Biology 101
23 January, 1998
Life on Planet Earth

The Development of Cell Theory

An Overview of

• The plasma membrane isolates the cell but allows interactions with the environment.


• The genetic material provides a cellular "blueprint. "


• Cytoplasm fills each cell and surrounds the nucleus.

Cell Function Limits Cell Size

Types of Cells:
Prokaryotic & Eukaryotic

Characteristics of Prokaryotes and Eukaryotes

There are two general classes of cells: prokaryotic and eukaryotic. The evolution
of prokaryotic cells preceded that of eukaryotic cells by 2 billion years.

• Streptococcus pyogenes, the bacterium that causes strep throat, is an example of a prokaryote.


• Yeast, the organism that makes bread rise and beer ferment, is an example of an unicellular eukaryote.


• Humans, of course, are an example of a multicellular eukaryote.

1.They both have DNA as their genetic material.
2.They are both membrane bound.
3.They both have ribosomes.
4.They have similar basic metabolism
5.They are both amazingly diverse in forms.

Additional obvious differences between prokaryotes and eukaryotes include:

Size
Eukaryotic cells are, on average, ten times the size of prokaryotic cells.

Genomic composition and length
The DNA of eukaryotes is much more complex (in both size and organisation) and than the DNA of prokaryotes.

Cell Wall
Prokaryotes have a cell wall composed of peptidoglycan, a single large polymer of amino acids and sugar. Many types of eukaryotic cells also have cell walls, but none made of peptidoglycan.

• Prokaryotic cells are relatively simple.
Staphylococcus aureus

• Eukaryotic cells are more complex.

The Nucleus:
Control Center of the Cell

• The nuclear envelope allows selective exchange of materials.

• Chromatin consists of DNA and its associated proteins.


• The nucleolus is the site of ribosome assembly.

• Membranes consist of a double layer of phospholipids in which proteins are embedded.








• The cell's membrane system includes the plasma membrane, endoplasmic reticulum, the Golgi complex, and lysosomes.

Chloroplasts & Mitochondria:
Energy Capture and Extraction

• Chloroplasts are the site of photosynthesis.



• Mitochondria produce ATP using energy stored in food molecules.

Plastids and Vacuoles:
Storage and Elimination

• Many types of plastids store food in plants.


• Vacuoles serve many functions, including:
• support
• storage
• and elimination of food and wastes.

The Cytoskeleton:
Shape, Support, and Movement

• Microfilaments allow cells to change shape and guide the movement of organelles.


Helicobacter pylori
• Intermediate filaments provide a supportive framework.



Association of plectin with myosin II. (a) REF-52 cytoskeleton with actin
filaments decorated by myosin S1 (black arrow). An intermediate filament (white
arrow) associates with an actin filament bundle via plectin sidearms (10 nm
immunogold labeling, white arrowheads). (b-e) Gelsolin-treated cells immunogold
labelled for plectin (b,e) or myosin II (d). After actin depletion, intermediate
filaments (white arrows) with plectin sidearms (white arrowheads) remain
associated with myosin-rich remnants of stress fibers (b, black arrowhead), and
also bind to clusters of myosin filaments (c, black arrowheads) and individual
myosin bipolar filaments (d, black arrowheads). (e) Nocodazole-treated cell with
actin depleted by gelsolin. Individual plectin molecules in lamella (white
arrowheads) cross-link myosin filaments (black arrowheads) and their clusters.
Some plectin molecules bind to each other (asterisk). Bars, 0.1 µm.


• Microtubules help position, anchor, and move organelles and alter the shapes of cells.

Microtubules & Mitotic Spindles

The mitotic spindle is the structure responsible for the segregation of chomosomes
at mitosis. How is the spindle assembled? What factors regulate the dynamics of
microtubules during the cell cycle?


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Microtubules in Cells

The following images show the localization of microtubules and DNA in interphase
and mitotic Xenopus XL-177 cells. Tubulin (detected with an anti-tubulin antibody)
is shown in green; DNA (detected with DAPI) is shown in blue. Images by Claire
Walczak.














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Microtubules In Vitro

This image shows the localization of microtubules and DNA in a mitotic spindle
assembled in vitro in a cytoplasmic extract from Xenopus laeviseggs. Tubulin
(detected with an anti-tubulin antibody) is shown in red; DNA (detected with
DAPI) is shown in blue. Image by Claire Walczak.





People and Projects

Claire Walczak and Ann Yonetani are studying the role of kinesin-like proteins in
spindle assembly. Lisa Belmont is characterizing a protein that regulates the
catastrophe rate of microtubules. Heather Deacon is looking for novel factors that
regulate microtubule dynamics. Mimi Shirasu is looking for novel factors involved
in spindle assembly. Jennifer Frazier is looking for molecular and structural
evidence of microtubules in prokaryotes and Archae.

Last updated: March 27, 1996
Questions or comments?? Send mail to
swedlow@cgl.ucsf.edu

Link to the Mitchison Lab Home Page.







• Cilia and flagella move the cell or move fluid past the cell.

Cilia from a Hamster's throat The direction of rotation determines whether the bacterial flagella swims or tumbles.


Cross-section of cilia

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Last modified on: 31 January, 2000 by Dave Ussery