BISC 101 Cell Cycle Student Notes PDF

Overview: The key roles of cell division The ability of organisms to reproduce best distinguishes living things from non-living matter The continuity of life is based on the reproduction of cells, or cell division Basic steps in cell division: – Copying of DNA – Separating DNA copie...

Overview: The key roles of cell division The ability of organisms to reproduce best distinguishes living things from non-living matter The continuity of life is based on the reproduction of cells, or cell division Basic steps in cell division: – Copying of DNA – Separating DNA copies – Dividing cytoplasm to create two complete cells Concept A: Cell division that results in genetically identical daughter cells Most cell division results in daughter cells with identical genetic information (i.e., DNA) when compared to mother cell – this describes asexual reproduction (so no genetic diversity!) – In eukaryotes, this type of cell division = mitosis – In prokaryotes, this type of cell division = binary fission  A special type of division produces genetically non-identical daughter cells (involves the gametes, also known as sperm and egg cells) when compared to mother cell – this describes sexual reproduction (so there is genetic diversity!) – In eukaryotes, this type of cell division = meiosis – In prokaryotes, there are three categories of this type of cell division = – Transformation = prokaryote takes up DNA found within the environment that has originated from other prokaryotes – Transduction = prokaryote is infected by a virus which injects short pieces of chromosomal DNA from one bacterium to another – Conjugation = DNA is transferred between prokaryotes by means of a sex pilus Sex pilus  Cellular organization of the genetic material All the DNA in a cell constitutes the cell’s genome (a.k.a. cell’s total genetic information) A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) DNA molecules in a cell are packaged into chromosomes When cell is not dividing, chromosome is in form of long, A chromosome is a single long double thin chromatin fiber helix of DNA wrapped around proteins called histones  Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Somatic cells (non-reproductive cells) have two sets of chromosomes – mitosis leads to the generation of somatic cells! Gametic cells (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells – meiosis leads to the generation of gametes!  Distribution of chromosomes during eukaryotic cell division In preparation for cell division, DNA is replicated and the chromosomes condense Each replicated chromosome has two sister chromatids, which separate during cell division The centromere is the narrow “waist” of the duplicated chromosome, Sister chromatids are held together with where the two chromatids assistance of protein rings called cohesins are most closely attached (mentioned later in lecture) Condensins are protein rings along length of chromosome for the purpose of compacting chromosome just before cell division  Concept B: Mitotic phase alternates with interphase in cell cycle The cell cycle consists of – Mitotic (M) phase (mitosis and cytokinesis) – Interphase (cell growth and copying of chromosomes in preparation for cell division) Interphase (about 90% of Cell grows & carries out DNA replication normal metabolism; the cell cycle) can be organelles duplicate & chromosome duplication divided into sub-phases: Cell grows & prepares – G1 phase (“first gap”) for mitosis – S phase (“synthesis”) – G2 phase (“second gap”) Mitosis (M) divided into 5 The cell grows during all three phases, but phases: Prophase, prometaphase, chromosomes are duplicated only during metaphase, anaphase and the S phase telophase (PPMAT!)  The mitotic spindle: a closer look Mitotic spindle = apparatus of microtubules controlling chromosome movement during mitosis (includes the centrosomes, spindle microtubules, and asters) Centrosome – Organelle that serves as a microtubule organizing center (MTOC) – Replication leads to two centrosomes that migrate to opposite ends of cell, as spindle microtubules grow out from them Aster (or astral microtubules) – A radial array of short microtubules that extends from each centrosome, which connect to Centrosomes contain centrioles proteins on inner surface of cell membrane (perpendicular to each other)  Spindle microtubules – Kinetochore microtubules = capture sister chromatids by binding to kinetochore proteins – Non-kinetochore microtubules (also known as polar microtubules) = do not capture sister chromatids Kinetochores are protein structures on sister chromatids (at centromere) where kinetochore microtubules attach during cell division to pull sister chromatids apart  Mitosis is broken down into five phases G2 interphase: nuclear envelope intact and nucleoli present; centrosome is duplicated; chromatin duplicated during S-phase Prophase (early): nucleolus disappears; chromatin fibers become tightly coiled and condense to form chromosomes (which first become visible using light microscope); centrosomes move away from each other as microtubules lengthen Prometaphase (late prophase): nuclear envelope breaks down; chromosomes become completely condensed; microtubules (kinetochore and non-kinetochore) invade nuclear space [kinetochore microtubules attach to chromosomes]  Metaphase: centrosomes at opposite ends of cell (held in place now by astral microtubules); formation of spindle apparatus is now complete; chromosomes settle at metaphase (equatorial) plate Anaphase: sister chromatids move to opposite poles of cell as kinetochore microtubules shorten; non-kinetochore microtubules grow longer which helps elongate cell Telophase: chromosomes decondense into chromatin; nucleolus and nuclear envelope reappears; spindle microtubules disappear  Cytokinesis In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow on cell surface Separation of cytoplasm into two daughter cells In plant cells, a cell plate forms during cytokinesis – Vesicles (containing cell wall materials) from Golgi move along microtubules to middle of cell Mitosis in a plant cell  Binary fission Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission (asexual reproduction)  Concept C: The eukaryotic cell cycle is regulated by a molecular control system The frequency of cell division varies with the type of cell These cell cycle differences result from regulation at the molecular level The cell cycle is driven by specific chemical signals present in the cytoplasm Sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received  For many cells, the G1 checkpoint seems to be the most important one If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a non-dividing state called the G0 phase  G2 checkpoint – Second checkpoint between G2 and M phases – Chromosomes must be replicated successfully and DNA is not damaged to pass checkpoint – This leads to activation of MPF (mentioned in slides to come later) M checkpoints – Third and fourth checkpoints during M phase – Third checkpoint = between metaphase and anaphase – Fourth checkpoint = between anaphase and telophase (make sure that sister chromatids have fully separated)  Frequency of cell divisions – and importance of molecular checkpoints – varies with the type of cell Human skin cells divide frequently throughout life Some cells do not divide but retain the ability to divide, e.g. if damaged like liver Fully formed nerve and heart muscle cells do not divide at all in mature humans (= G0 phase) These cell cycle differences result from regulation at the molecular level  Cell cycle clock: cyclins and cyclin-dependent kinases Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) Level of cyclin fluctuates during the cell cycle while level of Cdk remains fairly constant MPF (M phase-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past G2 checkpoint into M phase  Cyclins and cyclin-dependent kinases at the G2 checkpoint 1 Cyclin synthesis begins in 1 late S phase; cyclin 4 accumulates 2 Cyclin and Cdk bind to form MPF; with enough MPF the cell passes G2 checkpoint 3 2 3 MPF promotes mitosis via activation of various proteins such as condensins to help further compact chromosomes before M-phase 4 During anaphase MPF and cyclins are degraded; cell enters G1 phase; Cdk recycled