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Chromosomes and Cell Division
Chromatin and Chromosomes
Chromosome Terms
• chromatid
• centromere
• p arm
• q arm
• telomere
• kinetochore
Chromosome Structure
Telomeres
Kinetochores
Role of Kinetochores in Cell Division
Circular versus Linear Chromosomes
Prokaryotic Chromosome Replication
• Chromosome (DNA) replication begins at a single site called the origin of replication
Eukaryotic Chromosome Replication • Chromosome (DNA) replication begins at
multiple points along the DNA molecule
Cell Division
1. Binary Fission
2. Mitosis
3. Meiosis
Binary Fission
• Cell division in prokaryotic cells (bacteria)
• Asexual reproduction of unicellular eukaryotes
Complete the t-chart to explain three differences between mitosis and
meiosis:
Cell Cycle and Mitosis
Cell Cycle and Mitosis
• Growth, repair, replacement of dead or damaged cells, development, and asexual reproduction
• Occurs in the formation of somatic (body) cells
• Resulting daughter cells are genetically identical
Cell Cycle
Cell Cycle (Watch this 6 minute video to review the Cell Cycle)
• Complex set of stages that is highly regulated with checkpoints (determine the ultimate fate of the cell)
• Consists of interphase (G1, S, G2), mitosis and cytokinesis
• Results in two genetically identical daughter cells
G0
• Exit point from the cell cycle
• Resting or non-dividing stage, cell has reached final stage of specialization
Examples of cells:
• Neurons or cardiac muscle cells – permanent (terminal differentiation)
• Liver and kidney – semi-permanent
• Epithelial cells (lining) - rarely
Cell Cycle Checkpoints
Regulatory Molecules (Watch this 3 minute video to learn about regulation of the cell cycle)
• Cell cycle is regulated by cyclins and cyclin-dependent kinases (CDKs) which pace the sequential events of the cell cycle
1. cyclins – proteins that activate kinases
2. kinases (CDKs) – enzymes that activate or deactivate other proteins by phosphorylation
• Form CDK-cyclin complexes to stimulate the cell cycle
Fluctuations in Cyclin Levels
MPF (Mitosis Promoting Factor)
• Example of a cyclin-CDK complex
• Acts at the G2 checkpoint to trigger the events of the cell cycle (internal cellular cue)
PDGF (Platelet-Derived Growth Factor)
• Example of a growth factor that affects cell division primarily in blood vessels
• Protein released by platelets that stimulate fibroblasts to divide (external cellular cue)
• PDGF binds to receptors on fibroblasts
Other Regulatory Processes
Tumor Suppressor Genes (Watch this 2 minute video about tumor suppressor genes)
• Genes that inhibit the cell cycle or promote apoptosis (programmed cell death)
Examples include:
• p53 and Rb – act at the G1 checkpoint
• ATM and BRCA1 – act at the S checkpoint
• p53 – acts at the G2 checkpoint
Healthy versus Cancer Cell Division
p53 Tumor Suppressor Gene
• Acts at the G1 and G2 checkpoints
• Determines whether damaged DNA will be repaired during cell division (arrest cell division)
OR
• Damaged DNA cannot be repaired or is too excessive – triggers cell death (apoptosis)
• “Guardian of the Genome”
p53 Tumor Suppressor Gene
S Checkpoint: ATM and BRCA1
• Both act to inhibit the cell cycle at the S checkpoint
• If DNA is broken during replication, ATM protein is activated, halts the cell cycle
• ATM then activates other proteins such as BRCA1 to mediate DNA repair or trigger apoptosis
M Checkpoint: Anaphase-Promoting Complex/Cyclosome (APC/C) and
Mitotic Arrest Deficient (MAD)
• APC/C stimulates the destruction of proteins at the centromere to allow the sister chromatids to separate (anaphase)
• MAD acts to inhibit APC/C if chromosomes are not properly attached to the spindle
Consequences?
Errors at the M Checkpoint
Cell Cycle Checkpoints
• Watch this 6 minute video to review cell cycle regulation