TCA cycle

Examples of TCA cycle in the following topics:

• Acetyl CoA to CO2

  • The acetyl carbons of acetyl CoA are released as carbon dioxide in the citric acid cycle.
  • Acetyl CoA links glycolysis and pyruvate oxidation with the citric acid cycle.
  • The TCA cycle is named for tricarboxylic acids (TCA) because citric acid (or citrate) and isocitrate, the first two intermediates that are formed, are tricarboxylic acids.
  • Additionally, the cycle is known as the Krebs cycle, named after Hans Krebs, who first identified the steps in the pathway in the 1930s in pigeon flight muscle.
  • Describe the fate of the acetyl CoA carbons in the citric acid cycle

• The Carbon Cycle

• Regulator Molecules of the Cell Cycle

  • Cyclins regulate the cell cycle only when they are tightly bound to Cdks.
  • The second group of cell cycle regulatory molecules are negative regulators.
  • Negative regulators halt the cell cycle.
  • Rb halts the cell cycle by binding E2F.
  • The concentrations of cyclin proteins change throughout the cell cycle.

• The Lytic and Lysogenic Cycles of Bacteriophages

  • Bacteriophages, viruses that infect bacteria, may undergo a lytic or lysogenic cycle.
  • Bacteriophages may have a lytic cycle or a lysogenic cycle, and a few viruses are capable of carrying out both.
  • An example of a bacteriophage known to follow the lysogenic cycle and the lytic cycle is the phage lambda of E. coli.
  • A temperate bacteriophage has both lytic and lysogenic cycles.
  • In the lytic cycle, the phage replicates and lyses the host cell.

• Citric Acid Cycle

  • The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate.
  • Each turn of the cycle forms three NADH molecules and one FADH2 molecule.
  • One GTP or ATP is also made in each cycle.
  • Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is amphibolic (both catabolic and anabolic).
  • Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants.

• Regulation of the Cell Cycle at Internal Checkpoints

  • The cell cycle is controlled by three internal checkpoints that evaluate the condition of the genetic information.
  • A checkpoint is one of several points in the eukaryotic cell cycle at which the progression of a cell to the next stage in the cycle can be halted until conditions are favorable (e.g. the DNA is repaired).
  • Cyclins are cell-signaling molecules that regulate the cell cycle.
  • The cell cycle is controlled at three checkpoints.
  • Explain the effects of internal checkpoints on the regulation of the cell cycle

• Proto-oncogenes

  • The genes that code for the positive cell cycle regulators are called proto-oncogenes.
  • Consider what might happen to the cell cycle in a cell with a recently-acquired oncogene.
  • The result is detrimental to the cell and will likely prevent the cell from completing the cell cycle; however, the organism is not harmed because the mutation will not be carried forward.
  • In addition to the cell cycle regulatory proteins, any protein that influences the cycle can be altered in such a way as to override cell cycle checkpoints.
  • An oncogene is any gene that, when altered, leads to an increase in the rate of cell cycle progression.

• Female Hormones

  • The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle governs the preparation and maintenance of the uterine lining.
  • These cycles occur concurrently and are coordinated over a 22–32 day cycle, with an average length of 28 days.
  • The first half of the ovarian cycle is the follicular phase.
  • Following ovulation, the ovarian cycle enters its luteal phase and the menstrual cycle enters its secretory phase, both of which run from about day 15 to 28.
  • Rising and falling hormone levels result in progression of the ovarian and menstrual cycles.

• Life Cycles of Sexually Reproducing Organisms

  • The main categories of sexual life cycles in eukaryotic organisms are: diploid-dominant, haploid-dominant, and alternation of generations.
  • Fertilization and meiosis alternate in sexual life cycles.
  • Within haploid-dominant life cycles, the multicellular haploid stage is the most obvious life stage.
  • Most fungi and algae employ a life cycle type in which the "body" of the organism, the ecologically important part of the life cycle, is haploid.
  • Fungi, such as black bread mold (Rhizopus nigricans), have haploid-dominant life cycles.

• Regulation of the Cell Cycle by External Events

  • External factors can influence the cell cycle by inhibiting or initiating cell division.
  • Most cells in the body exist in the state of interphase, the non-dividing stage of the cell life cycle.
  • Moving forward from this initiation point, every parameter required during each cell cycle phase must be met or the cycle cannot progress.