Download Review of Cell Structure

Important terminology
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• Gene: a segment of DNA that codes for a protein (or an
RNA)
• Locus: the place on a chromosome where the gene is located
• Allele: an alternative form of a gene (e.g. codes for the same
enzyme, but a different form of the enzyme).
• Diploid: having two complete sets of genetic information.
(2N)
• Haploid: having one half the normal amount of DNA, for
example, one complete set of genetic information. (N)
• Genome: the total genetic information contained in a
haploid set of genes.
Chromosomes are at the heart of cytogenetics
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• Chromosomes:
– Consist of nuclear DNA packaged with histone proteins
– DNA not normally visible (chromatin) until mitosis.
– During mitosis, DNA is condensed; chromosomes can be
visualized by staining.
– Each chromosome has a characteristic appearance so you
can tell them apart.
– In diploid organisms, chromosomes come in pairs (one
from each parent), are called homologous chromosomes.
Difference between chromatin and chromosome
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Chromosome structure
• The centromere is an area
of highly condensed DNA
to which the spindle
attaches during mitosis.
• The telomere is the area at
the end of the “arms” of the
chromosome.
• Dark bands consist of
condensed DNA; patterns
are unique to each
chromosome.
kidney.niddk.nih.gov/.../ images/chromosome.gif
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Chromosome labeling
P arm is always the smaller arm, and by convention
drawn on top w/ q arm below centromere.
Each section is numbered, numbers moving outward from
centromere. Examples: “1” is a region, “11” is a band in that
area. A particular sub-band in an area (e.g. 21) separated by
decimal point (“21.3”).
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Classification of chromosomes by
centromere location
• Metacentric: near middle
• Submetacentric: offset
from middle
• Acrocentric: near an end
• Telocentric: at the very
end.
Acentric: no centromere;
dicentric: 2 centromeres.
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Karyotype Analysis:
Tool of the cytogeneticist
• Karyotype analysis: chromosomes are spread, stained and
viewed.
• Reveals improper number of chromosomes
– E.g. Down syndrome, with 3 copies of #21
• Reveals improper chromosome structure
– Odd structures linked to potential health problems
– Damage due to toxins or radiation
• Determination of species or gender
• Purity check of cells lines used in research
• Tests may be run on fetal cells or cancer cells
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Karyotype methods
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• From fetus
– Amniocentesis or chorionic villus sampling
• From adult
– use blood cells (WBC), easily obtained
– Treat with phytohemagglutinin, cells replicate
• A mitogen (“mitosis” + “genesis”)
– add colcemid (microtubule inhibitor related to
colchicine)
– some cells “frozen” in metaphase when chromosomes are
most easily seen.
Karyotype continued
– squash cells (chromosomes spill
out), stain them, and take pictures.
– In old days, cut pictures with
scissors; now digitally take pictures
and process with software.
– Chromosomes lined up in order of
size
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Ye olde G-banding
Stain with Giemsa stain.
Condensed DNA stains
darkly, reveals banding
patterns unique to each
chromosome.
Chromosomes can be distinguished on the basis of size,
position of centromere, and banding pattern.
Other stains can be used, create other banding patterns.
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Chromosome painting with FISH
Fluorescent in situ hybridization
(FISH)
Fluorescently-labeled DNA
probes are prepared from each
chromosome. After preparing
chromosome squash, probe is
added, confocal laser microscope is used to view.
Digital imaging, cut and pasting.
http://www.sun.ac.za/medlabs/fish.jpg
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Cell Division
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Reasons for cell division
• Asexual reproduction of unicellular organisms
• Development and growth of multicellular organisms
• Wound healing
• Maintenance and replacement
– Blood cells, GI tract lining
http://www.esg.montana.edu/esg/kla/ta/inthist.jpg
Mitosis, a stage in cell division
• Mitosis, also called Karyokinesis, is nuclear
division, an orderly division of genetic material
between 2 daughter cells
– DNA must first be accurately copied during cell cycle
– Chromosomes must be carefully divided up
• Cells go through a cycle; mitosis is the part of the
cycle in which the DNA is divided up.
– Cytokinesis occurs near the end of mitosis, cytoplasm
divided up as one cell becomes two.
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The Cell Cycle
•G1: a time of cell
growth and general
functioning.
•S: all the DNA in the
cell is doubled to
prepare for division.
•G2: cell prepares for
division.
•G1 – G2 = interphase;
Shows early biologists’ focus on mitosis.
http://www.med.unibs.it/~marchesi/cellcycle.gif
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Cell cycle, continued
• G0: cells not dividing; may
never divide again, or may reenter cycle when needed.
– E.g., nerve cells
• M: mitosis, actual dividing up
of the copied chromosomes and
distribution to daughter cells.
– A continuous process, but
separated into steps for
convenience of discussion.
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Cell cycle (continued)
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• From cancer research, we have learned that the cell cycle is
tightly regulated!
– Checkpoints exist: at G1/S, at G2/M, and late in Mitosis
(the “M” checkpoint)
• At each checkpoint, progress evaluated.
– G1/S checkpoint: is DNA in good condition? Has cell
grown sufficiently in size?
– G2/M checkpoint: is DNA synthesis completed? Is DNA
in good condition?
– M checkpoint: are spindle fibers formed? Are
chromosomes attached to the spindle?
Molecular regulation
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• Two kinds of proteins work together:
– cdc kinases and cyclins (“cdc” = “cell division cycle”)
– Kinases are proteins that phosphorylate other molecules
• Adding a PO4 turns molecules “on” or “off”
• Kinases always present in the cell
– Cyclins are proteins that come and go with the cell cycle
• Specific cyclins accumulate at different times.
• Kinase combines with cyclin
– Kinase is activated, given directions
– Kinase phosphorylates proteins controlling cell division
Kinase/cyclin combination acts during checkpoint
DNA damage at this
checkpoint leads to
activation of apoptosis by
p53, resulting in cell death.
http://www.portfolio.mvm.ed.ac.uk/studentwebs/session2/
group28/apoptosis.html
Picture based on Hartl & Jones, 5th edition.
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