THE CELL CYCLE AND MITOSIS http://www-users.york.ac.uk/~mal503/common/thesis/c3.html

PART 1: THE CELL CYCLE

3 stages of the cell cycle: Interphase – cell grows and carries out its normal functions, replication of chromosomes Mitosis – genetic material in the nucleus is divided equally into two new nuclei Cytokinesis – rest of the cell divides -each ‘daughter’ cell is identical to the parent cell

Cell Cycle Animation: http://www.cellsalive.com/cell_cycle.htm The Cell Cycle Interphase: in between stages of dividing G1—beginning cell growth S—DNA duplicates (makes a copy) DNA is in the form of chromatin G2—cell undergoes its function Purpose: to replenish dead or dying cells, to allow an organism to grow and develop - results in somatic cells such as internal organs, skin, bones, blood, etc. Cell Cycle Animation: http://www.cellsalive.com/cell_cycle.htm

Checkpoints regulatory points at which the cell examines internal and external cues and decides whether or not to move forward with division. The three most important ones are: The G1​ checkpoint: at the G1/S transition The G2 checkpoint​​: at the G2/M transition The spindle checkpoint: at the transition from metaphase to anaphase.

G1 Checkpoint What is being checked? Size: Is the cell large enough to divide? Nutrients: Are all needed nutrients present in the environment? Molecular signals: Is the cell receiving positive cues (such as growth factors) from neighbors? Mechanical signals: Is the cell attached to a support? Is it crowded by neighbors? DNA integrity: Is any of the DNA damaged? If cell doesn’t pass G1 checkpoint it may enter G0 resting phase Once cell passes G1 it is committed to dividing

G0 Phase Resting phase E.g., nerve cells in the brain stop dividing when the brain is developed and cannot re- enter the cell cycle. Liver cells can be in G0 but re-enter the cell cycle if the liver is damaged or needs repaired

G2 Checkpoint What is being checked? DNA integrity: Is any of the DNA damaged? DNA replication: Was the DNA completely copied during S phase? If errors or damage are detected, the cell will pause for repairs. If the problem is fixed, the cell will re-enter the cell cycle and divide If the damage is irreparable, however, the cell may instead undergo apoptosis, or programmed cell death This ensures that damaged DNA is not passed onto daughter cells and is important in preventing cancer

Cell Death - 2 Types Apoptosis: Programmed and targeted. Necrosis: E.g., White blood cells divide to fight viral infections. When they are no longer required, they undergo apoptosis. Necrosis: Cells may die due to external factors. E.g., Toxins, infections, trauma.

Spindle Checkpoint What is checked? Alignment: chromosomes are lined up at the metaphase plate. Attachment: Each chromosome is attached to a spindle fibre Cells look for "straggler" chromosomes that are in the wrong place cell will pause mitosis, allowing time for spindle to capture stray chromosome. If a cell fails to pass the spindle checkpoint, it may undergo apoptosis.

Part 2: Mitosis

Vocabulary Diploid – contains 2 complete sets of chromosomes – 46 in humans (2n) Body cells (somatic cells) are diploid Haploid – contains a single set of unpaired chromosomes – 23 in humans (n) Sex cells (gametes – sperm and egg) are haploid

Vocabulary a chromatid is one strand of a chromosome 2 chromatids that are the same are called sister chromatids

Mitosis Mitosis animations: Beginning of second class (1:22 min) http://www.youtube.com/watch?v=2WwIKdyBN_s&feature=related (1:30 min) http://www.youtube.com/watch?v=VlN7K1-9QB0 Beginning of second class 14

Interphase occurs just before Mitosis begins: DNA is replicated along with organelles and other cellular components and the cell prepares for division. http://www.bioweb.uncc.edu/1110Lab/notes/notes1/lab6.htm 15

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Interphase Animal cell Plant cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

Prophase (preparation phase) DNA recoils and the chromosomes condense and become shorter and thicker the nuclear membrane dissolves and disappears the spindle fibres (which attach to chromosomes during cell division) begin to form http://www.synapses.co.uk/genetics/index.html

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Prophase Animal cell Plant cell NOTE: In animal cells, centrioles are involved in the formation and organization of spindle fibres. Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

Metaphase (organizational phase) chromosomes line up in the middle of the cell with the help of spindle fibers attached to the centromere of each replicated chromosome (sister chromatids) http://www.synapses.co.uk/genetics/index.html

Metaphase Animal cell Plant cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

Anaphase (separation phase) chromosomes split in the middle (centromeres divide) and the sister chromatids are pulled by the spindle fibres to opposite poles of the cell http://www.synapses.co.uk/genetics/index.html

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Anaphase Plant cell Animal cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

Telophase chromosomes reach opposite poles of the cell and begin to unwind spindle fibres dissolve and nuclear membrane forms around the chromosomes http://www.synapses.co.uk/genetics/index.html

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Telophase Animal cell Plant cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

After Mitosis: Cytokinesis cytokinesis in a plant: cell plate forms The actual splitting of the cytoplasm and all the organelles into two separate cells called daughter cells. Occurs differently in both plant and animal cells. cytokinesis in an animal cell: cleavage furrow forms http://www.bioweb.uncc.edu/1110Lab/notes/notes1/lab6.htm