BIO 121 Chapter 12: The Cell Cycle

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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
- 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
Haploid cells
- contain one copy of each chromosome - individuals have just one copy of each allele
Diploid cells
- contain two copies of each chromosome - individuals have two copies of each allele
- 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)
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
- 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
- a section of a chromosome that codes for a particular protein or nucleic acid, which affect traits
- one double-stranded DNA copy of a replicated chromosome (+ its associated proteins)
Sister chromatids
- chromatids attached at the centromere
- proteins that attach sister chromatids along their entire lengths - once mitosis begins, these connections are removed except for at the centromere
- 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
S (synthesis) phase
- stage within interphase in which DNA replication occurs - chromosome replication occurs ONLY during interphase
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
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
- division of the cytoplasm - separates the mother cell into two daughter cells
Cell Cycle (IPPMAT)
- Interphase - mitosis: Prophase, Prometaphase, Metaphase, Anaphase, Telophase (+ cytokinesis)
- sister chromatids condense, and the mitotic spindle begins to form - the nuclear envelope begins to dissociate into vesicles - nucleolus is no longer visible
- 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
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
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
- cylindrical structures consisting of microtubule triplets - located inside animal centrosomes
Astral microtubules
- microtubules that position the spindle in the cell - extend from the MTOCs - interact with proteins on the plasma membrane
Polar microtubules
- microtubules that separate the two poles and push away during anaphase - extend from each spindle pole and overlap with one another
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
- sister chromatids align along the metaphase plate - polar microtubules overlap in the middle of the cell, forming a pole-to-pole connection
- 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
- chromosomes decondense and the nuclear envelope reforms
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
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
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
G0 Phase
- resting phase of the cell cycle in which the cell continues to function but does not divide
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
- 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
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
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
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
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)
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)
M-phase checkpoint 1
- regulates transition from metaphase to anaphase - pass if chromosomes have attached properly to the spindle apparatus (occurs during metaphase)
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
- 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
Malignant tumors
- cancerous and invasive tumors - metastasize - can spread throughout the body via the blood or lymph and initiate secondary tumors
Benign tumors
- noncancerous, noninvasive tumors
Social signals
- cells respond to signals from other cells - social control is based on growth factors, which allow cells to pass the G1 checkpoint
Growth factors
- small proteins that stimulate division - found in serum (the liquid portion of blood) - cancer cells divide without growth factors