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26.4: Nucleic Acids

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Lesson Objectives

  • Describe the structure of DNA.
  • Describe the structures of messenger RNA and transfer RNA.
  • Describe the process of protein synthesis.

Lesson Vocabulary

  • DNA
  • nucleotide
  • RNA
  • transcription
  • translation

Check Your Understanding

Recalling Prior Knowledge

  • What do we mean when we refer to the primary structure of a protein?

Classes of Nucleic Acids

The final type of biomolecules that we will be looking at are the nucleic acids. Like carbohydrates and proteins, nucleic acids are complex polymers of a few simple building blocks. Nucleic acids provide the molecular blueprints for all proteins produced in living systems. We will explore the process by which this information is translated into functional structures later in this lesson. First, we will look at the structures of nucleic acids.

Nucleotides

The subunits from which nucleic acids are generated are referred to as nucleotides. A nucleotide consists of a pentose molecule connected to a nitrogen-containing base and one or more phosphate groups. Biological systems make use of two different pentoses for the construction of nucleic acids: ribose and deoxyribose. Long polymers of ribose-derived nucleotides are referred to as ribonucleic acids (RNA), and and polymers of deoxyribose nucleotides are referred to as deoxyribonucleic acids (DNA).

In a given strand of DNA or RNA, the only difference between each subunit is the choice of base. The five common bases are shown in the Figure below. Based on the number of rings, they are classified as either purines or pyrimidines.

Nucleotide structures.

In general, DNA does not make use of the uracil base, and RNA does not contain thymine. As a result, each strand of a nucleic acid will use only four of these bases.

DNA

Individual nucleotides can be linked together through their phosphate groups to form nucleic acid polymers. Once constructed, DNA generally exists as two strands that are linked together by hydrogen bonds, producing a double-helical structure (see Figure below).

DNA structure.

In order for the structure in Figure above to form, the bases that line up across from each other must be complementary to one another. Based on their structures, adenine will make favorable hydrogen bonding interactions with thymine, and vice versa. Guanine and cytosine form another complementary pair. Mismatched bases will form unfavorable interactions in the center of the helix.

Base pairing in DNA.

If we know the sequence of one strand in a DNA double helix, we can predict the sequence of the opposite strand based on the required pairings.

RNA

RNA is nearly identical to DNA, except that the sugar has an extra OH group and uracil is used instead of thymine. Due to the additional hydrogen bonding opportunities generated by the extra OH group, RNA is found more often as a single strand that is folded in on itself. However, RNA can still form a double helix with a strand of either DNA or RNA if the bases form a complementary sequence.

RNA structure.

RNA is sometimes classified based on the function that it is performing. The types most relevant to the production of proteins are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). We will look at the role of each type of RNA in the following section.

Protein Synthesis

The process of protein synthesis is summarized in the Figure below. DNA produces an RNA template that directs the amino acids to be introduced into the growing protein chain in the proper sequence. A specific transfer RNA attaches to each amino acid and brings it to the RNA for incorporation.

Overview of protein synthesis.

The first step in the process is transcription - the unfolding of DNA and the production of an mRNA strand. This step takes place in the nucleus of the cell.

Formation of RNA from DNA.

The DNA uncoils and provides the pattern for the formation of a single strand of mRNA. After production of the RNA, the DNA refolds into the original double helix. The mRNA is exported to the cytoplasm (outside the nucleus) for further processing.

Amino acids will link with specific transfer RNA molecules for proper placement in the protein chain. The tRNA is a small coiled molecule that accepts an amino acid on one end and matches up to a specific three-base portion of the mRNA on the other end. The tRNA interacts with the mRNA so as to put the amino acids in the proper sequence for the developing protein. After adding the amino acid to the sequence, the tRNA is cleaved from the amino acid and recycled for further participation in the process. Because we are converting from one "language" to another (a sequence of mRNA bases into a sequence of amino acids), this part of the process is referred to as translation.

The process of assembling amino acids into a protein takes place in the ribosome. This structure consists of two subunits, each of which is composed of both proteins and ribosomal RNA (rRNA). The two subunits clamp together on the mRNA and catalyze the formation of the amide linkages in the growing protein. When protein synthesis is complete, the two subunits dissociate and release the completed protein chain.

The process of protein synthesis is fairly fast. Amino acids are added to the growing peptide chain at a rate of about 3-5 amino acids per second. A smaller protein (100-200 amino acids) can be produced in a minute or less.

Role of ribosome in protein synthesis.

Lesson Summary

  • Nucleotides contain a pentose sugar, a nitrogen-containing base, and one or more phosphate groups.
  • Nucleic acids, such as DNA and RNA, are composed of long strands of nucleotides bonded together through their phosphate groups.
  • DNA generally exists as two complementary strands connected by hydrogen bonds into a double helix structure.
  • DNA contains the genetic information for the production of proteins.
  • The transcription process produces a strand of messenger RNA from DNA.
  • During translation, messenger RNA interacts with the ribosome and transfer RNA to produce a protein.

Lesson Review Questions

  1. Describe the differences in structure between DNA and RNA.
  2. Describe the process of transcription.
  3. Describe the structure of transfer RNA.
  4. Describe how translation leads to synthesis of a new protein.

Further Reading/Supplementary Links

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