Biol111 Week 11 Lecture 3 Cell Junctions PDF

Sex Chromosomes All chromosomes have a pair Exception = sex chromosomes. In mammals In birds In platypuses Females = XX Females = ZW Females = XXXXXXXXXX Males = XY Males = ZZ Males = XXXXX...

Sex Chromosomes All chromosomes have a pair Exception = sex chromosomes. In mammals In birds In platypuses Females = XX Females = ZW Females = XXXXXXXXXX Males = XY Males = ZZ Males = XXXXXYYYYY XX XY ZW ZZ  2. Haploid and diploid cells haploid (n) – one set of chromosomes. In hymenoptera (bees, ants etc) diploid (2n) – two sets of chromosomes. Females = diploid Males = haploid Why is Meiosis Necessary? In animals, meiosis produces haploid In plants, meiosis produces haploid gametes (sperm and eggs). These fuse spores. These spores can divide by to generate diploid offspring that grow mitosis before producing gametes that through mitosis. undergo fertilisation. Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.06  3. Phases of Meiosis  Meiosis I Mitosis   Meiosis II Meiosis I Prophase I Metaphase I Anaphase I Telophase I Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.08 Meiosis I Chromosomes condense Spindle formation Nuclear envelope break down A difference to mitosis: Homologous chromosomes join together Joined at regions called chiasmata Crossing over: regions of homologous chromosomes are swapped Prophase I Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.08 Meiosis I Chromosomes attach to kinetochores Chromosomes align in the middle of the cell A difference to mitosis: Homologous chromosomes aligned together. Prophase I Metaphase I Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.08 Meiosis I Chromosomes move to poles A difference to mitosis: Homologous chromosomes separate Sister chromatids remain attached at centromere Prophase I Metaphase I Anaphase I Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.08 Meiosis I Prophase I Metaphase I Anaphase I Telophase I Two haploid cells form, each chromatid is still paired with its sister chromatid Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.08 Meiosis II Prophase II Metaphase II Anaphase II Telophase II A difference to mitosis: Four haploid cells produced. Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.08 Meiosis summary Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.07  4. Origins of Genetic Diversity Crossing over = a process where homologous chromosomes pair up during meiosis and exchange segments. Early in prophase I: Duplicated chromosomes condense. Homologous chromosomes pair in a process called synapsis Held together by proteins Becker et al (2002) The World of the Cell. 5th ed. Fig 18.07  4. Origins of Genetic Diversity Crossing over = a process where homologous chromosomes pair up during meiosis and exchange segments. Middle of prophase I: Crossing over occurs This results in new combinations of genes in each chromatid. Sister chromatids no longer identical. Becker et al (2002) The World of the Cell. 5th ed. Fig 18.07  4. Origins of Genetic Diversity Crossing over = a process where homologous chromosomes pair up during meiosis and exchange segments. Late in prophase I: Synapsis complex disassembles. Homologues held together at the site of crossing over (chiasma, pl. chiasmata) Sister chromatids held together at centromere. Becker et al (2002) The World of the Cell. 5th ed. Fig 18.07 Origins of Genetic Diversity Independent assortment of chromosomes. Reece et al. (2011) Campbell Biology, 9th ed, Fig. 13.10  Origins of Genetic Diversity Sexual organisms generate diversity through: 1) Crossing over Meiosis 2) Independent assortment of chromosomes 3) Random fertilisation Fertilisation Each gamete has 8.4 million possible combinations of chromosomes. A random egg combines with a random sperm: gives 70 trillion possible combinations Meiosis Key Points Meiosis functions in sexual reproduction. Diploid organisms are 2n and receive one set of chromosomes from each parent. Meiosis produces four haploid (n) daughter cells. Fusions of haploid cells will produce a diploid (2n) offspring. Meiosis has two consecutive divisions (meiosis I and meiosis II) and no DNA synthesis occurs between two divisions. Genetic diversity results from crossing-over during synapsis, independent assortment of chromosomes and random fertilisation. The Differences Between Mitosis and Meiosis MITOSIS MEIOSIS What is the role of each process in the organism? How many divisions occur? How many daughter cells are produced? Are daughter cells haploid or diploid? Is crossing over present or absent? Are daughter cells genetically identical to parent cell? Cells Junctions Biol111: Cellular Biology and Biochemistry Lecture 32 Organelles: Nucleus Endoplasmic reticulum Golgi apparatus Mitochondria Plastids (incl. Chloroplasts) Structure: Extracellular matrix (outside of cell) Cytoskeleton (inside of cell) Cell division: Mitosis (cellular growth) Meiosis (reproduction) Communication: Cells junctions Cells exchanging molecules Cells reacting to extracellular signals Today’s Outline 1. Cellular connections and communication: why and what? 2. Junctions in plant cells 3. Connecting junctions in animals cells 4. Communicating junctions in animal cells Tunnelling Nanotubes Vesicles move through nanotubes between cells, demonstrating traffic between them. Gousset et al (2009) Nature Cell Biology. 11: 328-336 1. Cell junctions: connections and communication Connections: Cells in multicellular organisms need to be stably attached to each other. Communication: Cells especially within unicellular organisms need to communicate and coordinate. These can be through signalling molecules sent through the environment, or direct cell to cell contact. 1. Cell junctions: connections and communication Connections: Animals: Tight junctions and adherens junctions Communication: Plants: plasmodesmata Animals: gap junctions and tunnelling nanotubes 2. Cell junctions in plants Do plants need junctions to join cells together? Reece et al. (2011) Campbell Biology, 9th ed, page 291  Plasmodesmata First observed in 1879, called “cytoplasmic bridges” Membrane-bound tubes of cytoplasm that penetrate through the plant cell wall Cytoplasm of most cells in plants is continuous Reece et al. (2011) Campbell Biology, 9th ed, Fig. 6.31  Plasmodesmata structure Channels are about 20 – 50 nm diameter Each cells has ~1000 of these channels Alberts et al (2008) Molecular Biology of the Cell, 5th ed. Fig. 19.27 Plasmodesmata function Injected dyes showed a size exclusion limit around 1.5 kDa. Only small signalling molecules could pass from cell to cell. Gene expression tests showed proteins up to 30 kDa and pieces of mRNA move Plasmodesmata can close or open under various conditions https://www.cshl.edu/plants-rna-notes-to-self/  Plasmodesmata function Involved in plant development and cell fate determination Plant viruses hijack plasmodesmata to infect neighbouring cells  Plasmodesmata function Involved in plant development and cell fate determination Plant viruses hijack Adenovirus plasmodesmata to infect neighbouring cells  Plasmodesmata function Involved in plant development and cell fate determination Plant viruses hijack Adenovirus plasmodesmata to infect neighbouring cells