RNA

RNA stands for ribonucleic acid. It is made up of a series of nucleotides joined by 3'-5'phosphodiester bonds. RNAforms a polynucleotide strand with a sugar-phosphate backbone. Unlike DNA, RNA has a ribose sugar, which means that it has a 2` hydroxyl group. The phosphodiester bonds that make up the backbone have a negative charge, this ensures it cannot be hydrolysed by nucleophilic attack, for example by hydroxide ions, as the negative charges repel each other.

Attached to the backbone are 4 bases, in a similar way to DNA, in whichcytosine (C) pairs with guanine (G) and thymine (T) pairs with adenine (A). However in RNAC pairs with G, but A pairs with uracil (U) instead of T. RNA is typically single-stranded, although regions can form where the RNA loops back on itself, to produce "hairpin"secondary structures.

RNA involved in gene expression

1. mRNA – messenger RNA

• Single polynucleotide strand madein the nucleus during transcription
• DNA is transcribed into mRNA by an RNA polymerase, therefore the mRNA and the DNA are complementary
• mRNA carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm
• This mRNAis then used as a template for translation into a functional protein
• mRNAis also used to make copy DNA(cDNA)
• The short-lived, unprocessed or partially processed product is termed precursor mRNA, or pre-mRNA; once completely processed, it is termed mature mRNA.

2. tRNA – transfer RNA

• Single polynucleotide strand whichis folded into a clover shape, held together by hydrogen bonds
• Consists of a specificsequence of three unpairedbases boundto a complementary codon(anticodon)and anamino acid bindingsite
• Found in the cytoplasm, where it is involved in translation
• This moleculecarriesamino acids tothe ribosomes where a polypeptide is formed, the sequence of which was determined by the mRNA.

3. rRNA – ribosomal RNA

• This is the RNAwhich formsribosomes
• It acts as a catalyst for protein synthesis
• It is synthesised in the nucleolus
• rRNA molecules do not code for protein
• tRNA has two subunits: large subunit (LSU) and small subunit (SSU). Large subunit acts as ribozymes which catalyse peptide bond formation.
• It is used to wrk out evoutionary pattterns between species since they are all form of lifes.

The three RNAs all work together to convert the initial DNA molecule into a protein. All three of these types of RNA are synthesised by RNA Polymerase.

4. snRNA - small nuclear RNA

• commonly known as U-RNA
• function in various nuclear processes
• function in the splicing of pre-mRNA
• transcribed by either RNA polymerase II or RNA polymerase III

5. snoRNA - small nucleolar RNA

• used to process and modify rRNA chemically

6. scaRNA - small cajal RNA

• a class of snoRNAs
• locate at the Cajal body
• to modify snoRNA and snRNA

7. miRNA - microRNA

• non-coding RNA molecule
• containing approximately 22 nucleotides
• regulate gene expression by blocking translation of selective mRNA

8. siRNA - small interfering RNA

• also known as silencing RNA
• double stranded RNA molecules
• turn off gene expression by directing degradation of selective mRNA and the establishment of compact chromatin structures
  

RNA can also exist in non coding forms. These non-coding RNAs function in diverse cell processes, such as telomere synthesis, transport of proteins intot the endoplasmic recticulum and X-chromosome inactivation. Beasides, non-coding RNAs also have many applications but many revolve around regulation of gene expression, such as riboswitchesin bacteria and miRNAs involved in RNAi(RNA interference) in animals.