Topic 3 - Cell Division 1311cb874681481abe8e118f7bb5dbfd.pdf

➗ Topic 3 - Cell Division Lecture 3 Purpose of cell division: development from a fertilized egg, growth & repair Mitosis: two diploid daughter cells with identical genetic information Meiosis:...

➗ Topic 3 - Cell Division Lecture 3 Purpose of cell division: development from a fertilized egg, growth & repair Mitosis: two diploid daughter cells with identical genetic information Meiosis: division that can produce sperm and egg cells (4 unidentical haploid daughter cells) What is chromatin The entire complex of DNA and proteins is called chromatin, which condenses during cell division What are Cohesin proteins? proteins that bind to DNA to keep strands/sister chromatins together number of chromosomes in Somatic cells & Gametes Somatic cells (nonreproductive cells) have two sets of chromosomes (46 in human) Gametes have half as many chromosomes as somatic cells (23 in human) Cellular organisation of genetic material in a chromosome p arm = short arm q arm = long arm centromere - specialised DNA sequence which links the sister chromatids Topic 3 - Cell Division 1  Explain the Cell Cycle 90% Interphase, 10% Mitotic Phase Interphase: G0 -The G0 phase, resting phase, is the time when the cell is neither dividing nor preparing to divide. During this time, the cell is performing maintenance and its other functions. G1 - cell grows in size, duplicated organelles & synthesises proteins S phase - DNA Replication/Chromosome Duplicates , 2 G2 - cell increases in size, proteins synthesises Mitotic Phase: Mitosis + Cytokinesis Explain each phase of Mitosis Topic 3 - Cell Division 2  Prophase chromatin condenses into condensed chromosomes + becomes visible spindle fibres emerge from centrosomes nuclear envelope breaks down nucleolus disappears Prometaphase nuclear envelope dissolves and spindle fibres attach to the kinetochore (found on the centromere). Metaphase chromosomes line up along the equator of the cell. each sister chromatid is attached to a spindle fiber originating from opposite poles Topic 3 - Cell Division 3  Anaphase Cohesin proteins binding the sister chromatids together break down sister chromatids are split and are pulled to opposite poles of the cell. non-kinetochore spindle fibres lengthen, elongating the cell Telophase nuclear envelope reforms, cytoplasm begins to cleave chromosomes arrive at opposite poles + decondense Cytokinesis splitting of the cytoplasm which results in the formation of 2 daughter cells. contractile ring is a cellular machine that constricts and divides cells into two during cytokinesis, the final stage of the cell cycle. Difference between nonkinetochore microtubules and kinetochore microtubules nonkinetochore microtubules - a spindle fibre that elongates cell during anaphase, extend between the poles of the cell without attaching to the chromosomes. Kinetochore microtubules - a type of spindle fibre that binds to kinetochores on centromers + pulls them apart Cell Cycle Checkpoints Topic 3 - Cell Division 4  G1 Checkpoint - checks for DNA + Genome Damage S checkpoint - DNA replication halted if genome is damaged G2 - entrance into M phase blocked if DNA replication is not completed M checkpoint - anaphase blocked if chromosomes are not assembled on the mitotic spindle What are the 2 types of regulatory proteins cyclins and cyclin-dependent kinases (Cdks) The activity of a Cdk rises and falls with changes in concentration of cyclin Topic 3 - Cell Division 5  PDGF stimulation of cell cycle entry Platelet-Derived Growth Factor (PDGF) is a potent mitogen, which means it stimulates cell division and promotes the entry of cells into the cell cycle. mitogen binds to external receptor Topic 3 - Cell Division 6  intracellular signalling events initiated Ras activates signalling complexes, then triggers protein kinsases. protein kinases activate transcription factors, which promote expression of genes for the cell cycle DNA Damage induced cell-cycle arrest DNA damage recruits kinases like ATM or ATR. Chk1 and Chk2 kinases are activated. They phosphorylate p53. Phosphorylated p53 accumulates. p53 triggers p21 production. p21 inactivates Cdks. Cell cycle halts in G1 phase. Topic 3 - Cell Division 7  The tumour suppressor gene p53 DNA Damage Response: It monitors DNA integrity and becomes active in response to DNA damage. Cell-Cycle Regulation: p53 can halt the cell cycle to allow for DNA repair and trigger cell death Transcriptional Regulator: It acts as a transcription factor, controlling genes involved in DNA repair, cell-cycle control, and apoptosis. Cancer Connection: Mutations in the p53 gene are common in cancer What is Rb phosphorylation of RB increases its affinity toward the E2F transcription factor, represses gene expression, and delays cell-cycle progression. Lecture 4 What is a karyotype A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in a pair are called homologous chromosomes Chromosomes in a homologous pair are the same shape + size, carry genes but are alleles of each other Topic 3 - Cell Division 8  Meiosis Overview Diagram occurs in germ line cells to produce gametes Explain Meiosis 1 & 2 1. The process begins with a stem cell containing 46 chromosomes in 23 pairs. 2. DNA replication occurs, resulting in 46 chromosomes, each with two chromatids, totallimg 92 chromatids. 3. Meiosis I divides the homologous chromosome pairs, creating two cells, each with 23 chromosomes (comprising 46 chromatids). 4. Meiosis II splits the sister chromatids, resulting in four daughter cells, each with 23 individual chromosomes, Topic 3 - Cell Division 9  State 3 ways of Genetic Variations within gametes Crossing Over Independent assortment Random fertilisation Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis Topic 3 - Cell Division 10  each pair of chromosomes undergoes independent assortment independently of the other pairs The number of combinations possible = 2^n For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes Crossing Over Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent, since genetic material is exchanged Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome Random Fertilisation any sperm can fuse with any ovum fusion of two gametes (each with 2^23 possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations (2^23 x 2^23) Compare Mitosis + Meiosis (synapsis = fusion of chromosome pair at the start of meiosis) Topic 3 - Cell Division 11  meiosis has the same preparatory steps as somatic cells, but they stop at S phase and don’t go through the G2 phase Define Aneuploidy Topic 3 - Cell Division 12  condition characterized by the presence of an abnormal number of chromosomes. Meiotic Non-disjunction + Define Mosaicism N-D: failure of homologous chromosomes to segregate properly during anaphase 1 and/or sister chromatids in anaphase 2 → results in the production of gametes that have an improper chromosome number → aneuploid Mosaicism - some of the cells are aneuploid Define Polyploidy Gain of a whole haploid set of chromosomes How can triploidy occur Triploidy can be caused by nondisjunction or by polyspermy (multiple sperm penetrating one egg). Trisomy 21 - Down syndrome Down Syndrome – (autosome) - three copies of chromosome 21 - Karyotype shows 47 chromosomes Distinguished facial features mental disabilities Higher incidence of certain diseases Organ defects Trisomy 18 Edward’s Syndrome – (autosome) three copies of chromosome 18 Karyotype shows 47 chromosomes Results in failure of all organ systems, death after a few months Trisomy 13 – Patau syndrome Cleft lip polydactyly (extra finger(s)) majority die in neonatal period Topic 3 - Cell Division 13  Turner’s syndrome Turner’s Syndrome – (sex chromosome) – only one X chromosome is inherited Karyotype 45, X Results in sterile female, short in stature, heart and kidney defect Trisomy X Klinefelter’s syndrome Topic 3 - Cell Division 14  Jacob’s syndrome CBG - Cell Cycle.pdf Topic 3 - Cell Division 15  Cell Cycle Questions.pdf CBG - Group work Session 4.pdf Topic 3 - Cell Division 16