17 - Sexual Reproduction

Genetics Fundamentals

  • Genes: DNA sequences encoding proteins.

  • Mutations: Alterations in DNA nucleotide sequences (A, T, G, C).

    • Impact on protein amino acid sequences derived from DNA may affect protein structure, function, and ultimately, traits in cells

Meiosis vs. Mitosis

  • Meiosis: Produces four genetically distinct haploid (cell only containing one set of chromosomes) (n) gametes from a diploid (cell containing two sets of chromosomes) (2n) parent cell

  • 1 Diploid cell ——> 4 haploid gametes

    • involving two rounds of division (Meiosis I and II).

      • Key differences from mitosis include:

      • 2 divisions instead of 1

      • 4 daughter cells instead of 2

      • Prophase I involves synapsis and crossing over between homologous chromosomes.

      • Independent assortment occurs, creating genetic diversity— genetically unique identity due to recombination

Sexual Reproduction and Genetic Diversity

  • Meiosis leads to genetic diversity essential for evolution and adaptability:

    • Asexual reproduction: Produces clones (identical offspring).

      • Prokaryotes: binary fission / Eukaryotes: mitosis

    • Sexual reproduction: involves a combination of genetic material; offers genetic recombination via gamete fusion.

Chromosomes and Karyotype

  • Types of Chromosomes:

    • Sex chromosomes: XX (female) or XY (male) in most mammals

      • involved in biological sex determination

    • Autosomes: Non-sex chromosomes

      • Humans have 22 pairs of homologous chromosomes, plus 1 pair of sex chromosomes derived paternally

    • Somatic cells: Diploid (2n = 46 chromosomes) — (2 copies of each DNA chromosomal molecule = 46 chromosomes total)

    • gametes: haploid (n = 23 chromosomes) — (1 copy of each DNA chromosomal molecule = 23 chromosomes total)

Genetic Variability from Viral DNA

  • Human-Virus Relationship:

    • Exposure to viruses can incorporate viral DNA into human genomes

      • contributing around 1-8% of the human genetic material.

    • Some viral DNA regulates genome

    • Some integrated viral sequences fulfill critical regulatory functions.

Sexual Life Cycles Overview

  • Somatic vs. germline cells:

    • Somatic cells: Diploid (2 copies of each chromosome)

      • perform mitosis.

      • most cells in the body

    • Gametogenic cells undergo meiosis to produce haploid gametes.

      • diploid cells that produce haploid gametes

    • Fertilization leads to the formation of a zygote,

      • which develops into an adult via mitosis and differentiation.

        • diploid zygote grows into an adult human through the developmental process

Meiosis Process Detailed

  • Meiosis Stages:

    • Interphase: Similar to mitosis with chromosome duplication.

    • Meiosis I: Homologous chromosomes separate into two cells.

    • Meiosis II: Sister chromatids separate.

    • Resulting in four haploid cells, with substantial genetic variation due to independent assortment and crossing over.

  • Step 1: (Interphase I) DNA replication occurs; each chromosome duplicates— forming two sister chromatids

  • Step 2: (Prophase I) so basically the chromosomes condense, enabling each chromosome to match their partner in synapsis. The chromosomes then undergo recombination (at the chiasma), where segments of DNA are exchanged between chromosomes. Spindle fibers also begin to form in this process

  • Step 3: (Metaphase I) The homologous chromosome pairs line up along the metaphase plate, fully attached to spindle fibers, preparing for separation. This is knolwn as independent assortment.

  • Step 4 (Anaphase I) Spindle fibers seperate each homologous chromosome pair, distributing one chromosome from each pair to opposite poles of the cell. Differs from mitosis because the sister chromatids are not seperated yet.

  • Step 5 (Telophase I and Cytokinesis) Once chromosomes have reached opposite poles, nuclear membrane begins to form around each set of chromosomes, allowing chromosomes to de-condense. The, cytokinesis takes place, with the cell splitting into two— each cell is haploid with one chromosome from each homologous pair.

  • Step 6 (Prophase II) Haploid cells prepare to divide again. Chromosomes condense, spindle fibers form, and the nuclear envelope is broken down.

  • Step 7 (Metaphase II) Chromosomes line up at the metaphase plate in each of the two haploid cells and are attached to spindle fibers at their kinetochores, ensuring that each chromosome is properly aligned and ready for separation.

  • Step 8: (Anaphase II)  Sister chromatids of each chromosome is pulled apart by the spindle fibers and moved to opposite poles of the cell. Leaving each chromatid to become an individual chromosome.

  • Step 9 (Telophase II and cytokinesis) Once chromatids have reached opposite poles of the cell, nuclear membranes reform around each set of chromosomes, allowing chromosomes to decondense. Cytokinesis then occurs, resulting in the division of the cytoplasm and the formation of four genetically distinct haploid daughter cells, each with a unique combination of genetic material. Each cell has one copy of each chromosome — no pairs, no duplicates

Independent Assortment and Crossing Over

  • Independent Assortment: Refers to the random distribution of maternal and paternal chromosomes into gametes,

    • leading to numerous combinations:

      • Example: 23 chromosome pairs yield about 8 million combinations.

  • Crossing Over: Occurs at chiasmata during Prophase I and involves DNA breaks and reattachment

    • contributing to genetic diversity.

Summary of Mitosis vs. Meiosis

  • Mitosis results in two genetically identical diploid cells

  • Meiosis creates four genetically diverse haploid cells:

    • Key differences include

      • The occurrence of synapsis in meiosis

      • the number of resulting daughter cells

      • genetic composition

      • chromosomal number reduction.

Fertilization and Genetic Diversity

  • Gametogenesis produces genetic variability, enhancing the diversity of resulting offspring:

    • Zygote formation after fusion of gametes combines genetic material from both parents

      • leading to unique genetic combinations and characteristics.

Twin Formation & Genetic Implications

  • Zygotic Splitting can lead to identical or fraternal twins:

    • Identical (monozygotic): Occurs from a single zygote splitting.

    • Fraternal (dizygotic): Result from two separate zygotes.