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26.4: Nucleic Acids

Created by: CK-12

Lesson Objectives

  • Identify the functions of DNA and RNA.
  • Describe the structure of a single nucleotide and how that structure leads to the overall structure of DNA and RNA.
  • Describe how the base sequence of DNA codes for specific amino acids during protein synthesis.

Lesson Vocabulary

  • gene
  • nucleic acid
  • nucleotide

Check Your Understanding

Recalling Prior Knowledge

  • What constitutes a hydrogen bond?
  • What are amino acids, and how many amino acids are used as the building blocks of proteins?

What is it that makes a frog a frog, a sunflower a sunflower, and a human a human? Genetic material that is passed on from generation to generation carries the instructions that allows the cells of every organism to produce the specific proteins that an organism needs. In this lesson, you will learn about nucleic acids, the molecules that perform this vital function.

DNA and RNA

Swiss biochemist Friedrich Miescher first discovered nitrogen-containing compounds in the nuclei of cells in 1869. The term nucleic acid was used to describe these molecules because they were originally discovered within the nucleus of a cell, and, in their neutral form, the phosphate groups are attached to acidic hydrogens. A nucleic acid is a large biopolymer consisting of many nucleotides. The two primary nucleic acids found in living cells are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the carrier of genetic information and is ultimately responsible for how cells produce proteins in order to carry out all the functions necessary for life. RNA is a related molecule that is involved in the mechanism by which the information stored in DNA is eventually converted into protein molecules.

The basic units of nucleic acids are called nucleotides. A nucleotide is a molecule that contains a five-carbon sugar, a phosphate group, and a nitrogen-containing base called a nucleobase. The five-carbon sugar is either ribose, in the case of RNA, or deoxyribose, in the case of DNA. The only difference between the two sugar molecules is that one of the hydroxyl groups of ribose is replaced by a hydrogen atom in deoxyribose (Figure below).

The sugars ribose and deoxyribose are components of RNA and DNA, respectively.

The three parts of a DNA nucleotide are assembled as shown in Figure below.

Nucleotides are composed of a phosphate group, a sugar, and one of five different nitrogenous bases.

Every DNA and RNA polymer consists of multiple nucleotides strung together into extremely long chains. The only variation in each nucleotide is the identity of the nitrogenous base. Figure above shows one example of a nitrogenous base, called adenine. There are only four different nitrogenous bases found in all nucleic acids. The four bases of DNA are adenine, thymine, cytosine, and guanine, abbreviated A, T, C, and G respectively. In RNA, the base thymine is not found, but it is replaced by a different base called uracil, abbreviated U. The other three bases are present in both DNA and RNA.

The specific structure of DNA proved elusive to scientists for many years. In 1953, James Watson and Francis Crick proposed that the structure of DNA consists of two side-by-side polynucleotide chains wrapped into the shape of a double helix. One aspect of this structure is that each nitrogenous base on one of the DNA strands must be paired up with another base on the opposite strand. Figure below shows how this works. Each adenine base is always paired with a thymine, while each cytosine is paired with a guanine. The bases fit together perfectly from one strand to the other and are also held together by hydrogen bonds. The A-T pairing contains two hydrogen bonds, while the C-G pairing contains three hydrogen bonds. The ends of each strand are labeled either 3’ or 5’ based on the numbering of the deoxyribose sugar ring.

Each strand of a DNA molecule consists of a backbone of phosphate and deoxyribose, to which is attached one of four bases: adenine (A), thymine (T), guanine (G), or cytosine (C). Each strand is then paired to a complimentary strand in such a way that A is paired with T and G is paired with C. The bases are held in place via hydrogen bonds. The double helical structure of DNA is shown on the right.

The Genetic Code

Each particular organism contains many protein molecules that are specific to that organism. The particular base sequence of DNA is responsible for the production of all of the different proteins that are present in each and every living thing that has ever inhabited the Earth. How does that work? Cells use the unique sequence of DNA bases to decide which proteins to synthesize. A gene is a segment of DNA that carries a code for making a specific polypeptide chain. The cell essentially decodes the DNA in order to make whatever peptides and proteins are needed by that organism.

The genetic code works as a series of three-letter codes. Each sequence of three letters, called a triplet, corresponds to one of the twenty common amino acids. The triplets are read by the cell, one after the other, in the process of protein synthesis. Table below shows all of the possible triplets and the amino acid that results from each three-letter code.

DNA Triplet Codes for Amino Acids
1st Base: A 1st Base: G 1st Base: T 1st Base: C
AAA Phe GAA Leu TAA Ile CAA Val
AAG Phe GAG Leu TAG Ile CAG Val
AAT Leu GAT Leu TAT Ile CAT Val
AAC Leu GAC Leu TAC Met CAC Val
AGA Ser GGA Pro TGA Thr CGA Ala
AGG Ser GGG Pro TGG Thr CGG Ala
AGT Ser GGT Pro TGT Thr CGT Ala
AGC Ser GGC Pro TGC Thr CGC Ala
ATA Tyr GTA His TTA Asn CTA Asp
ATG Tyr GTG His TTG Asn CTG Asp
ATT End GTT Gln TTT Lys CTT Glu
ATC End GTC Gln TTC Lys CTC Glu
ACA Cys GCA Arg TCA Ser CCA Gly
ACG Cys GCG Arg TCG Ser CCG Gly
ACT End GCT Arg TCT Arg CCT Gly
ACC Trp GCC Arg TCC Arg CCC Gly

For example, Table above indicates that the DNA code word GCA corresponds to the amino acid arginine, while the DNA code word TCG corresponds to the amino acid serine. Most amino acids are represented by more than one possible triplet code, but each triplet code yields only one amino acid. Three of the DNA code words (ATT, ATC, and ACT) are end or termination code words. When one of those codons is reached, the chain of amino acids is released, and no more are added. Recall that proteins consist of up to a hundred or more amino acids. A protein that contains 120 amino acids would be encoded by a gene that contains 363 DNA base pairs (3 for each amino acid, plus a stop codon).

Even with only four different bases, the number of possible nucleotide sequences in a DNA chain is virtually limitless. The DNA sequence of a particular organism constitutes the genetic blueprint for that organism. This genetic blueprint is found in the nucleus of each cell of the organism and is passed on from parents to offspring. The incredible diversity of life on Earth stems from the differences in the genetic code of every living thing.

Lesson Summary

  • DNA and RNA are biopolymers found in the nuclei of cells. DNA is the carrier of genetic material, while RNA takes part in the translation of that genetic code into specific proteins.
  • Nucleotides consist of a phosphate, a 5-membered sugar, and a nitrogen-containing base.
  • All DNA nucleotides contain one of four bases: adenine, thymine, guanine, or cytosine. In RNA, uracil is substituted for thymine.
  • DNA adopts the structure of a double helix, with hydrogen bonds between base pairs of each of the two strands. The base pairs are A-T (or A-U in RNA) and G-C.
  • The genetic code of DNA provides the information needed to generate all of the proteins produced by a particular organism. Each three-member sequence of nucleotides codes for the addition of a specific amino acid to a peptide chain.

Lesson Review Questions

Reviewing Concepts

  1. What two types of nucleic acids are present in all living cells?
  2. What are the three components of a nucleotide?
  3. How does ribose differ from deoxyribose?
  4. What are the base pairs of DNA? What are the base pairs of RNA?
  5. What connects the base of one nucleotide to its complementary base on the other strand?
  6. How many bases specify an amino acid in the genetic code?

Problems

  1. A segment of a DNA strand has the base sequence G−C−A−C−T−G−G−A−C. Write the base sequence that would be found on the other strand.
  2. Using the genetic code from Table above, write the amino acid sequence formed by translation of the following sequence into a peptide: T−A−G−G−A−C−C−C−T−A−G−C
  3. Use Table above to write a DNA base sequence that codes for the peptide Ala-Gln-Trp. Is there more than one possible answer to this question? Explain.

Further Reading / Supplemental Links

Points to Consider

DNA must be able to reproduce itself as cells divide so that the genetic material can be passed on. That process is called replication. Next, DNA is transcribed into RNA. Finally, a form of RNA is used to make proteins in a process called translation.

  • How does DNA replicate itself during cell division?
  • What is the mechanism by which proteins are built in the cell from the genetic code of the DNA?

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