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Chapter8

Cell Division Overview

  • Fundamental qualities of life: Growth and reproduction.

  • During cell division, one cell becomes two new cells.

  • Types of cell division:

    • Binary fission in prokaryotes.

    • Mitosis in eukaryotes for growth and some reproduction.

    • Meiosis in eukaryotes for sexual reproduction.

  • All cell division is preceded by DNA replication.

Importance of Cell Division

  • Essential for both growth and reproduction:

    • Asexual reproduction via binary fission and mitosis in single-celled organisms.

    • In multicellular organisms, mitosis:

      • Increases cell number.

      • Replaces lost cells.

      • Repairs injuries.

Meiosis

  • Involves donation of genetic information from two parents.

  • Each parent donates half of the genome, generating gametes (sperm and egg) prior to sexual reproduction.

  • Zygote formation occurs via fertilization, resulting in unique offspring.

The Cell Cycle in Eukaryotes

  • Cells transition through various stages from birth to division.

  • Includes:

    • Interphase: Major cellular activities and preparations for division.

    • Mitosis: Division of the nucleus.

Interphase Stages

G1 Phase

  • Cells gather nutrients and perform regular functions.

  • Commitment to dividing occurs, with some cells ceasing division (Go phase).

S Phase

  • DNA replication occurs; chromatin becomes sister chromatids at centromeres when complete.

G2 Phase

  • Final preparations for mitosis including protein synthesis to assist with chromosome separation.

Mitosis Overview

  • Involves two events:

    • Mitosis: Separation of chromosome copies into new nuclei over four phases.

    • Cytokinesis: Division of cytoplasm into new cells.

Phases of Mitosis

Prophase

  • Chromatin condenses into visible chromosomes with sister chromatids.

  • The nucleus disassembles; nucleolus is no longer visible.

Late Prophase

  • Spindle fibers attach to chromosomes at centromeres.

  • Asters form in animal cells, essential for movement of chromosomes.

Metaphase

  • Chromosomes aligned at the equatorial plate, centromeres begin to dissolve, sister chromatids prepare for separation.

Anaphase

  • Sister chromatids move towards opposite poles, now termed daughter chromosomes.

  • Movement facilitated by kinetochore proteins pulling on spindle fibers.

Telophase

  • Spindle fibers disassemble, nuclear membranes reform around the two sets of chromosomes.

  • Chromatin uncoils, nucleolus reappears, and cells re-enter interphase.

Cytokinesis Process

  • Separates nuclei into distinct cells:

    • Animal cells: Membrane forms a cleavage furrow, pinching the cell.

    • Plant cells: Cell plate forms a new cell wall.

Cell Division Regulation

  • Cells utilize checkpoints to determine whether to proceed through the cell cycle, influenced by genetic health, location, and environmental needs.

Genetic Regulation

  • Checkpoint proteins decide if the cell can continue.

  • Proto-oncogenes promote division, whereas tumor-suppressor genes inhibit it.

Role of p53 Gene

  • During G1, p53 evaluates DNA health to allow or inhibit cell division. If damaged, it initiates repair or apoptosis.

  • Mutations in p53 lead to unregulated cell cycles and cancer.

Cancer Overview

  • Caused by the failure to control cell division, leading to excessive cellular proliferation (tumors).

    • Benign tumors: Do not spread.

    • Malignant tumors: Can metastasize, invading surrounding tissues.

Causes of Cancer

  • Mutagens damage DNA, while carcinogens can cause mutations and cancer specific to compounds like cigarette smoke.

Treatment Strategies

  • Surgery: Removal of tumors if localized.

  • Chemotherapy: Targets rapidly dividing cells, affecting healthy cells, thus causing side effects like hair loss.

  • Radiation Therapy: Directs x-rays or radiation at tumors, often resulting in severe side effects.

Determination and Differentiation

  • Following fertilization, the zygote undergoes mitosis

    • Cells must express specific genes to differentiate into various types.

  • Determination: Process by which cells choose which genes to express.

  • Differentiation: Process whereby a cell becomes a specific type.

Gamete Production in Meiosis

  • Results in four haploid cells via two divisions:

    • Meiosis I: Reduces chromosome number from diploid to haploid.

    • Meiosis II: Separates sister chromatids without further reduction in chromosome number.

Chromosomes and Gametes

  • Humans have 46 chromosomes; gametes have 23 due to meiosis.

  • Diploid (2n) for somatic cells versus haploid (n) for gametes.

Genetic Diversity in Sexual Reproduction

  • Genetic diversity arises from independent assortment, mutations, and crossing-over during gamete formation, enhancing survival chances in variable environments

  • Genetic combinations from independent assortment and fertilization lead to nearly limitless offspring variations.

Nondisjunction and Chromosome Abnormalities

  • Nondisjunction can result in abnormal chromosome numbers:

    • Monosomy: One copy of a chromosome.

    • Trisomy: Three copies of a chromosome lead to conditions like Down's syndrome, which results from an extra chromosome 21 (47 instead of 46 chromosomes).

Karyotyping

  • A karyotype provides a visual representation of an individual's chromosomes, used to identify chromosomal abnormalities such as trisomy.