YR1 Lecture 1H - Cell Cycle & Cell Division 2021 PDF

The Cell Cycle, Mitosis & Meiosis Dr Morven Cameron [email protected] Learning Objectives Explain the progression of events that occurs in a somatic cell life cycle which leads from a single mother cell to two identical daughter cells Define the terms chromosome, centromere, chromatid, somatic ce...

The Cell Cycle, Mitosis & Meiosis Dr Morven Cameron [email protected] Learning Objectives Explain the progression of events that occurs in a somatic cell life cycle which leads from a single mother cell to two identical daughter cells Define the terms chromosome, centromere, chromatid, somatic cell, germ cell and gamete Describe the significant events that take place in the production of gametes via meiosis 2 The Cell Cycle G1 G2 S Interphase 3 Cell cycle G1 – Gap phase 1 – growth of the cell following cell division. – Cell can be diverted to G0 where it can remain for long periods of time/indefinitely. Osteocyte Different types of cell Permanent cells such as neurons are unable to proliferate and leave the cell cycle. Stable cells such as liver hepatocytes have a low level of replication. Labile cells such as epithelium inside the mouth continuously divide and regenerate. The Cell Cycle G1 G2 S Interphase 6 Cell cycle G1 – Gap phase 1 – growth of the cell following cell division. – G1 checkpoint – check for favourable environmental factors. S – Synthesis phase – DNA synthesis. G2 – Gap phase 2 – spindle pole duplication. INTERPHASE – Cell can be diverted to G0 where it can remain for long periods of time/indefinitely. – G2 checkpoint – check for proper DNA replication. M – M checkpoint – metaphase – check correct spindle attachment. – Cytokinesis MITOSIS Chromosomes Chromosome: DNA carrying a linear array of genes Fig 2.4; p27 8 Chromosome condensation Chromosomes 2 Metacentric Centromere in the middle Acrocentric Centromere towards one end Short Arm Long Arm Telocentric Centromere at the end 10 Chromosomes 2 Metacentric Centromere in the middle Acrocentric Centromere towards one end p q Telocentric Centromere at the end 11 Karyotype Karyotype Karyotype Definitions Paternal Maternal Homologous chromosomes: occurring in pairs they are similar in shape and size, one being inherited from the female parent and the other from the male parent. Such chromosomes contain the same array of genes. Chromatid: one of the two identical strands, resulting from self duplication of a chromosome Chromatid 15 Definitions Somatic Cell: A cell that is a component of the body (NOT a reproductive cell) Germ Cell: A reproductive cell capable, when mature, of being fertilised and reproducing an entire organism Gamete: A mature male or female reproductive cell Mitosis: Separation of duplicated chromosomes and division of the cytoplasm to produce two genetically identical daughter cells. (mitos = a thread, chromosomes look like threads during cell division) 16 Mitosis – overview Prophase Metaphase Anaphase Telophase Pass Me Another Tequila Mitosis - Prophase Chromosomes condense http://2.bp.blogspot.com/-dtLdrHW63bk/TZNnwDxUqII/AAAAAAAAAKw/N_SN1HGy4m8/s320/mitosis_cartoon_1.jpg 18 Mitosis - Metaphase Nuclear envelope breaks down. Chromosomes line up along the middle of the cell. M checkpoint – correct spindle attachment. or “prometaphase” 19 Microtubules Microtubule motors in mitosis. Sharp, et al., Nature 2000, 407, 41-47 doi:10.1038/35024000 Mitosis - Anaphase Chromosomes are separated at the centromere (one chromatid moves to one pole, the other to the other pole). NOTE: as soon as the chromatids separate they are called chromosomes 21 Mitosis - Telophase Spindles disassemble. Nuclear envelope reforms. Chromosomes decondense. 22 Cytokinesis Division of the cytoplasm occurs 23 Mitosis https://www.youtube.com/watch?v=3CfvmWMzF2A Mitosis bling! https://www.youtube.com/watch?v=6bK6VFLDz1g 24 Mitosis summary Starts with 23 pairs of chromosomes in the cell – diploid. Separation of sister chromatids to form new chromosomes Ends with 23 pairs of chromosomes per cell – diploid. Daughter cells are identical to the parent cell. Before mitosis After mitosis The Cell Cycle G1 G2 S Interphase 28 Meiosis The process by which the chromosome number of a reproductive cell is halved. Results in the formation of gametes – sperm or ovum – haploid cells. Important source of variability through recombination. Meiosis involves two successive divisions. 1. Reductional division - chromosome no. is halved 2. Equational division - sister chromatids separate 29 Variation How many combinations? 223 = 8,388,608 different combinations Recombination (Crossing Over) Paternal Maternal 31 Recombination (Crossing Over) Paternal Maternal 32 Meiosis Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Prophase I http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-20/2007.jpg 34 Metaphase I Chromosome pairs align across the middle of the cell. Fig 2.10; p33 35 Anaphase I The homologous pairs separate. One member moving to one pole the other to the opposite pole Each chromosome still consists of two chromatids Fig 2.10; p33 36 Telophase I Chromosomes reach opposite poles of the cell. 46 chromosomes (diploid) à 23 chromosomes (haploid) Telophase I is followed by cytokinesis and then a brief interphase Fig 2.10; p33 37 Meiosis II - Prophase II Meiosis II resembles Mitosis except starts with half the number of chromosomes per cell (one member of each homologous pair / cell) Prophase II - The chromosomes condense and start moving to the cell equator. Fig 2.10; p33 38 Metaphase II Chromosomes line up along the midline of the cell Fig 2.10; p33 39 Anaphase II The centromeres divide and the daughter chromosomes move to opposite poles Fig 2.10; p33 40 Telophase II & Cytokinesis Telophase II - The chromosomes stop moving and a nuclear membrane forms. Cytokinesis - Cytoplasm divides to produce 4 haploid cells, each containing one copy of one of the two homologous chromosomes. Fig 2.10; p33 41 Meiosis https://www.youtube.com/watch?v=BVO-Ram1L2M Diploid/haploid Mitosis Meiosis Occurs in somatic cells Occurs in germ cells One cell division resulting in 2 daughter cells Two cell divisions resulting in 4 products Pairing of homologs during prophase I No pairing of homologues No crossing over Sister chromatids divide at anaphase Crossing over occurs during prophase I Products are identical and diploid. Sister chromatids do not divide at anaphase I but at anaphase II. Products are not identical and cells are haploid. 44 Spermatogenesis Germ cell X Y Gamete X X Y Y 45 Oogenesis Germ cell Before ovulation Ovulation Asymmetric cytokinesis Fertilisation Gamete Apoptosis (cell death) 46 Ovulation http://faculty.sunydutchess.edu/Scala/Bio102/PDF/Oogenesis.jpg 47 Primary oocyte à secondary oocyte Yi, et al., Sequential actin-based pushing forces drive meiosis I chromosome migration and symmetry breaking in oocytes. J. Cell Biol.: 2013, 200(5);567-76 Gametogenesis Fertilisation Promotes meiosis II – polar body produced. Two haploid nuclei fuse to form one diploid nucleus. haploid haploid 46 chromosomes total - diploid Cell cycle, mitosis, meiosis Cell Cycle – events that lead to cell division Mitosis – division of cells into 2 genetically identical diploid cells (46 chromosomes) Meiosis – involves recombination & results in (up to) 4 genetically non-identical haploid cells (23 chromosomes) http://www.thetwentyfirstfloor.com/main/wp-content/uploads/2011/03/cell-division.jpg 51 Questions?