Since we weren't around to observe the beginnings of life, we must use clues found in the structures and functions of today's biological molecules to answer this question.

Every living thing is descended from its ancestors. Evolution results when changes occur along the way.

At a molecular level, the same idea applies. Each genetic information molecule must be copied and passed on to future generations. These molecules also evolve when they are changed along the way.

The molecules we know today are descended from the first molecules that formed life on Earth. The behavior of today's biological molecules reveals clues about their potential roles in the origin of life.

What do these molecules do today?

RNA

What Is It And What Does It Do?

RNA is a polymer made up of four repeating subunits called nucleotides. Each nucleotide contains a sugar, a phosphate group and a nitrogenous base. In RNA, that sugar is ribose (In DNA, it's deoxyribose).

The nucleotides that comprise each RNA strand occur in a specific sequence. In cells, the sequence of an RNA strand is specified by a corresponding sequence in the cell's DNA.

RNA bases can pair with one another if they are complementary:
  U (uracil) pairs with A (adenine)
  C (cytosine) pairs with G (guanine)

Even so, RNA generally occurs as a single strand. A single RNA molecule may fold in such a way that complementary bases are brought together to pair along short stretches.

The sequence of RNA bases along the strand determines how it will fold. In this way, the sequence of bases determines the strand's three-dimensional structure. What an RNA molecule can do depends in part on its three-dimensional shape.

1. RNA can store genetic information

   A genome is the catalog of genetic material in a living system. For most organisms, the genome is made of DNA. But in some viruses, the genome is made of RNA, not DNA. Human immunodeficiency virus type 1 (HIV1), the virus that causes AIDS, is an RNA virus.

2. RNA can be a temporary copy of genetic information

   Messenger RNA (mRNA) is a short-lived intermediary molecule that contains the information for making a specific protein. This information is first copied from a cell's DNA genome, and then the mRNA is taken to the cell's protein synthesis machinery, called the ribosome. By "reading" the information encoded in the mRNA, the ribosome knows which specific protein to make.

3. RNA can be a carrier molecule

   One role of RNA is to transport other molecules needed for specific biological reactions. Transfer RNA (tRNA) plays this role in protein synthesis. There are 20 different tRNA molecules, each of which is programmed to attach specifically to one of the 20 amino acids found in proteins. tRNAs brings these amino acids to the ribosome, where they are incorporated into the growing protein chain according to specifications laid out in the mRNA.

4. RNA can be a working part of cellular machinery

   The ribosome is a large structure composed of more than 50 different proteins and several ribosomal RNA (rRNA) molecules. rRNAs are crucial to protein synthesis because they form the structural core of the ribosome.

5. RNA can be a catalyst

   Catalysts drive reactions between molecules. They reduce the amount of energy required for the reaction to occur, and they make sure the reaction is specific and accurate. Protein catalysts are known as enzymes, and RNA can also behave as a catalyst. In fact, the ribosome relies on the catalytic functions of rRNA to assemble proteins.

   RNA catalysts are called ribozymes. Ribozymes can "cut and paste" links between nucleotides in a strand, a process known as splicing. Some mRNA molecules must splice themselves before they can be accurately "read" by the ribosome during protein synthesis.

DNA

What Is It And What Does It Do?

DNA is a polymer made up of four repeating subunits called nucleotides. Each nucleotide contains a sugar, a phosphate group and a nitrogenous base. In DNA, that sugar is deoxyribose (In RNA, it's ribose).

DNA can occur as a single or double-stranded molecule. The double stranded form, which twists into a helix, is a very stable structure. Complementary bases along the two strands pair in a specific way:
  A (adenine) pairs with T (thymine)
  C (cytosine) pairs with G (guanine)

• DNA can store genetic information

  Nucleotides occur in a specific sequence within each strand of a DNA molecule. A relatively short stretch of DNA that encodes a protein or RNA molecule is called a gene. DNA serves as a stable repository of genetic information of organisms in all five kingdoms.

PROTEINS

What Are They And What Do They Do?

Proteins are polymers made of amino acids. There are 20 different amino acids commonly found in proteins. To make a protein, the ribosome strings amino acids together into a chain. This chain, called a polypeptide, folds into a specific three-dimensional shape.

The final shape of a protein determines its function. For example, many enzymes are globular, while structural proteins often take a long, fibrous shape.

• Proteins are products of genetic information

  Amino acids are held together in a protein chain by strong bonds. The specific order of amino acids in the chain is specified by information encoded in genes, which are relatively short stretches of DNA within the genome of a living system. An intermediate copy of this information is made in the form of messenger RNA (mRNA). mRNA directs synthesis of a protein by listing the order in which amino acids will be added to the protein strand.

• Proteins can be catalysts

  Catalysts drive reactions between molecules. They reduce the amount of energy required for the reaction to occur, and they make sure the reaction is specific and accurate. Protein catalysts are known as enzymes. Enzymes make the huge variety of biochemical reactions in a cell possible by keeping the processes very specific. In addition, enzymes help reactions take place in reasonable time at the temperatures found in living systems.

What does this tell us about the first molecules of life?

1. Information stored in DNA is needed to replicate DNA.
2. Information stored in DNA is needed to make proteins.
3. RNA can serve as
   • The Messenger - mRNA is an intermediate copy of genetic information that guides protein synthesis
   • The Carrier - tRNA carries amino acids to the ribosome
   • The Machinery - rRNA is an essential component of the ribosome
4. Protein catalysts (enzymes) are needed to interpret genetic information, but they are also the product of this interpretation.

Based on the modern roles and behaviors of DNA, RNA and proteins, which molecule would you elect as the most likely candidate for the molecular origin of life? Why?

Life science researchers have been pondering this question for years. What do they think? Explore this topic in depth in the monograph Bringing RNA Into View: RNA and its Roles in Biology.