Archive for the ‘ Obj 4-RNA ’ Category

Obj 4-Structure of RNAs & its Functions


=Ribonucleic acid


Difference between DNA & RNA Structure

Why is DNA 2′-deoxy and RNA is not?

  • -OH groups in RNA make it more susceptible to hydrolysis
  • DNA, lacking 2′-OH is more stable
  • This makes sense – the genetic material must be more stable
  • RNA is designed to be used and then broken down


Types : formed by different RNA polymerases

  1. mRNA – Carries the message from DNA  /  Type II polymerase
  2. tRNA   – Cloverleaf-like structure  /  Type III polymerase
  3. snRNA – Used in splicing  /  Type II and III polymerase
  4. rRNA  –  Component of ribosome                                                        40S+60S subunits combine to form a functional 80S ribosome.

(60S) subunit – 28S, 5S, and 5.8S  rRNAs

(40S) subunit – 18S rRNA

Type I polymerase – 28S, 18S, and 5.8S  rRNAs

Type III polymerase – 5S  rRNAs

1) Messenger RNA (mRNA)

– Constitutes about 5% of all RNA in the cell.

– Formed by transcription where only one strand of DNA act as a template

– Each 3 successive bases in mRNA form a codon that code for certain a.a

– The sequence of codons will determine the order of in the formed protein

-Genetic code

1. There are 64 codons code for 20 amino acids

2. One start codon (AUG) and three stop codons (UAA), (UAG)& (UGA).

2) Transfer RNA (tRNA)

-15 % of RNA

-smallest RNA

-clover leaf arrangment

-the anticodon loop base pair with mRNA codon

-adaptor mol.

-CCA sequence at its free 3′ end

-31 tRNA in the cell

-each carries a specific amino acid on its 3′ end

-charged or uncharged

3)Small Nuclear RNA (snRNA)

-Constitutes less than 1% of all RNA in the cell

-about 30 different types

-Involved in splicing process

4) Ribosomal RNA (rRNA)

Constitutes about 80% of all RNA in the cell




1)mRNA as DNA messenger of coding information for protein synthesis

-need to undergo process


RNA Processing

2)snRNA + protein molecules = small nuclear ribonucleoproteins (snRNPs Complexes) + add protein = Spliceosome

-splice exons (joining)  / excise introns (remove)


3)tRNA deliver the amino acid to the ribosome where it will be incorporated into the polypeptide chain that is being produced according to the sequence of mRNA’s Codon by matching it with anticodon

4)rRNA provide a mechanism for decoding mRNA into amino acid and to interact with the tRNA’s during translation by providing peptidyl transferase activity.