BIO211 General Biology Lecture 5: Cell Division PDF

BIO211 (General Biology) Lecture 5: Cell division Robert Belshaw [email protected] We will look at cell division in 2 sections a) Cell division for growth by mitosis – cell cycle and how errors can lead t...

BIO211 (General Biology) Lecture 5: Cell division Robert Belshaw [email protected] We will look at cell division in 2 sections a) Cell division for growth by mitosis – cell cycle and how errors can lead to cancer (next PPTs) b) Cell division for reproduction by meiosis – its functions © McGraw Hill 1  Cellular division All cells come from cells Cellular division is necessary both for both … 1. … growth (and repair) of multicellular organisms 2. … to make new organisms (reproduction) © McGraw Hill 2  Reminder: DNA is arranged into genes All genes are on chromosomes (long polymer) Number of genes and chromosomes in organism varies. Humans have around 21,000 genes on 46 chromosomes https://my.clevelandclinic.org/health/body/23064-dna-genes--chromosomes © McGraw Hill 3  Both new cells need a copy of all the genes so the long strings of DNA copy and then condense to form visible chromosomes DNA is wound around proteins called histones to form nucleosomes These thin threads are called chromatin These thin threads copy Before cell division, they condense (compact; folds) into short fat chromosomes These can be more easily passed into daughter cells (so that each gets a full set of the DNA and the genes) © McGraw Hill (nucleosomes): ©Don W. Fawcett/Science Source 4  How can 2 daughter cells have the same DNA as each other and the parent cell? Before division, each chromosome duplicates (copies) itself to make 2 sister chromatids (joined at the centromere) Each sister chromatid has identical DNA (genetic information) The sister chromatids will separate and be called daughter chromosomes All genes in the parent cell are thus passed to the 2 daughter cells © McGraw Hill 5  Chromosomes condense: more images DNA double DNA and Chromatin Supercoiled helix histones DNA  Each human cell has 46 chromosomes. When condensed and arranged by size, they look like this (called a karyotype) https://www.mun.ca/biology/scarr/Human_Karyotype.html © McGraw Hill 7  I. Division for growth © McGraw Hill 8  Mitosis Mitosis traditionally divided into four phases: 1. Prophase — chromosomes are visible under microscope in condensed pairs 2. Metaphase — chromosomes line up along equatorial plate (middle) 3. Anaphase — chromatids are pulled to opposite poles of cell (apart) 4. Telophase and Cytokinesis — two distinct cells are visible under the microscopes Remember, before mitosis the chromosomes have already copied to make 2 chromatids © McGraw Hill 9  The microtubule spindle pulls apart the chromosomes (or chromatids) Kinesin motor proteins slide the microtubules past each Microtubules other to push poles apart shorten by depolymerising and are pulled by motor proteins on kinetochore These microtubules attach towards pole. This to opposite ends of the pulls chromatids cell apart Ignore names https://www.nature.com/scitable/topicpage/mitosis-14046258/ Watch videos at https://www.youtube.com/watch?v=IvJrDsRuWxQ watch out for signalling on the microtubules and https://www.youtube.com/watch?v=o1eWJZPDmxE © McGraw Hill 10  Mitosis & Cytokinesis 1. During prophase, chromosomes condense and spindle fibers form  Mitosis & Cytokinesis 2. During metaphase, chromosomes line up in the middle of the cell  Mitosis & Cytokinesis 3. During anaphase, sister chromatids separate to opposite sides of the cell  Mitosis & Cytokinesis 4. During telophase, the new nuclei form and chromosomes begin to uncoil  Mitosis & Cytokinesis After chromosomes separate, the whole cell divides – called Cytokinesis Animal cells use actin and myosin filaments to contract and ‘pinch off’ 2 new cells Plant cells build a new cell wall separating the daughter cells Good video on this at https://www.youtube.com/watch?v=mzeowbIxgwI  The cell cycle: Interphase Majority of the cell cycle Cell performs its usual functions (time varies widely depending on cell) Three stages: G1 — Growth Cell doubles its organelles Accumulates materials for DNA synthesis Makes decision whether to divide or not Can go into G0 — arrested — and not divide Nerves are permanently in Go S — DNA Synthesis Results in each chromosome being composed of two sister chromatids G2 — Growth Extends to beginning of mitosis Synthesizes (makes) proteins needed for cell division © McGraw Hill 16  Cells divide at different rates The rate of cell division varies with the need for those types of cells Some cells are unlikely to divide (in G0), e.g. Nerve cells  The cell cycle control system (goes wrong in cancer) Cell cycle must be controlled Ensures that the stages occur in order and that the cycle continues only when the previous stage is successfully completed Main cell cycle checkpoints G1 checkpoint. DNA integrity checked — if repair is not possible, apoptosis occurs or goes into G0 (no division) G2 checkpoint. Checks that DNA has been copied Mitotic stage checkpoint (M). Between metaphase and anaphase. Checks that chromosomes are attached to the microtubule spindle © McGraw Hill 18  Control of cell cycling Signal — a molecule that stimulates or inhibits an event Cell cycling signals either help push cells through the cell cycle or help block it External signals come from outside the cell: Examples Epidermal growth factor (EGF) stimulates skin near an injury to go through the cell cycle and produce new cells to repair the injury. Platelets in a blood clot PDGF secrete PDGF (Platelet Derived Growth Factor) Hormone estrogen stimulates lining of the uterus to divide and prepare for egg implantation Contact inhibition — cells stop dividing when they touch Internal signals come from inside the cell Cyclins are proteins present only during certain stages of the cell cycle. A cyclin activated in one stage of the cycle will help the cell pass into the next stage © McGraw Hill 19  Cell signals: e.g. growth factors and cyclins (We shall look at signaling pathways in the next PPT) © McGraw Hill 20  Some signals help cell cycling; others inhibit it (more on this later – in the cancer PPT) Concentration of different cyclins changes through the cycle: broken down in the next phase Inhibiting (stopping) Activating (helping) Detailed video explaining all the steps in the cell cycle at https://www.youtube.com/watch?v=nEMMKzYQf9A Also KahnAcademy video at https://www.youtube.com/watch?v=542CMooowNY © McGraw Hill 21  Apoptosis (’programmed cell death’) © McGraw Hill (photo): ©Steve Gschmeissner/Science Source 22  Apoptosis (’programmed cell death’) Caused by internal or external signals (= “ commanded to die: commit suicide”) Helps keep correct number of cells Normal part of growth and development Final cell fragments are engulfed by white blood cells © McGraw Hill (photo): ©Steve Gschmeissner/Science Source 23  II. Division for reproduction © McGraw Hill 24  Reproduction is easy for single-cell organisms https://byjus.com/biology/studying-binary-fission-in-amoeba-and-budding-in-yeast-with-the-help-of-prepared-slides/ © McGraw Hill 25 More complicated for multicellular organisms Multicellular organisms produce gametes, e.g. humans Egg (gamete) from woman (female) meet sperm (gamete) from man (male) Female reproductive system © McGraw Hill 26  Second form of cell division: meiosis Meiosis reduces the chromosome number from diploid (2n) to haploid (n) Gametes (egg and sperm) have only one member of each homologous pair 1. 1. Spermatogenesis produces sperm in the testes. 2. 2. Oogenesis produces eggs in the ovaries Egg and sperm join to make a diploid zygote © McGraw Hill 27  This is why we (and most Eukaryotes) have 2 sets of chromosomes (called ‘Diploid’) Meiosis Get XX you are female Get XY you are male https://www.genome.gov/genetics-glossary/Diploid © McGraw Hill 28 © McGraw Hill 29 © McGraw Hill 30  Meiosis (creating gametes) Remember, chromosomes are paired = Homologous chromosomes are members of a pair of chromosomes Have the same size, shape, and construction (location of centromere) Contain the same genes for the same traits © McGraw Hill 31  Meiosis Remember, each chromosome has duplicated before meiosis Two divisions: Meiosis I Homologous pairs line up in center of cell Homologous chromosomes of each pair separate (anaphase 1). Meiosis II Sister chromatids separate (anaphase 2) Four haploid daughter nuclei (gametes) are produced All gametes from female mammals have X In mammals, if gamete has Y chromosome it will make male offspring (XY); if has X chromosome it will make female offspring (XX) © McGraw Hill 32  Meiosis compared with mitosis Meiosis Mitosis Anaphase 1: Homologues Homologous separate chromosomes do not pair-up Anaphase 2: sister chromatids separate No proper interphase between meiosis I and meiosis II Telophase: Telophase: Daughter nuclei Daughter nuclei are are different from parent genetically identical and have only half the to parent cell chromosomes (haploid) (diploid) © McGraw Hill 33  Moving dots are the centrioles These organize the microtubule spindle First, homologous chromosomes are separated (Meiosis I) Second, sister chromatids are separated (Meiosis II) © McGraw Hill 34  Second function of meiosis – create genetic variation (3 ways) 1. Fusing gametes mixes DNA from 2 parents, which differ because of mutation 2. Shuffling chromosomes in the cell to produce genetically different gametes Mother Father Individual Gametes 1. Crossing-over (shuffling genetic material on the same chromosome) © McGraw Hill 35  3. Crossing-Over: When homologous chromosomes pair (so 4 chromatids), chromatids from different chromosomes (= non-sister chromatids) exchange genetic material © McGraw Hill 36 37  Variation This variation is important as the raw material for Natural Selection (we will cover this later) – only the “most fit” survive to reproduce and pass on their DNA 38  Some animal reproduce without meiosis and fertilization (= Parthenogenesis), e.g some species of whiptail lizards Sexual species Asexual species Sexual species https://en.wikipedia.org/wiki/Parthenogenesis © McGraw Hill 39  Why did sexual reproduction (and Meiosis) evolve? Agreement that there is a long-term benefit to sexual reproduction from creating more variation (disagree over details) Environment changes and kills many organisms Some different ones may survive © McGraw Hill 40  Even bacteria have ‘sex’ (not Meiosis) Some DNA is passed between the 2 bacteria https://bacterialsex.com/bacterial-sex-gene-transfer/ © McGraw Hill 41 © McGraw Hill 42