Bio 178 Lecture 20Chromosomes and the Cell Cycle

©2003 Howard Hughes Medical Institute. http://www.hhmi.org/news/page_pix.html

Reading

• Chapter 11

Quiz Material

• Questions on P 226

• Chapter 11 Quizzes on Text Website (www.mhhe.com/raven7)

Outline

• Cell Division

Chromosomes

Eukaryotic Cell Cycle

Eukaryotic Chromosomes

http://cellbio.utmb.edu/cellbio/nucleus2.htm

Chromosomes• Chromosome NumberVaries according to species:

Incorrect chromosome number:

Monosomy

Trisomy

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Chromosome Number (Cntd.)• PloidyThe number of sets of chromosomes.

Haploid (n)

Diploid (2n)

Gametes - 1 set (of each type of chromosome).

Somatic cells - 2 sets (of each chromosome type -2 homologues, 1 from each parent).

Chromosome Organization

Chromosomes (Cntd.)• Chromatin Composition~ 40% DNA, 60% protein• Chromosome StructureNucleosome

A complex composed of a stretch of ~ 200 nucleotides coiled around 8 histones.Solenoid

Nucleosomes are further coiled into solenoids. Solenoids are 30 nm and are the basis of the structure of interphase chromatin.Heterochromatin - Chromatin (condensed) that is not expressed.Euchromatin - Chromatin (threadlike) that is expressed.

Chromatin Structure

http://cellbio.utmb.edu/cellbio/nucleus2.htm

Chromatin

http://cellbio.utmb.edu/cellbio/nucleus2.htm

10 nm beads on a string: DNA wrapped around 8 histones

Chromosome Structure (Cntd.)• During Mitosis and Meiosis

Further compacted by arranging around a protein scaffold.

Each homologue duplicates during S phase of the cell cycle, so that it is composed of 2 sister chromatids joined at the centromere.

Karyotype

Organized profile of an individual’s chromosomes arranged by size and other properties.

Chromosomes vary in size, centromere position, staining properties, length of chromosome arms, and position of constrictions.

Cells in metaphase are used, fixed, prepared for microscopy, and stained. They are photographed and the images placed in order.

• What is a Karyotype?

• Chromosome Appearance

• Karyotype Preparation

Karyotype Preparation

http://arbl.cvmbs.colostate.edu/hbooks/genetics/medgen/chromo/cytotech.html

Karyotype ExamplesWhat sex is this individual?

http://author.senescence.info/thoughts/genetics.html

What Disorders do these Individuals have?

http://www.slh.wisc.edu/cytogenetics/cases/aug1996/karyotype.php

Eukaryotic Cell Cycle

Eukaryotic Cell Cycle• Why is the Cell Cycle more complex in eukaryotes than prokaryotes? More DNA and the packaging of that DNA is more complex.

• Stages of the Cell CycleInterphase ~ 90% of cell cycle. Composed of 3 stages:1. G1 (Gap1) - Initial growth (long stage)

2. S (Synthesis) - DNA replication

3. G2 (Gap 2) - Continued growth and preparation for separation of chromosomes

Eukaryotic Cell Cycle (Cntd.)• Stages of the Cell Cycle (Cntd.) M PhaseMitosisC PhaseCytokinesis

• Duration of the Cell CycleVaries with developmental stage, species and cell type:

Embryonic development

Fast (eg. 20 mins) - cells divide without growth.

Mature Cells

Slower (eg. 24 hrs for most mammalian cells) - cells grow prior to division.

http://www.sidwell.edu/us/science/vlb/seaurchin/

Sea Urchin Development

Duration of the Cell Cycle (Cntd.)

• Where in the cell cycle does the variation occur?

G1 - often enter G0. Mature muscle and nerve cells remain in G0 permanently.

Interphase(G1, S, and G2 phases)

• S PhaseChromosome duplicates 2 sister chromatids connected at the centromere (DNA).Kinetochore (protein disc) connects to the centromere. During mitosis the microtubules attach here.

Metaphase Chromosome

Interphase (Cntd.)

• G2 Phase

In S phase the chromosomes are extended and uncoiled, but in G2 they begin to condense - this is achieved via motor proteins (condensins).

Organelles replicate (including centrioles in animals).

Tubulin is synthesized - microtubules begin to assemble at the spindle.

Overview of Mitosis• DescriptionNuclear division in somatic cells.

• FunctionSeparation of daughter genomes.

• Stages**Continuous process but scientists have divided it into 4 stages to better understand it:

Prophase

Metaphase

Anaphase

Telophase

Mitosis and Cytokinesis

McGraw-Hill Video

Interphase

Prophase

• Chromosomes finish condensing

Prophase in a cell is determined by the point at which the chromosomes first become visible by LM.

• Spindle Apparatus is Assembled

Animal Cells

Centrioles move to opposite poles, forming spindle fibers between them.

Plant Cells

Although they do not possess centrioles, spindle fibers do form between opposite poles.

Prophase (Cntd.)

• Nuclear Envelope Breaks Down

ER reabsorbs nuclear envelope lipids & proteins. Pore components dispersed in cytoplasm.

Spindle fibers extend across cell - determine plane of division.

• Kinetochore Microtubules Attach to Kinetochores

Search and capture of chromosomes by microtubules - attachment to a chromosome stabilizes the microtubule.

Prophase

Prophase

http://io.uwinnipeg.ca/~simmons/1115/cm1503/mitosis.htm

Prophase

http://micro.magnet.fsu.edu/cells/fluorescencemitosis/prophasesmall.html

Metaphase• Both Kinetochores are Attached to Kinetochore Microtubules

• Chromosomes Align on the Metaphase Plate

Chromosomes are tugged between the kinetochore microtubules until they reach the metaphase plate.

Metaphase

Metaphase

http://io.uwinnipeg.ca/~simmons/1115/cm1503/mitosis.htm

Metaphase

http://micro.magnet.fsu.edu/cells/fluorescencemitosis/metaphasesmall.html

Anaphase

• Centromeres Divide Simultaneously

Cohesin proteins linking sister chromatids are cleaved enzymatically Chromosomes.

• Poles Move Apart

Polar microtubules elongate and slide past each other Poles move apart. Chromosomes move towards poles.

• Kinetochore Microtubules shorten

Kinetochore microtubules shorten at the kinetochore end Chromosomes move towards poles.

Anaphase

Anaphase

©Biodidac. http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=19-3C&File_type=GIF

Anaphase - Microtubule Sliding

Telophase

• Chromosomes reach opposite poles of cell

• Spindle fibers disassemble

• Nuclear envelopes form

• Chromosomes decondense

• Contractile Belt is assembled

Telophase

Telophase

http://www.uoguelph.ca/zoology/devobio/miller/

Cytokinesis• Animal CellsCell is pinched in 2 by the contractile belt (actin filaments). The cleavage furrow forms until it slices the cell in 2.

• Plant Cells

Cell walls too rigid to be deformed by actin filaments.

Use a cell plate :

Vesicles fuse across the middle of the cell to form 2 membranes, which eventually unite with the plasma membrane.

Cellulose is laid down between the 2 membranes Cell wall.

Cytokinesis

Cytokinesis in Animal Cells

http://www.dundee.ac.uk/biocentre/SLSBNewsarchivefeb03.htm

Red = Spindle

White = DNA

Green = Protein Kinase