Biol 121 225 Freeman 227-240 (Ch. 11)
Biol 121 225 Freeman 227-240 (Ch. 11) How many chromosomes do human (and potato plants) have? What is the general structure of eukaryotic chromosomes? Chromatids
At start of M phase, each chromosome...
Jan. 25, 10 -46 chromosomes in each cell
-eukaryotic chromosomes normally exist as extremely long, threadlike strands consisting of DNA associated with globular proteins called histones -in eukaryotes, the DNA-protein material is called chromatin -each of the DNA copies in a replicated chromosome is called a chromatid -chromatids from the same chromosome are called sister chromatids – they represent exact copies of the same genetic material -each chromatid contains one long DNA double helix -at the start of M phase, each chromosome consists of two sister chromatids that are attached to each other at the centromere -as mitosis begins, chromatin condenses to form a much more compact structure
Prophase
-during mitosis, the two sister chromatids separate to form independent chromosomes, and one copy of each chromosome goes to each of the two daughter cells -first step of mitosis -chromosomes and centrosomes have already replicated during interphase -during prophase, the chromosomes condense into compact structures -chromosomes first become visible in light microscope during prophase -in cytoplasm, prophase is marked by formation of the mitotic spindle – a structure that produces mechanical forces that pull chromosomes into the daughter cells during mitosis (via depolymerisation)
Prometaphase
-the mitotic spindle consists of an array of microtubules – components of the cytoskeleton -groups of microtubules attach to the chromosome and are called spindle fibres -in all eukaryotes, spindle fibres originate from a microtubule organizing center – the nature of which varies among species -in animal cells, this microtubule organizing center is a centrosome – a structure that contains a pair of centrioles -during prophase in all eukaryotes, the mitotic spindle either begin moving to opposite sides of the cell or form on opposite sides -once chromosomes have condensed, nucleolus disappears and the nuclear envelope fragments or breaks down -after the nuclear envelope has disintegrated, spindle fibres from each mitotic spindle attach to one of the two sister chromatids of each chromosome at the kinetochore -the kinetochore -the attachment between the spindle fibres and each chromatid is made at a structure called the kinetochore -kinetochores are located at the centromere region of the chromosome, where sister chromatids are attached to each other -each chromosome has two kinetochores where spindle fibres attach – one on each side -during prometaphase in animals, the centrosomes continue their movement to opposite poles of the cell -in all groups, the microtubules attached to the kinetochores begin moving the
Biol 121 225 Freeman 227-240 (Ch. 11) Metaphase
Anaphase
Telophase
Cytokinesis
Jan. 25, 10 chromosomes to the middle of the cell -animal centrosomes complete their migration to the opposite poles of the cell -in all eukaryotes, the kinetochore microtubules finish moving the chromosomes to the middle of the cell -when metaphase finishes, the chromosomes are lined up along an imaginary plane called the metaphase plate -at this point, the formation of the mitotic spindle is complete -each chromatid is attached to spindle fibres that run from its kinetochore to one of the poles of the cell -each chromosome is held by kinetochore spindle fibres reaching to opposite poles and exerting the same amount of tension or pull -at the start of anaphase, the centromeres that are holding sister chromatids together split -because they are under tension, sister chromatids are pulled apart equally – with the same amount of force – to create independent chromosomes -the kinetochore spindle fibres then begin to shorten, and motor proteins pull the chromosomes to opposite poles of the cell -the two poles of the cell are also pushed away from each other by motor proteins associated with microtubules that are not attached to chromosomes -during anaphase, replicated chromosomes split into two identical sets of unreplicated chromosomes – separation of sister chromatids -nuclear envelope begins to form around each set of chromosomes -the mitotic spindle disintegrates and the chromosomes begin to de-condense -once two independent nuclei have formed, mitosis is complete -usually occurs immediately following mitosis -during cytokinesis, cytoplasm divides to form two daughter cells, each with its own nucleus and complete set of organelles -in animals, fungi, and slime molds, cytokinesis begins with the formation of a cleavage furrow -the furrow appears because a ring of actin filaments forms just inside the plasma membrane, in a plane that bisects the cell -a motor protein called myosin binds to these actin filaments -when myosin binds to ATP or ADP, part of the protein moves in a way that causes actin filaments to slide -as myosin moves the ring of actin filament on the inside of the plasma membrane, the ring shrinks in size and tightens, pulling the membrane with it -the actin and myosin filaments continue to slide past each other, tightening the ring further, until the original membrane is pinched in two and the cell division is complete
Definitions of structures involved in mitosis: 1) Chromosome 2) Chromatin 3) Chromatid 4) Sister chromatids
5) Centromere 6) Kinetochore
-chromosome – a structure composed of a DNA molecule and associated proteins -chromatin – material making up eukaryotic chromosomes, consists of DNA molecule complexed with histone proteins -chromatid – one strand of a replicated chromosome, with its assoc. pns -sister chromatids – two strands of a replicated chromosome (identical genetically) and when sister chromatids separate during mitosis, become independent chromosomes -centromere – structure that joins sister chromatids -kinetochore – structure on sister chromatids where spindle fibres attach
Biol 121 225 Freeman 227-240 (Ch. 11) 7) Microtubule organizing center 8) Centrosome 9) Centriole Length of cell cycle
G0
Changes in conditions and how they affect division rate
MPF
Composition/subunits Role of subunits
Jan. 25, 10 -any structure that organizes microtubules -the microtubule organizing center in animals -cylindrical structures that comprise microtubules, located inside animal centrosomes - can vary enormously among different cell types, even in the same individual -in humans, intestinal cells divide more than twice a day to renew tissue lost during digestion; mature human nerve and muscle cells do not divide at all -most of these differences are due to variation in the length of the G1 phase -in rapidly dividing cells, G1 is essentially eliminated -most non-dividing cells in contrast are permanently stuck in G1 -as said before, most non-dividing cells in contrast are permanently stuck in G1 -this arrested stage is called the G0 state (“G zero”) -cells in G0 have effectively exited the cell cycle and sometimes referred to as postmitotic -nerve cells, muscle cells, and many other cell types enter G0 once matured -changes in conditions can affect rate -human liver cells divide once a year -but if part of liver is damaged/lost, remaining cells divide every one or two days until repair is accomplished -cells of unicellular organisms like yeasts, bacteria and archaea divide rapidly only if the environment is rich in nutrients; otherwise they enter a quiescent/inactive state -mitosis-promoting factor -a molecule that induces mitosis in all eukaryotes -made of two distinct polypeptide units -one is a protein kinase (enzyme that catalyzes transfer of phosphate group from ATP to a target protein) -because addition of a phosphate group changes target protein’s shape and activity, protein kinases frequently act as regulatory elements in the cell -the other subunit belongs to the family of proteins called cyclins -concentration of cyclins fluctuates throughout cell cycle -builds up in [ ] in interphase and peaks during M phase -this increase is important b/c the kinase subunit in MPF can be active only when bound to the cyclin subunit -therefore, the protein kinase subunit is called a cyclin-dependent kinase, or Cdk
Fluctuation of MPF Cyclin
Why doesn’t the increasing [MPF] trigger the onset of M phase? (in interphase)
Mechanism MPF uses to trigger
-MPF is a dimer consisting of a cyclin subunit and a cyclin-dependent kinase subunit – the cyclin subunit functions as a regulatory protein; the kinase subunit is the part that catalyzes the phosphorylation of other proteins to start mitosis -affects fluctuation of complete MPF dimmers -MPF Cdk remains constant throughout cell cycle -MPF Cyclin increases regularly from G1 to M phase, and then decreases to the end of M phase, and then increases from the start of G1 to M phase and so on -MPF’s Cdk subunit becomes phosphorylated at two sites after it binds to cyclin -when Cdk is phosphorylated, its conformation changes in a way that renders the protein inactive -late in G2, enzymes cause one of the phosphate groups on the Cdk subunit to drop off – this dephosphorylation reaction changes MPF’s shape in a way that activates it -once MPF activated, it binds to specific proteins and catalyzes their phosphorylation
Biol 121 225 Freeman 227-240 (Ch. 11) mitosis (M phase)
Negative feedback of MPF
Other regulatory protein complexes
Cell-cycle checkpoint
G1 checkpoint
What determines whether cell passes G1 checkpoint? (4)
Jan. 25, 10 -chromosomal proteins activated by MPF cause chromosomes to condense into the threads visible during M phase -also, microtubule-assoc proteins phosphorylated by MPF may be involved in assembling the mitotic spindle apparatus -MPF also activates an enzyme complex that promotes the degradation of MPF`s own cyclin subunit -by activating this enzyme complex, MPF triggers its own destruction -this is negative feedback inhibition of MPF -therefore, in response to MPF activity, cyclin declines rapidly -slowly, it builds up again during interphase -MPF is only one of the many protein complexes involved in regulating the cell cycle -ex. A different cyclin and protein kinase are involved in triggering the passage from G1 to S, and several regulatory proteins are involved in maintaining the G0 state of quiescent cells -critical point in cell cycle that is regulated -there are three distinct checkpoints during the four phases of the cell cycle -in effect, interactions among regulatory molecules at each checkpoint allow a cell to “decide” whether to proceed with division -if these regulatory molecules are defective, checkpoint may cell and cells may start growing in an uncontrolled manner -checkpoints prevent division of cells that are damaged/have other problems -if one checkpoint fails, affected cells may begin growing uncontrollably cancer -first checkpoint in cell cycle, occurring late in G1 -this checkpoint is most important for most cells in deciding whether cell will continue cycle or go into G0 -in general, components of the G1 checkpoint are simply to ensure the cell is healthy and should replicate its DNA and divide -b/c cell must reach certain size before division, may be some mechanism to arrest cell cycle if cell is too small -unicellular organisms arrest at G1 checkpoint if nutrient conditions are poor -cells in multicellular organisms pass or do not pass through the G1 checkpoint in response to signalling molecules from other cells – “social signals”
G2/M checkpoint
-if DNA physically damaged, protein p53 activates genes that either stop cell cycle until damage repaired or lead to cell’s programmed, controlled destruction – apoptosis -cancer may develop if p53 molecule defective (p53 is a regulatory protein and is a tumour suppressor) -also known as G2 checkpoint -2nd checkpoint occurs after S phase, at boundary b/t G2 and M -specifically, cells appear to arrest at G2 checkpoint if chromosome replication has not been completed properly or if DNA damaged -b/c MPF is key signal triggering onset of M phase, it is involved in checkpoint -if DNA damaged or if chromosomes not replicated correctly, dephosphorylation and activation of MPF are blocked -when MPF not activated, cells remain in G2 -cells at this checkpoint may also respond to signals from other cells and to internal signals relating to size
Biol 121 225 Freeman 227-240 (Ch. 11) M checkpoint
Jan. 25, 10 -also known as metaphase checkpoint -occurs during mitosis -if not all chromosomes are properly attached to mitotic spindle, M phase arrests at metaphase -specifically, anaphase is delayed until all kinetochores are properly attached to mitotic spindle fibers -if this checkpoint DNE, some chromosomes might not separate properly, and daughter cells would receive an incorrect number of chromosomes during anaphase
Biol 121 225 Freeman 227-240 (Ch. 11)
Jan. 25, 10
At start of M phase, each chromosome...
Jan. 25, 10 -46 chromosomes in each cell
-eukaryotic chromosomes normally exist as extremely long, threadlike strands consisting of DNA associated with globular proteins called histones -in eukaryotes, the DNA-protein material is called chromatin -each of the DNA copies in a replicated chromosome is called a chromatid -chromatids from the same chromosome are called sister chromatids – they represent exact copies of the same genetic material -each chromatid contains one long DNA double helix -at the start of M phase, each chromosome consists of two sister chromatids that are attached to each other at the centromere -as mitosis begins, chromatin condenses to form a much more compact structure
Prophase
-during mitosis, the two sister chromatids separate to form independent chromosomes, and one copy of each chromosome goes to each of the two daughter cells -first step of mitosis -chromosomes and centrosomes have already replicated during interphase -during prophase, the chromosomes condense into compact structures -chromosomes first become visible in light microscope during prophase -in cytoplasm, prophase is marked by formation of the mitotic spindle – a structure that produces mechanical forces that pull chromosomes into the daughter cells during mitosis (via depolymerisation)
Prometaphase
-the mitotic spindle consists of an array of microtubules – components of the cytoskeleton -groups of microtubules attach to the chromosome and are called spindle fibres -in all eukaryotes, spindle fibres originate from a microtubule organizing center – the nature of which varies among species -in animal cells, this microtubule organizing center is a centrosome – a structure that contains a pair of centrioles -during prophase in all eukaryotes, the mitotic spindle either begin moving to opposite sides of the cell or form on opposite sides -once chromosomes have condensed, nucleolus disappears and the nuclear envelope fragments or breaks down -after the nuclear envelope has disintegrated, spindle fibres from each mitotic spindle attach to one of the two sister chromatids of each chromosome at the kinetochore -the kinetochore -the attachment between the spindle fibres and each chromatid is made at a structure called the kinetochore -kinetochores are located at the centromere region of the chromosome, where sister chromatids are attached to each other -each chromosome has two kinetochores where spindle fibres attach – one on each side -during prometaphase in animals, the centrosomes continue their movement to opposite poles of the cell -in all groups, the microtubules attached to the kinetochores begin moving the
Biol 121 225 Freeman 227-240 (Ch. 11) Metaphase
Anaphase
Telophase
Cytokinesis
Jan. 25, 10 chromosomes to the middle of the cell -animal centrosomes complete their migration to the opposite poles of the cell -in all eukaryotes, the kinetochore microtubules finish moving the chromosomes to the middle of the cell -when metaphase finishes, the chromosomes are lined up along an imaginary plane called the metaphase plate -at this point, the formation of the mitotic spindle is complete -each chromatid is attached to spindle fibres that run from its kinetochore to one of the poles of the cell -each chromosome is held by kinetochore spindle fibres reaching to opposite poles and exerting the same amount of tension or pull -at the start of anaphase, the centromeres that are holding sister chromatids together split -because they are under tension, sister chromatids are pulled apart equally – with the same amount of force – to create independent chromosomes -the kinetochore spindle fibres then begin to shorten, and motor proteins pull the chromosomes to opposite poles of the cell -the two poles of the cell are also pushed away from each other by motor proteins associated with microtubules that are not attached to chromosomes -during anaphase, replicated chromosomes split into two identical sets of unreplicated chromosomes – separation of sister chromatids -nuclear envelope begins to form around each set of chromosomes -the mitotic spindle disintegrates and the chromosomes begin to de-condense -once two independent nuclei have formed, mitosis is complete -usually occurs immediately following mitosis -during cytokinesis, cytoplasm divides to form two daughter cells, each with its own nucleus and complete set of organelles -in animals, fungi, and slime molds, cytokinesis begins with the formation of a cleavage furrow -the furrow appears because a ring of actin filaments forms just inside the plasma membrane, in a plane that bisects the cell -a motor protein called myosin binds to these actin filaments -when myosin binds to ATP or ADP, part of the protein moves in a way that causes actin filaments to slide -as myosin moves the ring of actin filament on the inside of the plasma membrane, the ring shrinks in size and tightens, pulling the membrane with it -the actin and myosin filaments continue to slide past each other, tightening the ring further, until the original membrane is pinched in two and the cell division is complete
Definitions of structures involved in mitosis: 1) Chromosome 2) Chromatin 3) Chromatid 4) Sister chromatids
5) Centromere 6) Kinetochore
-chromosome – a structure composed of a DNA molecule and associated proteins -chromatin – material making up eukaryotic chromosomes, consists of DNA molecule complexed with histone proteins -chromatid – one strand of a replicated chromosome, with its assoc. pns -sister chromatids – two strands of a replicated chromosome (identical genetically) and when sister chromatids separate during mitosis, become independent chromosomes -centromere – structure that joins sister chromatids -kinetochore – structure on sister chromatids where spindle fibres attach
Biol 121 225 Freeman 227-240 (Ch. 11) 7) Microtubule organizing center 8) Centrosome 9) Centriole Length of cell cycle
G0
Changes in conditions and how they affect division rate
MPF
Composition/subunits Role of subunits
Jan. 25, 10 -any structure that organizes microtubules -the microtubule organizing center in animals -cylindrical structures that comprise microtubules, located inside animal centrosomes - can vary enormously among different cell types, even in the same individual -in humans, intestinal cells divide more than twice a day to renew tissue lost during digestion; mature human nerve and muscle cells do not divide at all -most of these differences are due to variation in the length of the G1 phase -in rapidly dividing cells, G1 is essentially eliminated -most non-dividing cells in contrast are permanently stuck in G1 -as said before, most non-dividing cells in contrast are permanently stuck in G1 -this arrested stage is called the G0 state (“G zero”) -cells in G0 have effectively exited the cell cycle and sometimes referred to as postmitotic -nerve cells, muscle cells, and many other cell types enter G0 once matured -changes in conditions can affect rate -human liver cells divide once a year -but if part of liver is damaged/lost, remaining cells divide every one or two days until repair is accomplished -cells of unicellular organisms like yeasts, bacteria and archaea divide rapidly only if the environment is rich in nutrients; otherwise they enter a quiescent/inactive state -mitosis-promoting factor -a molecule that induces mitosis in all eukaryotes -made of two distinct polypeptide units -one is a protein kinase (enzyme that catalyzes transfer of phosphate group from ATP to a target protein) -because addition of a phosphate group changes target protein’s shape and activity, protein kinases frequently act as regulatory elements in the cell -the other subunit belongs to the family of proteins called cyclins -concentration of cyclins fluctuates throughout cell cycle -builds up in [ ] in interphase and peaks during M phase -this increase is important b/c the kinase subunit in MPF can be active only when bound to the cyclin subunit -therefore, the protein kinase subunit is called a cyclin-dependent kinase, or Cdk
Fluctuation of MPF Cyclin
Why doesn’t the increasing [MPF] trigger the onset of M phase? (in interphase)
Mechanism MPF uses to trigger
-MPF is a dimer consisting of a cyclin subunit and a cyclin-dependent kinase subunit – the cyclin subunit functions as a regulatory protein; the kinase subunit is the part that catalyzes the phosphorylation of other proteins to start mitosis -affects fluctuation of complete MPF dimmers -MPF Cdk remains constant throughout cell cycle -MPF Cyclin increases regularly from G1 to M phase, and then decreases to the end of M phase, and then increases from the start of G1 to M phase and so on -MPF’s Cdk subunit becomes phosphorylated at two sites after it binds to cyclin -when Cdk is phosphorylated, its conformation changes in a way that renders the protein inactive -late in G2, enzymes cause one of the phosphate groups on the Cdk subunit to drop off – this dephosphorylation reaction changes MPF’s shape in a way that activates it -once MPF activated, it binds to specific proteins and catalyzes their phosphorylation
Biol 121 225 Freeman 227-240 (Ch. 11) mitosis (M phase)
Negative feedback of MPF
Other regulatory protein complexes
Cell-cycle checkpoint
G1 checkpoint
What determines whether cell passes G1 checkpoint? (4)
Jan. 25, 10 -chromosomal proteins activated by MPF cause chromosomes to condense into the threads visible during M phase -also, microtubule-assoc proteins phosphorylated by MPF may be involved in assembling the mitotic spindle apparatus -MPF also activates an enzyme complex that promotes the degradation of MPF`s own cyclin subunit -by activating this enzyme complex, MPF triggers its own destruction -this is negative feedback inhibition of MPF -therefore, in response to MPF activity, cyclin declines rapidly -slowly, it builds up again during interphase -MPF is only one of the many protein complexes involved in regulating the cell cycle -ex. A different cyclin and protein kinase are involved in triggering the passage from G1 to S, and several regulatory proteins are involved in maintaining the G0 state of quiescent cells -critical point in cell cycle that is regulated -there are three distinct checkpoints during the four phases of the cell cycle -in effect, interactions among regulatory molecules at each checkpoint allow a cell to “decide” whether to proceed with division -if these regulatory molecules are defective, checkpoint may cell and cells may start growing in an uncontrolled manner -checkpoints prevent division of cells that are damaged/have other problems -if one checkpoint fails, affected cells may begin growing uncontrollably cancer -first checkpoint in cell cycle, occurring late in G1 -this checkpoint is most important for most cells in deciding whether cell will continue cycle or go into G0 -in general, components of the G1 checkpoint are simply to ensure the cell is healthy and should replicate its DNA and divide -b/c cell must reach certain size before division, may be some mechanism to arrest cell cycle if cell is too small -unicellular organisms arrest at G1 checkpoint if nutrient conditions are poor -cells in multicellular organisms pass or do not pass through the G1 checkpoint in response to signalling molecules from other cells – “social signals”
G2/M checkpoint
-if DNA physically damaged, protein p53 activates genes that either stop cell cycle until damage repaired or lead to cell’s programmed, controlled destruction – apoptosis -cancer may develop if p53 molecule defective (p53 is a regulatory protein and is a tumour suppressor) -also known as G2 checkpoint -2nd checkpoint occurs after S phase, at boundary b/t G2 and M -specifically, cells appear to arrest at G2 checkpoint if chromosome replication has not been completed properly or if DNA damaged -b/c MPF is key signal triggering onset of M phase, it is involved in checkpoint -if DNA damaged or if chromosomes not replicated correctly, dephosphorylation and activation of MPF are blocked -when MPF not activated, cells remain in G2 -cells at this checkpoint may also respond to signals from other cells and to internal signals relating to size
Biol 121 225 Freeman 227-240 (Ch. 11) M checkpoint
Jan. 25, 10 -also known as metaphase checkpoint -occurs during mitosis -if not all chromosomes are properly attached to mitotic spindle, M phase arrests at metaphase -specifically, anaphase is delayed until all kinetochores are properly attached to mitotic spindle fibers -if this checkpoint DNE, some chromosomes might not separate properly, and daughter cells would receive an incorrect number of chromosomes during anaphase
Biol 121 225 Freeman 227-240 (Ch. 11)
Jan. 25, 10
