Lecture 7 – Cell Division PDF

Lecture 7 – Cell division Cytoskeleton - cytoskeleton of a cell consists of o Microtubules o Microfilaments = actin filaments o Intermediate filaments Functions of cytoskeleton - Ensures ability of a eukaryotic cell to: o Resist deformat...

Lecture 7 – Cell division Cytoskeleton - cytoskeleton of a cell consists of o Microtubules o Microfilaments = actin filaments o Intermediate filaments Functions of cytoskeleton - Ensures ability of a eukaryotic cell to: o Resist deformation, maintain cell’s shape o Transport intracellular cargo (e.g. vesicles) o Change shape during movement (e.g. cell division; organelle migration; cell migration) o Assists in the transportation of communication signals between cells Microtubules - Largest type of filament, composed of a protein tubulin, diameter approx. 25 nanometers (nm) - Tubulin is composed of α-tubulin and β-tubulin subunits assembled into linear protofilaments - A single microtubule contains 10 – 15 protofilaments (14 in mammalian cells) that wind together to for a 24 nm wide hollow cylinder - Are structures that can rapidly grow (via polymerization) or shrink (via depolymerization) in size, depending on how many tubulin molecules they contain 23 Microfilaments - smallest type of filament, composed of the contractile protein actin, diameter approx. 8 nm - particularly prevalent in muscle cells Intermediate filaments - Composed from different protein subunits, diameter approx. 10 nm - Associated with specific cell types – neurofilaments in neurons; desmin filaments – muscle cells; keratins – epithelial cells Motor proteins - Use energy derived from ATP hydrolysis to generate movement and force - Motor proteins involved in cell movement: o Kinesin – moves along microtubules to pull organelles towards cell membrane o Dynein – used to pull cellular components inward, work to slide microtubules o Myosin – interact with actin to perform muscle contractions, involved in cytokinesis, endocytosis and exocytosis Cell division in eukaryotes Cell cycle in eukaryotes Mitosis - Somatic cell division that gives rise to two genetically identical daughter cells - Refers only to nuclear division (karyokinesis) - Ensures tissue growth, regeneration or asexual (vegetative) reproduction - Consists of five phases: o Prophase (pro=before) – the preparative phase o Prometaphase – the transition phase to the next phase o Metaphase (meta=mid) – the middle phase o Anaphase (ana=upper) – the separation phase o Telophase (telos=end) – the final phase 24 - All five phases are called karyokinesis – division of the nucleus, karyokinesis is usually followed by cytokinesis, first step of M phase of the cycle - Cytokinesis (kytos=hollow vessel=cell, kinesis=movement) – the two daughter cells become independent by division of cytoplasm and the rest of organoids, second step of M phase of the cycle - Both steps partly overlapping Regulation of mitosis - The key cell cycle (including M phase) regulation proteins – cyclin-dependent kinases (CDKs) – ensure accurate cell cycle progression - Kinase – an enzyme, adds phosphate groups (PO43-) to other molecules, kinases can phosphorylate the amino acids serine, threonine and tyrosine - CDKs do not have kinase activity unless they are associated with cyclin proteins - Concentration of cyclins in the cell fluctuate, CDKs concentration is stable - Plks (Polo-like kinases) and Aurora kinases control phosphorylation - Major regulators of centrosome function, spindle assembly, chromosome segregation and cytokinesis 25 Prophase - chromosomes condense into compact structures, condensins attach to chromosomes that coil chromosomes into highly compact forms o H1 histone phosphorylation and attachment of condensins are mediated by Cdk1 o H3 phosphorylation is mediated by Aurora B kinase - cohesin forms rings that hold the sister chromatids together - nuclear envelope breaks down for form a number of small vesicles, nucleolus disintegrates (nuclear membrane), transcription and synthesis stop - beginning of formation of the mitotic spindle - centrosomes gradually move to take up positions at the poles of the cell - process is mediated by Plks and Aurora A kinases Centrosome - In cells, the minus ends of microtubules are anchored in structures called microtubule organizing centres (MTOCs), primary MTOC in a cell = centrosome - Consists of two centrioles - Duplicated during S phase of the cell cycle 26 Centrioles - Occur as paired cylindrical organelles together with pericentriolar material (PCM – containing more than 100 different proteins) - Constructed of microtubules - Organise PCM to produce microtubules including mitotic spindle fibers Prometaphase - chromosomes are completely attached to the mitotic spindle, spindle fibers bind to kinetochore - chromosomes, led by their centromeres, start migrate to equatorial plane in the mid-line of the cell, this region of mitotic spindle is known as the metaphase plate 27 Metaphase - Chromosomes line up along the metaphase plate - Spindle fibres attach to the centromeres of the sister chromatids, fibres act as tow cables to separate sister chromatids - A key cell cycle checkpoint – the mitotic spindle checkpoint can be activated in the case of mistake in the mitotic spindle assembly - Mediated by APC/C complex – anaphase-promoting complex/cyclosome – and cyclins A and B Microtubules in mitosis - role of astral microtubules in mitosis = ensure correct positioning and orientation of the mitotic spindle apparatus based on cell polarity, in anaphase they pull the cell further apart - Kinetochore microtubules attach to kinetochore of chromatids, in anaphase they shorten and draw chromosomes towards the spindle poles - Interpolar microtubules extend from spindle pole across the equator, in anaphase they slide past each other, exerting additional pull on the chromosomes Anaphase - Each chromosome’s sister chromatids separate and move to opposite poles of the cell - Enzymatic breakdown of cohesin (links the sister chromatids together during prophase) causes this separation to occur - The separated sister chromatids are now referred to as daughter chromosomes 28 Telophase - chromosomes arrive at the cell poles - mitotic spindle disassembles - vesicles that contain fragments of the original nuclear membrane assemble around the two sets of chromosomes - Phosphate dephosphorylates the lamins at each end of the cell, this dephosphorylation results in the formation of a new nuclear membrane around each group of chromosomes Cytokinesis - physical process that finally splits the parent cell into two identical daughter cells - signal for the start of cytokinesis is dephosphorylation of proteins, which are targets of Cdks - cell membrane pinches in at the cell equator, forming a cleft called the cleavage furrow, the position of the furrow depends on the position of the astral and interpolar microtubules during anaphase, the action of a contractile ring of overlapping actin and myosin filaments forms cleavage furrow 29 Biological role of mitosis - Growth of the organism - Repair - Replacement - Asexual reproduction (in plants – vegetative multiplication) Meiosis - process by which diploid cells give rise to haploid gametes, involves two rounds of cell division with only one round of DNA replication - A type of cell division unique only to germ cells - Within the gonads, the germ cells proliferate by mitosis until they receive the right signals to enter meiosis - Meiosis involves two cell divisions – Meiosis I and Meiosis II Meiosis I - Consists of four stages: o Prophase I, Metaphase I, Anaphase I, Telophase I Prophase I - Prophase of meiosis 1 Is a complicated process with several defined stages o Leptotene – chromosomes begin to condense o Zytogene – chromosomes become closely paired, homologous chromosomes (i.e. chromosomes having the same genes at the same loci, each chromosome binds its 30  homologous counterpart → hold equivalent genetic information: one set from father/one from mother) begin to align along their entire length – pairing or forming synapsis, chromosomes are held together by synaptonemal complex - Pachytene – synapsis completed, and each pair of homologous appears as a bivalent. Crossing over occurs - Diplotene – after recombination, synaptonemal complex begins to break down, Homologous chromosomes begin to separate but remain attached by chiasmata - Diakinesis – chromosomes condense and separate until terminal chiasmata only connects two chromosomes Crossing over - exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis, which results in new allelic combinations in the daughter cells - Genetic variability is produced by genetic recombination through the process of crossing over, crossing over is ensured by homologous recombination - Homologous recombination – process in which DNA molecules are broken and fragment are re-joined in new combinations Homologous recombination process - Involves double stranded breaks (DSB) followed by homologous reparation mediated by recombination complex - formation of DSBs is catalysed by highly conserved proteins with topoisomerase activity - In the absence of recombination, chromosomes often fail to align properly for the first meiotic division – as the result there is high incidence of chromosomal loss, called nondisjunction 31  - A failure in homologous recombination is often reflected in poor fertility Metaphase I - Nuclear membrane disappears - A spindle forms - paired chromosomes align themselves on the equatorial plane with their centromeres oriented towards different poles Anaphase I - The two members of each bivalent move apart, and their respective centromeres with the attached sister chromatids are drawn to opposite poles of the cell = disjunction, each maternal and paternal chromosome in a homologous pair segregates randomly into a daughter cell in meiosis I - original paternal and maternal chromosome sets are sorted into random combinations, possible number of combinations of the 23 chromosome pairs can be present in the gametes is 2^23 (more than 8 million) Telophase I and cytokinesis - Telophase I – the two haploid sets of chromosomes have normally grouped at opposite poles of the cell - Cytokinesis – cell divides into two haploid daughter cells and enters meiotic interphase - In contrast to mitosis, interphase is brief – interkinesis, and meiosis II begins Meiosis II - second meiotic division is similar to an ordinary mitosis except that the chromosome number of the cell entering meiosis II is haploid, no DNA replication before next division 32 Cell division and human pathology Aneuploidy and haploidy - most common form of polyploidy in human is triploidy (3n), caused by fertilization of an egg by two sperm (dispermy) - Meiotic failure – a diploid sperm or egg cell is produced, can also produce a triploid zygote - most common cause of aneuploidy (2n +/-1) = nondisjunction (failure of chromosomes to disjoin normally during meiosis) The most common aneuploidy – Down syndrome - A particular combination of phenotypic features that includes mental retardation and characteristic face features - Trisomy 21 is seen in approx. 1 of every 800 to 1000 live births - Approx. 95% of Down syndrome cases are caused by nondisjunction, and most of the remainder are caused by chromosome translocations - extra chromosome is contributed by the mother in 90 – 95% of cases - Mosaicism is seen in approx. 2-4% of live births Somatic and germ line mosaicism - Somatic mosaicism – refers to the occurrence of two genetically distinct populations of cells within an individual, derived from a postzygotic mutation - Germ line mosaicism – refers to the presence of genetically distinct groups of cells within germ line tissues Mechanism and UPD formation mechanism 33 A case report of mosaic trisomy 14 and maternal uniparental disomy 14 - A 15-year-old Guinean girl, fourth children of healthy parents, was referred to clinical genetics center in a context of short stature, obesity, intellectual disability and precious puberty - Clinical characteristics of upd(14)mat are caused by dysregulation of the expression of imprinted genes - Surprisingly, in patient, T14 cells were found in blood, saliva and urine but not significantly in skin → suggests that rescue mechanism could have occurred at a very early stage of embryogenesis Mitochondrial mosaicism - Mitochondrial populations are often heterogenous, due to an innately higher mutation rate for the mitochondrial genome - When a cell divides, its mitochondria are distributed to the two daughter cells, mitochondrial segregation occurs randomly and is not nearly as organized as the highly regulated process of mitotic chromosome segregation - Cells will receive similar, but not identical, mitochondrial DNA populations Tumorigenesis - Tumour cells typically characterized by widespread alteration in DNA, chromosome breaks and aneuploidy, this condition is termed genomic instability - Aneuploidy can contribute to tumorigenesis by creating extra copies of oncogenes or by deleting tumour suppressor genes 34