Unit 3 Chapter 10

Source of Cells:
- All cells arise from pre-existing cells.
- This statement is a foundational aspect of cell theory.
Purpose of Cell Division:
- Primary aim: To generate new cells.
Functions of Cell Division:
- Growth: Facilitates the increase in size of the organism.
- Repair: Replaces damaged or dead cells.
- Reproduction:
- Asexual Reproduction: Involves mitosis.
- Sexual Reproduction: Involves meiosis which produces gametes.

Genome:
- Defined as the complete set of DNA within a cell.
Chromatin:
- Composed of DNA and proteins in a loose form.
- Allows for the use of DNA in gene expression.
Chromosomes:
- Structures formed from condensed chromatin.
- Become visible during cell division.
Comparison of Chromosome Structure:
- Prokaryotic Cells:
- Singular, circular chromosome.
- Located within the nucleoid region (absence of nucleus).
- Eukaryotic Cells:
- Multiple linear chromosomes.
- Enclosed within a nucleus.
Human Chromosome Count:
- Somatic cells: Contain 46 chromosomes (23 pairs).
- Gametes: Contain 23 chromosomes.
Sister Chromatids:
- Identical copies of a chromosome.
- Formed post DNA replication.
Centromere:
- The region where sister chromatids are connected.

Mitosis:
- Defined as the process of nucleus division.
- Result: Production of two identical nuclei.
Cytokinesis:
- Defined as the division of the cytoplasm following mitosis.
Comparison of Mitosis and Meiosis:
- Mitosis:
- Involves one division.
- Produces two identical diploid cells.
- Function: Growth and repair.
- Meiosis:
- Involves two divisions.
- Produces four non-identical haploid cells.
- Function: Formation of gametes.

Phases of the Cell Cycle:
- Interphase (longest phase):
- G1 phase: Growth occurs.
- S phase: DNA replication takes place.
- G2 phase: Prepares for division.
- Mitotic Phase (shorter):
- Consists of mitosis and cytokinesis.
Duration of Phases:
- Cells typically spend most of their time in Interphase.
- Shortest duration is during Mitosis, particularly anaphase.
- Longest duration is typically during the G1 phase.

Definition: A structure responsible for moving chromosomes during cell division.
Composition: Made of microtubules.
Function: Essential for separating sister chromatids.
Mechanism of Chromosome Movement:
- Microtubules attach to kinetochores on chromosomes.
- The microtubules shorten, pulling chromosomes apart.
Key Structures:
- Centrosome: Acts as a microtubule organizing center that forms the spindle.
- Kinetochore: A protein structure at the centromere where microtubules attach.
Anaphase Separation Process:
- Cohesins hold sister chromatids together.
- Separase cuts cohesins, facilitating chromatid separation.

Animal Cells:
- Formation of a cleavage furrow that leads to the pinching of the cell membrane.
Plant Cells:
- Formation of a cell plate which develops into a new cell wall.
Reason for Difference:
- The presence of a rigid cell wall in plant cells necessitates a different cytokinesis mechanism.

Cell Cycle Definition: The process regulating the timing of cell division.
Control System: Utilizes checkpoints to determine if a cell proceeds in the cycle.
Major Checkpoints:
- G1 checkpoint: Considered the most crucial checkpoint.
- G2 checkpoint:
- M checkpoint:
Internal Signals:
- Examples include DNA damage and chromosome attachment issues.
External Signals:
- Include growth factors and cell density.
Cyclin-Dependent Kinases (Cdks):
- Enzymes that drive the cell cycle; activated by cyclins.
G0 Phase:
- Represents a non-dividing state for cells that exit the cycle.
- Typical cells in G0 include nerve and muscle cells.
Experiments Demonstrating Cyto-Control:
- Cell fusion experiments indicated cytoplasmic signals regulate the cell cycle.

Growth Factors:
- Defined as proteins that stimulate cell division.
Density-Dependent Inhibition:
- Cells cease to divide upon crowding.
Anchorage Dependence:
- Cells require attachment to a surface to proliferate.
In Vitro Experiments:
- Normal cells halt division when they come into contact, unlike cancer cells which continue dividing.

Definition: Proteins that inhibit cell division.
Examples:
- Tumor suppressor genes such as p53.
Functions:
- Repair DNA damage.
- Halt the cell cycle if damage is detected.
- Induce apoptosis if repair is unfeasible.

Definition: Characterized by uncontrolled cell division.
Origins: Arises from mutations in genes regulating the cell cycle.
Differences Between Cancer Cells and Normal Cells:
- Cancer cells divide continuously and ignore checkpoints.
- They disregard density-dependent inhibition.
Transformation: Refers to the conversion of a normal cell into a cancerous one.
”Immortal” Cells: Cells that can divide indefinitely.
Tumors:
- Benign Tumors: Remain localized.
- Malignant Tumors: Have the capacity to spread and invade other tissues.
Metastasis: The process by which cancer spreads to other parts of the body.
Angiogenesis: The formation of new blood vessels, often associated with tumor growth.

Proto-Oncogenes:
- Normal genes that promote cell growth.
Oncogenes:
- Mutated versions of proto-oncogenes causing excessive cell division.

Process of Bacterial Cell Division: Known as binary fission.
Steps Involved:
- DNA replication occurs.
- The cell elongates.
- The cell divides into two daughter cells.
Comparative Differences with Eukaryotes:
- Bacteria lack a nucleus.
- They do not undergo mitosis.
- Division is faster and simpler than eukaryotic cell division.