Questions & explanations
1. Compare the evolution of mitosis in single-celled eukaryotes versus multicellular organisms.
In single-celled eukaryotes like yeast, mitosis is relatively simple and fast because the cell does not need to coordinate with other cells. They often have a closed mitosis where the nuclear envelope does not break down. In multicellular organisms, mitosis is more complex and regulated by signals from other cells. They typically have open mitosis where the nuclear envelope disassembles to allow the spindle to reach chromosomes. Multicellular organisms also have more checkpoints to ensure proper division in different tissues. The evolution of multicellularity required tighter control of the cell cycle to prevent uncontrolled growth. Thus, mitosis became more elaborate and regulated in multicellular organisms.
2. Compare the eocyte hypothesis with the three-domain tree in terms of where eukaryotes are placed.
In the three-domain tree, eukaryotes are a separate domain equal to bacteria and archaea, all branching from a common ancestor. In the eocyte hypothesis, eukaryotes are nested within the archaea, specifically among the eocytes. So the three-domain tree has three primary branches, while the eocyte hypothesis has only two primary branches (bacteria and archaea), with eukaryotes as a subgroup of archaea. This difference affects how we view the evolution of complex cells: the eocyte hypothesis suggests that many eukaryotic features evolved within archaea before eukaryotes emerged. The three-domain tree implies that eukaryotes have been separate for just as long as bacteria and archaea.
3. What is the membrane invagination hypothesis for the origin of the nucleus?
The membrane invagination hypothesis suggests that the nucleus formed when the cell membrane folded inward, creating a separate compartment. This infolding would have surrounded the DNA, forming a double membrane similar to the nuclear envelope. The inner membrane became the nuclear envelope, while the outer membrane stayed connected to the endoplasmic reticulum. This idea explains why the nucleus has a double membrane and is connected to other membrane systems. It also accounts for the presence of nuclear pores, which are similar to structures in the endoplasmic reticulum. However, this hypothesis does not explain how the nuclear lamina or other nuclear-specific proteins evolved.
4. Give an example of a cell feature that supports the eocyte hypothesis over the three-domain tree.
One example is the presence of certain membrane lipids. Eocyte archaea and eukaryotes both have lipids called isoprenoids in their cell membranes, while bacteria have different fatty acid lipids. Also, the way these lipids are attached (ether bonds in archaea and eukaryotes vs. ester bonds in bacteria) is similar between eocytes and eukaryotes. This shared feature suggests a closer relationship. Additionally, some genes involved in DNA replication and transcription are more alike between eocytes and eukaryotes than between bacteria and eukaryotes. These molecular similarities support the idea that eukaryotes branched off from within the archaea, specifically from eocytes.
5. Compare the ribosome in bacteria, archaea, and eukaryotes. What does this tell us about their evolutionary relationships?
All ribosomes have a similar core structure, but there are differences in size and composition. Bacterial ribosomes are smaller (70S) and have different antibiotic sensitivities than archaeal and eukaryotic ribosomes. Archaeal ribosomes are more similar to eukaryotic ribosomes in terms of protein content and antibiotic resistance, even though archaea are prokaryotes. This suggests that archaea and eukaryotes share a more recent common ancestor than either does with bacteria. The ribosome's features support the two-domain tree, where eukaryotes evolved from within archaea. Thus, ribosome comparisons help confirm the close relationship between archaea and eukaryotes.
6. What is a major challenge for the RNA world hypothesis that the metabolism-first hypothesis might address?
A major challenge for RNA world is the formation of the first RNA molecules under prebiotic conditions. RNA nucleotides are complex and difficult to synthesize in the early Earth environment. Metabolism-first suggests that simpler organic molecules could form first and then assemble into more complex ones. For example, the formose reaction produces sugars, which are components of RNA. However, the assembly of nucleotides and their polymerization into RNA remains a hurdle. Metabolism-first might explain how the building blocks of RNA could accumulate, but it doesn't explain how they organized into functional RNA. Thus, both hypotheses have strengths and weaknesses.
7. What is a major weakness of the membrane invagination hypothesis for nuclear origin?
A major weakness is that it does not explain how the nuclear lamina and other nuclear-specific proteins evolved. The nuclear lamina is a dense network of proteins inside the nucleus that gives it structure. The invagination hypothesis only accounts for the membrane, not the unique protein components. Also, it does not explain why the nucleus has a distinct composition of lipids and proteins compared to the cell membrane. Another problem is that it does not address the origin of the nuclear pore complex, which is highly complex. Finally, the hypothesis cannot easily explain the origin of the nucleolus or the organization of chromosomes inside the nucleus.
8. How does the SELEX process relate to the concept of natural selection in the RNA world hypothesis?
SELEX directly demonstrates how RNA molecules can undergo Darwinian evolution in a test tube. In the RNA world, natural selection would have acted on RNA molecules that could replicate and perform useful functions. SELEX applies selective pressure (binding to a target) to a diverse RNA pool, and the best binders are amplified. This is analogous to how early RNA molecules might have been selected for their ability to catalyze reactions or bind other molecules. The success of SELEX shows that RNA can evolve new functions quickly, supporting the plausibility of an RNA world. It provides a model for how early life could have emerged from non-living RNA.
9. How does the membrane invagination hypothesis compare to the endosymbiotic theory for the origin of the nucleus?
The membrane invagination hypothesis says the nucleus came from infoldings of the cell's own membrane. In contrast, the endosymbiotic theory says mitochondria and chloroplasts came from bacteria that were engulfed. For the nucleus, endosymbiosis is less likely because no modern prokaryote has a nucleus-like structure. The invagination hypothesis is simpler because it only requires changes in the existing cell membrane. However, some evidence suggests the nucleus might have originated from a virus or a fusion of archaea and bacteria. Both ideas try to explain the double membrane, but invagination is more widely accepted for the nucleus.
10. Compare serpentinization with the Miller-Urey experiment. How do they differ in producing organic molecules?
Serpentinization produces hydrogen through rock-water reactions, while the Miller-Urey experiment used electrical sparks to break down gases. Miller-Urey required a reducing atmosphere with methane and ammonia, which may not have been present on early Earth. Serpentinization occurs naturally in the Earth's crust and does not need special atmospheric conditions. Both can produce organic molecules, but serpentinization provides a continuous, long-term source of hydrogen. The Miller-Urey experiment was a one-time spark, while serpentinization is ongoing at vents. Serpentinization is now considered more relevant to early Earth conditions.
11. How does the existence of ribozymes (RNA enzymes) support the idea that the genetic code originated in an RNA world?
Ribozymes are RNA molecules that can catalyze chemical reactions, like cutting or joining other RNA strands. The ribosome, which translates the genetic code into proteins, contains a ribozyme that forms peptide bonds. This shows that RNA can perform the key catalytic step in protein synthesis. In an RNA world, such ribozymes would have been crucial for translating genetic information into functional molecules. The fact that modern translation still relies on an RNA catalyst supports the idea that the genetic code evolved from RNA-based systems. Thus, ribozymes provide a direct link between the RNA world and the current genetic code.
12. What does isotopic fractionation mean?
Isotopic fractionation is the process where different forms of the same element, called isotopes, are separated based on their weight during chemical reactions. For example, carbon has two stable isotopes: carbon-12 and carbon-13. Living things prefer the lighter carbon-12, so their organic matter has less carbon-13 than the environment. This difference in isotope ratios can be measured in ancient rocks. Scientists use this as evidence for early metabolism because it shows that life was using carbon dioxide. The pattern of fractionation can tell us what type of metabolism was happening, like photosynthesis or methane production.