The Cell Cycle Chapter 12

Cell division - process cells reproduce; necessary to living things. Cell division due to cell cycle (life of cell from origin in division of parent cell until own division into 2) Unicellular organisms - results in many new members.

Multicellular organisms - division helps in development of organism and repair and renew preexisting cells Requires distribution of identical genetic material (DNA) to 2 daughter cells.

Genome - cell’s genetic information packaged as DNA. DNA molecules packaged into chromosomes. Body cells - somatic cells; sex cells - gametes. DNA has proteins – maintains structure; helps control gene activity.

Duplicated chromosome - 2 sister chromatids (identical copies of chromosome’s DNA) Region where strands connect shrinks to narrow area (centromere)

Processes continue every day to replace dead and damaged cells. Produce clones - cells with same genetic information.

Cloned cells

Mitotic (M) phase of cell cycle alternates with much longer interphase. M phase includes mitosis, cytokinesis. Interphase - 90% of cell cycle.

Interphase - cell grows by producing proteins and cytoplasmic organelles, copies chromosomes, prepares for cell division; 3 subphases. 1 G 1 phase (“first gap”) - growth. 2 S phase (“synthesis”) - chromosomes copied. 3 G 2 phase (“second gap”) - cell completes preparations for cell division.

Mitosis – 5 subphases. End interphase - centrosomes duplicated, begin to organize microtubules into aster (“star”).

1 Prophase - chromosomes tightly coiled, with sister chromatids joined together. Nucleoli disappear; mitotic spindle forms, appears to push centrosomes away toward opposite ends (poles) of cell.

2 Prometaphase - nuclear envelope fragments and microtubules from spindle interact with chromosomes. Microtubules from 1 pole attach to 1 of 2 kinetochores (special regions of centromere), microtubules from other pole attach to other kinetochore.

3 Metaphase - spindle fibers push sister chromatids until all arranged at metaphase plate (imaginary plane equidistant between poles)

4 Anaphase - centromeres divide, separating sister chromatids. Each pulled toward pole to which it is attached by spindle fibers. 2 poles have equivalent collections of chromosomes.

5 Telophase - cell elongates; free spindle fibers from each centrosome push off each other. 2 nuclei form, surrounded by fragments of parent’s nuclear envelope. Cytokinesis (division of cytoplasm) begins.

Animals - cytokinesis (cleavage) - appearance of cleavage furrow in cell surface near old metaphase plate. Cytoplasmic side of cleavage furrow contractile ring of actin microfilaments and motor protein myosin form. Contraction of ring pinches cell in 2.

Plants, cytokinesis - cell plate between dividing cells. Plate enlarges until membranes fuse with plasma membrane at perimeter; contents vesicles forming new wall material in between.

Bacteria Prokaryotes - binary fission. DNA of bacteria coiled, highly packed. Binary fission - chromosome replication begins at 1 point in circular chromosome, (origin of replication). Copied regions move to opposite ends of cell.

As chromosome replicates and copied regions move to opposite ends of cell, bacterium grows until it reaches 2x original size. Cell division involves inward growth of plasma membrane, dividing parent cell into 2 daughter cells with complete genome.

Regulation of cell cycle Some cells divide frequently in life (skin cells), others can divide (reserve - liver cells) mature nerve, muscle cells do not divide at all. Some control over when cells divide/how often they divide in lifetime.

Cycle driven by specific chemical signals in cytoplasm. Events of cell cycle directed by cell cycle control system. Checkpoint in cell cycle is critical control point where stop/go signals regulate cycle. 3 major checkpoints found in G 1, G 2, and M phases.

G 1 checkpoint (most important), cell either get go ahead to finish cycle and divide, or receive stop signal. If stop signal - goes into G 0 phase (remains in limbo waiting to start). Most human cells in this mode.

Proteins, kinases, can activate/deactivate other proteins. Kinases always present in cell; need cyclins (protein) to activate. Complex of kinases and cyclin - cyclin-dependent kinases (Cdks).

MPF (“maturation-promoting factor”) triggers cell’s passage past G 2 checkpoint to M phase. G 1 checkpoint regulated by at least 3 Cdk proteins and several cyclins.

M phase checkpoint makes sure chromosomes are attached to spindle so each cell ends up with right amount of chromosomes. Cell division influenced by growth factors, proteins released by 1 group of cells that stimulate other cells to divide.

Platelet growth driven by growth factors. Presence of injury - released to stimulate division of platelet cells to seal wound. Density of cells too high - cell division inhibited.

Cancer Cancer cells divide out of control - no regulation. Can either produce own growth factors or have problem in signaling pathway. Can divide indefinitely if they have continual supply of nutrients.

Starts when single cell undergoes transformation to change it into cancer cell. If immune system does not destroy, can form tumor (gathering of cells). If tumor does not invade other areas - benign. If it does - malignant. If cells get into blood stream, travel throughout body (metastasis).

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