# 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="" %}