Chapter 25: Sexual Reproduction, Meiosis & Genetic Recombination

Chapter 25: Sexual Reproduction, Meiosis & Genetic Recombination

1. Definitions and Descriptions

A. Sexual Reproduction

  • Definition: The production of genetically unique offspring through the fusion of two haploid gametes (egg and sperm in humans) to form a diploid zygote.

B. Homologous Chromosomes

  • Definition: Chromosome pairs (one maternal, one paternal) with the same gene loci (the order and location of genes) but possibly different alleles.

  • Commonly referred to as "homologs."

C. Alleles

  • Definition: Alternative versions of a gene at the same locus; these variations produce differences in a trait.

D. Dominant Alleles

  • Definition: An allele that is expressed in the phenotype even when only one copy is present (characterized as heterozygous state).

E. Recessive Alleles

  • Definition: An allele that is "masked" by a dominant allele and expressed only when both copies are present (homozygous state).

F. Homozygous vs. Heterozygous Alleles

  • Homozygous: Having two identical alleles for a gene (e.g., AA or aa).

  • Heterozygous: Having two different alleles for a gene (e.g., Aa).

  • Important Note: Heterozygous conditions can mask the effects of recessive alleles and increase genetic fitness (referred to as "heterozygote advantage").

G. Genotype vs. Phenotype

  • Genotype: The genetic makeup of an organism, specifically the allele combinations present.

  • Phenotype: The observable traits or characteristics resulting from genotype and environmental interaction.

H. Chromosome Ploidy

  • Diploid (2n): Cells containing two sets of chromosomes (e.g., 46 total chromosomes in humans), with one set from each parent.

  • Haploid (n): Cells containing one set of chromosomes, such as gametes (sperm and egg).

  • Significance: The formation of a zygote must occur without exceeding the proper amount of DNA.

I. Chromosomal Structure and Replication

  • Chromosome Definition: A chromosome is defined as one DNA molecule with its associated proteins and one centromere.

    • Before S phase (DNA replication): One chromosome consists of one chromatid.

    • After S phase: One chromosome consists of two sister chromatids held together at a centromere.

  • Important Clarification: DNA replication (formation of sister chromatids) does not change the ploidy; having two homologs is diploid (2n), while one version of a chromosome constitutes haploidy (n).

J. Gametes

  • Definition: Haploid reproductive cells (egg and sperm) produced by meiosis.

    • Gamete types: Egg and Sperm.

    • Fertilization leads to the formation of a zygote.

K. Alternation of Generations

  • Definition: A life cycle found in plants and some algae in which multicellular diploid (sporophyte) and multicellular haploid (gametophyte) stages alternate.

  • Significance: This alternation provides organisms with two different modes of survival and reproduction, each with distinct advantages.

  1. Diploid Sporophytes:

    • Undergo meiotic division, promoting genetic variability essential for survival amidst environmental changes (e.g., pathogens, climate).

    • Spores formed are lightweight (promoting easy dispersal), resilient, and produced in significant quantities.

  2. Haploid Gametophytes:

    • Produce gametes via mitosis (a complete gene copy with only one set of alleles).

    • The produced gametes then fuse (fertilize) to create diploid zygotes (sporophytes).

    • All alleles from haploid gametophytes are expressed, leading to the rapid extinction of harmful mutations while beneficial mutations spread quickly across populations.

2. Meiosis

Description

  • Definition: A specialized form of cell division that reduces chromosome number from diploid (2n) to haploid (n) to produce gametes that enhance genetic diversity.

  • It involves two sequential divisions (Meiosis I and II) occurring after a single round of DNA replication.

Step 1: Interphase

  • The phase where DNA is replicated prior to meiosis.

Meiosis I
Step 1: Prophase I

  • Divided into five substages:

    1. Leptotene:

    • Chromosomes condense, each containing two sister chromatids due to prior DNA replication.

    1. Zygotene:

    • Synapsis (pairing) of homologous chromosomes occurs, known as the formation of a synaptonemal complex (like a protein "zipper") that aligns homologs to nanometer precision.

    1. Pachytene:

    • Full synapsis forms a bivalent/tetrad (four chromatids).

    • Crossing Over: This is the physical exchange of DNA between non-sister homologous chromatids at corresponding locations, resulting in new allele combinations.

    • Mechanism of Crossing Over:

      • Spo11 Protein: Induces double-strand breaks in one chromatid.

      • Holliday Junctions: Structures formed where broken DNA searches for homologous sequences on opposing chromatids.

      • Enzymatic Repair: Enzymes cut and re-ligate the DNA, facilitating separation of the chromosomes once crossing has occurred.

    1. Diplotene:

    • The synaptonemal complex disassembles.

    • Homologs begin to separate but remain linked at chiasmata (cross-over sites).

    1. Diakinesis:

    • Chromosomes further condense and prepare for spindle attachment.

    • The nuclear envelope breaks down, and chiasmata move toward the ends of chromosomes (terminalization).

Step 2: Metaphase I

  • Chromosomes align at the metaphase plate (equator of the cell).

  • Microtubules attach to kinetochores on the same side of each pair.

Step 3: Anaphase I

  • Homologous chromosomes separate and migrate to opposite poles.

  • Shugoshin Protein: Protects centromeric cohesion, maintaining sister chromatids coherence.

Step 4: Telophase I & Cytokinesis

  • Cell division results in two haploid (n) daughter cells.

  • Each daughter cell contains duplicated chromosomes (two sister chromatids).

Meiosis II (New Division Stage)
Step 1: Prophase II

  • Chromosomes condense if they decondensed after meiosis I.

  • New spindle apparatus forms in each haploid cell.

Step 2: Metaphase II

  • Chromosomes align again at the metaphase plate.

  • Microtubules now attach to sister kinetochores.

Step 3: Anaphase II

  • Sister chromatids separate (centromeric cohesion is cleaved).

  • Each chromatid is now an independent chromosome.

Step 4: Telophase II & Cytokinesis

  • Chromosomes arrive at opposite poles.

  • The nuclear envelopes reform, and chromosomes decondense.

  • The cell divides, resulting in four genetically distinct haploid daughter cells.

3. Gametogenesis and Nondisjunction

A. Gametogenesis in Males

  • In human males, all four haploid cells formed from meiosis become sperm (gametes).

B. Gametogenesis in Females

  • In females, only one of the four haploid cells develops into an egg, while the other three are designated as polar bodies and degenerate.

  • Oocyte development arrests in prophase I from before birth until ovulation (puberty) and again at metaphase II until fertilization.

  • Rationale: To ensure production of one large nutrient-rich gamete that can support an early embryo.

C. Nondisjunction

  • Definition: Improper separation of chromosomes during meiosis, occurring in anaphase I or II, leading to gametes or daughter cells with an abnormal number of chromosomes (a condition known as "aneuploidy").

Consequences of Aneuploidy

  • Includes conditions such as:

    • Monosomy: Missing one chromosome.

    • Trisomy: Having an extra chromosome.

Examples of Trisomy Conditions:

  • Trisomy 18: Three copies of chromosome 18.

    • Turner Syndrome: 45 chromosomes with one X missing (only viable monosomy in humans).

    • Trisomy 21: Down syndrome.

    • Trisomy 18: Edwards syndrome.

    • Trisomy 13: Patau syndrome.

    • Trisomy X (47, XXX): In females.

    • XYY Syndrome: In males.

D. Transformation, Transduction, and Conjugation in Prokaryotes

  • Transformation: The uptake of DNA molecules from the environment by prokaryotes.

  • Transduction: The introduction of DNA via a virus into a host cell.

  • Conjugation: The transfer of DNA from one cell to another through a tube called a pilus.

  • Mechanism:

    • Pilus Structure: A thin, protein tube that connects donor and recipient bacterial cells to facilitate DNA transfer, often involving mobile plasmids.

  • Process and Terminology:

    • Relaxosome: Protein complex that processes the DNA for transfer.

    • Transferosome: The assembly involved in the DNA transfer between cells.