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

1. Prophase: Chromatin condenses into visible chromosomes; mitotic spindle begins to form.

2. Prometaphase: Nuclear envelope fragments, spindle microtubules attach to kinetochores.

3. Metaphase: Chromosomes align at the metaphase plate; kinetochores face opposite poles.

4. Anaphase: Sister chromatids separate and move to opposite poles; cell elongates.

5. 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

1. G1 Checkpoint: Decides entry into S phase or G0.

2. G2 Checkpoint: Checks for chromosome duplication and DNA damage.

3. 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.