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10.13: Metabolism and Replication

Difficulty Level: At Grade Created by: CK-12
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What is an RNA world?

Life must self-replicate, a task that is mostly accomplished with DNA today. Some scientists think that the first replicator was not DNA but RNA. They call this the RNA world hypothesis.


Organic molecules must also carry out the chemical work of cells; that is, their metabolism. Chemical reactions in a living organism allow that organism to live in its environment, grow, and reproduce. Metabolism gets energy from other sources and creates structures needed in cells. The chemical reactions occur in a sequence of steps known as metabolic pathways. The metabolic pathways are very similar between unicellular bacteria that have been around for billions of years and the most complex life forms on Earth today. This means that they evolved very early in Earth's history.


Living cells need organic molecules, known as nucleic acids, to store genetic information and pass it to the next generation. Deoxyribonucleic acid (DNA) is the nucleic acid that carries information for nearly all living cells today and did for most of Earth's history. Ribonucleic acid (RNA) delivers genetic instructions to the location in a cell where protein is synthesized.

RNA World

Many scientists think that RNA was the first replicator. Since RNA catalyzes protein synthesis, most scientists think that RNA came before proteins. RNA can also encode genetic instructions and carry it to daughter cells, such as DNA.

The idea that RNA is the most primitive organic molecule is called the RNA world hypothesis, referring to the possibility that the RNA is more ancient than DNA. RNA can pass along genetic instructions as DNA can, and some RNA can carry out chemical reactions like proteins can.

Pieces of many scenarios can be put together to come up with a plausible suggestion for how life began.



  • An organism's metabolism is the chemical reactions that allow it to live, grow, and reproduce.
  • Nucleic acids pass genetic information to the next generation: DNA for living cells, and RNA for protein synthesis.
  • The RNA world hypothesis suggests that RNA was the first nucleic acid to evolve and DNA came later.


  1. What is the purpose of an organism's metabolism?
  2. Why is the fact that metabolic pathways are similar between organisms significant?
  3. Explain the RNA world hypothesis.

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Use this resource (watch up to 6:23) to answer the questions that follow.

  1. What does DNA do? What does that mean?
  2. How many chromosomes does every cell in a human have? How many DNA molecules does each cell have? Where are the DNA molecules in human cells?
  3. What are the two nucleic acids?
  4. What are the four bases called and what are their letter symbols?
  5. What is the structure of the DNA molecule? What connects the two strands of the structure?
  6. What are the base pairs? How many are in every cell?
  7. What are the differences between RNA and DNA?

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The chemical work of cells; the chemical reactions a living organism needs to live, grow, and reproduce.

nucleic acid

Biological molecules necessary for life; includes DNA and RNA

RNA world hypothesis

Hypothesis that RNA was the first nucleic acid to evolve and that early life was based on RNA, rather than DNA or protein.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 06, 2016
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