Chapter+7+Cell+Cycle+and+Cell+Division

The Cell Cycle and Cell Division - Key Concepts

Concept 7.1: Reproduction May Be Asexual or Sexual

  • Cell Division: One cell divides to form two daughter cells.

    • Asymmetrical Division: Mother cell produces one daughter cell and remains unchanged (e.g., yeast).

    • Symmetrical Division: Mother cell divides to produce two identical daughter cells (e.g., bacteria and protists).

    • Multicellular Organisms: Exhibit both types of reproduction.

  • Asexual Reproduction:

    • Offspring are clones, genetically identical to the parent; variations arise from mutations (changes in DNA sequences).

    • Prokaryotes: Reproduce by binary fission.

    • Eukaryotes: Single-celled reproduce by mitosis; many multicellular reproduce asexually.

  • Sexual Reproduction:

    • Occurs in eukaryotes through the production of gametes (sperm and egg), resulting in genetically diverse offspring.

    • Gametes are formed by meiosis which halves the genetic material.

    • This increases genetic diversity, which drives natural selection and evolution.

Concept 7.2: Asexual Reproduction Results in Genetically Identical Daughter Cells

  • Key Events in Cell Division:

    1. Initiation Signals: Start the process of cell division.

    2. DNA Replication: Duplication ensures each new cell has the full amount of genetic information.

    3. DNA Segregation: Distribution of DNA into daughter cells.

    4. Cytokinesis: Division of the cytoplasm leading to separate new cells.

  • Binary Fission in Prokaryotes:

    • Environmental factors can initiate division.

    • Replication begins at a single origin (ori) and moves towards the terminus (ter).

    • DNA copies segregate to opposite ends of the cell via cytoskeletal filaments.

  • Eukaryotic Cell Cycle:

    • Stages:

      • G1 Phase: Normal cell functions; can enter G0 (quiescent state).

      • S Phase: DNA is replicated.

      • G2 Phase: Prepares for mitosis.

      • M Phase: Mitosis and cytokinesis occur.

    • Chromosomes are compacted, becoming visible in the M phase.

Concept 7.3: Sexual Reproduction by Meiosis

  • Meiosis Overview:

    • Produces haploid gametes; consists of two division phases but replicates DNA only once, resulting in four genetically different nuclei.

  • Meiosis I:

    • Homologous chromosomes pair up and undergo crossing over to exchange genetic material.

    • At the end, two haploid nuclei are formed.

  • Meiosis II:

    • Similar to mitosis, where sister chromatids separate to produce four haploid cells.

Concept 7.4: Errors during Cell Division Can Result in Changes in Chromosome Number

  • Nondisjunction: Failure of chromosomes to separate properly during cell division.

    • Results in aneuploidy (abnormal chromosome number).

    • Aneuploidy can lead to conditions like Down syndrome.

  • Polyploidy: Presence of extra sets of chromosomes; common in plants but rare in animals. Causes can include errors in DNA replication and fertilization.

Concept 7.5: The Cell Cycle and Cell Death

  • Prokaryotic Cell Cycle: Initiated when DnaA proteins bind to the replication origin in the presence of sufficient nutrients. Cytokinesis follows replication completion.

  • Eukaryotic Cell Cycle Regulation:

    • Cells require internal signals (e.g., cyclin-dependent kinases, CDKs) for progression.

    • The G1-S transition is a crucial control point where cells commit to division.

  • Apoptosis vs. Necrosis:

    • Apoptosis is programmed cell death; necessary for normal development and to eliminate damaged cells.

    • Necrosis is uncontrolled cell death, often due to injury, leading to inflammation.

  • Cancer: Cells can proliferate uncontrollably by evading checkpoints in the cell cycle, often due to mutations in cell cycle regulatory proteins._