Editing Macromolecule (section)

== Linear biopolymers ==
All [[organism|living organisms]] are dependent on three essential [[biopolymers]] for their biological functions: [[DNA]], [[RNA]] and [[proteins]].<ref name="isbn1-4292-2936-5">{{cite book |author1=Berg, Jeremy Mark |author2=Tymoczko, John L. |author3=Stryer, Lubert |title = Biochemistry, 7th ed. (Biochemistry (Berg))|publisher = [[W.H. Freeman & Company]]|year = 2010|isbn = 978-1-4292-2936-4}} Fifth edition available online through the NCBI Bookshelf: [https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=stryer link]</ref> Each of these molecules is required for life since each plays a distinct, indispensable role in the [[cell (biology)|cell]].<ref name="isbn0-8153-4111-3">{{cite book |author1=Walter, Peter |author2=Alberts, Bruce |author3=Johnson, Alexander S. |author4=Lewis, Julian |author5=Raff, Martin C. |author6=Roberts, Keith |title = Molecular Biology of the Cell (5th edition, Extended version)|publisher = [[Garland Science]]|location = New York|year = 2008|isbn = 978-0-8153-4111-6}}. Fourth edition is available online through the NCBI Bookshelf: [https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4 link]</ref> The simple summary is that [[central dogma of molecular biology|DNA makes RNA, and then RNA makes proteins]].

DNA, RNA, and proteins all consist of a repeating structure of related building blocks ([[nucleotide]]s in the case of DNA and RNA, [[amino acids]] in the case of proteins). In general, they are all unbranched polymers, and so can be represented in the form of a string. Indeed, they can be viewed as a string of beads, with each bead representing a single nucleotide or amino acid monomer linked together through [[covalent bond|covalent chemical bonds]] into a very long chain.

In most cases, the monomers within the chain have a strong propensity to interact with other amino acids or nucleotides. In DNA and RNA, this can take the form of [[base pair|Watson–Crick base pairs]] (G–C and A–T or A–U), although many more complicated interactions can and do occur.

=== Structural features ===
{| class="wikitable floatright"
!  
! DNA 
! RNA 
! Proteins
|-
| Encodes genetic information 
| Yes 
| Yes 
| No
|-
| Catalyzes biological reactions 
| No 
| Yes 
| Yes
|-
| Building blocks (type) 
| Nucleotides 
| Nucleotides 
| Amino acids
|-
| Building blocks (number) 
| 4 
| href="dendrimer" | 4 
| 20
|-
| Strandedness 
| Double 
| Single
|-
| Structure 
| Double helix 
| Complex 
|  Complex
|-
| Stability to degradation 
| High 
| Variable 
| Variable
|-
| Repair systems 
| Yes 
| No 
| No
|}

Because of the double-stranded nature of DNA, essentially all of the nucleotides take the form of [[base pair|Watson–Crick base pairs]] between nucleotides on the two complementary strands of the [[nucleic acid double helix|double helix]].

In contrast, both RNA and proteins are normally single-stranded. Therefore, they are not constrained by the regular geometry of the DNA double helix, and so fold into complex [[biomolecular structure|three-dimensional shape]]s dependent on their sequence. These different shapes are responsible for many of the common properties of RNA and proteins, including the formation of specific [[Binding site|binding pockets]], and the ability to catalyse biochemical reactions.

==== DNA is optimised for encoding information ====
[[DNA]] is an information storage macromolecule that encodes the complete set of [[nucleic acid sequence|instructions]] (the [[genome]]) that are required to assemble, maintain, and reproduce every living organism.<ref name="isbn978-0062730992">{{cite book |author1=Golnick, Larry |author2=Wheelis, Mark. |title=The Cartoon Guide to Genetics |publisher=Collins Reference |isbn=978-0-06-273099-2 |date=1991-08-14 |url-access=registration |url=https://archive.org/details/cartoonguidetoge00larr }}</ref>

DNA and RNA are both capable of encoding genetic information, because there are biochemical mechanisms which read the information coded within a DNA or RNA sequence and use it to generate a specified protein. On the other hand, the sequence information of a protein molecule is not used by cells to functionally encode genetic information.<ref name="Stryer_2002"/>{{Rp|5}}

DNA has three primary attributes that allow it to be far better than RNA at encoding genetic information. First, it is normally double-stranded, so that there are a minimum of two copies of the information encoding each gene in every cell. Second, DNA has a much greater stability against breakdown than does RNA, an attribute primarily associated with the absence of the 2'-hydroxyl group within every nucleotide of DNA. Third, highly sophisticated DNA surveillance and repair systems are present which monitor damage to the DNA and [[DNA repair|repair]] the sequence when necessary. Analogous systems have not evolved for repairing damaged RNA molecules. Consequently, chromosomes can contain many billions of atoms, arranged in a specific chemical structure.

==== Proteins are optimised for catalysis ====
Proteins are functional macromolecules responsible for [[enzyme catalysis|catalysing]] the [[Metabolism|biochemical reaction]]s that sustain life.<ref name="Stryer_2002"/>{{Rp|3}} Proteins carry out all functions of an organism, for example photosynthesis, neural function, vision, and movement.<ref name="isbn978-1593272029">{{cite book |author = Takemura, Masaharu|title = The Manga Guide to Molecular Biology|publisher = [[No Starch Press]]|year = 2009|isbn = 978-1-59327-202-9}}</ref>

The single-stranded nature of protein molecules, together with their composition of 20 or more different amino acid building blocks, allows them to fold in to a vast number of different three-dimensional shapes, while providing binding pockets through which they can specifically interact with all manner of molecules. In addition, the chemical diversity of the different amino acids, together with different chemical environments afforded by local 3D structure, enables many proteins to act as [[enzymes]], catalyzing a wide range of specific biochemical transformations within cells. In addition, proteins have evolved the ability to bind a wide range of [[Cofactor (biochemistry)|cofactors]] and [[coenzymes]], smaller molecules that can endow the protein with specific activities beyond those associated with the polypeptide chain alone.

==== RNA is multifunctional ====
[[RNA]] is multifunctional, its primary function is to [[Protein synthesis|encode proteins]], according to the instructions within a cell's DNA.<ref name="Stryer_2002"/>{{Rp|5}} They control and regulate many aspects of protein synthesis in [[eukaryote]]s.

RNA encodes genetic information that can be [[Translation (biology)|translated]] into the amino acid sequence of proteins, as evidenced by the messenger RNA molecules present within every cell, and the RNA genomes of a large number of viruses. The single-stranded nature of RNA, together with tendency for rapid breakdown and a lack of repair systems means that RNA is not so well suited for the long-term storage of genetic information as is DNA.

In addition, RNA is a single-stranded polymer that can, like proteins, fold into a very large number of three-dimensional structures. Some of these structures provide binding sites for other molecules and chemically active centers that can catalyze specific chemical reactions on those bound molecules. The limited number of different building blocks of RNA (4 nucleotides vs >20 amino acids in proteins), together with their lack of chemical diversity, results in catalytic RNA ([[ribozymes]]) being generally less-effective catalysts than proteins for most biological reactions.