microbio

Last Universal Common Ancestor (Luca)

  • Definition: The last universal common ancestor is often referred to as Luca. It is considered the point from which all life on Earth originated.
  • Evolutionary Perspectives:
    • Life initially evolved from this common ancestor, leading to the development of bacteria.
    • It is hypothesized that Archaea and Eukarya diverged from the lineage following bacteria.
    • Phylogenetic Tree Insights: Eukarya are believed to be more closely related to Archaea than to Bacteria.

Hypotheses on the Origins of Life

  • Primordial Soup Hypothesis:
    • Earlier thought to be a primary model of life's origin.
    • Presents challenges as it's difficult for molecules to consistently interact in a liquid medium (low chances of successful reactions).
  • Thermal Vents Theory:
    • Current hypothesis suggests life originated in hydrothermal vents in the ocean.
    • Concentrated environments facilitate interaction among molecules, which promotes the replication of genetic material (e.g., genomes).
  • RNA World Hypothesis:
    • Describes early life forms as RNA-based organisms, highlighting RNA's ability to combine small molecules.
    • Non-coding and coding RNAs, as well as CRISPR systems, are all RNA-based.
    • Catalytic Activity of RNA:
    • The ribosome, composed of rRNA and proteins, acts as an enzyme to form covalent bonds by adding amino acids during protein synthesis.
    • RNA played a significant role in protein translation even in the absence of proteins, suggesting early molecular interactions.
  • DNA Evolution:
    • Eventually, DNA took precedence over RNA due to its increased stability, leading to modern genetic material structures with DNA, RNA, and proteins evolving into diverse life forms.

Divergence of Life Forms

  • **Key Divergence Events:
    • Formation of the Earth:** Approximately 4.5extbillionyearsago4.5 ext{ billion years ago}
    • Cellular Life Emergence: Around 4extbillionyearsago4 ext{ billion years ago}
    • Divergence of Archaea and Bacteria: Approximately 2.7extbillionyearsago2.7 ext{ billion years ago}
    • First Endosymbiotic Event: About 2extbillionyearsago2 ext{ billion years ago}—critical for Eukaryotic evolution.
    • Development of First Eukaryotes: Roughly 1.7extbillionyearsago1.7 ext{ billion years ago}
    • Multi-cellular Organisms: Emerged around 1extbillionyearsago1 ext{ billion years ago}

Endosymbiotic Theory

  • This theory suggests that certain organelles, specifically mitochondria and chloroplasts, originated from earlier prokaryotic organisms due to symbiotic events.
    • Evidence includes:
    • Mitochondria evolving from a engulfed bacterium (likely related to Rickettsia) by an Archaea ancestor.
    • Chloroplasts likely originated from cyanobacteria through a similar process.
    • Mitochondrial Genome:
    • Contains approximately 17,00017,000 base pairs and encodes 13 proteins.
    • Contains an RNA gene akin to that of Rickettsia, reinforcing the endosymbiotic theory.
    • Chloroplast Genome:
    • About 12,00012,000 base pairs and encodes proteins involved in photosynthesis (nearly 2020 proteins).
    • Similarities between the 16S gene of chloroplasts and cyanobacteria provide additional support.

Rickettsia as Case Study

  • Characteristics:
    • Tick-borne, gram-negative, non-spore forming bacterium requiring a host for survival, can't be isolated from soil.
    • Symptoms of infection include rash, fever, and muscle aches; usually treated with tetracycline.

Lactose Operon (Lac Operon) Overview

  • Implication for Exam: Focused study on the lac operon structure and regulation will be critical for upcoming assessments.
  • Comprised of:
    • Structural Genes: Z, Y, and A, essential for lactose metabolism.
    • Regulatory Elements: CAP and others crucial for operon regulation.

Transport of Lactose

  • Lactose enters the cell using an active transporter (permease encoded by Y) powered by proton motive force.
  • Transport Characteristics:
    • Saturable and specific to lactose transport, facilitated even in low external concentrations.
  • Regulated by:
    • Operon components activating in response to glucose levels and the presence of lactose in the environment.

Understanding Operons

  • Definition of an Operon:
    • A cluster of genes organized in the same direction and co-transcribed into mRNA, with a defined promoter region.
    • Example in the lac operon: genes Z, Y, and A are transcribed together in a single mRNA strand.

Regulating the Lac Operon

  • Role of Lac I:
    • Acts as a repressor that binds to the operator region, blocking transcription when lactose is absent (inhibits RNA polymerase binding).
  • Diauxic Growth:
    • Early growth on glucose, followed by growth on lactose when glucose is depleted.
  • Importance of Constant Low-Level Expression:
    • The lac operon is never fully off to ensure a basic level of permease is available for lactose uptake when needed.

Conclusion

  • Review of key points leading up to the next class covering the lac operon regulation in detail.
  • Upcoming lab exams and the importance of differentiation between glycolysis and lactose metabolism will be emphasized as students prepare for assessments.