AP Biology

Lecture #20Mitosis

Mitosis

• Prophase• Prometaphase• Metaphase• Anaphase• Telophase

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

Mitosis in whitefish blastula

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

Mitosis in plant cell

onion root tip

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

Bacterial Fission

2007-2008

Any Questions??