BIOL1110 M2 Lecture 5 Mitosis (HKU)

BIOL1110 From Molecules to Cells (Lecture 5) Mitosis Dr. Gary Ying Wai Chan Office: Rm4N11 KBSB Email: [email protected] Eukaryotic chromosomes - Chromosomes are made of chromatin, a material consisting of DNA and associated proteins (histones)...

BIOL1110 From Molecules to Cells (Lecture 5) Mitosis Dr. Gary Ying Wai Chan Office: Rm4N11 KBSB Email: [email protected] Eukaryotic chromosomes - Chromosomes are made of chromatin, a material consisting of DNA and associated proteins (histones) 2nd order - Histone proteins are among of DNA the most highly conserved compactio proteins in eukaryotes, n emphasizing their important role in the biology of the nucleus - Core histones: H2A, H2B, H3, H4. They form histone octamer: 2 H2A-H2B dimer + a tetramer H32-H42, ~146bp DNA wrap a histone octamer - H1 histone: linker histone, mediates packing of nucleosomes 1st order of DNA compaction  3rd order of DNA compaction - Chromatin fibers are further compacted into loop domains. - CTCF and cohesin play an important role in the loop extrusion. - Chromosomes are present in an extended, partially unraveled form in interphase (left hand side of the upper picture). - During cell division, the chromatin fibers condense, and chromosomes become visible as distinct structure (right hand side) - Condensin complexes are involved in chromosome condensation during mitosis (also meiosis) Eukaryotic chromosomes - Each species has a characteristic chromosome number in the nuclei of its somatic cells e.g., Human somatic cells have a diploid number (2n) of chromosomes Human somatic cells have two sets of 23 chromosomes, one set from father (paternal) and one set from mother (maternal).  46 chromosomes e.g., Gorillas somatic cells 48 chromosomes e.g., Turkey somatic cells 80 chromosomes e.g., Carp somatic cells 104 chromosomes e.g., A fern called Adders Tongue has 1260 chromosomes - The number of chromosomes and the amount of DNA in a species do not directly relate to the complexity or the intelligence of the species Cell cycle- around 24 hour/cycle - Cell cycle is an orderly sequence of events through which a cell duplicates its content (interphase) and then divides into two (M phase) - The number of cell division of a cell lineage is defined. - Timing of cell cycle varies widely in different actively growing cells, e.g., crypt cells in intestine have a cycle of 9-10 hrs, and skin stem cells is about 200 hrs. Interphase - Cells spend most of their lifetime in interphase, but duration varies widely among different cell types - The Cells aregap two metabolically phases (G1 active and G2) provide time for cells - Cells actively synthesize materials and grow to monitor the environment to see whether it is suitable for cells to commit itself to S and M phases Mitosis - A nuclear division mechanism is taken place in somatic cell - Results in producing two daughter nuclei containing identical sets of chromosome as their parental - It is divided into five phases: prophase, prometaphase, metaphase, anaphase and telophase Interphase Prophase Prometaphase Metaphase Anaphase Telophase https://www.youtube.com/watch?v=of  Prophase Duplicated chromosom Mitosis Interphase kinetochore centromere Two sister chromatids 1. Long chromatin condense to form mitotic chromosomes 2. Each consists of two chromatids attached at their centromeres 1. Cell carries out normal 3. Mitotic spindle forms between activities centrioles and centrioles moves to 2. DNA becomes duplicated if cell opposite poles of the cell is prepared for division 4. Nuclear envelope breaks down Mitosis Metaphase Prometaphase 1. Spindle microtubules bind to kinetochores 1. Chromosomes line up at the (search and capture mechanism) equator of the cell 2. Chromosomes begin to move toward 2. Spindle microtubules attach each midplane (equator) of the cell chromosome to both poles Mitosis Telophase Anaphase Central spindle 1. Sister chromatids separate at their 1. Chromosomes are grouped at poles centromeres 2. Chromosomes decondense 2. Each set of chromosomes moves 3. Nuclear envelopes begin to form toward opposite pole 4. Cytokinesis is followed to produce 3. Spindle poles move further apart two identical daughter cells https://cnx.org/contents/[email protected]:Vbi92lHB@10/The-Cell-C ycle  Untreated + Kinesin 5 inhibitor How kinesin-5 drives spindle pole separation in prophase/prometaphase? Kinesin-5 crosslinks the antiparallel microtubules. The movement of kinesin-5 leads to the antiparallel sliding of the spindle microtubules and therefore spindle poles separation How dynein drives spindle poles closer to each other ? Dynein concentrates at the spindle poles, mediates depolymerization of the spindle microtubules. As a result, dynein brings two mitotic spindle poles closer to each other.  Question time True or False Q: Prokaryotic cells have histone proteins Q: Histone H1 holds sister chromatids together Q: Kinetochores interact with spindle microtubules Q: Nuclear envelop breaks down at prophase Q: In anaphase, spindle poles move apart from each other before chromosomes move to opposite poles Q: Dynein is a plus-end directed motor protein