Mitosis & Meiosis
Mitosis
just stage before interphase
1. Prophase – when chromosomes
becomes shortened, thicker & more
dense
2. Metaphase – chromosome line
up on equator of cell on metaphase plate
3. Anaphase - chromatids are pulled apart to become separate chromosomes, moved
to opposite poles of cells
4. Telophase – sperate into daughter cells
Sister chromatids – when held together but whole thing is chromosome, pulled a part in
anaphase (when pulled apart become two sperate chromosome)
Spindle fibres attach to each chromosome, pull them to equator of cell, only when cell is
ready attached to pole of cell, can cell go into anaphase – mustn’t happen early
Colchicine – poisons spindle fibres, keeps chromosomes in metaphase
how are chromosomes held together
(sister chromatids)?
Cohesin – a protein complex to stick
chromosomes together
• Two alpha helices wound round each
other to make a chain, which link together
two heads at hinge domain
• SMC = Structural Maintenance of
Chromosomes
• PCNA= Proliferating cell nuclear antigen – a replication factor DNA clamp
• MSH2-6 = Mismatch repair protein (also MutS)
When need to be held together, globular heads come together
Accessory proteins – help join two heads together to keep lock & help it come apart
• multiple subunits forming a ring
• Keeps products of replication together
• Broken open at anaphase checkpoint
• Promotes attachment of spindle
• Promotes repair of damage by recombination
• Regulates gene expression
In an individual chroma? Involved in shortening etc
Also involved in meiosis
, Cohesinopathies
• Roberts Syndrome (RBS)
• Cornelia de Lange Syndrome (CdLS) – problem with cohesive proteins
• Multi-spectrum birth defects
• RBS: increased mitotic failure and apoptosis (programmed cell death)
• CdLS: transcriptional dysregulation, yet normal mitosis
Importance of centromere
• Site of attachment for spindle fibres
• Segment of centromere important for – allowing chromatids to separate (sit of
kinetochore formation)
• Transcriptionally inert (heterochromatin)
• Structure ranges from simple to complex
• CNEP-A replaces histone H3 in nucleosomes
Kinetochore
^ complex of more than 40 proteins, that assembles on centromere
Provides link to spindle microtubules
Provides directionality to chromosome movement
Complex layered structure with
• Inner plate - Makes contact with the centromere, interacting
with CENP-A
• Outer plat – makes contact with spindle microtubules, contains cell cycle check point
proteins
• Fibrous corona – outermost layer & site of microtubule attachment, recognizes the +
end of microtubules to promote MT depolymersiation
Subunits constantly added at one end & lost at other
• Microtubules = Repetitive cylindrical structure made from α and β tubulin subunits.
• Assembly is nucleated at the Microtubule Organising Centre (MTOC), e.g.
centrosome.
• Microtubules associate to form a heterodimer, associate longitudinally =
protofilaments = stack them all together you get cylindrical structure
• Dynamic structure, with subunits added at +ve end and falling off at –ve end (in
vitro) – like a treadmill
+ve end of microtubule which recognizes the
kinetochore, it stabilizes the microtubule and stops it
depolymerizing
Makes contact with both of sister chromatids
• Main MTOC in animals (other eukaryotes have
equivalent)
• Replicates in G1/S phase
• Initiates microtubule assembly
• Anchors microtubules to poles
Spindle microtubules – orients the chromosome
Mitosis
just stage before interphase
1. Prophase – when chromosomes
becomes shortened, thicker & more
dense
2. Metaphase – chromosome line
up on equator of cell on metaphase plate
3. Anaphase - chromatids are pulled apart to become separate chromosomes, moved
to opposite poles of cells
4. Telophase – sperate into daughter cells
Sister chromatids – when held together but whole thing is chromosome, pulled a part in
anaphase (when pulled apart become two sperate chromosome)
Spindle fibres attach to each chromosome, pull them to equator of cell, only when cell is
ready attached to pole of cell, can cell go into anaphase – mustn’t happen early
Colchicine – poisons spindle fibres, keeps chromosomes in metaphase
how are chromosomes held together
(sister chromatids)?
Cohesin – a protein complex to stick
chromosomes together
• Two alpha helices wound round each
other to make a chain, which link together
two heads at hinge domain
• SMC = Structural Maintenance of
Chromosomes
• PCNA= Proliferating cell nuclear antigen – a replication factor DNA clamp
• MSH2-6 = Mismatch repair protein (also MutS)
When need to be held together, globular heads come together
Accessory proteins – help join two heads together to keep lock & help it come apart
• multiple subunits forming a ring
• Keeps products of replication together
• Broken open at anaphase checkpoint
• Promotes attachment of spindle
• Promotes repair of damage by recombination
• Regulates gene expression
In an individual chroma? Involved in shortening etc
Also involved in meiosis
, Cohesinopathies
• Roberts Syndrome (RBS)
• Cornelia de Lange Syndrome (CdLS) – problem with cohesive proteins
• Multi-spectrum birth defects
• RBS: increased mitotic failure and apoptosis (programmed cell death)
• CdLS: transcriptional dysregulation, yet normal mitosis
Importance of centromere
• Site of attachment for spindle fibres
• Segment of centromere important for – allowing chromatids to separate (sit of
kinetochore formation)
• Transcriptionally inert (heterochromatin)
• Structure ranges from simple to complex
• CNEP-A replaces histone H3 in nucleosomes
Kinetochore
^ complex of more than 40 proteins, that assembles on centromere
Provides link to spindle microtubules
Provides directionality to chromosome movement
Complex layered structure with
• Inner plate - Makes contact with the centromere, interacting
with CENP-A
• Outer plat – makes contact with spindle microtubules, contains cell cycle check point
proteins
• Fibrous corona – outermost layer & site of microtubule attachment, recognizes the +
end of microtubules to promote MT depolymersiation
Subunits constantly added at one end & lost at other
• Microtubules = Repetitive cylindrical structure made from α and β tubulin subunits.
• Assembly is nucleated at the Microtubule Organising Centre (MTOC), e.g.
centrosome.
• Microtubules associate to form a heterodimer, associate longitudinally =
protofilaments = stack them all together you get cylindrical structure
• Dynamic structure, with subunits added at +ve end and falling off at –ve end (in
vitro) – like a treadmill
+ve end of microtubule which recognizes the
kinetochore, it stabilizes the microtubule and stops it
depolymerizing
Makes contact with both of sister chromatids
• Main MTOC in animals (other eukaryotes have
equivalent)
• Replicates in G1/S phase
• Initiates microtubule assembly
• Anchors microtubules to poles
Spindle microtubules – orients the chromosome
