Cell Cycle, DNA Structure, Replication, and Cancer: Study Notes for Anatomy & Physiology

Cell Cycle and Cell Division

Overview of the Cell Cycle

The cell cycle is the series of changes a cell undergoes from the time it forms until the time it divides. It is essential for growth, development, and tissue repair in multicellular organisms.

• Main Stages: Interphase, Mitosis, and Cytokinesis

• Interphase: Divided into G1, S, and G2 phases

Phases of Interphase

• G1 phase (Gap 1): Cell grows and carries out normal functions. The restriction (G1) checkpoint determines if the cell will divide.

• S phase (Synthesis): DNA replication occurs; each chromosome is duplicated.

• G2 phase (Gap 2): Further growth and preparation for division; synthesis of proteins required for mitosis.

• G0 phase: Some cells exit the cycle and enter a non-dividing state (e.g., neurons, cardiac myocytes); this is called terminal differentiation.

Mitosis

Mitosis is the process of nuclear division that results in two genetically identical daughter cells. It is essential for growth, repair, and asexual reproduction.

• Prophase: Chromatin condenses into visible chromosomes; nuclear envelope breaks down; spindle apparatus forms.

• Metaphase: Chromosomes align at the cell's equator (metaphase plate).

• Anaphase: Sister chromatids are pulled apart toward opposite poles.

• Telophase: Chromosomes decondense; nuclear envelope reforms.

Cytokinesis is the division of the cytoplasm, resulting in two separate cells.

Regulation of Cell Division

• Cell division is tightly regulated by cell cycle checkpoints and various proteins.

• Growth stimulatory proteins (proto-oncogenes): Promote cell division.

• Growth inhibitory proteins (tumor suppressor genes): Inhibit cell division; mutations can lead to cancer.

• Checkpoints: G1/S, G2/M, and spindle checkpoints ensure proper division and DNA integrity.

DNA Structure and Replication

DNA Structure

DNA (deoxyribonucleic acid) is a double-stranded helical molecule that stores genetic information.

• Composed of nucleotides: phosphate group, deoxyribose sugar, and nitrogenous base (A, T, G, C).

• Phosphate backbone: Covalently bonded sugar and phosphate groups form the backbone of each strand.

• Base pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C) via hydrogen bonds.

• Antiparallel strands: The two DNA strands run in opposite directions.

Chromosomes and Genes

• Humans have 46 chromosomes (23 pairs) in each somatic cell.

• DNA is packaged with histone proteins to form nucleosomes.

• Gene: A sequence of DNA that codes for a protein or functional RNA.

DNA Replication

DNA replication is the process by which DNA makes an exact copy of itself before cell division. It is semiconservative, meaning each new DNA molecule consists of one old strand and one new strand.

• Steps of DNA Replication:

  1. DNA helicase unwinds the double helix.

  2. Single-stranded binding proteins stabilize the unwound DNA.

  3. DNA polymerase synthesizes a new complementary strand using the original strand as a template.

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized in short fragments (Okazaki fragments) that are later joined.

Equation:

$\text{DNA}_{\text{parent}} \xrightarrow{\text{replication}} \text{DNA}_{\text{daughter}} + \text{DNA}_{\text{daughter}}$

Gene Expression: Transcription and Translation

Transcription

Transcription is the process of synthesizing RNA from a DNA template.

• Initiation: RNA polymerase binds to the promoter region of the gene.

• Elongation: RNA polymerase synthesizes the RNA strand by adding complementary RNA nucleotides.

• Termination: RNA polymerase reaches a terminator sequence and releases the RNA transcript.

Translation (Protein Synthesis)

Translation is the process by which mRNA is decoded by ribosomes to produce a specific protein.

• Initiation: mRNA attaches to the ribosome; translation begins at the start codon (AUG).

• Elongation: tRNA brings amino acids to the ribosome, which are joined to form a polypeptide chain.

• Termination: The process ends when a stop codon is reached.

Equation:

$\text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Transfer RNA (tRNA)

• tRNA molecules bring the correct amino acids to the ribosome during translation.

• Each tRNA has an anticodon that pairs with the corresponding codon on the mRNA.

Cell Senescence and Aging

Cell Senescence

Cell senescence is the process by which cells lose the ability to divide and grow, often as a result of DNA damage or telomere shortening. It is a key factor in aging and the prevention of cancer.

• Most cells can only divide a limited number of times (~50-60 divisions).

• Telomeres: Repetitive DNA sequences at the ends of chromosomes that shorten with each division.

• Telomerase: An enzyme that can extend telomeres; active in stem cells and cancer cells.

Cancer: Causes, Progression, and Therapies

Overview of Cancer

Cancer is a group of diseases characterized by uncontrolled cell growth due to mutations in genes that regulate the cell cycle.

• Oncogenes: Mutated proto-oncogenes that promote excessive cell division.

• Tumor suppressor genes: Genes that normally inhibit cell division; mutations can lead to cancer.

• Metastasis: The spread of cancer cells to distant sites in the body.

• Anaplasia: Loss of normal cell structure and function.

Types of Cancer

• Carcinoma: Cancer of epithelial cells (most common type).

• Sarcoma: Cancer of connective tissue (e.g., bone, muscle).

Stages of Tumor Progression

Stage	Description
Stage 0	Cancer in situ (in its normal place)
Stage I	Small and confined tumor
Stage II	Larger, but still confined tumor
Stage III	Cancer cells found in nearby lymph nodes; may require chemotherapy, etc.
Stage IV	Metastasis to distant sites; aggressive treatment required

Genes and Proteins Involved in Cancer

• Proto-oncogenes: Encode proteins that stimulate cell division.

• Tumor suppressor genes: Encode proteins that inhibit cell division (e.g., p53 gene).

• Cyclins and cyclin-dependent kinases (CDKs): Regulate the cell cycle.

Anti-Cancer Therapies

• Surgery: Removal of the tumor if possible.

• Chemotherapy: Use of drugs to kill rapidly dividing cells; may be cell cycle specific or non-specific.

• Radiation: Kills cancer cells by damaging DNA.

• Hormone therapy: Used for hormone-sensitive cancers (e.g., breast, prostate).

• Immunotherapy: Use of monoclonal antibodies or immune system stimulation.

• Personalized medicine: Drugs tailored to the genetic makeup of the tumor.

Summary Table: Key Terms and Definitions

Term	Definition
Cell Cycle	Series of events that cells go through as they grow and divide
Mitosis	Division of the nucleus resulting in two identical daughter cells
DNA Replication	Process of copying DNA before cell division
Oncogene	Mutated gene that promotes uncontrolled cell division
Tumor Suppressor Gene	Gene that inhibits cell division and prevents cancer
Apoptosis	Programmed cell death
Telomere	Protective DNA sequence at the end of chromosomes

Additional info: Some explanations and context have been expanded for clarity and completeness, including the summary tables and the stepwise breakdown of mitosis and cancer therapies.