THE CELL CYCLE Chapter 12 YOU MUST KNOW

THE CELL CYCLE Chapter 12

YOU MUST KNOW • The structure of a duplicated chromosome. • The events that occur in the cell cycle (G 1, S, G 2). • 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 • G 1 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!!!) • G 2 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

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

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

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

4. 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

5. Telophase • Nuclear envelopes reform around sets of chromosomes at each pole • Chromosomes start to decondense 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

G 1 phase checkpoint • Most important – if cell passes this checkpoint, will likely complete cell cycle • Cells that do not pass this checkpoint go to G 0 phase – non-dividing • Most adult cells remain in G 0 (muscle, nerve cells) but some can respond to signals to move back to G 1 (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