chromosomes into two sets • Each gamete receives one of the pair. • During fertilization haploid gametes fuse to produce a new unique diploid individual.

  20. Meiosis Highlights • Replicate DNA • Crossing Over of Homologous pairs • Homologous Pairs Align • Homologous Pairs Separate • Align sister chromatids • Separate sister chromatids • 4 haploid gametes are produced • Sperm or egg

  21. Meiosis overview:

  22. Crossing over of homologous pairs • Homologous chromosomes exchange genetic material. • This increases genetic variation. • The closer genes are on a chromosome to each other, the more likely they are to be inherited together (liked traits)

  23. During the first alignment homologous pairs are aligned to form a tetrad. • Orientation of the paternal and maternal chromosomes is random • Homologous pairs are separated • Sister chromatids are separated • The resulting gamete is haploid • Genome is a mixture of paternal and maternal genomes (about 50% from each)

  24. Fertilization combines two haploid gametes • Fertilization ensures genetic variation in sexually reproducing organisms. • Combination of two haploid gametes restores the diploid number of chromosomes

  25. Most genetic traits are inherited from nuclear DNA. There is one major exception of non-nuclear genetic inheritance: • Mitochondria and Chloroplasts assort independently from the nuclear DNA • In animals mitochondrial DNA is transmitted by the egg, not by the sperm. • Mitochondrial traits are inherited from the mother

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