RNA (Ribonucleic acid)

Transcription

1 RNA (Ribonucleic acid) Structure: Similar to that of DNA except: 1- it is single stranded polunucleotide chain. 2- Sugar is ribose 3- Uracil is instead of thymine There are 3 types of RNA: 1- Ribosomal RNA (rrna) 2- Messenger RNA (mrna) 3- Transfer RNA (trna) RNA are copies from DNA sequences formed by a process called transcription. After transcription some modifications occur to obtain the three types of RNA. 1

2 1- Ribosomal RNA (rrna): 80 % of total RNA in the cells are rrna. rrna are found in combination with several proteins ( about 82 proteins) as component of the ribosome Which is the site of protein synthesis. In Eucaryotic ( mammals). There are 4 size types of rrna (5S, 5.8S, 18Ss and 28S) representing 2/3 particle mass of the ribosome. N.B In procaryotics ( bacteria), there are 3 size species of rrna. 2- Messenger RNA (mrna): comprised only 5% of total cellular RNA. Function: Carry genetic information from DNA in the nucleus to ribosomes (in cystol) where it is used as template for protein biosynthesis. 2

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4 3- Transfer RNA (trna): trna represents 15% of total RNA in the cell. Structure: 1- amino acid attachment site or amino acid acceptor: which terminates with the triplet CCA. 2- Anticodon loop or anticodon triplet 3- D loop and T loop: contain unusual bases e.g. dihydrouracil, ribothymidine or methyl guanine Functions of trna: 1- transport amino acids to ribosome for protein synthesis. Each trna carry only one amino acid. The specific amino acid is attached enzymatically to 3' end of trna. 2- recognize the specified codon on mrna to ensure the insertion of the correct amino acid in the growing polypeptide chain. This function is due to anticodon triplet which binds to codon on mrna by base pairing. NB: Three nucleotide bases on mrna form a codon which is then 4 translated into specific amino acid.

5 Structure of trna Funtions of trna 5

6 Transcription RNA is a complementary copy from DNA template except uracil is instead of thymine. The process of RNA synthesis from DNA template is called: transcription and occurs in nucleus. The enzyme responsible for transcription is RNA polymerase Steps in RNA synthesis: (Watch Movie) 1) Initiation: In contrast to DNA polymerase, RNA polymerase does not need a primer to begin synthesis. Here the transcription is initiated by the binding of RNA polymerase to a specific region of DNAdouble helix. This site is called promoter site or promoter region. This region is recognized by sigma factor (subunit) of RNA polymerase. When RNA polymerase recognizes this region, it binds to it leading to a local unwinding (separation) of the promoter region into 2 single strands: 6

7 a- DNA strand that is transcripted into mrna and called template strand or antisense strand. b- The other strand is coding strand or sense strand that contains gene to be translated ( This strand not transcripted, not used) Direction of transcription: RNA polymerase will read the information sequence on DNA template from 3 5 direction, so RNA is synthesized antiparallel to DNA template i.e. from 5 3 direction. 2) RNA elongation: Once RNA polymerase recognizes promoter region, it begins to synthesize a transcript (copy) of DNA template. 3) Termination: Process of elongation of RNA continues until reach what is called : termination region which is recognized by rho factor (subunit in RNA polymerase) resulting in release of the enzyme, and the 7 synthesized RNA

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9 Notes: 1- The synthesized RNA will have the sequence of the sense strand except for U instead of T. 2- In prokaryotic (bacteria) all types of RNA are synthesized by only one species of RNA polymerase. 3- In Eukaryotic ( mammaians), there are 3 classes of RNA polymerase a- RNA polymerase I: synthesizes the precursor of rrna named : pre rrna b- RNA polymerase II: synthesizes pre mrna c- RNA polymerase III: synthesizes pre trna All these enzymes synthesize what is called primary transcript or immature RNAs (pre form) which by some modifications occur after 9 transcription, will give the mature rrna, mrna and trna.

10 Examples on post-transcriptional modifications of RNA: 1-Transcritptinal modifications of mrna: After transcription, the formed immature mrna will undergo the following modifications to be mature and functioning: 1) 5 -capping: 5 - end in the first nucleotide is blocked by 7-methyl guanosine triphosphate (7 methyl-gtp). Role of cap: a- help to stabilize mrna b- Permit initiation of translation (specifies, where translation should begin). 2) Poly A tail: chain of about AMP is attached to 3 - end is added after transcription Role of poly A: Help to stabilize mrna and facilitate its exit from the nucleus. 10

11 2- In rrna: pre rrna is a long RNA that is cleaved into the normal sized rrna species ( 5.8s, 18s and 28s). 5s species is synthesized separately. See hand out. 3- In trna: a- addition of CCA sequence to 3 end of pre trna b- modification of bases to give unusual base 11