What did the 1953 Miller-Urey experiment demonstrate?
AThat living cells can form from nonliving chemicals in a week
BThat amino acids and other organic molecules can form from simple inorganic gases and water when energy such as electric sparks is supplied
CThat DNA was the first genetic molecule
DThat life arrived on Earth from Mars
Stanley Miller, working with Harold Urey, passed electric sparks through a mixture of methane, ammonia, hydrogen and water vapor. Within days the water contained amino acids and other organic compounds. The experiment showed that building blocks of life can form without living things.
Question 2 True / False
Louis Pasteur's swan-neck flask experiments proved that life could never have arisen from nonliving matter at any point in Earth's history.
TTrue
FFalse
Answer: False
Pasteur showed that sterilized broth stays free of microbes when dust and microbes from the air cannot reach it. This disproved spontaneous generation under everyday conditions. It does not address how the first life arose from chemistry on a very different early Earth over long periods.
Question 3 Short Answer
Why is RNA, rather than DNA or protein, proposed as the first genetic molecule?
Think about your answer, then reveal below.
Model answer: RNA can store information in its base sequence, like DNA, and some RNA molecules (ribozymes) can also catalyze reactions, like proteins. A molecule that does both could, in principle, copy itself without needing separate genes and enzymes.
Ribozymes were discovered by Thomas Cech and Sidney Altman, who shared the 1989 Nobel Prize in Chemistry. Modern cells still show signs of an RNA past: the catalytic site of the ribosome that forms peptide bonds is made of RNA.
Question 4 Multiple Choice
Which observation supports the idea that membrane-bounded protocells could form without cells?
ALipids added to water spontaneously form vesicles with a bilayer membrane
BDNA spontaneously forms cell walls
CProteins cannot fold without a cell
DWater cannot dissolve any organic molecules
Amphipathic lipids in water assemble into bilayer vesicles because of the hydrophobic effect. In experiments, such vesicles can grow, divide and take up molecules from their surroundings, which are basic properties a protocell would need.