AP Biology Lecture #20 Mitosis
Dec 27, 2015
The Mitotic Spindle
• The mitotic spindle is made of microtubules and proteins which help move the chromosomes around.
• It is made in the centrosome (an organelle)• Asters are short microtubules that extend out from the
centrosome• Kinetochores are where the asters attach to chromosomes
Prophase
• Chromatin condenses – visible chromosomes
• chromatids• Centrioles move to opposite
poles of cell – animal cell
• Protein fibers cross cell to form mitotic spindle– microtubules
• Nucleolus disappears• Nuclear membrane breaks down
Prometaphase
– spindle fibers attach to centromeres
• creating kinetochores
– microtubules attach at kinetochores
• connect centromeres to centrioles
– chromosomes begin moving
Metaphase
• Centrosomes at opposite poles
• Centromeres are aligned
• Kinetochores of sister chromatids attached to microtubules (spindle)
Fig. 12-7
Microtubules Chromosomes
Sisterchromatids
Aster
Metaphaseplate
Centrosome
Kineto-chores
Kinetochoremicrotubules
Overlappingnonkinetochoremicrotubules
Centrosome 1 µm
0.5 µm
Anaphase
Paired centromeres separate; sister chromatids liberated
Chromosomes move to opposite poles
Each pole now has a complete set of chromosomes
Separation of chromatids
• In anaphase, proteins holding together sister chromatids are inactivated– separate to become individual chromosomes
2 chromosomes1
chromosome2 chromatids
single-strandeddouble-stranded
• Kinetochores use motor proteins that “walk” chromosome along attached microtubule– microtubule shortens
by dismantling at kinetochore (chromosome) end
Chromosome movement
Telophase
• Daughter nuclei form• Nuclear envelopes arise• Chromatin becomes less
coiled• Two new nuclei complete
mitosis• Cytokinesis begins
– cell division
Cytokinesis
• Cytoplasmic division• Animals
– constriction belt of actin microfilaments around equator of cell
• cleavage furrow forms• splits cell in two• like tightening a draw
string
Cytokinesis in Plants• Plants
– cell plate forms• vesicles line up at
equator– derived from Golgi
• vesicles fuse to form 2 cell membranes
– new cell wall laid down between membranes
• new cell wall fuses with existing cell wall
**In plant cells, cytokinesis begins when a new cell wall forms between the two new cells. **In animal cells, the two new cells pinch and pull apart
21
Mitosis in Animals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Animal cell(Early prophase, Prophase, Metaphase, Anaphase, Telophase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.;Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase, Anaphase): © R. Calentine/Visuals Unlimited; Plant cell(Telophase): © Jack M. Bostrack/Visuals Unlimited;
Plant Cellat Interphase
centromere
aster
kinetochore
polar spindle fiber
chromosomescell wall25µm
centrosome
lacks centrioles
MIT
OSI
S
centrosomehas centrioles
Animal Cellat Interphase
nuclearenvelopefragments
chromatincondenses
nucleolusdisappears
Early ProphaseCentrosomes have duplicated.Chromatin is condensing into
chromosomes, and the nuclearenvelope is fragmenting.
ProphaseNucleolus has disappeared, and
duplicated chromosomes are visible.Centrosomes begin moving apart,
and spindle is in process of forming.
ProphaseNucleolus has disappeared, and
duplicated chromosomes are visible.Centrosomes begin moving apart,
and spindle is in process of forming.
20 µm duplicatedchromosome
20 µm
spindlefibers forming
spindlepole
9 µm
kinetochorespindle fiber
cleavage furrow
spindle fibers
20µm 16µm
kinetochorespindle fiber
AnaphaseSister chromatids part and become daughterchromosomes that move toward the spindle
poles. In this way, each pole receives the samenumber and kinds of chromosomes as the parent cell.
MetaphaseCentromeres of duplicated chromosomes
are aligned at the metaphase plate (centerof fully formed spindle). Kinetochore spindle
fibers attached to the sister chromatidscome from opposite spindle poles.
chromosomes atmetaphase plate
6.2µm6.2µm20µm6.2µmspindle pole lackscentrioles and aster
TelophaseDaughter cells are formingas nuclear envelopes and
nucleoli reappear. Chromosomes willbecome indistinct chromatin.
daughter chromosome 20µm
nucleolus
cell plate 6.6µm