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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)
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).
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
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 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• 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.
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
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.
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.
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
©Biodidac. http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=19-3C&File_type=GIF
Telophase
• Chromosomes reach opposite poles of cell
• Spindle fibers disassemble
• Nuclear envelopes form
• Chromosomes decondense
• Contractile Belt is assembled
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.