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

Cells: The Living Units – DNA, Cell Cycle, and Cancer

DNA Structure and Function

Deoxyribonucleic acid (DNA) is the hereditary material in almost all living organisms. It serves as the blueprint for life, containing the instructions necessary for the development, functioning, and reproduction of cells.

• DNA Structure: DNA is a double helix composed of two strands wound around each other. Each strand is made up of repeating units called nucleotides.

• Nucleotide: Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.

• Nitrogenous Bases: There are four types: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

• Base Pairing: Adenine always pairs with Thymine (A-T), and Guanine always pairs with Cytosine (G-C).

• Gene: A segment of DNA that contains the instructions for making a specific protein.

• Chromosome: DNA is packaged into structures called chromosomes, which are found in the nucleus of the cell.

Example: The gene for hemoglobin is a specific sequence of DNA that codes for the hemoglobin protein in red blood cells.

DNA Organization in the Cell

DNA is highly organized within the cell to fit inside the nucleus and to regulate gene expression.

• Chromatin: DNA is wound around proteins called histones, forming a compact structure known as chromatin.

• Location: DNA is always located in the nucleus of eukaryotic cells.

• Gene Expression: Specific regions of DNA unwind when a gene is expressed or transcribed to make a protein.

The Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It ensures that new cells are produced for growth, repair, and replacement of dying cells.

• Purpose: To replace dying cells, heal wounds, and allow for growth.

• Frequency: Many human cells go through one cycle approximately every 24 hours.

Main Phases of the Cell Cycle

• Interphase: The cell grows and prepares for division. It consists of three sub-phases:

  • G1 phase (Gap 1): Cell growth and normal functions.

  • S phase (Synthesis): DNA replication occurs; chromosomes are duplicated.

  • G2 phase (Gap 2): Further growth and preparation for mitosis.

• M phase (Mitosis): The cell divides its copied DNA and cytoplasm to form two new cells.

• G0 phase: A resting state where the cell is not actively dividing.

Diagram: The Cell Cycle

The cell cycle is often represented as a circular diagram with arrows indicating progression through G1, S, G2, and M phases, with G0 as a possible exit from the cycle.

Mitosis: Steps of Cell Division

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets, resulting in two daughter cells.

1. Prophase: Chromosomes condense and become visible. The nuclear envelope begins to break down. Spindle fibers start to form.

2. Metaphase: Chromosomes align at the cell's equator, attached to spindle fibers.

3. Anaphase: Sister chromatids are pulled apart to opposite poles of the cell.

4. Telophase: New nuclear envelopes form around each set of chromosomes, which de-condense back into chromatin.

5. Cytokinesis: The cytoplasm divides, resulting in two separate daughter cells.

Example: Skin cells divide by mitosis to replace cells lost due to injury.

Cell Cycle Regulation and Checkpoints

The cell cycle is tightly regulated by checkpoints to ensure proper division and prevent errors.

• Checkpoints: Critical points in G1, G2, and M phases where the cell assesses whether to proceed with division.

• Regulatory Proteins: Proteins such as cyclins and cyclin-dependent kinases (CDKs) control progression through the cell cycle.

Cancer and the Cell Cycle

Cancer is a disease characterized by uncontrolled cell division due to failures in cell cycle regulation.

• Causes: Mutations in genes that regulate the cell cycle, exposure to carcinogens (e.g., tobacco, radiation, chemicals), inherited genetic mutations, and errors in DNA replication.

• Oncogenes: Genes that, when mutated, can promote uncontrolled cell division.

• Tumor Suppressor Genes: Genes that normally inhibit cell division or cause apoptosis (programmed cell death) when necessary.

• HeLa Cells: An example of 'immortal' cancer cells derived from Henrietta Lacks in 1951, widely used in research.

Common Cancer Types (by Sex)

Males	Females
Prostate	Breast
Non-Hodgkin Lymphoma	Non-Hodgkin Lymphoma
Leukemia	Leukemia
Colon & Rectum	Ovary
Lung	Lung

Additional info: Table entries inferred and summarized from provided data.

Key Terms and Definitions

• Mutation: A change in the DNA sequence that can affect gene function.

• Carcinogen: A substance capable of causing cancer in living tissue.

• Apoptosis: Programmed cell death, a normal process to remove damaged or unnecessary cells.

Summary Table: DNA, Chromosomes, and Genes

Structure	Description
DNA	Double helix of nucleotides; contains genetic instructions
Gene	Segment of DNA coding for a protein
Chromosome	DNA packaged with proteins; found in the nucleus

Key Equations

• Base Pairing Rule:

$\text{A} = \text{T}$ $\text{C} = \text{G}$

• Cell Cycle Progression:

$\text{G1} \rightarrow \text{S} \rightarrow \text{G2} \rightarrow \text{M}$

Additional info: Some explanations and table entries have been expanded for clarity and completeness.