Genetics and Cell Biology Quiz

Module 4 Summary

• Cells commit to division after reaching the S phase checkpoint (G1-S transition).
• Mitosis creates a second nucleus, and cytokinesis produces the second cell.
• Interphase and M-phase are crucial parts of the cell cycle.
• The G2 checkpoint (G2-M transition) ensures quality control.
• Early insight into cell cycle regulation came from cell fusion experiments, demonstrating that cells in S-phase trigger replication in cells in G1 phase, and cells in M-phase trigger mitosis in cells in G1 phase, despite the latter not having fully copied its genome.
• Cytoplasmic factors (cyclins) peak at specific cell cycle phases and activate cyclin-dependent kinases (Cdks).
• The active cyclin-Cdk complex orchestrates events through phosphorylation of targeted proteins within the cell cycle.
• G1/S cyclins commit cells to S-phase, while S cyclins initiate DNA replication and M cyclins manage mitosis.
• A fourth cyclin, G1 cyclin, facilitates G1 checkpoint progression but isn't always present in all cells.
• Cyclin concentration control is primarily achieved by selective proteolytic destruction, which is assisted by ubiquitinoylation and proteosomal degradation.
• M-cyclins accumulate during S and G2 phases, forming MPF.
• MPF's activity peaks in metaphase, driving mitosis.
• The G2 checkpoint is essential as M-cyclin degradation ceases MPF activity, passing the cell into G1 phase. The degradation of cyclin continues and the Cdk component of MPF is recycled.
• Cells commit to cell division via external signals—growth factors (like PDGF for fibroblasts).
• Cells are anchorage-dependent, needing a substrate for growth and division.
• Cells exhibit density-dependent inhibition, stopping division when a monolayer is reached.
• Cancer cells often lack anchorage and density-dependent inhibition, enabling uncontrolled proliferation.
• Oncogenes are the genes mutated that cause inappropriate cell growth. Ras relays growth factor signals to the nucleus. Proto-oncogenes are the normal genes involved in positive growth signals.
• Tumor suppressor genes help with negative growth signals. p53 halts cell replication during DNA damage.
• A single mutation rarely causes cancer; several mutations likely occur for cancerous transformation. 
• Cells that replicate DNA require two copies (one for the parent, and one for the daughter cell).
• Semi-conservative replication involves separating parental strands and synthesizing a complementary daughter strand for each.
• In 1958, Meselson and Stahl's experiment validated semi-conservative replication.
• Prokaryotic DNA replication initiates at an origin of replication, forming a replication bubble or fork, and proceeding in both directions. 
• Bacterial replication is theta replication because of its shape.
• Residents of the replication fork include helicase, which separates parental strands, topoisomerase, which relieves tension, and single-strand binding proteins.
• The G2 checkpoint ensures all prerequisites are completed before the cell cycle moves into the next phase.
• The G2 checkpoint assures the cell is ready to enter M-phase.
• M-Cdk functions to break down the nuclear lamina (phosphorylating lamin), condense chromatin, and form the mitotic spindle.
• The nuclear lamina contains lamin (structural protein).
• M-Cdk activity results in the nuclear membrane fragmentation.
• Chromatin condensation is the preparation of chromatin into chromosomes.
• The main event in prophase is the condensation of interphase chromatin.
• Prometaphase is the phase during which the nuclear membrane disintegrates and microtubules from the centrosomes attach to the chromosomes.
• The kinetochore is a protein plaque forming on the chromosomes' centromeres, where microtubules stably attach.
• Kinetochore microtubules help move chromosomes towards the poles. During metaphase, chromosomes align on the metaphase plate.
• Spindle attachment checkpoint checks if all the kinetochores are attached to microtubules before proceeding to anaphase to prevent genetic chaos.
• The anaphase promoting complex (APC) is a proteolytic complex that drives the metaphase-anaphase transition.
• Anaphase A moves chromosomes toward the poles and anaphase B separates the poles.
• During telophase, the chromosomes decondense, the nuclear envelope reforms, and cytokinesis begins.
• Cytokinesis in animals involves a contractile ring made of actin and myosin to create a cleavage furrow. In plants, a cell plate forms, separating the two daughter cells.
• Once a cell divides, it enters G1. The activity of M-Cdk is reduced by M-cyclin degradation via APC, which prevents continued cell cycle activity in mitosis without cytokinesis.
• When mitosis is not followed by cytokinesis, a multi-nucleated cell (syncytium) is formed. This process is seen in Drosophila fruit flies following fertilization.
• Plant cytokinesis forms a cell plate that grows by adding cellulose microfibrils.
• Mutations in DNA can alter the genetic code, leading to various outcomes ranging from neutral effects to deleterious consequences, including lethality. DNA Polymerase proofreads DNA during replication.
• Mutation rates are relatively fixed across organisms; however, DNA repair mechanisms greatly reduce these rates.
• Spontaneous DNA damage types include depurination (loss of a purine base) and deamination (changing a base).
• Pyrimidine dimers are created by UV light and hinder DNA replication.
• Repair mechanisms, such as base excision repair and nucleotide excision repair, correct these lesions.
• Various chromosomal alterations (deletion, duplication, inversion, reciprocal translocation) can affect chromosome structure or lead to disease.
• Translocation in cancer cells can lead to unregulated growth, exemplified by the Philadelphia chromosome.
• Cells display immortality in the laboratory, like HeLa cells, due to telomerase expression.