Chromosome Structure, Classification, and the Cell Cycle

Comprehensive vocabulary flashcards covering chromosome architecture, centromere-based classification, telomere biology, phases of the cell cycle, and molecular checkpoint controls.

Centromere

A constricted region of a chromosome where sister chromatids are joined by cohesin proteins, responsible for the assembly of the kinetochore.

Kinetochore

A specialized protein structure assembled at the centromere that serves as the attachment site for mitotic spindle microtubules and directs chromosome movement during cell division.

Telomere

A specific non-coding DNA sequence located at the tips of chromosomes that stabilizes and protects chromosomal ends, prevents thread entangling or fusion, and acts as an aging clock.

End-Replication Problem

The inability of DNA polymerases to completely replicate chromosomal ends, causing progressive telomere shortening with each division until a critical length triggers cellular senescence or apoptosis.

Telomerase

An enzyme responsible for synthesizing new telomeric sequences; typically low in somatic cells but highly active in germ cells, stem cells, and cancer cells.

Homologous Chromosomes

Pairs of matching chromosomes (one maternal and one paternal) that are identical in size, structure, and genetic loci across both sister chromatids.

Metacentric Chromosome

A chromosome with its centromere located precisely at the center, resulting in equal arm lengths.

Submetacentric Chromosome

A chromosome with its centromere positioned nearer to the center, creating unequal short (p) and long (q) arms.

Acrocentric Chromosome

A chromosome where the centromere is situated near one end, yielding very short p arms.

Telocentric Chromosome

A chromosome whose centromere is positioned at the terminal end; this type is not present in human cells.

Interphase

The interval between two successive mitotic divisions, constituting approximately $90\%$ of the cell cycle, during which a cell performs specialized functions and prepares for division.

$$G_1\text{ Phase (Gap 1)}$$

The initial growth phase of interphase, variable in duration, marked by restoration of cell volume, intense RNA and protein synthesis, and the presence of $46$ single (S.) chromosomes.

$$S\text{ Phase (Synthesis)}$$

An $8\text{-hour}$ phase of interphase characterized by DNA synthesis, duplication of chromosomes, histone synthesis, and the initiation of centrosome duplication.

$$G_2\text{ Phase (Gap 2)}$$

A $2.5\text{ to }3\text{-hour}$ postsynthetic phase involving tubulin and ATP synthesis, completion of centrosome duplication, and containing $46$ double (D.) chromosomes ($4n$).

$$G_0\text{ Phase}$$

A quiescent state outside the replicative cycle where cells no longer divide, but can re-enter upon stimulation by growth factors.

Labile Cells (Continuous Renewable Cells)

Cells that continuously proliferate and regenerate tissues throughout life, such as epithelial cells and blood cells.

Static Cells (Non-Renewable Cells)

Terminally differentiated ($G_{TD}$) cells that permanently exit the cell cycle and cannot undergo further division, such as cardiac muscle and nerve cells.

Stable Cells (Potentially Renewable Cells)

Quiescent cells residing in $G_0$ that divide slowly or episodically to maintain tissue architecture, capable of resuming division in response to injury or demand (e.g., hepatocytes and fibroblasts).

Restriction Checkpoint ($G_1\text{-}S\text{ Checkpoint}$)

A primary cell cycle regulatory checkpoint, termed the 'point of no return,' where the cell assesses environmental conditions, size, nutrition, and DNA integrity to commit to division or cycle exit.

$$G_2\text{-}M\text{ Checkpoint}$$

A control checkpoint that verifies whether DNA replication has been completed without damage prior to permitting cell entry into mitosis.

Spindle Assembly Checkpoint

A metaphase-to-anaphase checkpoint verifying that all chromosomes are correctly aligned and attached to mitotic spindle microtubules before initiating sister chromatid separation.

Cyclin-Dependent Kinases (CDKs)

Protein kinases that, when complexed with regulatory cyclins, phosphorylate specific downstream target proteins needed to drive transitions through phases of the cell cycle.

p53 Gene

A tumor suppressor gene activated in response to persistent DNA damage that redirects the cell toward programmed cell death (apoptosis) if repairs cannot be made.

Retinoblastoma (Rb) Gene

A tumor suppressor gene that encodes a regulatory protein active at the $G_1$ restriction checkpoint, arresting cycle progression until a mitogenic signal arrives.

Proto-oncogenes

Normal regulatory genes responsible for promoting cell growth and division, which upon mutation become oncogenes leading to uncontrolled proliferation and malignant transformation.