Bio 162: Creating and Maintaining Genotypic Diversity

Bio 162: Creating and Maintaining Genotypic Diversity

Heritable Information

  • Heritable information is primarily carried in genetic material.
  • Chromatin is a combination of DNA and proteins.
  • Chromatin can exist in different states:
    • Heterochromatin: condensed, densely packed form of chromatin.
    • Euchromatin: uncondensed, diffuse form of chromatin.

Chromosomes

  • Chromosomes are discrete structures that are composed of chromatin.
  • Chromosomes can exist in various states:
    • They can be completely condensed or may have "extended" portions depending on the requirement.
  • The number of chromosomes varies between different organisms.
  • DNA (deoxyribonucleic acid):
    • Structure: double helix of base pairs that contains the fundamental hereditary information of a living organism.

Chromatid and Chromosome Structure

  • Chromosomes can be in two states regarding replication:
    • Un-replicated state:
    • 1 chromosome, 1 chromatid, resulting in 1 copy of information.
    • Replicated state:
    • 1 chromosome, 2 chromatids, resulting in 2 copies of information.
  • In Eukaryotes, chromosomes are linear, while in most Prokaryotes, mitochondria, and chloroplasts, they are circular.

Mechanisms of Variation in Prokaryotes

  1. DNA Replication
  2. Horizontal Gene Transfer:
    • Definition: The transfer of genetic information that is not from parent to offspring.
    • Resulting in unique genotypes and allows favorable combinations of genes to proliferate.
    • Genetic recombination: Any exchange of genetic information between individuals to create new combinations of genes.

Binary Fission

  • Definition: Cell division in bacteria and archaea that results in two genetically identical individual cells.
  • Errors in DNA replication can change the genome, potentially creating new genes.
  • Factors affecting cell replication include environmental conditions and nutrient levels.
  • Process Breakdown:
    • DNA replicates as the cell grows.
    • Daughter DNA molecules move apart as the cell elongates.
    • A ring of proteins forms to constrict about the middle of the cell, initiating cytokinesis.
    • Upon completing cytokinesis, two identical cells are formed.
  • Note: Methods of DNA replication, separation, and cytokinesis in Eukaryotes during mitosis differ from Prokaryotes.

Chromatids and Chromosomes

  • Chromatids represent copies of chromosomes during replication.
    • Un-replicated chromosomes have 1 chromatids (1 chromosome = 1 chromatid).
    • Replicated chromosomes have 2 chromatids (1 chromosome = 2 chromatids).
  • Eukaryotes typically have multiple different chromosomes, each containing a different subset of genetic information.

Chromosomal Ploidy

  • Ploidy: The number of sets of chromosomes in a cell.
    • Haploid (N): One set of chromosomes, representing a minimal set of instructions, with one version of each gene necessary to construct the organism.
    • Diploid (2N): Two complete and different sets of chromosomes, having two versions of each gene for organism construction.
    • Organisms may also be polyploid (e.g., Tetraploid (4N)), containing more than two versions of genetic information.
    • Example of Ploidy:
    • 3 chromosomes = 1 version of information.
    • 6 chromosomes = 2 versions of information.

Mechanisms of Variation in Eukaryotes

  1. DNA Replication.
  2. Meiosis:
    • Key processes include:
      • Crossing-over: The exchange of segments of DNA between homologous chromatids.
      • Segregation: Distribution of different alleles to gametes.
  3. Syngamy: The joining of gametes.

Mitosis vs. Meiosis

  • Mitosis:

    • Definition: Eukaryotic cell division resulting in two genetically identical daughter cells.
    • Stages include:Interphase, DNA replication, chromosomes lining up, chromosomes condensing, chromosomes moving, separating, and cytokinesis.
    • Mitosis maintains cell integrity across generations by producing identical copies.

  • Meiosis:

    • Definition: Eukaryotic cell division resulting in four genetically unique daughter cells.
    • Stages include:
    • Interphase: Initial DNA replication.
    • Meiosis I: Produces two genetically unique cells.
    • Meiosis II: Produces four unique cells.

Meiosis Process Overview

  • Meiosis I:

    • Events include condensation of 2N chromosomes, crossing over, and separating homologous pairs.
    • Cytokinesis results in two unique 1N cells.

  • Crossing Over:

    • Mechanism where homologous chromatids exchange corresponding DNA segments.
    • Each crossover event results in a 50-50 chance of information exchange:
    • Between sister chromatids from the same parent — no change in information.
    • Between chromatids from different parents — information on two chromatids changes.

  • Independent Assortment:

    • Homologous pairs randomly align during Meiosis I, leading to random combinations of chromosomes in the resulting daughter cells.

  • Meiosis II:

    • Similar to mitosis; involves the alignment of 1N chromosomes, their separation, and cytokinesis, resulting in four unique 1N cells.

Conclusion

  • Variation in both Prokaryotes and Eukaryotes is achieved through specific mechanisms, including DNA replication, horizontal gene transfer, meiosis with crossing over and independent assortment, and syngamy.
  • These processes contribute significantly to the maintenance and increase of genetic diversity within populations.