The Cell cycle Chapter 12

You must know The structure of a duplicated chromosome. The events that occur in the cell cycle (G1, S, G2). The role of cyclins and cyclin-dependent kinases in the regulation of the cell cycle. Ways in which normal cell cycle is disrupted to cause cancer, or halted in certain specialized cells, The features of mitosis that result in the production of genetically identical daughter cells including replication, alignment of chromosomes (metaphase), and separation of chromosomes (anaphase).

Cell Cycle – life of a cell from when it is first formed from a dividing parent cell until its own division into two cells

Somatic cells – all “body” cells except for gametes (eggs and sperm) Called diploid – contain two full sets of chromosomes (one from each parent) Gametes are haploid – contain only one set of chromosomes so that when they combine, the zygote formed will contain 2 complete sets again

The Chromosome Long sequence of DNA During interphase, they are found in chromatin form (thin, stringy) When replicated, consist of two sister chromatids attached by a centromere which is composed of protein kinetochores on each chromatid Each of the chromatids have identical DNA sequences

Interphase 90% of the cell cycle G1 phase – cell grows and carries out normal cell functions (protein synthesis, cellular respiration, etc.) S phase – DNA replication – every chromosome is copied to form sister chromatids (total chromosome # does not change!!!) G2 phase – more growth, cell functions, preparation for mitosis

Mitosis Division of genetic material (chromosomes) in preparation for cell division Note: AP does not require you to know the names of the different phases, only the sequence of events

1. Prophase Chromatin becomes tightly coiled into distinct chromosomes This also causes the nucleolus to disappear

Mitotic spindle begins to form in cytoplasm Prophase (continued) Mitotic spindle begins to form in cytoplasm Consists of microtubules extending from two centrioles in the centrosomes

Prometaphase The nuclear membrane breaks down, allowing microtubules to attach to chromosomes at centromeres

Metaphase Centrioles have migrated to opposite poles Microtubules move chromosomes to metaphase plate (equator of the cell) – aligned “single file”

Anaphase Sister chromatids are pulled apart by kinetochore microtubules Cell elongates as non- kinetochore microtubules push away from each other Opposite poles end up with complete, equal sets of chromosomes

Telophase Nuclear envelopes reform around sets of chromosomes at each pole Chromosomes start to de- condense to chromotin Cytokinesis (division of the cell) begins

Cytokinesis Physical division of the cell into two cells Animals – cleavage furrow forms and cell pinches in two Plants – cell plate forms to divide cytoplasm and then cell wall & membrane are built upon it

Binary Fission Prokaryotic version of cell division

Cell Cycle Regulation Involves a series of checkpoints in which molecular signals tell the cell whether to pause or continue on in the cell cycle

G1 phase checkpoint Most important – if cell passes this checkpoint, will likely complete cell cycle Cells that do not pass this checkpoint go to G0 phase – non-dividing Most adult cells remain in G0 (muscle, nerve cells) but some can respond to signals to move back to G1 (liver cells)

Checkpoint mechanisms Cyclin-dependent kinases (Cdks) Cyclins are proteins – production controlled by genes being on/off Cdks are activated when they bond to a cyclins Activation of Cdks allows them to produce MPFs – Mitosis Promoting Factors Accumulation of enough MPFs signal cell to continue on in the cell cycle During anaphase, MPFs start to “turn off” by initiating the break down of the cyclins – causes mitosis to end

Normal Cell Division Control Density-dependent inhibition – crowded cells don’t divide Anchorage dependency – cells must be attached to something, such as ECM of a tissue, to divide

Cancer – uncontrolled cell division Transformation – normal cells become cancerous Do no exhibit density-dependent or anchorage inhibition Tumor – mass of abnormal cells that divide uncontrollably Benign – stays in one place Malignant – tumor impairs organ function Metastasis – cells of tumor break off and travel to other parts of body