UNIT II Patterns of Inheritance Mendelian genetics also cell division and cell cycle 1 Describe a duplicated or replicated chromosome Replicated eukaryotic chromosome 2 sister chromatids joined at a centromere by cohesion proteins 2 To what does the cell cycle refer and what events stages steps etc are involved The Cell Cycle is the life of a cell it has two components cell growth cell division it is also a continuous process 1 Interphase 3 stages G1 S G2 G1 Cell is growing metabolizing i e genes are being regulated S DNA synthesis chromosomes replicate G2 After DNA replicates cell must synthesize proteins structures to prepare for cell division e g centrosome replicates This is a microtubule organizing center Each centrosome consists of a pair of cylindrical centrioles These synthesize microtubules needed for spindle formation Centrioles are absent in plant cells 2 M or Mitotic phase 2 parts mitosis cytokinesis In M the nucleus cytoplasm divide by mitosis cytokinesis into 2 daughter cells each with the original of chromosomes 3 Why do cells organisms need to regulate or control the cell cycle The cell cycle must be regulated so a cell knows when to divide to make new cells for example for growth or repair or when not to divide when it remains in G0 and simply serve its function depending on the type of cell Normal cells must also respond to signals that cause them to exhibit anchorage dependence so a cell stops dividing once it loses contact with it s usual substrate and to exhibit density dependent inhibition so a cell stops dividing once cell density reaches some maximum The cell cycle must also be regulated so that a cell doesn t replicate damaged DNA For example the changing concentration of a particular cyclin allows it to interact with an ever present particular CDK forming the MPF signal required to move a cell through the G2 checkpoint Another example when PDGF serves as an external growth factor signal stimulating a cell to divide Another example is when the P53 tumor suppressing gene is activated when DNA is damaged and passage into the S stage is restricted until DNA is repaired if possible 4 How do cells organisms regulate or control the cell cycle Explain the role of checkpoints Explain the chemical control system that gets cells through the G2 checkpoint There are molecular signals in the cytoplasm that regulate the cell cycle The cell cycle is regulate at various checkpoints G1 G2 and M checkpoints Whether cell cycle stops or proceeds through a checkpoint depends on what chemical signals are present 1 G1 restriction checkpoint cell cycle pauses i e cell hangs out just doing its job in G0 if directed to do so by signals or other signals can direct the cell to pass the checkpoint And enter S or even undergo apoptosis 2 G2 checkpoint if preparations for cell division are ok then signals molecules direct the cell to enter the M mitotic phase Cell cycle is controlled by regulatory proteins that act as signal molecules One particular signal needed to pass the G2 checkpoint is MPF MPF one particular cyclin Cdk complex cyclin Cdk MPF needed to get thru G2 checkpoint Cyclins proteins the fluctuate in concentration throughout the cell cycle Cdks various cyclin dependent kinases they bind to and activate cyclins to get though a checkpoint Cdk proteins are always present Kinases proteins in signaling transduction that phosphorylate activate other proteins Example passage through the G2 checkpoint Synthesis of G2 cyclin fluctuates cyclically through the cell cycle When the amount of the G2 cyclin gets high it binds to is activated by a specific Cdk forms a Maturation Promoting Factor MPF signal to proceed through the G2 checkpoint Once past the checkpoint the cyclin part of MPF is degraded but the Cdk part becomes available again and initiates mitosis Passage through the cell cycle involves two intertwined mechanism 1 The repeated internal synthesis and degradation of various cyclin clock molecules 2 Uses feedback from other internal signals i e hormones external signals i e growth factors that adjust the activity of cyclins as needed i e other signals can regulate the internal cyclin clock Example human fibroblast cell growth division Fibroblast cells connective tissue secrete an extracellular matrix e g collagen An external growth factor signal called PDGF platelet derived growth factor stimulates fibroblast cell division mitosis If no PDGF cells remain at G0 don t divide If PDGF is present it signals cells to proceed through G1 checkpoint enter S to divide e g a growth factor signal that initiates a signal transduction pathway that activates a transcription factor to express a gene 5 What is the general purpose of mitotic cell division Describe the step wise process of mitotic cell division Compare the number of chromosomes before the start of mitosis at each step and at the end Mitosis division of the nucleus that preserves the parental chromosome in both daughter cells 1 Prophase Centrosomes move to opposite poles microtubules start to lengthen Spindle Nuclear membrane starts to dissolve Nucleolus disintegrates Diffuse DNA that already replicated during S begins to condense into distinct chromosomes A replicated chromosome 2 sister chromatids joined at a centromere joined by cohesion proteins 2 Pro metaphase Nuclear membrane is gone Chromosomes are clearly distinct A kinetochore appears at each centromere Spindle apparatus is now complete Non kinetochore polar microtubules pole to pole Kinetochore microtubules pole to kinetochore Aster microtubules from centrosome to cell edge Spindle microtubules move chromosomes Chromosomes line up SINGLE FILE at the midline or cell equator metaphase plate Centromere splits Spindle microtubules pull apart sister chromatids toward opposite Tubulin subunits of a microtubule breakdown Each separated chromatid is now called a distinct chromosome 3 Metaphase 4 Anaphase poles 5 Telophase Spindle disassembles Nuclear membrane reforms Nucleolus reappears Mitosis or nuclear division is now complete Cytokinesis Cell membrane pinches in invaginates at cleavage furor Cytoplasmic division 2 daughter cells result Each with same chromosome as parent cell Each with identical DNA as parent cell is now complete 6 What do cancer and the cell cycle have to do with one another What do cancer and gene regulation have to do with one another Cancer results when there are errors in the control of the cell cycle Cancer is caused by mutations to genes Normal
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