Ch 8 Cellular Reproduction1
Reproduction at the Cellular Level
Overview
Generation of daughter cells from a parent cell.
Transfer of genetic material from parent to daughter cells.
Importance of Cell Division
Reproduction: Necessary for prokaryotic organisms.
Repair: Involves healing processes for cuts, skin cells, and intestinal lining.
Growth: Refers to the increase in size of an organism.
Types of Cell Division
Asexual Reproduction
Produces offspring that are identical to the parent cell or organism.
Occurs in almost all organisms but not in vertebrate animals.
Utilizes basic cell division processes.
Sexual Reproduction
Combines DNA from two organisms to create a new, non-identical organism.
Involves unique cell division processes discussed in the next chapter.
The Genome
Learning Goals (Chapter 8.1)
Describe bacterial and eukaryotic genomes.
Differentiate between a whole genome, individual chromosomes, and plasmids.
Genomic DNA
Genome: Cell's full complement of DNA.
Bacterial Genome
Composed of a single circular loop of DNA, known as the "Bacterial Chromosome."
Often contains small loops of DNA called plasmids, which do not carry essential cell activities but may carry resistance genes.
Eukaryotic Genome
More DNA content than prokaryotic cells, structured in multiple linear chromosomes
Some DNA exists in organelles such as mitochondria and chloroplasts.
Each species has a specific number of chromosomes:
Example: Humans have 46, chimps have 48, and ferns have 300.
Each chromosome contains multiple genes.
Example: Human chromosome 9 carries approximately 800-900 genes.
The Eukaryotic Cell Cycle
Learning Goals (Chapter 8.2)
Explain G0 phase where most body cells are not actively growing or dividing.
List three stages of interphase and key events that occur in each.
Identify five stages of mitosis and describe key events.
Discuss behavior of chromosomes during mitosis and cytoplasmic division during cytokinesis.
Explain the three checkpoints of the cell cycle.
Overview of the Cell Cycle
An ordered sequence including:
Cell Growth
Cell Division
Interphase: Preparation for cell division including cell growth and copying of chromosomes.
Mitotic (M) Phase: Includes Mitosis (division of nucleus) and Cytokinesis (division of cytoplasm).
G0 Phase
An inactive stage where cells perform their normal functions without planning to divide.
Brain cells remain permanently in G0 phase.
Most cells can leave G0 phase when an external signal triggers G1
Interphase (not in G0)
Longest part of the cell cycle divided into:
G1 Phase (first gap): Cell growth.
S Phase (synthesis): DNA and centrosome duplication.
G2 Phase (second gap): Further cell growth and cytoskeleton breakdown.
Anatomy of a Chromosome
Each duplicated chromosome comprises:
Sister Chromatids: Two identical copies of the original chromosome.
Centromere: The narrow waist where sister chromatids are attached.
Chromatin vs Chromosome
Chromatin: Decondensed form of DNA visible during interphase, looks like a net.
Chromosomes: Highly condensed chromatin, visible during cell division for proper separation.
Mitotic Phase: Mitosis and Cytokinesis
Mitosis: Divides the nucleus into two nuclei, with five stages:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis: Divides cytoplasm into two cells; starts in late anaphase/early telophase.
Mitotic Spindle
Required for aligning and separating sister chromatids.
Composed of microtubules (cytoskeleton); aids in elongation of the cell.
In animal cells → centrosome splits into two and organize both halves of the mitotic spindle
Phases of Mitosis
Prophase: Chromatin condenses into visible chromosomes; mitotic spindle begins to form.
Prometaphase: Nuclear envelope fragments, spindle microtubules attach to kinetochores.
Metaphase: Chromosomes align at the metaphase plate; kinetochores face opposite poles.
Anaphase: Sister chromatids separate and move to opposite poles; cell elongates.
Telophase: Nuclear envelope reforms around daughter chromosomes; they decondense.
Cytokinesis
Cytoplasm divides into two daughter cells:
Starts during the mitotic phase → late anaphase or early telophase
Cytokinesis isn’t a part of mitosis → it is a part of Mitotic Phase
In animal cells, cytokinesis occurs via a cleavage furrow formed by microfilaments.
In plant cells, a cell plate forms due to vesicles merging to create new cell wall material.
Control of the Cell Cycle: Checkpoints
Key decision points in the cell cycle determine whether to progress or stop, ensuring proper sequencing and completion of cell division phases.
Major Cell Cycle Checkpoints
G1 Checkpoint: Decides entry into S phase or G0.
G2 Checkpoint: Checks for chromosome duplication and DNA damage.
M Checkpoint: Assesses spindle attachment during metaphase.
Cancer and the Cell Cycle
Unchecked cell division leads to cancer as cells fail to respond to normal regulatory mechanisms.
Tumor Formation
Tumors are masses of abnormally growing cells, often due to malfunctioning checkpoints or cells that should remain in G0, or dividing cells that have lost checkpoint control → increased mutation rate
Types of Tumors
Benign Tumors: Remain localized; generally harmless unless pressing on vital organs.
Malignant Tumors: Can spread to other areas (metastasis) and are classified as cancer.
Key Genes in Tumor Growth
Proto-oncogenes (gas pedal): Encourage growth; mutations (oncogenes) lead to uncontrolled cell growth (“stuck gas pedal”).
Tumor Suppressor Genes (brake pedals): Promote DNA repair and apoptosis; negative mutationscan lead to the inability to control growth (“failed brake pedal”).
p53 Tumor Suppressor
Affects G1 checkpoint regulation; mutated in approximately 50% of malignant tumors.
Plays a critical role in determining cell cycle arrest and initiating apoptosis when cellular damage occurs.
Summary
Prokaryotes have a single circular chromosome; eukaryotes have multiple linear chromosomes enclosed in a nuclear membrane.
The cell cycle is a closely regulated process monitored by checkpoints.
Cancer results from the failure of these regulatory mechanisms, leading to unchecked cell division.