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Nucleic Acids

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Nucleic Acids - Advanced

You may have heard that "it's in your DNA." What does that mean?

Nucleic acids. Essentially the "instructions" or "blueprints" of life. Deoxyribonucleic acid, or DNA, is the unique blueprints to make the proteins that give you your traits. Half of these blueprints come from your mother, and half from your father. And they come in different combinations every time. In fact, every couple - every man and woman that has every lived - together has over 64,000,000,000,000 combinations of their chromosomes, which is where the DNA is found. Therefore, every person that has ever lived - except for identical twins - has his or her own unique set of blueprints - or instructions - or DNA.

Nucleic Acids

Nucleic acids are organic compounds that contain carbon, hydrogen, oxygen, nitrogen, and phosphorus. They are made of smaller units called nucleotides . Nucleic acids are named for the nucleus of the cell, where some of them are found. Nucleic acids are found not only in all living cells but also in viruses. Types of nucleic acids include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) .

Structure of Nucleic Acids

A nucleic acid consists of one chain (in RNA) or two chains (in DNA) of nucleotides held together by chemical bonds. Each individual nucleotide unit consists of three parts:

  • a base (containing nitrogen)
  • a sugar (ribose in RNA, deoxyribose in DNA)
  • a phosphate group (containing phosphorus)

The sugar of one nucleotide binds to the phosphate group of the next nucleotide. Alternating sugars and phosphate groups form the backbone of a nucleotide chain, as shown in Figure below . The bases, which are bound to the sugars, protrude from the backbone of the chain. In DNA, pairs of bases-one from each of two nucleotides-form the middle section of the molecule.

Part of a Nucleic Acid. This small section of a nucleic acid shows how phosphate groups and sugars alternate to form the backbone of a nucleotide chain. The bases that jut out to the side from the backbone are adenine, thymine, cytosine, and guanine. Hydrogen bonds between complementary bases, such as between adenine and thymine, hold the two chains of nucleotides together.

An animation of DNA structure can be viewed at http://www.youtube.com/watch?v=qy8dk5iS1f0 (1:19).

An overview of DNA can be seen at http://www.youtube.com/watch?v=_-vZ_g7K6P0 (28:05).

RNA consists of a single chain of nucleotides, and DNA consists of two chains of nucleotides. Bonds form between the bases on the two chains of DNA and hold the chains together ( Figure above ). There are four different types of bases in a nucleic acid molecule: cytosine (C), adenine (A), guanine (G), and either thymine (T) (in DNA) or uracil (U) (in RNA). Each type of base bonds with just one other type of base. Cytosine and guanine always bond together, and adenine and thymine (or uracil) always bond with one another. The pairs of bases that bond together are called complementary bases .

The binding of complementary bases allows DNA molecules to take their well-known shape, called a double helix . Figure below shows how two chains of nucleotides form a DNA double helix. A simplified double helix is illustrated in Figure below . It shows more clearly how the two chains are intertwined. The double helix shape forms naturally and is very strong. Being intertwined, the two chains are difficult to break apart. This is important given the fundamental role of DNA in all living organisms.

DNA Molecule. Hydrogen bonds between complementary bases help form the double helix of a DNA molecule. The letters A, T, G, and C stand for the bases adenine, thymine, guanine, and cytosine. The sequence of these four bases in DNA is a code that carries instructions for making proteins. Shown is a representation of how the double helix folds into a chromosome. In this double-stranded nucleic acid, complementary bases (A and T, C and G) form hydrogen bonds that hold the two nucleotide chains together in the shape of a double helix. Notice that A always bonds with T and C always bonds with G. The hydrogen bonds help maintain the double helix shape of the molecule.

Simple Model of DNA. In this simple model of DNA, each line represents a nucleotide chain. The double helix shape forms when the two chains wrap around the same axis.

A brief overview of DNA, stressing the base-pairing rules, can be seen in the following animation: http://www.youtube.com/watch?v=cwfO6SzGaEg (1:28).

Role of Nucleic Acids

The order of bases in nucleic acids is highly significant. The bases are like the letters of a four-letter alphabet. These "letters" can be combined to form "words." Groups of three bases form words of the genetic code . Each code word, called a codon, stands for a different amino acid. A series of many codons spells out the sequence of amino acids in a polypeptide or protein ( Figure below ). In short, nucleic acids contain the information needed for cells to make proteins. This information is passed from a body cell to its daughter cells when the cell divides. It is also passed from parents to their offspring when organisms reproduce.

How RNA codes for Proteins

The letters G, U, C, and A stand for the bases in RNA, specifically mRNA or messenger RNA. Each group of three bases makes up a codon, and each codon represents one amino acid (represented here by a single letter, such as V (valine), H (histidine), or L (leucine)). A string of codons specifies the sequence of amino acids in a protein.

DNA and RNA have different functions relating to the genetic code and proteins. Like a set of blueprints, DNA contains the genetic instructions for the correct sequence of amino acids in proteins. RNA uses the information in DNA to assemble the amino acids and make the proteins. More about the genetic code and the role of nucleic acids will be discussed in Molecular Biology - Advanced .

Adenosine Triphosphate

Adenosine Triphosphate (ATP) , or Adenosine-5'-triphosphate, is another important nucleic acid. ATP is described as the "energy currency" of the cell or the "molecular unit of currency." One molecule of ATP contains three phosphate groups, and it is produced by ATP synthase from inorganic phosphate and adenosine diphosphate (ADP) or adenosine monophosphate (AMP). The structure of ATP consists of the purine base adenine, attached to the 1' carbon atom of the pentose sugar ribose. Three phosphate groups are attached at the 5' carbon atom of the pentose sugar. It is the removal of these phosphate groups that convert ATP to ADP (adenosine diphosphate) and to AMP (adenosine monophosphate). ATP is produced during cellular respiration, and will be further discussed in the Cellular Respiration - Advanced concepts.

ATP is used as a substrate in signal transduction pathways by kinases that phosphorylate proteins and lipids, as well as by adenylate cyclase, which uses ATP to produce the second messenger molecule cyclic AMP (cAMP). The ratio between ATP and AMP determines the amount of available energy. This regulates the metabolic pathways that produce and consume ATP. Apart from its roles in energy metabolism and signaling, ATP is also incorporated into DNA and RNA by polymerases during both DNA replication and transcription. When ATP is used in DNA synthesis, the ribose sugar is first converted to deoxyribose by ribonucleotide reductase.

ATP. The ATP molecule clearly shows the three phosphate groups.

See DNA and proteins are key molecules of the cell nucleus at http://www.dnaftb.org/15/animation.html for a description of early work (starting in 1869) on DNA and proteins.

Vocabulary

  • Adenosine Triphosphate ( ATP ): Energy-carrying molecule that cells use to power their metabolic processes; energy-currency of the cell.
  • codon : A sequence of three nucleotides within mRNA; encodes for a specific amino acid or termination (stop) sequence.
  • complementary bases : A pair of nucleotide bases that bond together—either adenine and thymine (or uracil) or cytosine and guanine; complementary base pair.
  • deoxyribonucleic acid ( DNA ): Double-stranded nucleic acid that composes genes and chromosomes; the hereditary material.
  • double helix : The double spiral shape of the DNA molecule; resembles a spiral staircase.
  • genetic code : The universal code of three-base codons; encodes the genetic instructions for the amino acid sequence of proteins.
  • nucleic acid : organic compound such as DNA or RNA
  • nucleotide : Monomer of nucleic acids, composed of a nitrogen-containing base, a five-carbon sugar, and a phosphate group.
  • ribonucleic acid ( RNA ): Single-stranded nucleic acid; involved in protein synthesis.

Summary

  • Nucleic acids are organic compounds that consist of carbon, hydrogen, oxygen, nitrogen, and phosphorus.
  • DNA, RNA and ATP are important nucleic acids.
  • DNA and RNA are made up of repeating units called nucleotides. They contain genetic instructions for proteins, help synthesize proteins, and pass genetic instructions on to daughter cells and offspring.

Practice

Use this resource to answer the questions that follow.

  1. What is DNA? Describe its shape.
  2. What makes the "sides" and the "bridges" of the double helix?
  3. What are the 2 base pairs?
  4. How many base pairs are in 1 cell?
  5. Briefly describe the process of transcription.
  6. How many chromosomes are in a human cell?

Additional information can be viewed at

Review

  1. What is a nucleic acid?
  2. Identify the three parts of a nucleotide.
  3. What is the structure of DNA?
  4. Bases in nucleic acids are represented by the letters A, G, C, and T (or U). How are the bases in nucleic acids like the letters of an alphabet.
  5. Describe the role and structure of ATP.

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