Cell Division and Reproduction: Understanding The Mechanisms

SBI3U – Grade 11 Biology – Cell Division and Reproduction Chapter 4: Cell Division and Reproduction Does not contain DNA function, RNA and protein synthesis, does not contain selective breeding and cloning (4.3) 4.1 – Cell Division and Genetic Material ● All new cells come from existing cells so traits = pass on genetic material from parent cell to daughter cell. Genetics: How the genetic info of living organisms = passed from one generation to next Cell Theory: 1. All living things are composed of one or more cells 2. Cells are the smallest units of living organisms 3. New cells come only from pre-existing cells by cell division The Cell Cycle: ● Cells reproduce through growth + division process called the cell cycle ● Somatic cells go through cell cycles Somatic cells: body cells of plants and animals excluding reproductive cells ● ● 1. 2. 3. ● Cell cycle = continuous production of new cells 3 functions of cell division: Growth of organism Repair of damaged tissues and organs Maintenance to replace dying/dead cells Duration of cell cycle depends on type of cell ○ Healthy actively dividing animal cells = 12-24 hrs. Stages of the Cell Cycle: 1. Cells go through their usual processes during the interphase, growing and duplicating their genetic material in order to get ready for the cycle's next phase. lengthiest phase. 2. Mitosis: Stage when cell’s nucleus + genetic material divide. NUCLEAR DIVISION 3. Cytokinesis: Begins near the end of mitosis, involves division of cell cytoplasm + creation of new cell. ● Cycle checkpoints monitor growth ○ Ensure that cycle continues when it should ○ If something interferes with signals = uncontrolled growth ○ EX: Cancer = uncontrolled , cancerous cells keep dividing = mass cells = tumor Interphase: ● Cell grows, develops into a mature, functioning cell, duplicates DNA, and prepares for division. G1 (Growth 1) ● Major growth period of cell ● Cell synthesize new molecules in preparation for next phase in cell cycle S (Synthesis) G2 (Growth 2) ● Cellular DNA = copied / replicated ● DNA exists in uncondensed fibers called chromatin ● Cells that complete this phase enter GS ● Final phase of interphase ● Cell synthesizes more molecules prior to mitosis/cell division Mitosis: ● Cell’s copied genetic material separates cell prepares to split into 2 ● Key idea of mitosis = separate cell’s replicated DNA accurately ○ Allow cell’s genetic information to be passed into new cells intact = 2 identical cells PROPHASE: ● Cell chromatin condense into chromosomes which contain DNA ○ Each DNA contains 2 copies (because of replication in interphase) ● Nuclear membrane breaks down ● Nucleolus disappears ● Spindle fibers form at centrosomes and move to opposite sides of the cell. METAPHASE: ● Spindle fibers guide chromosomes to the equator of the cell. ● Spindle fibers reach opposite sides of cell, attach to centromere of each chromosome ANAPHASE: ● ● ● ● Centromere splits apart, sister chromatids separate from each other Separated sister chromatids now = chromosomes Spindle fibers shorten, pull the chromosomes to opposite sides @ end of anaphase, one complete set of chromosomes = gathered @ each pole of cell TELOPHASE: ● ● ● ● ● Chromosomes have reached opposite poles of cell Chromosomes start to unwind into less visible chromatin Spindle fibers break down Nuclear membrane forms around new set of chromosomes Nucleolus reforms within each nucleus. Cytokinesis: ● Division of cytoplasm ● Indentation forms in the cell membrane along the equator of the cell. ○ Indentation deepens until cell = pinched in 2 ○ Cytoplasm divides equally between 2 halves of cell ○ Ends with separation of 2 genetically identical daughter cells ○ Daughter cells are now in G1 of interphase Animal cells: Cytokinesis = by microfilaments that pinch cytoplasm Plant cells: Plant cells have a stiff, too-strong cell wall that covers their membrane. Instead, a brand-new structure known as a cell plate develops between two daughter nuclei. Then, the cell plate's two sides begin to create cell walls. 2 genetically identical plant cells are formed after the new cell wall is finished. Prokaryotic cells: No nucleus, binary fission for cell division. Both copies of DNA that are duplicated adhere to the membrane. DNA molecules that are connected are pushed apart as a cell membrane expands. The Structures of Genetic Material: DNA ● ● ● ● ● ● ● = deoxyribonucleic acid Nucleic acid containing sugar caller deoxyribose = 5 carbon atoms Made up of long strands that form shape called double helix During most of cell cycle DNA = strands of chromatin fiber When mitosis begins, chromatin condenses= distinct chromosomes A polymer, made of monomers = nucleotides Individual units of each strand of DNA = nucleotides Nucleotide: 1. Sugar molecule: deoxyribose 2. Nitrogen base: A,T,C,G 3. Phosphate group SUGAR PHOSPHATE BACKBONE: ● Sugar + phosphate always same but base changes ● Since S and P = constant, they form backbone for bases to hang on. = Sugar phosphate backbone ● Most living cells, DNA = double-stranded molecule, 2 strands = parallel NITROGENOUS BASE (4): ● 2 categories: ○ Pyrimidines = 1 ring of atoms ■ C – cytosine ■ T – thymine ○ Purines = 2 rings of atoms ■ A- ademine ■ G- guanine ● Certain nitrogen bases pair up with others: ○ A always with T ○ C always with G ● Pyrimidines always with purines = complementary base pairs ● If DNA mutation / genetic mutation, = change in nucleotide sequence Genome: Complete DNA sequence in every cell of organism ● Type of bonding involved in base pairing = hydrogen bonds ● Weak enough so that molecule can be pulled apart when cell divides ● Strong enough to hold DNA strands together REPLICATION: ● When DNA = replicated during interphase, double helix unwinds, each strand of DNA = template for new strand ● When DNA = copied, each new double-strand of DNA molecules contains ● one original strand + one new = Semi conservative because the DNA molecule conserves half of original DNA. Chromosomes are Paired: ● # of individual chromosomes in cell varies between species ● # doesn’t reflect complexity of organ ● Human somatic cells = 46 chromosomes = 23 pairs ○ Each pair = 1 from mom, 1 from dad ○ 23rd chromosome pair = sex chromosomes Sex Chromosomes: organism an X or Y chromosome, determines the genetic sex of an Human Female = XX Human Male = XY ● Remaining 22 = autosomes Autosomes: Chromosome that is not involved in determining sex of organism ● Chromosomes = paired based on sharing similar characteristics Homologous Chromosomes Contain Alleles: Homologous Chromosomes: Pairs of chromosomes that appear similar in terms of length, centromere location, + banding pattern ● Not identical with each other, carry different genes for same traits (hair color, eye color etc.) ● = Allele Allele: Different forms of the same gene. Gene: Sections of DNA that contain genetic information for the inheritance of specific traits Examining Chromosomes: The Karyotype: Karyotype: Particular set of chromosomes that an individual has ● To prepare, cell sample = collected + treated to stop cell division during ● ● ● ● metaphase of mitosis Sample = stained, makes banding pattern on chromosomes visible Chromosomes = sorted + paired Autosomes = numbered 1 – 22, sex chromosomes labeled as X or Y Y chromosome = smaller than X chromosome. 4.2 Sexual Reproduction: ● When somatic cells = reproduced by mitosis, new daughter cells have same genetic info as parents Asexual Reproduction: Reproduction that only needs 1 parent (e.g. bacteria) ● If mitosis = only way of make more cells, we would be exact clones of ourselves ● Except for identical twins, no person = exact genetic copy of other Sexual Reproduction: Reproduction that needs 2 parents and produces genetically distinct offspring Haploid and Diploid Cells in Sexual Reproduction: ● Sexual Repro = fusion of male repro cell + female repro cell Gametes: Male/female reproductive cells. Male gamete = sperm. Female gamete = ovum Zygote: Result of when the 2 gametes fuse Fertilization: In humans, the joining of male and female haploid gametes ● Because 2 gametes = zygote, gametes must have half # of chromosomes as parent cells = haploid Haploid: Cell that has half number of chromosomes as parent cell. Haploid number of chromosomes in species = n (n also describes the pairs of chromosomes in an organism) Diploid: Cells that contain pairs of chromosomes (includes all somatic cells) ● ● ● ● ● Human gametes = haploid → n = 23 chromosomes After fertilization zygote cell = diploid → 2n chromosomes ( n from female, n from male) Diploid number in humans = 46 When 2 human gametes combine = 23 pairs of homologous chromosomes See page 169 figure 4.12 Meiosis – Producing Haploid Gametes: ● Process that produces gametes with haploid # of chromosomes = meiosis Meiosis: Cellular process that produces cells containing half the # of chromosomes as the parent cell ● 2 outcomes: 1. Genetic Reduction: Produces daughter cells with half the # of chromosomes of parent cell 2. Genetic Recombination: Different alleles = combined = offspring that are genetically different from one another / parents = genetic variation in population INTERPHASE ● Cells that will divide by meiosis go through growth/synthesis phase before divide ● = replication of chromosomes ● @ start of meiosis, cell contains duplicated chromosomes PHASES OF MEIOSIS ● Prophase, metaphase, anaphase, Telophase X 2 ● 2 cycles of 4 phases → meiosis I, meiosis II MEIOSIS I: PROPHASE 1 ● Each pair of homologous chromosomes (1 from each parent) lines up side by side) = synapsis Synapsis: The alignment of homologous chromosomes ● @ synapsis, homologous chromosomes = held tightly together along their ● ● lengths Exchange genetic information = genetic diversity Centrosomes move to poles of cell, spindle apparatus forms METAPHASE 1 ● Pairs of homo chromosomes line up along equator of cell (they have now exchanged genes) ● Spindle fibers attach to centromere of each homologous chromosome ANAPHASE 1 ● Homologous chromosomes separate, move to opposite poles of cell ● Sister chromatids = still held together so centromeres don’t split like in mitosis ● Result = single chromosome (2 sister chromatids) from each homologous ● pair moves to each pole of the cell. ○ Homologous pair = 2 chromosomes Chromosome number = 2n (diploid ) to n (haploid) TELOPHASE 1 ● ● ● ● ● Homologous chromosomes begin to uncoil Spindle fibers disappear Cytokinesis takes place Nuclear membrane forms around each group of homologous chromosomes = 2 cells Each new cell = haploid MEIOSIS II: ● Phases = similar to phases of mitosis ● Difference = cell that undergoes division in meiosis II = haploid instead of diploid METAPHASE 2: ● Haploid # of chromosomes line up @ equator ANAPHASE 2: ● Sister chromatids = pulled apart @ centromeres by spindle fibers, chromosomes move to opposite sides of cell TELOPHASE 2: ● Chromosomes reach poles ● Nuclear membrane + nuclei reform At the end of meiosis II, Cytokinesis occurs = 4 haploid cells with n # of chromosomes Comparison of Mitosis and Meiosis: ● Mitosis = 1 set of division phases = 2 diploid daughter cells that are identical ● Meiosis = 2 sets of divisions = 4 haploid daughter cells that are not identical ● Meiosis = important for humans because = genetic variation = genetic diversity Gamete Formation in Animals: Spermatogenesis: the process of producing male male gametes (sperm) in mammals Oogenesis: process of producing female gametes (eggs) in mammals Spermatogenesis: ● ● ● ● ● ● ● ● Most male animals, meiosis takes place in testes Process starts with diploid cell called spermatogonium @ puberty, spermatogonia reproduce by mitosis These cells go through meiosis make 4 haploid cells After meiosis 2, cell develops into mature sperm Nucleus + other molecules = organized into “head” of sperm Midsection holds mitochondria Tail = flagellum for locomotion Oogenesis: ● Most female animals, meiosis = ovaries ● Oogenesis starts with diploid cell called oogonium ● Before birth, oogonia reproduce by mitosis, start meiosis, but stop @ prophase 1 ● Meiosis I will continue for 1 cell each month @ beginning of puberty ● Oogenesis involves unequal division of cytoplasm ○ Cell that gets more cytoplasm after 1st division continues through meiosis I and II = viable egg ○ = more nutrients will support zygote after fertilization ○ Other smaller cell = polar body, will degenerate ● Final stages of meiosis II not completed until fertilization by sperm cell occurs ● When meiosis II = complete, mature egg + another polar body = produced. ● Haploid nucleus of the egg cell fuses with haploid nucleus of sperm cell to complete fertilization ○ = diploid zygote Multiple Births: ● If more than 1 egg = released = more than one baby @ once Fraternal Twins: 2 eggs released, both are fertilized = twins that are like siblings Identical Twins: Single zygote divides into 2 separate bodies in first few days of development = genetically identical twins The Importance of Meiosis for Genetic Variation: ● Outcome of meiosis forms genetically distinct haploid gametes (mitosis = identical) ● Each diploid cell has 2 copies of each chromosome (one from mom, one from dad) ● Genetic variation: ○ creation of gametes that carry different combinations of ○ maternal / paternal chromosomes = independent assortment exchange of genetic material between maternal and paternal chromosomes in crossing over Independent Assortment: ● @ metaphase 1, homologous pairs line up along equator of cell ● Chromosomes from mom/dad = oriented on random sides of pole, some facing this way, some that ● Orientation of chromosomes = independent of orientation of others ● Depending on how chromosomes line up, number of different combinations of chromosomes = found in gametes ● Number of genetically distinct gametes produced from diploid cell = 2n ● (n = # of chromosome pairs in diploid cell) ● Humans produce 223 or 8388608 genetically distinct gametes Crossing Over: ● During prophase 1 (synapsis), non-sister chromatids of homologous chromosomes exchange pieces of chromosome Crossing over: exchange of chromosomal segments between a pair of homologous chromosomes ● can occurs @ several points along non-sister chromatids ● section of chromosome that = crossed may have thousands of genes ● Result = individual chromosomes have some stuff from mom and some from dad ● = genetic diversity of gametes produced Errors During Meiosis: ● 2 processes that = genetic variation, (independent assor, + crossing) = potential chromosomal abnormalities ● Errors that occur during meiosis = gametes that don’t survive ● If survive + fertilized = zygote ● Every cell in offspring = from one zygote cell, all cells in offspring will contain the error ● 2 types of chromosomal errors in meiosis: 1. Changes in chromosome structure 2. Changes to chromosome number Errors Caused by Changes in Chromosome Structure: ● When crossover, chemical bonds that hold DNA in chromosome = broken + reformed ● Sometimes chromosome no reform proper ● Or non-homologous pairs crossover = chromosome that has genes no normally on that chromosome Error Chromosome Structure in Description Example of Genetic Disorder Deletion Piece of deleted chromosome = Cri du Chat Deletion in chromosome 5. Duplication Section of chromosome Charcot-Marie-Tooth appears 2 or more times in DiseaseDuplication on a row. 17 Inversion A section of a chromosome FG SyndromeInversion = inverted (upside down) chromosome Translocation A segment of one Chronic chromosome becomes LeukemiaTranslocation attached to a different chromosome 9 and 22 chromosome chromosome of X Myelogenous between Errors Caused by Changes in Chromosome Number: ● When homologous chromosome pairs / sister chromatids don’t separate proper = nondisjunction Nondisjunction: Failure of homologous chromosome pairs / sister chromatids to separate during meiosis ● occurs in anaphase I or II of meiosis ● Anaphase 1: occurs when homologous chromosome pairs do not separate to ● opposite poles ○ Instead 1 whole pair = pulled toward same pole Anaphase 2: occurs when sister chromatids no separate to opposite poles ○ both sister chromatids = pulled to same pole ○ produces gametes that have too few / too many chromosomes Genetic Disorders Associated with Chromosome Number: ● Many genetic disorders = incorrect # of chromosomes ● Down Syndrome = extra piece on chromosome 21 ● This incidence o non-disjunction increases with maternal age Trisomies and Monosomies: Monosomy: Loss of chromosome due to nondisjunction (missing one chromosome of a homologous pair) Trisomy: Gain of extra chromosome due to nondisjunction (most common in 21, 18, 13, sex chromo) Conditions Syndrome Number of Live Births Trisomy 21 Down 1 in 800 Trisomy 18 Edward 1 in 18 000 Trisomy 13 Patau 1 in 15 000 XXY Klinefelter 1 in 1000 males XYY Jacobs 1 in 1000 males XXX Triple X 1 in 1500 females X Turner 1 in 5000 females Prenatal Genetic Testing: ● = test performed on fetus (baby developing in womb)to find genetic-based abnormalities ● Before only for women in high-risk situations ○ women 35+ yrs ○ women with family history of genetic disorders ● Now, doctor can let any women go ● Ethical dilemmas: could end pregnancy, discrimination against ppl with disabilities Prenatal Testing Procedures: ● Blood tests + ultrasound can give info about potential physical + chromosomal abnormalities = non-invasive ● Non-invasive because do not require direct sampling of fetal cells ● Invasive test = collect DNA from sample of fetus (EX: Amniocentesis, Chorionic Villus Sampling) 4.3 Reproductive Strategies and Technologies: Reproductive Strategies in Agriculture: Artificial Insemination: Sperm is collected and concentrated before being introduced into the female’s reproductive system. Advantages: ● Makes semen from high-quality males more widely available through breeders and online sources ● Farmers can choose desirable traits for male parent ● Instead of shipping the whole cow, just ship semen Embryo Transfer: Process where an egg that has been fertilized = artificially transferred into a recipient female’s uterus. Advantages: ● Embryos = shipped easily (don’t need to ship the whole cow) ● Ensured quality of embryos Disadvantages: ● Animals born and raised in native environment do better than those that are imported Reproductive Technologies for Humans: ● Assisted Reproductive Technologies (ART) = for couples who not able to conceive child ● Artificial Insemination: Sperm collected + concentrated from women’s partner / sperm bank, introduced into woman’s reproductive system In Vitro Fertilization: ● 1. 2. 3. 4. for women who have blocked Fallopian tubes Immature egg = received from the woman Eggs = combined with sperm in laboratory glassware After fertilization, developing embryo = placed in uterus = Test tube babies Other Terms: Chromosome: A structure in the nucleus that contains DNA Sister Chromatid: one of the 2 chromosomes that are genetically identical and are held together at the centromere Centromere: The region where the 2 sister chromatids are held together in a chromosome Spindle fiber: a microtubule structure that facilitates the movement of chromosomes within a cell Centrosome: A structure that helps to form the spindle fibers