Structure and function of rna

MOLECULAR BIOLOGY

1. Structure and function of RNA Ankita Behera M.Sc Microbiology St. George College of Management & Science Bengaluru North University

2. RNA or Ribonucleic acid is a polymer of nucleotides which is made up of i )ribose sugar ii)phosphate iii)a bases such as Adenine, Guanine , Cytosine and Uracil. It is a polymer molecular essential in various biological roles in coding , decoding , regulation and expression of genes.

3. a)Ribonucleotides contain the pentose sugar ribose instead Of the deoxy ribose found in deoxyribonucleotides. b)RNA contains the pyrimidine uracil in place of thymine found in DNA

4. Like DNA , RNA is a long polymer consisting of nucleotides:- RNA is a single stranded helix. The strand has a 5’ end (phosphate group) and a 3’ end (hydroxyl group). It is composed of ribonucleotides . The ribonucleotides are linked together by 3’-> 5’ phosphodiester bonds. The nitrogenous bases that compose the ribonucleotides include Adenine , Cytosine ,Uracil and Guanine. Thus, the difference in the structure of RNA from that of DNA include: Adenine (A) ,Guanine(G) , Uracil (U) and Cytosine( C).So , thymine in DNA is replaced by Uracil in RNA .

5. RNA secondary structure Most RNA molecules are single stranded but an RNA molecule may contain regions which can form complementary base pairing where the RNA strand loops back on itself. If so , the RNA will have some double stranded regions. Ribosomal RNAs (rRNAs) and Transfer RNAs (tRNA) exhibit substantial secondary structure as do some messenger RNAs(mRNAs).

6. Types of RNA:- In both prokaryotes and eukaryotes , there are 3 main types of RNA:- i )rRNA (ribosomal RNA) ii)tRNA(transfer RNA) iii)mRNA (messenger RNA)

7. Transfer RNA (tRNA):- It delivers amino acids to ribosome and decodes the information of mRNA .each nucleotide triplet codon on mRNA represents an amino acids . It plays the roles of an adaptor and matches each codon to its particular amino acids in the cytoplasmic pool. It has 2 properties :- Its trinucleotide sequence is called anticodons which is complementary to codon of mRNA , the codon and anticodon forms base pairs. The other is amino acid binding site.

8. There are different kinds of tRNA molecules in a cell. Each tRNA is named after the amino acid it carries .for example , if tRNA carries amino acid tyrosine it is written as tRNA Tyr, Sometimes there are more than one tRNA for an amino acid then it is denoted as tRNA 1 Tyr and tRNA 2 Tyr . a minimum 32 tRNA are required to translate all 61 codon The tRNA charged with an amino acid is called amino acyl tRNA.

9. Clover leaf structure of tRNA:- The primary structure of all tRNA molecules is small ,linear , single stranded nucleic acid and ranging in size from 73-93 nucleotide. The tRNA due to its property having stretches of complementary base pairs forms secondary structure which is in the form of a clover leaf . Several regions of the single stranded molecule from double stranded stems or arms and single stranded loops due to folding of various regions of the molecule . These double stranded stems have complementary base paired typical tRNA has bases numbering from 1-76 using the stranded numbering convention where position 1 is the 5’ end and 76 is the 3’ end.

10. Clover leaf structure of tRNA:-

11. 3-D structure of tRNA:- X-ray crystallographic analysis of tRNA shows 3-D structure called tertiary structure . The molecule is folded and has a 2 helical double stranded branches one branch consists of acceptor arm and T ψ C arm and the other arm consists of DHU loop and anticodon arm with loop . The tRNA molecule is L-shaped and tertiary structure creates two double helices at right angle to each other .The amino acid binding site is opposite to the anticodon arm. tRNA constitutes about 10% of the total cellular RNA.

12. messenger RNA (mRNA) It is a linear molecular transcribed from one strand of DNA It carries the base sequence complementary to DNA template strand The base sequence of mRNA is in the form of triplet codons. Ribosome translates these triplet codons into amino acid sequence of polypeptide chain . LENGTH OF mRNA :- -length is depends upon the length of polypeptide chain it codes for . -polypeptide length varies from a chain of a few amino acids to 1000. amino acid . For example -600 nucleotides code for apolypeptide having a chain of 200 amino acids .

13. Life span of mRNA:- In bacteria , mRNA is transcribed and translated in a single cellular compartment and the two processes are linked as they occur simultaneously . Transcription begins when enzyme RNA polymerase binds to DNA and the moves along making a copy of one strand . As soon as the transcription begins , the ribosomes attach to 5’ end of the mRNA and start translation while the other end of mRNA is still under synthesis . This is known as coupled transcription and translation in prokaryotic . After , the translation of whole of mRNA is completed , the mRNA is then degraded is 5’->3’ direction . An individual mRNA molecule survives only for a minute or less.

14. In eukaryotes transcription occurs in the nucleus while translation takes place in cytoplasm .eukaryotic mRNA is stable and survive from a few minutes to more than a day . Eukaryotic mRNA constitutes only a small proportion of the total cellular RNA .it is only about 3% of the total RNA. Coding region consists of series of codons but the mRNA is longer than the coding regions. The coding regions are called “EXONS” and between the coding regions lie various non-coding called “INTRONS”.

15. Ribosomal RNA (rRNA):- Most of the RNA of the cell is in the form of ribosomal RNA which constitutes about 85% of the total RNA. Ribosomes consists of many types of RNA :- i )70s ribosomes of prokaryotes in its smaller subunits of 30 s has 16s rRNA. ii)The 50s larger subunit consists of 23s and 5s rRNA. iii)80s ribosome has 18s rRNA in its smaller subunit of 40s. iv)60s larger subunit has 28s ,5.8s and 5s rRNA.

16. The rRNA molecules form secondary structure of double stranded stems and single stranded loops by extensive complementary base pairings. It helps in protein synthesis because they interact with mRNA and tRNA at each step of translation or protein synthesis. The 3’ terminus of rRNA of 16s rRNA interacts with initiation site on mRNA which is called “SHINE-DALGARNO SEQUENCE” and just before the start codon AUG . The 23s rRNA plays an active role in peptidyl transferase activity .movement of tRNA between A and P site on ribosome is aided by 23 s rRNA. The RNA-protein complexes are called “Ribonucleoproteins”(RNP).

17. RNA play major role as enzymes as “RIBOZYMES” It has a active sites a binding site for substrate and a binding site for a Co-factor. They are mainly involved in splicing of introns present on RNA.

18. Functions of Ribosomal RNA(rRNA):- They bind proteins molecules and give rise to ribosome. 18s rRNA has nucleotides complementary to those of cap region of mRNA. 5s rRNA and surrounding protein complex provide binding site of tRNA.

19. Functions of transfer RNA (tRNA):- tRNA is adapter molecule which is meant for transferring amino acids to ribosomes for synthesis of polypeptides. They carry specific amino acids at particular points during polypeptide synthesis as per cidons of mRNA. They held peptidyl chains over the mRNA.

20. Function of messenger RNA (mRNA):- mRNA carries coded information for translation into polypeptide formation. It has a cap region for attachment to ribosome. Through reverse transcription it form compact genes which are used in GENETIC ENGINEERING.

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