Reproduction and Meiosis Notes

Reproduction and Meiosis

Sexual and Asexual Reproduction

• Learning Goals:

  • Differentiate between diploid and haploid genomes.

  • Discuss similarities and differences in homologous chromosomes.

  • Assess advantages and disadvantages of sexual and asexual reproduction.

  • Explain life-cycle strategies among sexual organisms and provide examples.

Cell Types

• Somatic Cells:

  • Non-reproductive cells, possess two sets of chromosomes (Diploid, 2n).

  • Homologous chromosomes : matched chromosomes from each parent.

• Gametes:

• Reproductive cells, contain one set of chromosomes (Haploid, 1n).

• NO homologous pairs present.

Chromosome Structure

• Homologous Chromosomes:

  • Matched pairs, one from each parent, same length and centromere position.

  • Somatic (body) cells of each species contain → specific number of chromosomes

  • Example:

    • Humans have 46 chromosomes → 23 pairs of homologous chromosome.

• Genes and Homologous Chromosomes:

• genes:

  • Stretches of DNA nucleotides telling ribosome how to make a protein

  • Each protein controls a → triat

    • Example: eye color, blood type

  • Different versions of the trait → alleles

    • Example: brown and blue eyes colors, A/B/AB/O blood type

  • A homologue of a pair carries the same genes, but may have different alleles.

  • Alleles are different version of the same gene that can result in variations of certain traits, contributing to genetic diversity within a population.

Asexual Reproduction

• Process:

  • Single parent produces genetically identical offspring (usually through mitosis).

  • Cell Division → usually mitosis

• Advantages:

  • Faster and cheaper, no need for mate.

  • All individuals capable of reproducing, suited for rapid colonization. (no mate requirement)

• Disadvantages:

  • Low genetic diversity, identical offspring struggle to adapt to environmental changes.

  • Unsuited for the environment. (slower evolution)

Example of Asexual Reproduction

• Seedless Fruit:

  • Original banana variety (Gros Michel) became extinct due to Panama disease (fungus).

  • Replaced by Cavendish variety, also propagated asexually and Panama disease started to adpat.

Sexual Reproduction

• Process:

  • Fusion of gametes from two parents results in genetically non-identical offspring.

  • Cell division → Meiosis

• Advantages:

  • Increased genetic diversity, adaptability to environmental changes (faster evolution).

• Disadvantages:

  • Slower and Expensive, more energy-consuming, only half the population can reproduce (females).

Life Cycles of Sexually Reproducing Organisms

• Definition of Life Cycle:

  • Sequence of developmental stages from birth to death, including → growth and reproduction.

• Meiosis and Fertilization:

  • Fertilization and meiosis alternate in sexual life cycle

  • Meiosis converts diploid cell → haploid cells

  • fertilization converts haploid cells → diploid state.

• Types of Life Cycles: Three types of life cycles in multicelluar organisms

  1. Diploid-Dominant:

  • Most multicellular stage is diploid most obvious (e.g., humans).

    • most body cells are diploid

    • Gametes (sperm and egg) are haploid

  • Lacks multicellular haploid stage

    • Animal haploid cells can NEVER do mitosis

  • Example → Humans and most other animals

  1. Haploid-Dominant:

  • Most prominent stage is haploid most obvious (e.g., fungi).

    • Two haploid fungi fuse to make → a diploid organism

    • Meiosis occurs soon after to make → haploid offspring

      • Haploid fungi CAN DO mitosis and grow bigger!

  • Lacks multicellular diploid stage

  • Example: fungi

  1. Alternation of Generations:

  • Alternation between multicellular diploid and multicellular haploid stages (e.g., plants).

  • Example → Plants, some algae

Meiosis Overview

• Definition:

  • Nuclear division reducing chromosome number from diploid to haploid.

  • Results in four unique haploid cells from a diploid parent cell.

Stages of Meiosis

1. Meiosis I:

   • Separates homologous chromosomes, reduces chromosome sets (Prophase, Metaphase, Anaphase, Telophase I).

2. Meiosis II:

   • Similar to mitosis, separates sister chromatids (Prophase II, Metaphase II, Anaphase II, Telophase II).

Genetic Variability in Meiosis

• Crossing Over:

  • Occurs during Prophase I; homologous chromosomes exchange genetic material, increasing diversity.

• Independent Assortment:

  • Orientation of chromosomes during Metaphase I leads to varied combinations of chromosomes in gametes.

• Results in numerous genetic combinations in offspring.

Differences Between Meiosis and Mitosis

• Meiosis:

  • Two rounds of nuclear division, produces four unique haploid cells.

• Mitosis:

  • One round of nuclear division, produces two identical somatic cells.

Meiosis and Gender

• Gender Determination in Humans:

  • XX (female) and XY (male) determine sex; females contribute X, males contribute either X or Y.

• Nondisjunction Events:

  • Failure of chromosomes to separate properly can lead to disorders.

  • Example Disorders:

  • Down Syndrome (Trisomy 21): Three copies of chromosome 21.

  • Turner Syndrome: Females with a single X chromosome (XO).

  • Klinefelter Syndrome: Males with an extra X chromosome (XXY).

Summary Points

• Diploid cells: two sets of chromosomes; haploid cells: one set.

• Asexual reproduction: rapid reproduction, less genetic diversity.

• Sexual reproduction: genetic diversity, various life cycle strategies.

• Meiosis is key for producing haploid cells and genetic variation; disorders can result from meiosis errors.