Download Biol 178 Lecture 20

Bio 178 Lecture 20
Chromosomes 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 Number
Varies according to species:
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Incorrect chromosome number:
Monosomy
Trisomy
Chromosome Number (Cntd.)
• Ploidy
The number of sets of chromosomes.
Haploid (n)
Gametes - 1 set (of each type of chromosome).
Diploid (2n)
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 Structure
Nucleosome
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
10 nm beads on a string: DNA
wrapped around 8 histones
http://cellbio.utmb.edu/cellbio/nucleus2.htm
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
• What is a Karyotype?
Organized profile of an individual’s chromosomes
arranged by size and other properties.
• Chromosome Appearance
Chromosomes vary in size, centromere position, staining
properties, length of chromosome arms, and position of
constrictions.
• Karyotype Preparation
Cells in metaphase are used, fixed, prepared for
microscopy, and stained. They are photographed and the
images placed in order.
Karyotype Preparation
http://arbl.cvmbs.colostate.edu/hbooks/genetics/medgen/chromo/cytotech.html
Karyotype Examples
What sex is this individual?
http://author.senescence.info/thoughts/genetics.html
What Disorders do these Individuals
have?
http://www.slh.wisc.edu/cytogenetics/cases/aug1
996/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 Cycle
Interphase
 ~ 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 Phase
Mitosis
C Phase
Cytokinesis
• Duration of the Cell Cycle
Varies 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.
Sea Urchin Development
http://www.sidwell.edu/us/science/vlb/seaurchin/
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 Phase
Chromosome 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
• Description
Nuclear division in somatic cells.
• Function
Separation 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=193C&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 Cells
Cell 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
Red = Spindle
White = DNA
Green = Protein Kinase
http://www.dundee.ac.uk/biocentre/SLSBNewsarchivefeb03.htm