UNIT 1 CH. 25 1. What are the 4 \"stages\" that are thought to have led to the d
ID: 196464 • Letter: U
Question
UNIT 1 CH. 25 1. What are the 4 "stages" that are thought to have led to the development of the first life? Explain each and describe the experiments and data used to support these ideas (25.1) i Ltee smc.mees to m 2. Are amino acids alive? Are RNA nucleotides alive? How many different amino acids and RNA nucleotides are there? Are these complex molecules? About how many atoms are needed to build a typical amino acid or RNA nucleotide: a few dozen, a few hundred, or a few thousand? (5.4-5.5) 3. What are the limitations of the fossil record? How can we know about ancient microbes from fossils? How are fossils radiometrically dated? How does carbon dating work? What are its limitations? (25.2) 4. What are the 5 eras of geologic history? About how long ago did each era start? How were these eras determined? (You may have to do a little research for this.) Using Table 25.1, summarize the key events of each er on a notecard a. (25.3) This group will need to write the era and when it occurred (in millions of years ago)Explanation / Answer
1.The abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nucleotides.
The synthesis and accumulation of small organic molecules, or monomers, such as amino acids and nucleotides.
• Production of glycine (an amino acid)
3 HCN + 2 H 2 O energy C 2 H 5 O 2N + CN 2 H 2.
• Production of adenine (a base):
5 HCN C 5 H 5 N 5,
• Production of ribose (a sugar):
5 H 2CO C 5 H10 O 5.
Evidences and theories:
Step 1 is possible in the early Earth’s atmosphere if it was was highly reducing as opposed to oxidizing (cf. Miller & Urey experiment). Later research cast doubt on the existence of a reducing atmosphere and pointed to a neutral atmosphere dominated by CO 2. More recent evidence is that H escaped very slowly on early Earth and its abundance wasn’t negligible after all. Supported by evidence from chondritic meteorites which were Earth’s building blocks. Discovery of highly reducing conditions near hydrothermal vents and in volcanos may make this debate irrelevant.
• Energy sources to drive initial chemical reactions available from UV solar radiation, radioactivity, electrical discharges (lightning), cosmic rays and solar wind (Earth’s magnetic field not yet formed). Volcanic and vent energy available near hydrothermal vents.
2. The joining of these small molecules into macromolecules, including proteins and nucleic acids.
Bernal showed that clay-like materials could serve as sites for polymerization.
Clay structure is that of alternating negatively charged sheets of Si O 4 and Al O 4 tetrahedra separated by positive cations (Ca, Na, Fe, or Mg).
• Clays are extremely common on the Earth and Mars.
• Charged layers and cations provide multitudinous sites for monomers to stick.
• Water can easily flow through the structure as the layers are separted by 1 mm or more, enabling dehydration.
• A cubic centimeter (thumb-tip) of clay has the net surface area of a football field.
• Many peptide bonds are catalyzed by clays; RNA strands up to 100 bases in length have been produced in laboratories; lipids can be polymerized into pre-cells, sometimes containing short RNA strands.
3. The packaging of these molecules into "protobionts", droplets with membranes that maintained an internal chemistry different from that of their surroundings.
• Certain materials are ambiphilic: they have a polar hydrophilic head and a hydrophobic tail. Hydrophilic materials can be dissolved in water.
• Ambiphilic molecules added to water tend to stay on the surface with hydrophilic heads in the water, creating a single (or mono-) layer, i.e., a membrane. Formation of spheres, or micells, permits surface area and free energy reduction. In sufficient concentrations, ambiphilic molecules will form a double-layer structure, or bilayer. Spheres, or bilayer vesicles, will form.This relies heavily on the formation of a semi-permeable membrane, one that allows only certain materials to flow one way or the other through it. Droplet formation requires a liquid with a large surface tension, such as water. Membrane formation naturally occurs if phospholipids are present.
• Phospholipids self-assemble into films forming semi-permeable membranes
• Concentration of polymers
• Existence of enzymes for growth
• Fission forms daughter drops
• Limited raw material, growth enzyme
• Random inheritance of important enzymes.
The droplet, consisting of protein and polysaccharide, contains the enzyme phosphorylase. Glucose-1-phosphate diffuses into the droplet and is polymerized to starch by the enzyme. The starch migrates to the wall, thickens it, and increases volume of droplet.
The enzyme, phosphorylase, polymerizes glucose-1-phosphate to starch. A second enzyme, amylase, degrades the starch to maltose. Droplets containing both enzymes do not grow because the starch disappears as fast as it is made. Maltose diffuses back into surrounding medium.
4. The origin of self-replicating molecules that eventually made inheritance possible.
The origin of heredity, or a means of relatively error-free reproduction. It is widely, but not universally, believed that RNA-like molecules were the first self-replicators — the RNA world hypothesis. They may have been preceded by inorganic self-replicators.
Reactive nucleotides made random polymers, and the first RNA molecules could have been short, virus-like sequences.
• Inside protobionts, some amino acid polymers could have had rudimentary catalytic properties, aiding in RNA replication.
• Under certain conditions, some RNA sequences are more stable and replicate faster and with fewer errors than other sequences.
• RNA-directed protein synthesis may have begun as a weak binding of specific amino acids to bases along RNA molecules which served as templates holding a few amino acids together long enough for them to link (rRNA does this today).
• Some RNA molecules may have produced short amino acid chains that were enzymes for RNA replication.
• Other RNA sequences may have enabled the use of high-energy molecules like H2S to provide energy.
• A protobiont with self-replicating, catalytic RNA would dominate the population of molecules.
• The first protobiont would have only limited genetic information, but because thier properties were heritable, they could be acted on by natural selection.
• Mutation (occasional copying errors) and natural selection leads to more stable and faster replicating varieties. • Natural refinement to replace RNA by DNA as the repository of genetic information; being double-stranded it is more stable and accurately reproduced.
RNA, nucleotides too complex, evolved from simpler systems
• Interdependency evolved by means of scaffolding
• First organism contained information only; material and machinery for replication provided by environment
• Organic molecules too varied, reversible bonds too weak, to be simple and self-assembling
• Simple inorganic self-assemblers with strong reversible bonds exist: soap bubbles, clay crystals
• Growth naturally controlled by supersaturation
• Life is information: crystal defects
• Information: control of environment (supersaturation, raw materials?)
• Evolution (natural selection) by direct genetic action
• Advantage gained by going to indirect genetic action
• Organic polymers have advantage of efficiency, flexibility, size
• Possible links: • Photosynthesis can control supersaturation
• Amino acids control concentration of metal ions (Al, Mg)
• Polysaccharides control consistency of solutions
• Gradually, structure & genetic information transferred to organic polymers
• Organic polymers take over. Efficiency and self-assembly work against inorganics
2. Are amino aicds alive?
No.Amino acids are not alive.They are simpler chemical structures that combine with other amino acids through peptide bonds.
Are RNA nucleotides alive?
No RNA nucleotides are not alive.
How many different amino acids and RNA nucleotides are there?
There are 4 RNA nucleotides.present they are:
Twenty amino acids:
Are these complex molecules?
Nucleotides are simpler molecules made of a ribose sugar connected to another through a phosphate backbone.
Amino acids are simpler molecules made of simpler monomer and they polymerise into a peptide chain there by folding into an entire protein.
There are 64 combinations of 4 nucleotides taken three at a time and only 20 amino acids, the code is degenerate (more than one codon per amino acid.
A few dozen atoms are needed to build an amino acid or a nucleotide
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.