Structure and Importance of RNA

Megha R (1RV14CH022)

BIOCHEMICAL ENGINEERING SELF STUDY

RNA – Ribonucleic Acid

• RNA is a polymer of ribonucleotides linked together by 3’-5’ phosphodiester linkage

• Usually single stranded. Forms double stranded structure when self complementary sequences exist

RNA – Ribonucleic Acid

• Ribonucleotide consists of a phosphate group, a ribose sugar and a nitrogenous base

• Phosphate group is linked with 3’ of nucleoside through phosphoester linkage

RNA – Ribonucleic Acid

• Nitrogenous bases are classified as – Purines – Adenine and

Guanine– Pyrimidines – Uracil and

Cytosine• Nitrogenous bases are

linked to pentose sugar through N-glycosidic linkage to form a nucleoside

RNA – Ribonucleic Acid

• 2 nucleotides are linked through 3’-5’-phosphodiester linkage to form a dinucleotide

• The polymer has a free phosphate group at 5’ end of ribose sugar and it is called as 5’-end of polynucleotide chain

• At other end, ribose has free 3’-OH group which is called as the 3’-end of polynucleotide chain

Types of RNA

Three main classes of RNA molecules exist-• Messenger RNA(m RNA)• Transfer RNA (t RNA)• Ribosomal RNA (r RNA)The other are small nuclear RNA (SnRNA), micro

RNA(mi RNA) and small interfering RNA(Si RNA)

mRNA – Messenger RNA

• Messenger RNA (mRNA) carries information about a protein sequence to the ribosomes

• It is coded so that every three nucleotides forms a codon and corresponds to one amino acid

• The mRNA is formed with the help of DNA during the process of transcription.

• The sequence of nucleotides in mRNA is complementary to the sequence of nucleotides on DNA.

mRNA

• In eukaryotic cells, once precursor mRNA (pre-mRNA) has been transcribed from DNA, it is processed to mature mRNA

• This involves removal of introns which are non-coding sections of the pre-mRNA

• The mRNA is then sent from the nucleus to the cytoplasm, where it is bound to ribosomes and translated into its corresponding protein form with the help of tRNA

mRNA

• The 5’ terminal end is capped by 7- methyl guanosine triphosphate cap.

• The cap is involved in the recognition of mRNA by the translating machinery

• The 3’end of most m-RNAs have a polymer of Adenylate residues

mRNA

• Synthesis– Catalyzed by an enzyme called RNA polymerase– A process known as transcription occurs using DNA as a

template – Initiation of transcription begins with the binding of the

enzyme to a promoter sequence in the DNA – The DNA double helix is unwound by DNA helicase – The enzyme then progresses along the template strand in the

3’ to 5’ direction, synthesizing a complementary RNA molecule with elongation occurring in the 5’ to 3’ direction.

– The DNA sequence also dictates where termination of RNA synthesis will occur.

rRNA – Ribosomal RNA

• A ribosome contains two subunits—a larger one 60S and a smaller subunit 40S.

• The 60S subunit contains a 5S rRNA, a 5.8S rRNA, and a 28S rRNA

• The 40S subunit contains a single 18S rRNA• It forms complexes with proteins forming

ribosomal subunits which provide space for protein synthesis

• Required for the binding of mRNA to ribosomes and its translation

rRNA

tRNA – Transfer RNA

• tRNA work in conjunction with mRNA to carry out the process of translation and transcription

• They ‘transfer’ the amino acids from cytoplasm to the protein synthesizing machinery

• There are at least 20 species of tRNA one corresponding to each of the 20 amino acids required for protein synthesis.

tRNA

• The nucleotide sequence of tRNA allows intrastand complimentarity that generates a secondary structure.

• Each tRNA shows extensive internal base pairing and acquires a clover leaf like structure.

tRNA

• The structure is stabilized by hydrogen bonding between the bases

• The L shaped tertiary structure is formed by further folding due to formation of hydrogen bonds between T and D arms.

tRNA

tRNA contain 5 main arms or loops:

• Acceptor • Anticodon arm• DHU arm• TΨC arm• Extra arm

tRNA

• Acceptor – The acceptor arm is at 3’ end and has 7 base pairs– The end sequence is unpaired Cytosine and

Adenine at the 3’ end– The 3’ OH group terminal of Adenine binds with

carboxyl group of amino acids– The tRNA bound with amino acid is called amino-

acyl tRNA

tRNA

• Anticodon arm– Lies at the opposite end of acceptor and is 5 base

pairs long– Base sequence of anticodon arm is

complementary to the base sequence of mRNA codon.

– Due to this it can bind specifically with mRNA by hydrogen bonds.

tRNA

• DHU arm acts as the recognition site for the enzyme that adds the amino acid to the acceptor arm.

• TΨC arm contains pseudo uridine and is involved in the binding of tRNA to the ribosomes

snRNA – Small Nuclear RNA

• Involved in the process of splicing or intron removal of primary transcript pre-mRNA to form mature mRNA

miRNA – Micro RNA

• Short, non-coding sequences of RNA• Play important role in gene silencing by

blocking mRNA and preventing translation• Regulate the expression of at least half of all

human genes.• Bind onto mRNAs preventing their translation

into proteins.

miRNA

siRNA – Small Interfering RNA

• They are involved in the RNA interference pathway

• Pairs to its corresponding RNA sequence in the target mRNA.

• This then activates the degrading mRNA. • Once the target mRNA is degraded, the mRNA

cannot be translated into protein

siRNA

Significance

• mRNA – Carries genetic formation of DNA for protein synthesis from nucleus to ribosome in the form of genetic code

• rRNA - Combines with proteins to form ribosomes• tRNA - Acts as adapter molecule. Carries amino

acid and drops it to particular location by recognising codon on mRNA with its anticodon

Significance

• snRNA – Processing of mRNA. U1, U2, U4, U5, and U6 are involved in intron removal

• miRNA – Gene silencing by blocking mRNA and preventing protein synthesis

• siRNA – Gene silencing by causing degradation of mRNA