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.