Unit C: Cell Division - Section 1.1 Cell Cycle -Cells are limited in size due to plasma membrane. Cytoplasm grows faster than membrane and requires more nutrients to be absorbed and waste excreted resulting in a cap; new growth must come from addition of new cells -“Theory of spontaneous generation”: living organisms can arise from non living matter (INCORRECT) - CORRECT: Rudolph Virchow discovered cell division was the cause for life STRUCTURES: -DNA: molecule of nucleic acid within chromosomes that governs processes of heredity stored -Haploid: Single, unpaired chromosomes (Gametes like egg and sperm n=23) -Diploid: Paired homologous chromosomes that carry same gene (2n=46 or 23 pairs) -Polyploid: sets of more than two homologous chromosomes -Chromosomes: a length of DNA and it’s proteins made from condensed chromatin -Sex chromosomes: determine sex (Male XY, Female XX) -Autosomes: chromosomes that don’t influence sex -Homologous chromosomes carry genes at the same location -not identical, carry different allele of the same genes, but share same length, centromeres location and banding pattern -Chromatin: long, intertwined strands of DNA that form fibres around histones -Centromere: pinched region on a condensed chromosome; responsible for movement of Chromosomes -Genes: Areas of DNA that contain specific genetic info -Alleles: Different forms of the same gene Karyotype: layout of a person’s individual set of chromosomes - Scientists use chemicals to halt cell cycle while they’re visible, stain, photo, and organize them To diagnose genetic conditions Cell Cycle (Interphase): “sequence of events from one cell div to the next” Growth Stage • Maintains regular metabolic function and prepares for next division • G1: Rapid growth and its processes (interphase ends) • Synthesis: DNA in chromatin replicates a second identical set (sister chromatids) • G2: energy depleted; rebuilds energy reserve make protein & structures required for next division Section 1.2 - Reproduction of Somatic Cells (MITOSIS) Cell Division: • Mitosis: division of genetic material and nucleus into 2 complete and separate sets • Cytokinesis: separation of cytoplasm and organelles into two separate cells ◦Result: 2 new daughter cells with identical info as parent ◦3 important functions: 1. Growth: enables organisms to grow from single cell zygote into a mature organism 2. Maintenance: produce new cells to replace worn out/dead cells 3. Repair: regenerate damaged cells 4 Major cellular events: - Genetic material of parent cell duplicated - Replicated chromatin condenses > chromosomes - One complete set of chromosomes divides into two new nuclei - Cell cytoplasm divides to create two complete daughter cells -ability to start/stop cell production ensures proper development. Uncontrolled leads to tumours/cancer 4 Main Phases: -Prophase: - chromatin condenses into visible chromosomes - centrioles move to opposite poles and spindle apparatus forms between them - spindle app: microtubules that facilitate movement of chromosomes -Metaphase: - spindle fibres attach to centromere and guide chromosomes to equator, sister chromatids aligned facing each pole -Anaphase: - centromeres split and sister chromatids pulled apart - spindle fibres shorten and pull chromatids to poles - other microtubules lengthen and force poles of each cell away from each other -Telophase -begins when chromatids reached opposite poles. -chromatin unwinds into longer, less visible strands of chromatin. -spindle fibres break down -nuclear membrane forms around each set of chromosomes; nucleolus forms within each nucleus Cytokinesis: Equal division of the cytoplasm/organelles where indentation forms & deepens until cell is pinched in two. New daughter cells are now in G1 of interphase. Structural differences in Plants: - Plants don’t have centrioles, but form spindle apparatus - Plant’s cell wall is much stronger and doesn’t pinch. A cell plate forms b/w daughter nuclei - Cell plate extends across and is reinforced by addition of cellulose and proteins to create a new cell wall Section 1.3 The Formation of Gametes (Meiosis) Sexual reproduction: a gamete from the male and female organism (aka germ cells) fuse to create a new cell/zygote that contains genetic info from both parents w/ same # of chromosomes. Meiosis is the process that produces haploid gametes from diploid cells in ovaries & testes. Genetic variation is ensured by two ways: 1. Creation of gametes carry different combinations of maternal and paternal chromosomes 2. Exchange of genetic material between maternal and paternal chromosomes 2 Key outcomes: 1. Reduction Division: cell division that produces daughter cells w/ fewer chromosomes than parent cells. 2. Recombination: product of having offspring w/ different combinations of genes Interphase: - Growth and synthesis phase where chromosomes are replicated. - Each chromosome is made up of a pair of identical sister chromatids held by a centromere Prophase I: - homologous chromosomes align side-by-side aka synapsis where crossing over occurs - Pair of homologous chromosomes = 4 chromatids called a tetrad - the middle portion of a tetrad two homologous but non-identical chromatids are called non-sister chromatids Metaphase I: - spindle fibre attaches to centromeres of each pair of sister chromatids and guide them to equator and are arranged as homologous pairs Anaphase I: - spindle fibres shorten causing homologous chromosomes to separate - single chromosomes from each pair are pulled to poles Telophase I: (may skip from A1 to Meiosis II) - homologous chromosomes uncoil and spindle fibres disappear; cytoplasm divides and nuclear membrane forms around each group of homologous chromosomes - Two haploid cells are formed (one set of sister chromatids) Meiosis II: cells entering are replicated -> end result is 4 unreplicated haploid cells Independent Assortment: cells are assorted individually from each other (Metaphase I) resulting in different combinations from both parental chromosomes Crossing Over: during Prophase I, non-sister chromatids can exchange pieces of chromosome resulting in an individual carrying both maternal/paternal genes Crossing Over Nondisjunction Non-disjunction: When chromatids don’t separate properly ◦Anaphase I: (homologous chromosome pairs) one entire pair is pulled toward same pole together ◦Anaphase II: (sister chromatids) pulled toward the same pole together ◦Monosomy: gamete is missing one chromosome of a homologous pair ‣ Turner’s syndrome: undeveloped female characteristics ◦Trisomy: gain of an extra chromosome ‣ Down Syndrome: physical growth impairment; decreased cognitive ability, other health defects Spermatogenesis: takes places in the testes beginning at puberty; undergoes typical meiotic process beginning with a spermatogonium (2n) that divide into two daughter cells • One replenishes spermatogonium population • 2nd develops into primary spermatocyte ◦Undergoes Meiosis I to form two secondary spermatocytes ‣ Undergoes Meiosis II to form four spermatids • Head: acrosome (nucleus & enzymes), • Mid: mitochondria for energy • Tail: flagellum for locomotion Oogenesis: takes place in the ovaries. Starts with an oogonium (2n) that undergoes mitosis to form two primary oocytes . 3 months after conception, two million oocytes are found in ovaries. These are arrested in prophase I and remain until puberty. • Every month after puberty, one primary oocyte undergoes Meiosis, however involves an uneven division of cytoplasm, asymmetrical cytokinesis. ◦Cytoplasm not evenly divided between daughter cells. The one that receives the most is called secondary oocyte, while the lesser is the first polar body ‣ Polar bodies are non-functional and degenerate, but may undergo a second division ◦Meiosis II occurs if egg is fertilized, but cytoplasm again is not evenly divided. This leaves one mature egg and second polar body ◦Mature egg contains the most nutrients that the zygote can use prior to implantation Meiosis I and II are not always continuous. In females, a decade separates Meiosis I and II. Before birth: Primary oocytes begin meiosis I, but cell division stalls in prophase I (suspended until puberty). Puberty: a hormone signal triggers a single primary oocyte to resume and complete Meiosis I. -Secondary oocyte is released from ovary -> Fallopian tube. - arrested at metaphase II until fertilization. -unfertilized = halt -fertilized > second meiotic division = ovum + second polar body (n) egg + (n) sperm = 2n zygote Fraternal twins: occur when secondary oocyte is released, fertilized, and both are implanted in uterus Identical twins: occur when a single zygote divides into separate bodies during embryonic development Section 1.4 Reproductive Strategies • Asexual Reproduction: parent organism produces genetically identical offspring Reproduction in Prokaryotes -bacteria have single Circular chromosome with no nucleus and cannot undergo mitosis. -uses Binary Fission to asexually reproduce exponentially -bacterial DNA attaches to cell wall > cell elongates and septum forms > separates & creates two identical daughter cells -Conjugation: bacteria can reproduce by transferring genetic material from cell-to-cell contact through a bridging structure, pilus. -creates cells with new genetic combination and provides a chance for environmental adaptation Types of Asexual Reproduction: • Budding: complete/miniature version of parent grows from parent’s body which separates to become and individual organism. (Ie. Hydra) • Vegetative reproduction: a new plant grows from a modified stem (ie. strawberry) • Fragmentation: New organisms formed from a fragment of parent (ie. potatoes) • Parthenogenesis (animals): Unfertilized egg develops into an adult (ie. bees, sea stars) • Spore: reproductive cells capable of developing into a new organism w/out fusion with another cell; contains genetic material and cytoplasm in a sheath or wall ◦May be n or 2n and may be produced asexually or by meiosis Alternation of Generations: consist of a haploid and diploid generation (only in plants) • sporophytes (2n) produce 1+ haploid (n) spores which self fertilized into a gametophyte (n) • gametophyte (n) produces male and female gametes (n) that fuse and fertilize into another sporophyte (2n) Vascular plants (plants that have tube transport system) Has a diploid sporophyte dominant generation. Non-vascular plants (moss) are haploid gametophyte dominant generation FERN Moss: • Green mat gametophyte (n) grows a stalk (sporophyte) where spores are cast and grows into leafy gametophyte (produce sperm that swim to eggs) → fertilization occurs and a new stalk develops Conifer: • tree = diploid sporophyte • microscopic structures in cones = haploid gametophytes • female gametophyte develops from a spore (produced by female cone structure at the top of each scale and stays there. • male gametophyte produced on the male cone (smaller) which is released in the pollen cast by male cones. • pollen dispersed by wind > reaches female cone and fertilizes > zygote forms a seed attached to scale of female cone MOSS Cnidaria: - alternates between asexual & sexual reproduction - adult forms: non motile polyp & free-swimming medusa - either may be dominant/exclusive in different species Advantages of Sexual Reproduction: CONIFER 1. Offers population to adapt to changing environment (offspring able to resist parasites/toxins, take advantage of new food sources) 2. Reduced competition among siblings if genetically diverse 3. Pairing and crossing over of chromosomes offer opportunities to replace and repair damaged chromosomes Advantages of Asexual Reproduction: 1. Quicker and doesn’t require second parent org Cnidaria 2. Requires less energy 3. Many forms like budding help to maximizes chances of offspring’s survival. Daughter cells don’t separate until fully capable
