BIO 121 Chapter 12: The Cell Cycle

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Binary Fission

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Binary Fission

  • Asexual cell division for unicellular organisms

  • Purpose: reproduction; division of one cell reproduces the entire organism

  • Occurs in bacteria, archaea, and protists

  • bacterial chromosome replication —> segregation (proteins bind to chromosomes and separate them) —> other proteins (tubulin homologs) divide the cytoplasm —> PG (peptidoglycan?) is synthesized

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Mitosis

  • a type of cell division where the daughter cells are identical to the parent cell (clones—same genetic content)

  • purpose: reproduction, growth and development, and tissue renewal within multicellular organisms; essential for the development of the zygote into the adult organism

  • number of chromosomes is conserved in division

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Haploid cells

  • contain one copy of each chromosome

  • individuals have just one copy of each allele

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Diploid cells

  • contain two copies of each chromosome

  • individuals have two copies of each allele

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Meiosis

  • a type of cell division where the daughter cells have half as much genetic material as a parent cell

  • not genetically identical to parents—recombinated DNA; used in gametes of diploid organisms

  • 2n —> n (chromosome number is halved)

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Cellular replication (basic)

  • copy the DNA

  • separate the copies

  • divide the cytoplasm to create two complete cells

  • main purpose is to transmit the mother cell's genetic information (usually DNA) to the daughter cells

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Chromosome

  • carrier of genes

  • a single, long double-helix of DNA wrapped around proteins called histones

  • two attached sister chromatids are still considered a single chromosome

  • unreplicated: consists of a single, long DNA double helix wrapped around proteins

  • replicated: consists of two copies of the same DNA double helix

  • condensed replicated: consists of DNA condensed around its associated proteins, resulting in a compact chromosome

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Gene

  • a section of a chromosome that codes for a particular protein or nucleic acid, which affect traits

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Chromatid

  • one double-stranded DNA copy of a replicated chromosome (+ its associated proteins)

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Sister chromatids

  • chromatids attached at the centromere

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Cohesins

  • proteins that attach sister chromatids along their entire lengths

  • once mitosis begins, these connections are removed except for at the centromere

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Interphase

  • nondividing phase of the cell cycle

  • phase in which cells spend most of their time

  • chromosomes are uncoiled or loosely coiled (chromatin)

  • cells are growing and preparing for division or are fulfilling their specialized functions

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S (synthesis) phase

  • stage within interphase in which DNA replication occurs

  • chromosome replication occurs ONLY during interphase

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Gap/Growth phases (G1 and G2)

  • G1 comes before S phase and G2 comes after

  • responsible for protein synthesis and organelle duplication

  • existence confirmed by pulse-chase experiments

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M (mitotic) phase

  • chromosomes are condensed into compact structures

  • division of replicated chromosomes to daughter cells; one copy of each chromatid goes to each daughter cell

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Cytokinesis

  • division of the cytoplasm

  • separates the mother cell into two daughter cells

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Cell Cycle (IPPMAT)

  • Interphase

  • mitosis: Prophase, Prometaphase, Metaphase, Anaphase, Telophase (+ cytokinesis)

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Prophase

  • sister chromatids condense, and the mitotic spindle begins to form

  • the nuclear envelope begins to dissociate into vesicles

  • nucleolus is no longer visible

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Prometaphase

  • the nuclear envelope has completely dissociated into vesicles and the mitotic spindle is completely formed

  • early in prometaphase, kinesin and dynein motors attached to the kinetochores "walk" the chromosomes up/down the microtubules until the chromosomes reach the plus ends, at which point the kinetochore proteins secure their attachment to the spindle

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Mitotic spindle (mitotic spindle apparatus)

  • ensures that each daughter cell will obtain the correct number and types of chromosomes

  • responsible for organizing and sorting the chromosomes during mitosis

  • composed of microtubules

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Microtubule organizing centers

  • centrosomes (in animals and certain plants and fungi) that duplicate at the beginning of the M phase

  • each defines a pole

  • animal cells have centrioles, while other eukaryotes do not

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Centrioles

  • cylindrical structures consisting of microtubule triplets

  • located inside animal centrosomes

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Astral microtubules

  • microtubules that position the spindle in the cell

  • extend from the MTOCs

  • interact with proteins on the plasma membrane

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Polar microtubules

  • microtubules that separate the two poles and push away during anaphase

  • extend from each spindle pole and overlap with one another

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Kinetochore microtubules

  • microtubules attached to the kinetochore bound to centromeres

  • play a central role in anaphase; remain stationary and shorten as subunits are lost from the + ends

  • proteins from the kinetochore attach to a ring that surrounds the kinetochore microtubule; as the + end disassembles, the ring moves along the microtubule

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Metaphase

  • sister chromatids align along the metaphase plate

  • polar microtubules overlap in the middle of the cell, forming a pole-to-pole connection

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Anaphase

  • cohesins that hold together sister chromotids at the centromeres split

  • individual chromosomes move toward the poles as kinetochore microtubules shorten

  • creates two identical sets of daughter chromosomes at each pole

  • both the shrinking of kinetochore microtubules and the movement of the poles away from each other due to the pushing of the polar microtubules' motor proteins pull the chromosomes apart

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Telophase

  • chromosomes decondense and the nuclear envelope reforms

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Cytokinesis in plants

  • vesicles containing cellulose from the Golgi apparatus bring membrane and cell wall components to the middle of the cell, which fuse to form a cell plate

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Cytokinesis in animals (and other eukaryotes)

  • a ring of actin and myosin filaments contracts inside the cell membrane, pinching inward to form a cleavage furrow

  • the ring shrinks and tightens until division is complete

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Cell cycle length variation

  • variation most commonly occurs in the G1 phase

  • rapidly dividing cells, such as epithelial skin cells, essentially eliminate the G1 phase while non-dividing cells get permanently stuck in the G1 phase

  • variation may also vary in response to different conditions, indicating that the cell cycle is regulated

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G0 Phase

  • resting phase of the cell cycle in which the cell continues to function but does not divide

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Mitosis promoting factor (MPF)

  • is present in the cytoplasm of M-phase cells and induces mitosis in all eukaryotes

  • consists of two units: cyclin and cyclin-dependent kinase (Cdk)

  • concentration increases during interphase and peaks in M phase before decreasing again

  • active when cyclin concentrations are high

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Cyclin

  • regulatory protein

  • component of MPF

  • CYCLES during the cell cycle

  • high concentration before/during the M phase; during anaphase, degradation proteins activate and decrease concentrations of cyclin

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Cyclin-dependent kinase

  • catalyzes the phosphorylation of other proteins to start the M phase

  • regulated by cyclin; active only when bound to the cyclin subunit

  • 2 phosphorylation sites (1 activation site, 1 inhibition site); activation site but not inhibition site must be phosphorylated for Cdk to be active

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MPF regulation

  • the enzyme complex that is activated during anaphase attaches proteins to the cyclin subunit, marking it for destruction and leading to the deactivation of MPF

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Cell cycle checkpoints

  • regulatory molecules at each checkpoint allow a cell to "decide" whether to proceed with division

  • if these regulatory molecules are defective, the checkpoint may fail

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G1 checkpoint

  • occurs late in the G1 phase

  • pass if:

  • cell size is adequate

  • nutrients are sufficient

  • social signals are present

  • DNA is undamaged (if damaged, p53 activates and either pauses the cell cycle so damage can be repaired, or it triggers apoptosis)

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G2 checkpoint

  • occurs between G2 and M

  • pass if:

  • chromosomes have replicated successfully

  • DNA is undamaged

  • activated MPF is present (only possible if first two criteria are met)

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M-phase checkpoint 1

  • regulates transition from metaphase to anaphase

  • pass if chromosomes have attached properly to the spindle apparatus (occurs during metaphase)

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M-phase checkpoint 2

  • regulates transition from anaphase to telophase

  • pass if chromosomes have properly segregated and MPF is COMPLETELY absent

  • if chromosomes do not fully separate during anaphase, remaining MPF activity will prevent the cell from entering telophase and undergoing cytokinesis

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Cancer

  • a complex family of diseases caused by cells that grow in an uncontrolled fashion, invade nearby tissues, and spread to other sites in the body

  • 200 types of cancers, all arising from cells in which cell-cycle checkpoints have failed (many are thought to arise from cells with defects in the G1 checkpoint)

  • arise from 2 types of defects: defects that activate the proteins required for cell growth when they should not be active, and defects that prevent tumor suppressor genes from shutting down the cell cycle

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Malignant tumors

  • cancerous and invasive tumors

  • metastasize

  • can spread throughout the body via the blood or lymph and initiate secondary tumors

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Benign tumors

  • noncancerous, noninvasive tumors

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Social signals

  • cells respond to signals from other cells

  • social control is based on growth factors, which allow cells to pass the G1 checkpoint

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Growth factors

  • small proteins that stimulate division

  • found in serum (the liquid portion of blood)

  • cancer cells divide without growth factors

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