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Nucleic acids

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DNA and RNA structure and function. Nucleotides and polynucleotides. mRNA, rRNA, tRNA, miRNA, and siRNA.

Introduction

Nucleic acids, and DNA in particular, are key macromolecules for the continuity of life. DNA bears the hereditary information that’s passed on from parents to children, providing instructions for how (and when) to make the many proteins needed to build and maintain functioning cells, tissues, and organisms.
How DNA carries this information, and how it is put into action by cells and organisms, is complex, fascinating, and fairly mind-blowing, and we’ll explore it in more detail in the section on molecular biology. Here, we’ll just take a quick look at nucleic acids from the macromolecule perspective.

Roles of DNA and RNA in cells

Nucleic acids, macromolecules made out of units called nucleotides, come in two naturally occurring varieties: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material found in living organisms, all the way from single-celled bacteria to multicellular mammals like you and me. Some viruses use RNA, not DNA, as their genetic material, but aren’t technically considered to be alive (since they cannot reproduce without help from a host).

DNA in cells

In eukaryotes, such as plants and animals, DNA is found in the nucleus, a specialized, membrane-bound vault in the cell, as well as in certain other types of organelles (such as mitochondria and the chloroplasts of plants). In prokaryotes, such as bacteria, the DNA is not enclosed in a membranous envelope, although it's located in a specialized cell region called the nucleoid.
In eukaryotes, DNA is typically broken up into a number of very long, linear pieces called chromosomes, while in prokaryotes such as bacteria, chromosomes are much smaller and often circular (ring-shaped). A chromosome may contain tens of thousands of genes, each providing instructions on how to make a particular product needed by the cell.

From DNA to RNA to proteins

Many genes encode protein products, meaning that they specify the sequence of amino acids used to build a particular protein. Before this information can be used for protein synthesis, however, an RNA copy (transcript) of the gene must first be made. This type of RNA is called a messenger RNA (mRNA), as it serves as a messenger between DNA and the ribosomes, molecular machines that read mRNA sequences and use them to build proteins. This progression from DNA to RNA to protein is called the “central dogma” of molecular biology.
Importantly, not all genes encode protein products. For instance, some genes specify ribosomal RNAs (rRNAs), which serve as structural components of ribosomes, or transfer RNAs (tRNAs), cloverleaf-shaped RNA molecules that bring amino acids to the ribosome for protein synthesis. Still other RNA molecules, such as tiny microRNAs (miRNAs), act as regulators of other genes, and new types of non-protein-coding RNAs are being discovered all the time.

Nucleotides

DNA and RNA are polymers (in the case of DNA, often very long polymers), and are made up of monomers known as nucleotides. When these monomers combine, the resulting chain is called a polynucleotide (poly- = "many").
Each nucleotide is made up of three parts: a nitrogen-containing ring structure called a nitrogenous base, a five-carbon sugar, and at least one phosphate group. The sugar molecule has a central position in the nucleotide, with the base attached to one of its carbons and the phosphate group (or groups) attached to another. Let’s look at each part of a nucleotide in turn.
Image of the components of DNA and RNA, including the sugar (deoxyribose or ribose), phosphate group, and nitrogenous base. Bases include the pyrimidine bases (cytosine, thymine in DNA, and uracil in RNA, one ring) and the purine bases (adenine and guanine, two rings). The phosphate group is attached to the 5' carbon. The 2' carbon bears a hydroxyl group in ribose, but no hydroxyl (just hydrogen) in deoxyribose.
_Image modified from "Nucleic acids: Figure 1," by OpenStax College, Biology (CC BY 3.0)._

Nitrogenous bases

The nitrogenous bases of nucleotides are organic (carbon-based) molecules made up of nitrogen-containing ring structures.
Each nucleotide in DNA contains one of four possible nitrogenous bases: adenine (A), guanine (G) cytosine (C), and thymine (T). Adenine and guanine are purines, meaning that their structures contain two fused carbon-nitrogen rings. Cytosine and thymine, in contrast, are pyrimidines and have a single carbon-nitrogen ring. RNA nucleotides may also bear adenine, guanine and cytosine bases, but instead of thymine they have another pyrimidine base called uracil (U). As shown in the figure above, each base has a unique structure, with its own set of functional groups attached to the ring structure.
In molecular biology shorthand, the nitrogenous bases are often just referred to by their one-letter symbols, A, T, G, C, and U. DNA contains A, T, G, and C, while RNA contains A, U, G, and C (that is, U is swapped in for T).

Sugars

In addition to having slightly different sets of bases, DNA and RNA nucleotides also have slightly different sugars. The five-carbon sugar in DNA is called deoxyribose, while in RNA, the sugar is ribose. These two are very similar in structure, with just one difference: the second carbon of ribose bears a hydroxyl group, while the equivalent carbon of deoxyribose has a hydrogen instead. The carbon atoms of a nucleotide’s sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”), as shown in the figure above. In a nucleotide, the sugar occupies a central position, with the base attached to its 1′ carbon and the phosphate group (or groups) attached to its 5′ carbon.

Phosphate

Nucleotides may have a single phosphate group, or a chain of up to three phosphate groups, attached to the 5’ carbon of the sugar. Some chemistry sources use the term “nucleotide” only for the single-phosphate case, but in molecular biology, the broader definition is generally acceptedstart superscript, 1, end superscript
In a cell, a nucleotide about to be added to the end of a polynucleotide chain will bear a series of three phosphate groups. When the nucleotide joins the growing DNA or RNA chain, it loses two phosphate groups. So, in a chain of DNA or RNA, each nucleotide has just one phosphate group.

Polynucleotide chains

A consequence of the structure of nucleotides is that a polynucleotide chain has directionality – that is, it has two ends that are different from each other. At the 5’ end, or beginning, of the chain, the 5’ phosphate group of the first nucleotide in the chain sticks out. At the other end, called the 3’ end, the 3’ hydroxyl of the last nucleotide added to the chain is exposed. DNA sequences are usually written in the 5' to 3' direction, meaning that the nucleotide at the 5' end comes first and the nucleotide at the 3' end comes last.
As new nucleotides are added to a strand of DNA or RNA, the strand grows at its 3’ end, with the 5′ phosphate of an incoming nucleotide attaching to the hydroxyl group at the 3’ end of the chain. This makes a chain with each sugar joined to its neighbors by a set of bonds called a phosphodiester linkage.

Properties of DNA

Deoxyribonucleic acid, or DNA, chains are typically found in a double helix, a structure in which two matching (complementary) chains are stuck together, as shown in the diagram at left. The sugars and phosphates lie on the outside of the helix, forming the backbone of the DNA; this portion of the molecule is sometimes called the sugar-phosphate backbone. The nitrogenous bases extend into the interior, like the steps of a staircase, in pairs; the bases of a pair are bound to each other by hydrogen bonds.
Structural model of a DNA double helix.
Image credit: Jerome Walker/Dennis Myts.
The two strands of the helix run in opposite directions, meaning that the 5′ end of one strand is paired up with the 3′ end of its matching strand. (This is referred to as antiparallel orientation and is important for the copying of DNA.)
So, can any two bases decide to get together and form a pair in the double helix? The answer is a definite no. Because of the sizes and functional groups of the bases, base pairing is highly specific: A can only pair with T, and G can only pair with C, as shown below. This means that the two strands of a DNA double helix have a very predictable relationship to each other.
For instance, if you know that the sequence of one strand is 5’-AATTGGCC-3’, the complementary strand must have the sequence 3’-TTAACCGG-5’. This allows each base to match up with its partner:
5'-AATTGGCC-3' 3'-TTAACCGG-5'
These two strands are complementary, with each base in one sticking to its partner on the other. The A-T pairs are connected by two hydrogen bonds, while the G-C pairs are connected by three hydrogen bonds.
When two DNA sequences match in this way, such that they can stick to each other in an antiparallel fashion and form a helix, they are said to be complementary.
Hydrogen bonding between complementary bases holds DNA strands together in a double helix of antiparallel strands. Thymine forms two hydrogen bonds with adenine, and guanine forms three hydrogen bonds with cytosine.
Image modified from OpenStax Biology.

Properties of RNA

Ribonucleic acid (RNA), unlike DNA, is usually single-stranded. A nucleotide in an RNA chain will contain ribose (the five-carbon sugar), one of the four nitrogenous bases (A, U, G, or C), and a phosphate group. Here, we'll take a look at four major types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and regulatory RNAs.

Messenger RNA (mRNA)

Messenger RNA (mRNA) is an intermediate between a protein-coding gene and its protein product. If a cell needs to make a particular protein, the gene encoding the protein will be turned “on,” meaning an RNA-polymerizing enzyme will come and make an RNA copy, or transcript, of the gene’s DNA sequence. The transcript carries the same information as the DNA sequence of its gene. However, in the RNA molecule, the base T is replaced with U. For instance, if a DNA coding strand has the sequence 5’-AATTGCGC-3’, the sequence of the corresponding RNA will be 5’-AAUUGCGC-3’.
Once an mRNA has been produced, it will associate with a ribosome, a molecular machine that specializes in assembling proteins out of amino acids. The ribosome uses the information in the mRNA to make a protein of a specific sequence, “reading out” the mRNA’s nucleotides in groups of three (called codons) and adding a particular amino acid for each codon.
Image of a ribosome (made of proteins and rRNA) bound to an mRNA, with tRNAs bringing amino acids to be added to the growing chain. The tRNA that binds, and thus the amino acid that's added, at a given moment is determined by the sequence of the mRNA that is being "read" at that time.
Image credit: OpenStax Biology.

Ribosomal RNA (rRNA) and transfer RNA (tRNA)

Ribosomal RNA (rRNA) is a major component of ribosomes, where it helps mRNA bind in the right spot so its sequence information can be read out. Some rRNAs also act as enzymes, meaning that they help accelerate (catalyze) chemical reactions – in this case, the formation of bonds that link amino acids to form a protein. RNAs that act as enzymes are known as ribozymes.
Transfer RNAs (tRNAs) are also involved in protein synthesis, but their job is to act as carriers – to bring amino acids to the ribosome, ensuring that the amino acid added to the chain is the one specified by the mRNA. Transfer RNAs consist of a single strand of RNA, but this strand has complementary segments that stick together to make double-stranded regions. This base-pairing creates a complex 3D structure important to the function of the molecule.
Structure of a tRNA. The overall molecule has a shape somewhat like an L.
Image modified from Protein Data Bank (work of the U.S. government).

Regulatory RNA (miRNAs and siRNAs)

Some types of non-coding RNAs (RNAs that do not encode proteins) help regulate the expression of other genes. Such RNAs may be called regulatory RNAs. For example, microRNAs (miRNAs) and small interfering RNAs siRNAs are small regulatory RNA molecules about 22 nucleotides long. They bind to specific mRNA molecules (with partly or fully complementary sequences) and reduce their stability or interfere with their translation, providing a way for the cell to decrease or fine-tune levels of these mRNAs.
These are just some examples out of many types of noncoding and regulatory RNAs. Scientists are still discovering new varieties of noncoding RNA.

Summary: Features of DNA and RNA

DNARNA
FunctionRepository of genetic informationInvolved in protein synthesis and gene regulation; carrier of genetic information in some viruses
SugarDeoxyriboseRibose
StructureDouble helixUsually single-stranded
BasesC, T, A, GC, U, A, G
Table modified from OpenStax Biology.

Explore outside of Khan Academy

Do you want to learn more about nucleotide base-pairing? Check out this scrollable interactive from LabXchange.
LabXchange is a free online science education platform created at Harvard’s Faculty of Arts and Sciences and supported by the Amgen Foundation.

Want to join the conversation?

  • leaf blue style avatar for user kind of blue
    How do mRNA and tRNA communicate with eachother during the formation of the proteins?
    (52 votes)
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    • piceratops sapling style avatar for user Evan Patev
      mRNA is like a recipe from a cookbook; a list of ingredients to make a protein. mRNA is a chain of nucleotides (A, U, C, and G, not T since this is RNA). A group of three nucleotides is called a codon. A codon matches with three nucleotides, called an anticodon, on a single tRNA molecule while in a ribosome. The tRNA carries an amino acid, our ingredient to make the protein.
      So mRNA is the recipe, tRNA matches to the recipe bringing an ingredient, and the line of ingredients become a protein.
      (147 votes)
  • starky ultimate style avatar for user Greacus
    If A-T bonds have 2 hydrogen bonds and G-C bonds have 3... Would it be true that longer periods of A-T bonds in DNA (so like: AATAATTATTTTAATTAAAA) are less stable parts of the DNA helix than parts that have more (or only) G-C bonds in them? And if this is true, are these parts (AT only parts) more prone to mutations?
    (27 votes)
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    • leaf yellow style avatar for user StephYakir87
      The first part is true, T-A bonds are less stable and more likely to come apart. The A-T bond strands also signal where DNA needs to separate for commonly transcribed genes, such as the TATA Box commonly found just before the beginning of gene sequences.

      I'm not sure if they are more prone to mutations though.
      (20 votes)
  • starky tree style avatar for user Ryan
    DNA is common to all organisms, all organisms use the same 4 nitrogenous bases, A T, C G

    is that right?
    (9 votes)
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  • duskpin ultimate style avatar for user Marwan
    Are all the 46 chromosomes present in a single cell?
    (6 votes)
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  • piceratops ultimate style avatar for user Alex Auvenshine
    Are the functions of nucleic acids guided only by molecular forces and just appear to have intention or are there other forces at work that I'm not aware of? How do these macromolecules "know" what to do?
    (5 votes)
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    • leaf green style avatar for user Jon Hill
      A creationist would say that this is part of the intelligent design. An evolutionist would say it's all down to chance. Two spanners to consider - 1) one molecule of hormone, once recognised by the cell, leads to prduction of thousands of times more molecules, and types of molecules, than a mere chemical would suggest, and such secretions can be brought about by tiny changes in brain activity. 2) DNA is just for storage. It is a molecularly inert form for the passing on of genes without having a massive effect upon the rest of the body - and so the active form is the sticky stuff of RNA and these determine how the proteins are folded together.
      (8 votes)
  • duskpin ultimate style avatar for user Katherine
    Why do some nitrogenous bases have two fused carbon rings while other have one? Would it be possible for there to be nitrogenous bases with more than two fused carbon rings? Could there ever be an instance where there are more than just five kinds of nitrogenous bases (Adenine, Thymine, Guanine, Cytocine and Uracil)? If it could be possible how would DNA and RNA have to rearrange themselves? Would it be possible for DNA and RNA to use other sugars aside from Deoxyribose and Ribose? If so, like what? If not, why?
    (6 votes)
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  • leaf green style avatar for user Prakriti Marwah
    When transcription takes place and the DNA is broken into two, and then mRNA is formed with one of the DNA strands or for BOTH the DNA strands?
    (3 votes)
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    • female robot grace style avatar for user tyersome
      Within a gene usually only one strand is transcribed, but there are many examples where transcription happens from the both strands. This is especially common in viruses.

      Also, the strand that is transcribed for one gene may not be the same as the strand being transcribed for a neighboring gene.

      Finally, the whole DNA double helix is not separated - just a small bubble is opened around each RNA polymerase as it works its way along the DNA.
      (4 votes)
  • marcimus pink style avatar for user Erica Guira
    In the first paragraph of the section "Regulatory RNA (miRNAs and siRNAs)" it says "They bind to specific mRNA molecules (with partly or fully complementary sequences) and reduce their stability or interfere with their translation, providing a way for the cell to decrease or fine-tune levels of these mRNAs."

    Does this mean that the purpose of microRNAs (miRNAs) and small interfering RNAs (siRNAs) it to make sure that there are not too many mRNA molecules producing proteins? They are regulating the amount of protein produced by the cell? Am I understanding this correctly?

    Please help! Thanks. :)
    (3 votes)
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  • aqualine tree style avatar for user ICE
    If the Deoxyribose in DNA is replaced with Ribose, what will happen? Will the molecular structure of DNA become like RNA?
    (3 votes)
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  • starky sapling style avatar for user Raian Ailanazar
    What is the role of U(uracil).Why we can't use uracil in DNA or use T(thymine) in RNA, what's the difference between them?
    (2 votes)
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