1. Endocrine Signaling -- An endocrine cell secrets regulatory molecules (hormones) that are released into the bloodstream where they travel to distant Target Cells that hasve specific Cell Surface Receptors that bind the molecules and transmit the signal into the cell.
2. Paracrine Signaling -- Cell releases regulatory molecules that diffuse to neighboring cells where specific cell surface receptors (integral membrane proteins) bind the molecules and transmit a signal to the Target Cell.
3. Autocrine Signaling -- A cell releases regulatory molecules that bind to receptors on the same cell stimulating a response.
4. Communication by direct contact between cells -Figure 15.18
1. Cell Surface Receptor -- binding of a signal molecule triggers a sequence of steps beginning with the activation of a second messenger that transmist the signal to subsequent molecules in the transduction pathway leading ultimately to a cellular response.
2. Internal Receptor -- signal molecule diffuses across the plasma membrane and binds to a receptor in the cytosol or nucleus; these signal molecules are generally hydrophobic.
3. Ligand-gated ion channel receptor -- binding of the signal molecule or ligand opens the ion channel allowing specific ions to enter the cell triggering a cellular response. Figure 15.6
4. Phosphorylation Cascade -- binding of a signal molecule by a cell surface receptor activates a protein kinase that then activates another protein kinase by transferring phosphate groups from ATP to the protein kinase; this process may be repeated several times amplifying the original signal before activating the protein that leads to a cellular response. Figure 15.17
between actual cell division cell grows approximately doubling its size and numbers of organelles and ribosomes and duplicating its DNA (chromosomes)
1. G1 Phase -- 1st gap; reproduction of cellular components; separation and replication of centrioles
2. S Phase -- chromosomes (DNA) are duplicated (S stands for synthesis)
a) 2 genetically identical sister chromatids are produced from each chromosome; they're held together at a special structure call the centromere
b) chromosomes are not distinguishable at this stage but are dispersed into a large mass of chromatin (DNA organized by bound proteins called histones). The dispersal of chromatin is probably necessary for replication of DNA
3. G2 Phase -- 2nd gap; continued reproduction of cellular material; synthesis of specific proteins necessary for cell division
1. Many enzymes are regulated by the attachment of phosphate groups to sidechains of specific Serine, Threonine, or Tyrosine amino acids by Protein Kinases, enzymes which transfer phosphate groups from ATP to proteins.
2. The Protein Kinases which regulate the Cell Cycle are themselves regulated by interaction with protein molecules called Cyclins.
a. These Protein Kinases are called Cyclin dependent kinases or Cdk's. There are several/many different Cdk's which control the Cell Cycle.b. Cyclins are synthesized during specific points in the Cell Cycle and are later degraded. There are several/many different types of Cyclins.
3. Cdk1 plus Cyclin B form a complex called M-Phase Promoting Factor (MPF) which initiates Mitosis by phosphorylating...
a. nuclear lamina causing it to disassemble allowing the nuclear membrane to disassembleb. Histone H1, one of the proteins which binds to the DNA in chromosomes, this causes the chromosomes to condense.
c. Cytoskeletal proteins allowing assembly of new cytoskeletal filaments to...
- form the mitotic spindle which separates the daughter chromosomes
- formation of the cleavage furrow by microfilaments; this allows cytokinesis constricting the cell at the center forming two new cells
4. If the cell is ready to initiate the cell division process during the G1 Phase, G1 Cyclins bind to Cdk1 and activate it taking the cell past the Restriction Point and leading to replication of the chromosomes (DNA Synthesis) during S Phase.
5. Many cells in multicellular organisms (like ourselves) cease dividing at some point; in this case we say that they are in the Go Phase.
6. Division of cells in multicellular organisms is controlled by Growth Factors which are secreted by other cells. Different types of cells require different growth factors or combinations of growth factors which take the cell past the Restriction Point and lead to cell division.
7. Loss of this control of cell division in multicellular organisms can lead to inappropriate cell division forming a tumor. Cancer is a condition in which cells from a tumor can escape and start new tumors in other tissues. Not all tumors are cancer however.
1. Prophase
a) nucleoli disappear
b) chromatin fibers more tightly coiled ==> discrete chromosomes Figure 9.7
c) mitotic spindle forms - MT's + associate proteins which will separate the 2 identical sister chromatids of each chromosome focused at MT organizing centers (centrioles in animal cells).
d) the number of chromosomes is characteristic of the type of cell
2. Prometaphase
a) Nuclear envelope fragments
b) bundles of microtubules extend from each pole to the center of the cell and interact with chromosomes
c) each of the two chromatids of a chromosome has a kinetochore at the centrosome and microtubules (kinetochore microtubules) attach there
3. Metaphase
a) centriole pairs at opposite ends (poles) of the cell; microtubules of the mitotic spindle radiate from opposite poles
b) chromosomes move to plane at cell center pulled by mitotic spindle, metaphase plate
c) each kinetochore attached to the microtubules from opposite poles.
4. Anaphase
a) daughter chromatids of each pair begin to move apart -- MT's from opposite poles pull them apart via the kinetochores
b) poles of the cell move further apart
5. Telophase and Cytokinesis -- reversal of prophase
a) daughter nuclei form at 2 poles of the cell surrounding 2 sets of daugher chromosomes
b) nuclear membrane reforms along with nuclear pores and nuclear lamina
c) nucleoli reappear and chromatin of each chromosome uncoils
d) cytokinesis - division of cytoplasm; appearance of 2 daughter cells after end of mitosis; cleavage furrow caused by constriction of actin/myosin filaments (microfilaments) at center (contractile ring) which pinches the plasma membrane forming two new cells. Figure 9.10
6. Plant cells
a) don't have centrioles -- they have
more diffuse microtubule organizing centers; mitotic spindle still
forms.
b) new membrane and cell wall forms between daughter
cells
