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11.3: Metabolism and Replication

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

DNA passes most genetic information today. But some scientists think that for the first life on Earth, genetic information was passed by RNA. They call this the RNA world hypothesis.


Organic molecules must carry out the chemical work of cells. This is their metabolism. Living organisms need chemical reactions to live, grow, and reproduce. The chemical reactions occur in sequences of steps. These sequences are known as metabolic pathways. The metabolic pathways are very similar for all life on Earth. That includes single celled bacteria that have been around for billions of years. It includes the most complex life forms on Earth today. What does it mean that the metabolic pathways are so similar for all life? This means that the metabolic pathways evolved very early in Earth's history.


Living cells need organic molecules, known as nucleic acids. Nucleic acids are molecules that store genetic information. They pass that genetic information to the next generation. Deoxyribonucleic acid (DNA) is the nucleic acid that carries information for nearly all living cells today. DNA has done this for most of Earth's history. Ribonucleic acid (RNA) delivers genetic instructions to the location in a cell where protein is synthesized. RNA regulates protein synthesis. Some scientists think that RNA was the first replicating molecule.

A segment of DNA that is inheritable is a gene. Every organism has a collection of genes that code for the traits that organism could have. Not all genes are expressed; therefore, knowing all of your genes does not tell you exactly who you are.


  • 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 passes genetic information for living cells. RNA passes genetic information for protein synthesis.


  1. What is the purpose of an organism's metabolism?
  2. What are metabolic pathways?
  3. Metabolic pathways are similar between very simple and very complex organisms. What do scientists think this means?
  4. What do nucleic acids do?
  5. What are the two nucleic acids and what does each do?
  6. If you have a gene for a purple nose, but you don't in fact have a purple nose, what do scientist say about that gene?

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Use the resource below to answer the questions that follow.

  1. Where are virtually all of your genes?
  2. What are genes?
  3. Describe the structure of a DNA molecule.
  4. What do the four bases—adenine (A), thymine (T), cytosine (C), and guanine (G)—do?
  5. What are chromosomes made of?
  6. How many chromosomes do humans have?
  7. How similar are we to chimps? How similar to other humans?

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gene Piece of DNA that passes genetic information from parent to offspring.
metabolism 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

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Difficulty Level:
6 , 7
Date Created:
Jan 04, 2013
Last Modified:
Aug 29, 2016
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