Biochemistry Lec. 2 PDF

Nucleic Acids 2. Nucleic Acids  Nucleic acids are required for the storage and expression of genetic information.  There are two chemically distinct types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). I. DNA Struct...

Nucleic Acids 2. Nucleic Acids  Nucleic acids are required for the storage and expression of genetic information.  There are two chemically distinct types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). I. DNA Structure  The structure of DNA was first described by James Watson and Francis Crick in 1953.  DNA exists as a double helix  DNA is a double-stranded polymer of deoxyribonucleotides (dAMP, dGMP, dCMP, dTMP) joined by phosphodiester bonds.  The phosphodiester bonds are bonds that are formed between the 3′-OH groups of the deoxyribose sugar on one nucleotide with the 5′ phosphate groups on the adjacent nucleotide.  The phosphate and sugar form the backbone of the DNA strand.  The nitrogenous bases projecting inwards.  The two complementary strands of DNA double helix run in antiparallel directions. The 5′ end of one strand is base-paired with the 3′ end of the other strand.  One type of noncovalent interaction within DNA includes hydrogen bonds that hold together the two strands of DNA within the double helical structure.  Adenine forms two hydrogen bonds with thymine, while guanine and cytosine are connected by three hydrogen bonds.  This type of base pairing in the interior of the helix stabilizes the interior of the double-stranded DNA. 1  Nucleic Acids II. RNA Structure  There are three major types of RNA that participate in the process of protein synthesis: rRNA, tRNA, and mRNA.  The three types of RNA are unbranched polymeric molecules composed of nucleoside monophosphates joined together by 3'→5'-phosphodiester bonds.  The three major types of RNA also differ from each other in size, function, and special structural modifications. A. Ribosomal RNA  rRNA accounts for approximately 80% of total RNA in the cell 2  Nucleic Acids  Function: Ribosomes are important during protein synthesis as they contain peptidyl transferase “activity,” an activity catalyzed by ribozymes. B. Transfer RNA  tRNA is the smallest of the three RNAs.  Function: It carries amino acids during the process of protein synthesis. C. Messenger RNA  Function: mRNA carries genetic information from DNA to cytosol for translation.  About 5% of the total RNA within a cell is mRNA. 3  Nucleic Acids Differences between DNA and RNA DNA RNA Nitrogenous Bases a- Purines Adenine & Guanine Adenine & Guanine b- Pyrimidines Cytosine Cytosine Thymine Uracil Sugar 2-Deoxyribose Ribose Shape of strand Double helix Single strand Types One type mRNAs tRNAs rRNAs Site Nucleus and Mainly in cytosol, less Mitochondria commonly in nucleus Function Genetic information Protein synthesis 4  Nucleic Acids DNA Replication DNA replication is the process of producing two identical copies from one original DNA molecule. The major enzymes involved in replication are DNA polymerases Transcription Synthesis of RNA from a DNA template is called transcription. Genes are transcribed by enzymes called RNA polymerases There are several distinct RNA polymerases in eukaryotic cells each principally responsible for one of the major classes of RNA. 1. RNA polymerase I: synthesizes most of the rRNA. 2. RNA polymerase II: synthesizes mainly mRNA. 3. RNA polymerase III: synthesizes mainly tRNA. 5  Nucleic Acids Post-transcriptional Modification of RNA: I. Capping of mRNA II. Addition of a poly(A) tail III. Splicing Translation  Proteins are produced by the process of translation, which occurs on ribosomes and is directed by mRNA.  The genetic message encoded in DNA is first transcribed into mRNA, and the nucleotide sequence in the coding region of the mRNA is then translated into the amino acid sequence of the protein. 6  Nucleic Acids  These genetic words are called codons.  Each codon is formed of 3 successive nitrogenous bases. Post-translational modifications: 1. Trimming 2. Covalent modifications 7