The Cell Cycle and Mitosis AP Biology

Chromatin Chromosomes VS.

Chromatin 2 m of DNA must fit in a 1x10-5 m nucleus. DNA wrapped around histone proteins to organize it and allow it fit into the nucleus Remember – it is condensed 200,000 x to fit in the nucleus It is still loosely coiled enough that enzymes can get into the DNA to copy it and make mRNA for protein synthesis It is the normal form of DNA during all phases of the cell cycle except mitosis

Chromosomes DNA compacted 12,000 times from chromatin Cannot read or copy the DNA in chromosomes – it is too tightly wound Formed solely during mitosis in order to divide the doubled DNA in ½ Also, in chromosome form, the DNA is protected from destructive enzymes since they can’t get into the tightly coiled structure

Formation of Chromatin and Chromosomes Chromatin Up Close

DNA Released from a single chromosome Coiling into Chromosomes

Structure of the Mitotic Chromosome Showing Sister Chromatids, Centromeres, Kinetochores, and Spindle Fiber Attachment Chromatid – ½ of a chromosome Sister chromatid – each half of the same chromosome Centromere – complex of proteins attached to DNA holding the sister chromatids together Kinetochore – complex of proteins attached to the outside surface of the chromosome at the centromeric region – where spindle fibers attach

The Cell Cycle G0 phase: cells do not divide Ex: Nerve cells Apoptosis

Apoptosis Programmed Cell Death Natural part of the cell cycle Nucleases and proteases are specifically activated  chop up the DNA and organelles Different from necrosis (premature death of cells that occurs when the cell doesn’t have access to blood supply) Can be time activated: Development of nervous system Immune system Hand and feet Leaf termination Cell death after irreparable damage

Cell Cycle – “The Hourly Life of a Cell” What happens when and how Why do cells divide? To make a new organism Growth Repair Replacement of normal cell loss Development

Stages of Mitosis S Stage Interphase

Interphase Interphase is not part of mitosis – it is the time between cell divisions Interphase includes G1, S, and G2 During interphase the cell is doing its normal metabolic activities like protein synthesis The cells are performing their duty as part of a tissue The DNA duplicates to get ready for mitosis The DNA is in chromatin form Animal Cell Plant Cell

Prophase

Prophase The chromatin begins to condense into chromosomes and become visible in the nucleus The nuclear membrane begins to break down Centrosomes duplicate, form spindles, & move to the poles Proteins attach to chromosomes forming kinetochores Spindle fibers attach to the kinetochores and chromosomes begin moving Animal Cell Plant Cell

Metaphase

Metaphase The chromosomes are lined up down the equator by the spindles Plant Cell Animal Cell

Anaphase

Anaphase The sister chromatids separate at the centromeres Each chromatid (now called a chromosome) heads to the pole of the cell The movement is due to kinetochore movement along the spindle fiber microtubules Animal Cell Plant Cell

Telophase http://www.bioweb.uncc.edu/biol1110/Stages.htm

Telophase The chromosomes are completely to the opposite poles New membranes start to form around the DNA The chromosomes begin to decondense back to chromatin Cytoplasm begins to pinch in animal cells and a cell wall begins to form in plant cells – This is cytokinesis Animal Cell Plant Cell

After telophase is complete, the cells reenter interphase and go about their normal business The DNA is totally decondensed, new nuclei reformed, and there are totally 2 new cells Interphase

Differences Plant vs. Animal Cell Mitosis Plant cells do not have centrioles in their centrosomes but animal cells do ????? Plant cells cannot pinch in due to the cell wall – a new cell wall forms down the middle from the endoplasmic reticulum Plant cells divide slower due to having to reform the cell wall Cytokinesis in a plant cell Cytokinesis in an animal cell

Mitosis Quiz – Animal Cells Interphase Metaphase Anaphase Interphase Prophase Telophase

Mitosis Quiz – Plant Cells Metaphase Telophase Anaphase Interphase Prophase Interphase http://biology.nebrwesleyan.edu/benham/mitosis/

Control of The Cell Cycle Regulation by Internal Signals There are checkpoints at the end of G1 and end of G2. Signal molecules cause the cycle to go on or stop. Protein kinases (static levels) + cyclins (concentration fluctuates) = active kinases Example: MPF is an activated kinase that promotes G2→M There is a checkpoint at the end of metaphase. Kinetochores produce a delay signal until spindles attach. After attachment another protein breaks down the proteins holding the sister chromatids together.

Control of the Cell Cycle Regulation by External Factors Growth factors GF’s bind to cell receptors activating the cell cycle Example: Platelet derived growth factor PDGF – in response to a wound, platelets release the GF which cause fibroblasts to proliferate. Attachment proteins relay a message via cytoskeleton to halt cell cycle

Immortality Why do cells cease to divide? When cells cease to divide, why do they deteriorate and die? Does this happen in-vivo and Can something change this?