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  1. Genetics alone is a relatively small part of Step 1 but shows up as part of questions in almost every organ system

  2. The cell cycle provides an important foundation to discuss genetics, as cell division errors lead to diseases like Down syndrome

  3. Here we'll review some important concepts that can get easy points on Step 1

  4. We also review important genetic diseases seen in previous chapters to highlight how genetics can show up in many organ systems

  5. Lysosomal storage diseases represent a final review of genetics and review of concepts discussed previously


  1. The cell cycle describes the common process of cell growth

  2. Phases can be divided into 3 main groups

  3. DNA Synthesis (S)

    1. The initial step toward cell division is replication of the DNA

    2. Each of the 23 pairs of chromosomes is duplicated, forming sister chromatids

  4. Mitosis/meiosis (M)

    1. After DNA synthesis and a period of rest, cells undergo division, and sister chromatids split to provide DNA for both daughter cells

    2. Germ cells meant for sexual reproduction undergo meiosis to produce gametes (sperm or ova)

    3. All other cells are somatic cells and undergo mitosis

  5. Gap/rest (G)

    1. G1: After recent division, the cell rests and carries out normal function

    2. G0: At this point the cell may enter a long-term rest phase if it does not need to regularly multiply

    3. G2: Between S and M phases, the cell grows and prepares organelles to support 2 new cells

    4. After M phase, the cell enters G1 and restarts the cycle, or may enter G0

Figure 10-1.

Overview of the cell cycle.


  1. The cell cycle is a crucial process that requires careful regulation

  2. Loss of regulation is a key mechanism underlying cancer

  3. There are 3 major types of regulation to discuss

    1. Cyclins

    2. Tumor suppressors

    3. Proto-oncogenes


  1. Checkpoints at each step in the cycle prevent unrestrained cell division

  2. The 2 protein classes to know are cyclins and cyclin-dependent kinases

  3. Cyclins are the “time keepers” of the cell cycle

    1. Cyclin concentration changes depending on the phase of the cycle

    2. High levels of active cyclins are required to activate the next step of cell division

  4. Cyclin-dependent kinases (CDKs)

    1. Kinase proteins that are inactive in their resting state

    2. Binding of cyclins is required to activate CDKs and drive cell maturation

    3. However, cyclin binding is not sufficient to activate CDKs: Phosphorylation of the CDK is also required

  5. Phosphorylation of CDK-cyclin complexes allows for many checks on cell cycle progression


  1. Many proteins that regulate the cell cycle were discovered because they are inactivated in cancer cells

  2. Tumor suppressors are a class of genes that work to limit cell ...

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