The cell grows, synthesizes proteins and RNA, and evaluates environmental signals. The cell decides whether to commit to division. The critical decision point is called the Restriction Point.
G1/S Checkpoint
Rb protein binds E2F transcription factors, blocking S phase entry. Growth signals activate CDK4/Cyclin D, which phosphorylates Rb, releasing E2F. The cell commits irreversibly.
Cancer Disruption
Cyclin D amplification overactivates CDK4, continuously hyperphosphorylating Rb. The restriction point disappears. Cells divide without any growth signal -- a hallmark of cancer.
Key Proteins
Cyclin D / CDK4/6 push past restriction point Rb is the brake pedal p21, p27 are CDK inhibitors p53 activates p21 when DNA is damaged
Checkpoint Gate -- Answer Correctly to Unlock S Phase
What happens to Rb protein when a growth signal allows the cell to enter S phase?
S
S Phase
Synthesis -- DNA Replication
6-8 hrs
Average
What Happens
Every one of the cell's 3 billion base pairs is copied. DNA content doubles from 2N to 4N. Each chromosome becomes two identical sister chromatids joined at the centromere.
Error Rate and Repair
DNA polymerase makes one error per 10 billion base pairs thanks to proofreading. Mismatch repair catches remaining errors. When MMR is disabled by cadmium, error rates skyrocket -- the mutator phenotype.
Where Carcinogens Strike
Benzo[a]pyrene forms bulky adducts on guanine. DNA polymerase inserts the wrong base at the lesion. If codon 12 of RAS is mutated, an oncogene is born in a single replication event.
Key Proteins
DNA Pol delta/epsilon with proofreading PCNA sliding clamp processivity factor BRCA1/2 repair stalled replication forks Cyclin E/CDK2 fire replication origins
Checkpoint Gate -- Answer Correctly to Unlock G2 Phase
What type of mutation does benzo[a]pyrene cause that can convert a proto-oncogene into an active oncogene?
G2
G2 Phase
Gap 2 -- Pre-Mitotic Verification
3-4 hrs
Average
What Happens
The cell verifies DNA replication is complete and checks for damage before committing to mitosis. The cell produces tubulin for spindle fibers and condensins for chromosome compaction.
G2/M Checkpoint
ATM and ATR kinases scan replicated DNA for double-strand breaks. When damage is found, ATM activates CHK2, which stabilizes p53. p53 transcribes p21, which blocks CDK1/Cyclin B -- division halts.
When p53 is Lost
Without p53 the G2/M checkpoint is blind. Cells with damaged DNA proceed into mitosis, distribute broken chromosomes to daughter cells, and accumulate genomic instability with every division.
Key Proteins
ATM/ATR detect DNA damage CHK1/CHK2 amplify the damage signal CDC25 activates CDK1 (inactivated during arrest) Cyclin B/CDK1 is the Mitosis-Promoting Factor
Checkpoint Gate -- Answer Correctly to Unlock M Phase
A cell with non-functional p53 has UV-damaged DNA at the G2/M checkpoint. What most likely happens?
M
M Phase
Mitosis -- Cell Division
1-2 hrs
Average
Four Sub-phases
Prophase: Chromosomes condense, spindle forms Metaphase: Chromosomes align at plate Anaphase: Sister chromatids pulled apart Telophase: Two nuclei form, cytokinesis begins
Spindle Assembly Checkpoint
Mad2 and BubR1 are released from any kinetochore not yet attached to spindle fibers. Free Mad2 inhibits APC/C ubiquitin ligase. All chromosomes must attach before Securin is degraded and Separase separates chromatids.
Aneuploidy in Cancer
Defective spindle checkpoints allow cells with unattached chromosomes to proceed. Daughter cells receive the wrong chromosome number -- aneuploidy. Over 90% of solid tumors are aneuploid.
Key Proteins
APC/C drives mitotic exit via ubiquitination Securin holds sister chromatids together Separase cleaves cohesin when released Mad1/Mad2/BubR1 spindle checkpoint
Checkpoint Gate -- Answer Correctly to Unlock Gene Deep-Dive
During metaphase, one chromosome has not yet attached to the spindle. What happens to APC/C activity?
GEN
Gene Deep-Dive
Proto-oncogenes, Oncogenes, and Tumor Suppressors
Proto-oncogenes
Normal Function
Proto-oncogenes are normal, healthy genes encoding proteins essential for regulated cell growth. They produce growth factors (EGF, PDGF), growth factor receptors (EGFR, HER2), intracellular signal transducers (RAS, RAF, PI3K), and transcription factors (MYC, FOS, JUN).
In a healthy cell, proto-oncogenes are under tight regulatory control -- switched on by external signals and switched off once the signal passes. They only activate when the cell genuinely needs to divide.
The key insight: proto-oncogenes are not inherently dangerous. Every cell requires them. Danger arises only when mutations cause constitutive activation.
350+
Known proto-oncogenes in humans
4 types
GFs, receptors, transducers, TFs
1 hit
Only one mutant copy needed (dominant)
Oncogenes
Gain-of-Function
Oncogenes are mutated proto-oncogenes that signal constant division even without growth factor binding.
Point mutation: A single base change at Gly12 of RAS locks the protein in active GTP-bound state permanently. Mutant KRAS cannot hydrolyze GTP -- found in 40% of all human cancers.
Gene amplification: Extra copies produce excess protein. HER2 is amplified in 20% of breast cancers -- targeted by Herceptin (trastuzumab).
Chromosomal translocation: BCR-ABL fusion in CML produces a constitutively active tyrosine kinase, targeted by Gleevec (imatinib).
40%
of cancers carry RAS mutations
Dominant
One mutant copy sufficient
3 routes
Point mut / amplification / translocation
Tumor Suppressor Genes
Loss-of-Function
Tumor suppressors are the brakes of the cell cycle -- they inhibit proliferation, trigger DNA repair, or activate apoptosis. Unlike oncogenes, they are recessive: both copies must be inactivated for cancer to develop.
Knudson Two-Hit Hypothesis (1971): In hereditary cancers one mutant copy is inherited -- only one somatic second hit is needed. In sporadic cancers both hits occur somatically.
Rb: Binds E2F when hypophosphorylated, blocking S phase. Loss of both Rb copies causes retinoblastoma in children.
BRCA1/BRCA2: Homologous recombination repair proteins. Without them, double-strand breaks route through error-prone NHEJ.
Recessive
Both copies must be lost
2 hits
Knudson two-hit hypothesis
70-80%
lifetime breast cancer risk with BRCA1 mut
p53 -- Guardian of the Genome
Most Critical Tumor Suppressor
p53 (encoded by TP53) sits at the center of the cell's damage response. Under normal conditions it is continuously produced but rapidly degraded by MDM2.
Activation: DNA damage activates ATM/ATR, which phosphorylate p53 at Ser15/Ser20, blocking MDM2 binding. p53 accumulates rapidly.
Three outputs p53 can trigger: 1. Cell cycle arrest: Transcribes CDKN1A (p21), which inhibits CDK2/Cyclin E -- blocks S phase entry. 2. DNA repair: Activates GADD45 and other repair pathway genes. 3. Apoptosis: If damage is irreparable, transcribes BAX, PUMA, NOXA -- activating mitochondrial apoptosis.
50%+
of all cancers have TP53 mutations
3 roles
Arrest / Repair / Apoptosis
MDM2
Normal p53 regulator -- active drug target
Live Cell Event Log
System
Cell cycle simulation initialized. All checkpoints active. Genome integrity: 100%.
