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