Lecture 7 - Cell Cycle PDF

Lecture 7 - Cell Cycle Chapters 12 Concept 12.1 -12.3 Rudolf Virchow, a German physician, put it this way in 1855: “Where a cell exists, there must have been a preexisting cell, just as the animal arises only from an animal and the plant only from a plant.” He summarized this concept...

Lecture 7 - Cell Cycle Chapters 12 Concept 12.1 -12.3 Rudolf Virchow, a German physician, put it this way in 1855: “Where a cell exists, there must have been a preexisting cell, just as the animal arises only from an animal and the plant only from a plant.” He summarized this concept with the Latin axiom “Omnis cellula e cellula,” meaning “Every cell from a cell.” When an unicellular cell divides, it is actually reproducing, since the process gives rise to a new organism (another cell) For multicellular eukaryotes, cell division enables each of them to develop from a single cell—the fertilized egg Lytechinus Sea urchin Cell division continues to function in renewal and repair in fully grown multicellular eukaryotes, replacing cells that die. Your body produces an astonishing 100 billion blood cells each day. This is necessary because immune cells and red blood cells have short half-lives  For example, dividing cells in your bone marrow continuously make new blood cells hematopoiesis occurs in the bone marrow—the central cavity of your bones. A cell’s DNA, its genetic information, is called its genome. Each human cell has a total of 6 billion base pairs of DNA. Each base pair is around 0.34 nanometers long (a nanometer is one-billionth of a meter), therefore, each diploid cell contains about 2 meters of DNA ! [(0.34 × 10-9) × (6 × 109)]. we have ~50 trillion cells—which works out to 100 trillion meters of DNA per human. That is enough DNA to go from here to the Sun and back more than 300 times! DNA is very long and needs to be properly packaged Chromatin refers to a mixture of DNA and proteins that form the chromosomes found in the cells of humans and other higher organisms. A nucleosome is the basic repeating subunit of chromatin packaged inside the cell's nucleus Nucleosome, with DNA in orange and histone proteins in blue. Identify the protein and the DNA molecules https://www.rcsb.org/3d-view/1AOI/1 DNA molecules are packaged into structures called chromosomes Before the cell can divide to form genetically identical daughter cells, all of this DNA must be copied, or replicated, A cell cycle is a series of events that takes place in a cell as it grows and divides. S phase is the period of DNA synthesis during which the cell replicates its genetic content Each duplicated chromosome consists of two sister chromatids, which are joined copies of the original chromosome Each chromatid has a centromere, a region in the chromosomal DNA where the chromatids are attached most closely Chromosomes duplicate during S phase When a cell is not dividing, each chromosome is in the form of a long, thin chromatin fiber. After DNA replication, the chromosomes condense as a part of cell division Mitosis is the division of the genetic material in the nucleus, is usually followed immediately by cytokinesis, the division of the cytoplasm Mitosis is just one part of the cell cycle, the life of a cell from the time it is first formed during division of a parent cell until its own division into two daughter cells The mitotic phase alternates with a much longer stage called interphase, Interphase can be divided into three phases: the G1 phase (“first gap”), the S phase (“synthesis”), and the G2 phase (“second gap”). Mitosis is conventionally broken down into five stages: prophase, prometaphase, metaphase, anaphase, and telophase The mitotic spindle is a structure consists of fibers made of microtubules and associated proteins  microtubules are hollow rods constructed from a globular protein called tubulin. Each tubulin protein is a dimer, a molecule made up of two subunits. A tubulin dimer consists of two slightly different polypeptides, alpha-tubulin and beta-tubulin the assembly of spindle microtubules starts at the centrosome, a subcellular region that organizes the cell’s microtubules. Remember the centrosomes microtubules grow out from a centrosome, a region that is often located near the nucleus. These microtubules function as compression-resisting girders of the cytoskeleton. Each of the two sister chromatids of a duplicated chromosome has a kinetochore, a structure made up of proteins that have assembled on specific sections of DNA at each centromere. microtubules chromatid chromatid  1 2 3 5 6 4 Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells. The place where Cytokinesis occurs is called the cleavage furrow.. On the cytoplasmic side of the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin Microfilaments are thin solid rods built from molecules of actin subunits. Myosin is a microfilament motor protein myosin → actin filament → CONCEPT 12.3 The eukaryotic cell cycle is regulated by a molecular control system (pp. 256–263) The frequency of cell division varies with the type of cell. Skin cells divide frequently Liver cells maintain the ability to divide but keep it in reserve until it is required. Neurons and muscle cells, do not divide at all. The eukaryotic cell cycle is regulated by a molecular control system composed of cyclins and cyclin-dependent kinases Cyclin-dependent kinase Cyclin ATP A kinase is an enzyme that adds phosphate groups to other molecules and modulate protein functions. To be active, cyclin-dependent kinases (Cdks) must be attached to a cyclin, a protein that gets its name from its cyclically fluctuating concentration in the cell. phosphorylation of various proteins of the nuclear lamina promotes fragmentation of the nuclear envelope during prometaphase of mitosis. Nuclear lamina The activity of a Cdk rises and falls with changes in the concentration of its cyclin partner. cells have built-in stop signals that halt the cell cycle at checkpoints until overridden by go-ahead signals. Cancer cells do not need the normal signals that regulate the cell cycle and so they do not stop dividing cells that divide uncontrollably will proliferate and form a benign tumour, a mass of abnormal cells within otherwise normal tissue. a malignant tumour has genetic and cellular changes enable it to spread to new tissues (Metastasis)  Test your knowledge 1. Through a microscope, you can see a cell plate beginning to 4. The decline of MPF activity at the end of mitosis is due to develop across the middle of a cell and nuclei forming on either (A) the destruction of the protein kinase Cdk. side of the cell plate. This cell is most likely (B) decreased synthesis of Cdk. (A) an animal cell in the process of cytokinesis. (C) the degradation of cyclin. (B) an animal cell in the process of DNA synthesis. (D) the accumulation of cyclin. (C) a bacterial cell dividing. (D) a plant cell in metaphase. 5. In the cells of some organisms, mitosis occurs without cytokinesis. This will result in 2. Vinblastine is a standard chemotherapeutic drug used to treat (A) cells with more than one nucleus. cancer. Because it interferes with the assembly of microtubules, (B) cells that are unusually small. its effectiveness must be related to (C) cells lacking nuclei. (A) disruption of mitotic spindle formation. (D) cell cycles lacking an S phase. (B) suppression of cyclin production. (C) myosin denaturation and inhibition of cleavage furrow 6. Which of the following does not occur during mitosis? formation. (A) condensation of the chromosomes (D) inhibition of DNA synthesis. (B) replication of the DNA (C) separation of sister chromatids 3. One difference between cancer cells and normal cells is that (D) spindle formation cancer cells (A) are unable to synthesize DNA. (B) are arrested at the S phase of the cell cycle. (C) continue to divide even when they are tightly packed together. (D) cannot function properly because they are affected by density-dependent inhibition.  Summary CONCEPT 12.1 Most cell division results in genetically identical daughter cells (pp. 247–249) CONCEPT 12.2 The mitotic phase alternates with interphase in the cell cycle (pp. 249–256) CONCEPT 12.3 The eukaryotic cell cycle is regulated by a molecular control system (pp. 256–263)