Nucleic Acids RNA PDF

NUCLEIC ACIDS RNA By: Khalda Mahgoub Berair, MSc Assistant Professor of Biochemistry  Objectives By the end of this lecture, a student should be able to: Define ribonucleic acid(RNA). Outline the general structure of RNA including the different. component units &...

NUCLEIC ACIDS RNA By: Khalda Mahgoub Berair, MSc Assistant Professor of Biochemistry  Objectives By the end of this lecture, a student should be able to: Define ribonucleic acid(RNA). Outline the general structure of RNA including the different. component units & the chemical bonds that join these units. Outline how the genetic information of DNA is copied into distinct forms of ribonucleic acid (RNA). Identify the various types of RNA & explain their cellular roles. Explain that one form of RNA, can be translated into proteins.  RiboNucleic Acid (RNA) RNA is a polymer of ribonucleotides joined by 3‘,5‘ phosphodiester bonds.  RNA The sugar unit in RNA is ribose. The four major bases in RNA are adenine (A), uracil (U), guanine (G) and cytosine (C). RNA  RNA RNA molecules are usually single stranded. Thus, the proportion of adenine differ from that of uracil & the proportion of guanine differ from that of cytosine, i.e.(A+G/C+U)≠1.  RNA RNA is synthesized from DNA in a process called Transcription. In eukaryotes transcription takes place in the nucleus, then RNA is exported to the cytosol. ❖In eukaryotes RNA is found in both nucleus & cytoplasm. ❑ The base sequence of RNA is complementary to DNA template. ❑ DNA directs the synthesis of RNA, which in turn directs protein synthesis.  Functions of RNA Role in protein synthesis. Catalytic RNA has enzymatic activity. Some viruses have RNA as genetic material(retroviruses).  Types of RNA Cells contain several kinds of RNA: Messenger RNA (mRNA) Transfer RNA (tRNA) Ribosomal RNA (rRNA) Small nuclear RNA (snRNA) …. Three major types of RNA mRNA carries genetic information to the ribosomes. rRNA along with protein, makes up the ribosomes. tRNA transfers amino acids to the ribosomes where proteins are synthesized. 16  Messenger RNA (mRNA) mRNA is a transcript from genes coding for proteins. Forms about 5% of the total RNA. It is very heterogeneous. The mRNA sequence is complementary to the DNA template strand from which it is synthesized. The sequence shown here is from the gene for the enzyme β-galactosidase, which takes part in lactose metabolism.  Proteins are not made directly from DNA, however, Instead, a complementary copy of the relevant part of the DNA is made in the form of messenger RNA (mRNA), which is used to direct the synthesis of protein. ❑ mRNA carries the genetic information that will be expressed ultimately as proteins.  mRNA The bases on mRNA are arranged as triplets Each triplet of bases on mRNA is called Codon. Each codon specifies a particular amino acid that is to be placed in the polypeptide chain. mRNA  OH A Codon NH2 HO P O O N N N Adenine CH2 N O O H O HO P O O N NH Guanine N N NH2 CH2 O HO O P H NH2 Arginine O O N N Adenine N N CH2 O OH H  mRNA In prokaryotes mRNA is short-lived and is turned over rapidly. Eukaryotic mRNA is more stable as a result of the modifications that are made to it after it is transcribed.  mRNA transcript is first made in a eukaryotic cell, as a pre-mRNA and need to be processed into a messenger RNA (mRNA). A 5' cap is added to the beginning of the RNA transcript, and a 3' poly-A tail is added to the end. Posttranscriptional modifications of Eukaryotic mRNA include: 5'Cap 7-methyl guanosine to 5` end. Protect from degradation Help to bind ribosome 3' Tail Polyadenylate(polyA) at the 3` end About 200 As Protect from degradation Help in transport to the cytoplasm Polyadenylation of mRNAs The 5'-Cap of Eukaryotic mRNAs  Ribosomal RNA (rRNA) 80% of the total RNA. rRNA Combines with proteins to form ribosomes, the site of protein synthesis. The ribosome  rRNA Has structural and Catalytic function : o Structural role of rRNA The ribosome is made up of ribosomal proteins & ribosomal RNA. o Catalytic role of rRNA The large subunit of the ribosome has enzymatic activity.  rRNA prokaryotic ribosomes have 3 rRNA molecules: 23S, 16S and 5S. Eukaryotic ribosomes have 4 rRNA molecules: 28S, 18S, 5.8S and 5S. 23S (and 28S) rRNA is the catalytic agent in protein synthesis.  Transfer RNA (tRNA) It constitutes 15% of the total RNA. Helps in decoding the mRNA into a protein by bringing the appropriate amino acids to the ribosome.  Transfer RNA (tRNA) It acts as adaptor molecule between the mRNA and the growing peptide during protein synthesis.  Transfer RNA (tRNA) Anticodon loop: used to recognize the codon (triplet of bases) of the mRNA. Acceptor stem: binds the appropriate amino acid covalently.  References: ❖ "Harper's Illustrated Biochemistry " 31st ed. by Rodwell, V. W., Bender, D. A., & Botham, K. M. (2018). McGraw-Hill Education. ❖ "Lippincott's Illustrated Reviews: Biochemistry“7ed. by Pamela C. Champe, Richard A. Harvey, and Denise R. Ferrier. ❖ "Biochemistry“5th ed. by Jeremy M. Berg, John L. Tymoczko, and Lubert Stryer. Thank You