Chapter3c

The Cytoskeleton

• A networked mesh of protein rods that support the cell, acting as scaffolding

• Also contain contractile units that allow for the movement of cells and for intracellular movement (organelle movement, vesicular transport)

• Three Types:– Microtubules– Microfilaments– Intermediate filaments

Microfilaments

• Thin (but dense), cross-linked strands of actin (contractile proteins)

• Involved in cell motility and changes in shape

Intermediate Filaments

• Tough, stable, permanent, rope-like protein fibers

• Have high tensile strength

• Helps cells resist mechanical stress and are found in desmosomes for cell-cell contact

Microtubules

• Long, hollow tubes composed of tubulin. Large in diameter

• Radiate from the centrosome

• Determine cell shape and organelle distribution

• Important in intracellular transport

Centrosome & Centrioles

• “microtubule organizing center”

• Composed of two centrioles (90o to each other) and a granular matrix

• Matrix generates microtubules

• Centrioles generate spindle fibers during cell division

Cellular Extensions: Cilia

Cellular Extensions: Flagella

• Sperm cells are the only cells in the human body with flagella

• Similar to cilia but are longer and allow cells to propel themselves

• Use a whip-like motion for locomotion

Cellular Extensions: Microvilli

• Finger-like plasma membrane projections• Increases plasma membrane surface area• Typically found in absorptive cells• Example: Lining of the small intestine

The Nucleus

• The “control center” of the cell containing genetic information

• The largest organelle of

the cell and is filled with nucleoplasm

• Composed of: Nuclear envelope, Nucleolus, and Chromatin

The Nuclear Envelope

• Double membrane barrier separated by a fluid-filled space

• Outer membrane is continuous with RER & is studded with ribosomes

• Inner membrane is lined with nuclear lamina

• Two membranes come together and form nuclear pores (entry and exit point of particles)

Nucleoli

• Small, dense body composed of RNA and protein, where ribosomes are synthesized

Chromatin

• Loosely coiled DNA fibers and histone proteins, condense to form chromosomes.

• DNA + histone = nucleosome

• Condense into chromosomes during cell division

The Cell Cycle

• Cell cycles form a multi-celled organism from one original cell

• A series of changes cells go through from the time they are formed until they divide

• Promote growth and tissue repair in organisms

• Involves phases and checkpoints for quality control

*Movie time!

Interphase• Cell growth and DNA

replication, “preparation period”

• G1 Phase: Growth Phase 1, structures are duplicated

• S Phase: Synthesis Phase, DNA is replicated

• G2 Phase: Growth Phase 2, more structural duplication, final preparations

DNA Replication

• Occurs before cell division• DNA is uncoiled & H-bonds broken by a helicase• DNA polymerase introduces complementary bases• Replicated DNA is semi-conservative• DNA condenses after replication

Mitosis

• Division of the nucleus: – Prophase– Metaphase– Anaphase– Telophase

• Cytokinesis: Division of the cytoplasm

• Exceptions: RBCs, nervous, skeletal & cardiac tissue

Mitosis

Mitosis

Protein Synthesis

• DNA provides the information for protein synthesis

• Two events:– Transcription

– Translation

Transcription

• Synthesis of complementary mRNA (messenger RNA)

• Transcribes the code for making proteins and transports it outside the nucleus

Translation

Translation

• Every mRNA codon corresponds to an amino acid

• Examples: – CAC = Histidine– CAA = Glutamine– AAA = Lysine