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10-18-12 cell cycle & meiosis.docx

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Department
Biology
Course Code
BIOL 1300
Professor
Carol Chaia Halpern

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Regulation Continued 10/18/2012 MPF = maturation promoting factors Which includes CdKs (cyclin dependent kinases involved in phosphorylation of proteins involved in mitosis) p53 is a protein that blocks progression into mitosis if the DNA is damaged. Mutations in this protein are linked to many cancers. Regulation • Cell growth factors (such as FGF, EGF, Erythropoietin) signal the cell to move through the cycle by  stimulating it to produce active CdKs. • One of the things required is to remove the inhibition on the expression of certain genes • The RETINOBLASTOMA protein is abundant in the nucleus of all cells and it prevents certain  proteins from stimulating transcription. • However, when it is phosphorylated (by a CdK which was activated by a growth factor), it  releases the protein needed for transcription and thus the cell will progress through the cycle. • The inhibition of a cell from continuing through the cell cycle is in part done by Cdk inhibitor proteins. One of the best understood checkpoints stops the cell in G1 if there is DNA damage (stops it from dividing). DNA damage ­­­> increase in p53 which is a gene regulatory protein p53 stimulates the transcription of p21 which is Cdk inhibitor protein (Cdk that promotes DNA replication) P21 binds to S­phase cyclin Cdk complexes (Cdks that allow cell to enter into S phase), blocking their  action, thus inhibiting DNA replication and possibly then leading down the path of DNA repair In addition to these signals, there are: Contact inhibition  Involves cell­cell communication­ when cells are healthy they stop dividing when they make contact with  each other  Cells that are malignant lose sensitivity of contact inhibition and pile on top of each other▯ tumors  Anchorage dependence Importance of the Extracellular Matrix = ECM in keeping a cell under regulation. Normal cells will stick to a petri dish, ECM Malignant cells can separate away and do fine without having an attachment  Cell Cycle Regulation by CdKs KINASES regulate the movement through these phases S G2 Kinases are activated by the presence of CYCLINS which vary in concentration at various times in the cycle Cyclins complex with the kinases to form CdKs which become active phosphorylating enzymes G1 Mitosis By phosphorylating specific proteins the cell cycle proceeds EXAMPLE 1: The Retinoblastoma protein – When phosphorylated, it releases a protein 2 that is bound to it which acts as a transcription factor thus allowing the progression through the cycle cells EXAMPLE 2: DNA damage -à increases in p53 p53 stimulates transcription of p21 p21 binds to and blocks S-phase CdK Meiosis  • Occurs in the Gonads: ovaries and testes • Fertilization happens­Zygote is very small­ cytoplasm is divided into many cells  • Eggs cells and sperm cells known as gametes (germ line cells) – have half of the amount of DNA, only  one chromosome of each kind, have 23 chromosomes  • Haploid is one chromosome of each kind (1N) where n is number of chromosomes (different kinds) • Diploid is having two of each kind of chromosome (2N) • Sperm cell is haploid, zygote is diploid  • Results in the generation of four cells that have only one copy of each chromosome (haploid or 1n  chromosomes) and therefore half the genetic material that a somatic cell has after the completion of  mitosis (which is diploid and has 2n chromosomes). These (1n cells) are the gametes ­ egg and sperm cells • Requires two consecutive divisions (Meiosis I and Meiosis II) without a significant gap between them. • The specifics are different in males and females in terms of timing and final gamete numbers. Comparing Meiosis in Males and Females  • Timing: •
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