Mitosis and Meiosis - notes PDF

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Cell Biology

Mitosis and Meiosis The process of cell division in eukaryotes – the cell cycle How DNA is replicated and packaged during cell division (mitosis) How the cell cycle is adapted to produce genetic variation during sexual reproduction (meiosis) Cell division in bacteria process: binary fission  Chromosomes replication begins  Replication continues  Replication finishes  Two daughter cells are created Cell division in eukaryotes  Processes are contained within the nucleus  Replicated DNA and other components  Regulation: so, proliferation occurs only when its needed Genetic information  Sored as DNA in the nucleus  Very large (2 metres long)  Replication requires a form of packaging to allow to be replicated – involves the wrapping round of DNA into nucleosomes which fold to from fibres -> chromosomes (condensed forms of the DNA and separate easily during cell division) The Cell Cycle Interphase: period of growth, DNA replicates // 90%  G1: 8-10h // cell prefers itself for division // cell growth, protein and RNA synthesis.  G0: resting state  S PHASE: 6-20h // DNA replication DNA is replicated in small units (replicons) // helicase unwinds the DNA // DNA polymerase working in one direction, creating a new complementary strand. = two identical copies of chromosomes Mitosis: replicated DNA is separated into two identical daughter cells Fluorescence microscope  DNA blue  Microtubules green  Active cytoskeleton red Mitotic spindle  Made of microtubules that control chromosomes movement during mitosis

Cell Biology   

Begins in centrosome – replicated two structures which migrate to opposite ends of the cell creating the mitotic spindle. Aster – extends from each centrosome Kinetochores – protein complexes associated with centromeres

Mitosis Prophase, Prometaphase, metaphase, anaphase, telophase, cytokinesis Prophase  Chromosomes condense and become visible (under light microscope)  Centromeres form  Mitotic spindle and aster form  Centrosomes move away from each other Prometaphase  Nuclear envelop fragments  Chromosomes more condensed  Spindle microtubules make contact with chromosomes  Kinetochore forms Metaphase  Centrosomes are at opposite poles  Chromosomes position at the metaphase plate  Kinetochores of each chromosomes attach to microtubules from opposite poles Anaphase  Shortest phase  Sister chromatids separate and move to opposite poles  Anaphase ends with each pole having equal number of chromosomes Telophase and cytokinesis  Daughter chromosomes arrive at poles of spindle  Daughter nuclei forms  Chromosomes uncoil  Spindle disassembles  Cytokinesis dived the cytoplasm in two - Cell division is complete – Asexual reproduction - MITOSIS  Reproduction for unicellular organisms – plants  Offspring is genetically identical to parents - Binary fusion - Budding - Sporulation Sexual reproduction – MEIOSIS  2 parents contribute genes via gametes (sperm and egg) to produce unique offspring  Genetic diversity  Promotes survival – reduce risk of species being destroyed by a single pathogen  Removes bad genes from the population Homologous pair: set of chromosomes which are similar; paternal and maternal Sister chromatids: both stands of the chromosomes Non-sister chromatids: a strand from each homologous pair Haploid gametes: both contain 23 chromosomes

Cell Biology Diploid zygote = 2n (46 chromosomes)

Meiosis   

Replication of chromosomes Meiosis I and meiosis II Results in 4 daughter cells that has half the number of chromosomes as the parent cell and are all genetically distinct – a haploid set of unreplicated chromosomes

Meiosis I Prophase I, Metaphase I, Anaphase I, Telophase I and cytokinesis  Prophase I: non-sister chromatids broken at a corresponding point, cross over and is repairs (chiasmata)  Metaphase I: Homologous pairs line up on the metaphase plate  Anaphase I: pairs of homologous chromosomes separate doing to each pole  Telophase I: each consists of two sister chromatids *No DNA replication occurs before meiosis II as they have already formed 2 s c* Meiosis II  Prophase II: Spindle forms, chromosomes move towards metaphase plate  Metaphase II: sister chromatids are arranged at the plate: due to crossing over they are no longer identical // Kinetochores of sister chromatids attach to the microtubules  Anaphase II: SC separate  Telophase II: chromosomes reach poles // nuclei forms // chromosomes begin to decondense  Cytokinesis: The separation of the cytoplasm

Cell Biology

Genetic diversity  Mutations in DNA – different versions of genes called alleles  Reshuffling of alleles during sexual reproduction  Independent assortment of chromosomes – chromosomes orientate randomly during metaphase I (meiosis) – large number of possible chromosome combinations  Crossing over – chiasmata // creates recombinant DNA (form both M and P) 1 to 3 per chromosomes  Random fertilisation of sperm...