# Nucleic Acids (DNA and RNA) ### Introduction Nucleic acids are polymers utilised by cells as information molecules. The genetic information is stored in the form of **deoxyribonucleic acid** (**DNA**) providing instructions for the synthesis of proteins needed to build and maintain functioning cells, tissues, and organisms. Notably, some viruses employ **ribonucleic acid** (**RNA**) for storing genetic information. Within cells, RNA serves to carry the instructions from the DNA to the protein-synthesising machines, known as ribosomes.

DNA is the template for the synthesis of RNA which is decoded by a ribosome to make a protein
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OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30131214

The choice of nucleic acids as information molecules is determined by their ability to drive their self-replication. The **replication** of DNA before cell division enables each cell to have its copy of the genetic material.

The structure of DNA enables a replication mechanism in which two identical copies of the parental molecule are produced
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Genomics Education Programme - Semi conservative replication of DNA, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=50542923

## Nucleic acids are made of nucleotides Nucleic acids are often referred to as **polynucleotides** since they consist of monomers called nucleotides. Each **nucleotide** comprises three components: a five-carbon sugar, a phosphate group and a nitrogenous base. In RNA, the sugar is **ribose**, while DNA incorporates **deoxyribose**. The nitrogenous bases are attributed to one of two families: purines and pyrimidines. **Purines**, distinguished by their larger size and two fused rings, include **adenine** (**A**) and **guanine** (**G**). **Pyrimidines** contain one ring and include **cytosine** (**C**), **thymine** (**T**) and **uracil** (**U**). Thymine is exclusive to DNA, while uracil is only found in RNA.

Nucleotides make up nucleic acids
(a) Each nucleotide contains a pentose sugar, a phosphate and a nitrogenous base
(b) Nitrogenous bases
(c) Fice-carbon sugars of DNA and RNA
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OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30131164

## DNA and RNA structure In a polynucleotide, nucleotides are joined by **phosphodiester linkages** formed through condensation reactions. This bonding creates the **sugar-phosphate backbone**, characterized by an alternating pattern of sugars and phosphates. The two ends of a polynucleotide molecule feature distinct chemical groups. The end with a phosphate group is termed the **5' end**, while the end with a hydroxyl group is called the **3' end**.

DNA structure
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Zephyris - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15027555

A DNA molecule is comprised of two polynucleotide strands that intertwine, forming a **double helix**. The strands are referred to as antiparallel as they run in opposite 5' to 3' directions. The nitrogenous bases are positioned inwardly towards the double helix from the sugar-phosphate backbone. Thus, the nitrogenous bases of the two strands face each other forming **complementary base pairs**. In DNA, adenine (A) always pairs with thymine (T), while guanine (G) pairs with cytosine (C). The complementary bases are held together by **hydrogen bonds**. There are two hydrogen bonds between A and T, and three hydrogen bonds between G and C.

Left: the regions of complementarity in tRNA
Right: tRNA 3D shape
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Kyle Schneider (SchneiderKD) (Transfered by BQmUB2010090/Original uploaded by SchneiderKD) - Schneider KD (Original uploaded on en.wikipedia), Public Domain, https://commons.wikimedia.org/w/index.php?curid=12309266

The majority of RNA molecules are **single-stranded**. However, the complementary base-pairing between regions of the same RNA molecule can make it fold into a complex three-dimensional shape. A striking example is the 3D L-shape of transfer RNA (tRNA) molecules that serve to carry amino acids to the ribosome. ## ## Summary The videos below provide the summary of the topic and some additional details: {% embed url="" %} {% embed url="" %} --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://edu.abi.am/molecular-biology/biological-molecules/nucleic-acids-dna-and-rna.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.