cell division

Two main phases of cell division

1. Interphase: The cell grows, performs metabolic processes, and replicates its DNA. 2. M Phase (Mitotic Phase): The cell divides its duplicated nucleus (mitosis) and cytoplasm (cytokinesis).

Comparison of mitosis and meiosis

Mitosis occurs in somatic cells for growth and repair, involving 1 division to produce 2 genetically identical diploid cells. Meiosis occurs in germ cells for sexual reproduction, involving 2 divisions to produce 4 genetically unique haploid gametes.

Phases of mitosis in sequential order

1. Prophase: Chromatin condenses, nuclear envelope breaks down, and spindle apparatus forms. 2. Metaphase: Chromosomes align at the metaphase plate. 3. Anaphase: Sister chromatids separate and move to opposite poles. 4. Telophase: Nuclear envelopes reform around separated chromosomes as they decondense.

Cell differentiation

The developmental process by which an unspecialized stem cell becomes a specialized cell type with specific structures and functions through selective gene expression.

Three major cell cycle checkpoints

1. $G_1$ Checkpoint: Checks for cell size, energy reserves, growth factors, and DNA damage before commitment to S phase. 2. $G_2$ Checkpoint: Ensures all DNA has been completely and accurately replicated before entering mitosis. 3. M (Spindle) Checkpoint: Verifies that all chromosomes are properly attached to spindle fibers at the metaphase plate prior to anaphase.

Comparison of benign and malignant tumors

Benign tumors stay localized, do not invade adjacent tissues, and do not metastasize. Malignant tumors invade surrounding tissues, disrupt organ function, and can spread through blood or lymph to form secondary tumors.

Cancer

A disease state caused by uncontrollable cell proliferation resulting from mutations that disrupt normal cell cycle regulation and checkpoint controls.

Metastasis

The spread of cancer cells from the primary tumor site to distant tissues and organs through the cardiovascular or lymphatic systems.

Proto-oncogene vs. Oncogene

A proto-oncogene is a normal cell gene that promotes controlled cell growth and division. An oncogene is a mutated, hyperactive version of a proto-oncogene that drives uncontrolled cell division.

Tumor suppressor gene

A gene that encodes proteins that normally inhibit cell division, repair damaged DNA, or trigger apoptosis when DNA damage cannot be fixed.

Action of the p53 gene

Acts as the 'guardian of the genome' by sensing DNA damage, halting the cell cycle at the $G_1$ checkpoint to activate repair machinery, or triggering programmed cell death (apoptosis) if damage is irreparable.

Cell cycle phase of DNA replication

S (Synthesis) phase of Interphase.

Process of DNA replication

1. Helicase unwinds the double helix at origins of replication. 2. Single-strand binding proteins stabilize template strands. 3. Primase lays down RNA primers. 4. DNA polymerase synthesizes new strands in the $5' \rightarrow 3'$ direction. 5. DNA ligase seals single-stranded nicks in the sugar-phosphate backbone.

Leading strand vs. lagging strand

The leading strand is synthesized continuously toward the replication fork in the $5' \rightarrow 3'$ direction. The lagging strand is synthesized discontinuously away from the replication fork in short sections.

Okazaki fragments

Short, discontinuous segments of newly synthesized DNA created on the lagging strand that are later joined together by DNA ligase.