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The RNA Three major classes of RNA: messenger (mRNA), transfer (tRNA) and ribosomal (rRNA). Minor classes of RNA include small nuclear RNA; small nucleolar RNA;………. RNA is a single stranded; the pyrimidine base uracil (U) replaces thymine and ribose sugar replaces deoxyribose. Dr./Salwa Hassan Teama 2012 Messenger RNA/ mRNA • Transcripts of structural genes. • Encode all the information necessary for the synthesis of a polypeptide of protein. • The 5' terminus is capped by 7 methyguanosine triphosphate. • Synthesis of the poly (A) tail involves cleavage of its 3' end and then the addition of about 200 adenine residues. • Intermediate carrier of genetic information; deliver genetic information to the cytoplasm. mRNA Transfer RNA/ tRNA All the tRNAs share a common secondary structure resembles a cloverleaf: They have four base- paired stems defining three stem-loops (the D loop, anticodon loop, and T loop) and the acceptor stem. tRNA carry correct amino acids to their position along the mRNA template to be added to the growing polypeptide chain. Dr./Salwa Hassan Teama 2012 tRNA Ribosomal RNA/ rRNA • The central component of the ribosome. • Ribosome; factory for protein synthesis; composed of ribosomal RNA and ribosomal proteins (known as a Ribonucleoproteinor RNP). • rRNA provides a mechanism for decoding mRNA into amino acids. Dr./Salwa Hassan Teama 2012 General Structure of Nucleic Acid DNA and RNA are long chain polymers of small chemical compound called nucleotides. Dr./Salwa Hassan Teama 2012 Nucleotides Nucleotides; ring shaped structures composed of: Nitrogenous base; these bases are classified based on their chemical structures into two groups: Purine; double ringed structure (Adenine and Guanine). Pyrimidine; single ring structures (cytosine and thymine). Sugar Phosphate group Dr./Salwa Hassan Teama 2012 Nucleotides • DNA: Four different types of nucleotides differ in nitrogenous base: A is for adenine; G is for guanine; C is for cytosine and T is for thymine. RNA: thymine base replaced by uracil base. Dr./Salwa Hassan Teama 2012 Nucleotides Jypx35. Source: Wikipedia The DNA Deoxyribonucleic Acid (DNA); the genetic material of all cellular organisms and most viruses. DNA; the gigantic molecule which is used to encode genetic information for all life on Earth. A human cell contains about 2 meters of DNA. DNA in the body could stretch to the sun and back almost 100 times. So it is tightly packed. DNA responsible for preserving, copying and transmitting information within cells and from generation to generation. Dr./Salwa Hassan Teama 2012 DNA Double Helix Linked as a twisted ladder. The curving sides of the ladder represent the sugar-phosphate backbone of the two DNA strands; the rungs are the base pairs. Possess antiparallel polarity. Stabilized by hydrogen bonds between the bases. Dr./Salwa Hassan Teama 2012 DNA Double Helix Madprime. Source: Wikipedia DNA Structure DNA is a nucleic acid. The building blocks of DNA are nucleotides, each composed of: – a 5-carbon sugar called deoxyribose – a phosphate group (PO4) – a nitrogenous base • adenine, thymine, cytosine, guanine 14 15 DNA Structure The nucleotide structure consists of – the nitrogenous base attached to the 1’ carbon of deoxyribose – the phosphate group attached to the 5’ carbon of deoxyribose – a free hydroxyl group (-OH) at the 3’ carbon of deoxyribose 16 17 DNA Structure Nucleotides are connected to each other to form a long chain phosphodiester bond: bond between adjacent nucleotides – formed between the phosphate group of one nucleotide and the 3’ –OH of the next nucleotide The chain of nucleotides has a 5’ to 3’ orientation. 18 19 DNA Structure Determining the 3-dimmensional structure of DNA involved the work of a few scientists: – Erwin Chargaff determined that • amount of adenine = amount of thymine • amount of cytosine = amount of guanine This is known as Chargaff’s Rules 20 DNA Structure Rosalind Franklin and Maurice Wilkins – Franklin performed X-ray diffraction studies to identify the 3-D structure – discovered that DNA is helical – discovered that the molecule has a diameter of 2nm and makes a complete turn of the helix every 3.4 nm 21 DNA Structure James Watson and Francis Crick, 1953 – deduced the structure of DNA using evidence from Chargaff, Franklin, and others – proposed a double helix structure 22 DNA Structure The double helix consists of: – 2 sugar-phosphate backbones – nitrogenous bases toward the interior of the molecule – bases form hydrogen bonds with complementary bases on the opposite sugar-phosphate backbone 23 DNA Structure The two strands of nucleotides are antiparallel to each other – one is oriented 5’ to 3’, the other 3’ to 5’ The two strands wrap around each other to create the helical shape of the molecule. 24 25 DNA Replication Matthew Meselson & Franklin Stahl, 1958 investigated the process of DNA replication considered 3 possible mechanisms: – conservative model – semiconservative model – dispersive model 26 27 DNA Replication • DNA replication is the process of copying a DNA molecule. Replication is semiconservative, with each strand of the original double helix (parental molecule) serving as a template (mold or model) for a new strand in a daughter molecule. This process consists of: • Unwinding (initiation): old strands of the parent DNA molecule are unwound as weak hydrogen bonds between the paired bases are “unzipped” and broken by the enzyme helicase. • Complementary base pairing (elongation): free nucleotides present in the nucleus bind with complementary bases on unzipped portions of the two strands of DNA; this process is catalyzed by DNA polymerase. • Joining (elongation): complementary nucleotides bond to each other to form new strands; each daughter DNA molecule contains an old strand and a new strand; this process is also catalyzed by DNA polymerase. • termination – replication is terminated differently in prokaryotes and eukaryotes Ends in prokaryotes when origin is reached Ends in eukaryotes when telomere is reached telomeres – repeated DNA sequence on the ends of eukaryotic chromosomes • DNA replication must occur before a cell can divide; in cancer, drugs with molecules similar to the four nucleotides are used to stop replication. 28 DNA replication 30 (2) Chromosomes Thousands of genes per cell – Genes are a segment of DNA that produce a biologically active product 2 of each chromosome per cell – one maternal, one paternal Histones have high contents of Arg, Lys 48-240 million base pairs per chromosome The Gene The gene; it is a segment within a very long strand of DNA. Genes are the basic units of hereditary. Genes located on chromosome on its place or locus. Allele; a variant of the DNA sequence at a given locus. Each allele inherited from a different parent. Dr./Salwa Hassan Teama 2012 The Gene Source: National Human Genome Research Institute. Gene Structure • Most of the genes consist of; short coding sequences or exons are interrupted by a longer intervening noncoding sequence or introns; although a few genes in the human genome have no introns. Dr./Salwa Hassan Teama 2012 Genes • Genes are the basic physical and functional units of heredity. Each gene is located on a particular region of a chromosome and has a specific ordered sequence of nucleotides (the building blocks of DNA). Exons vs Introns • Eukaryotic genes have introns and exons. Exons contain nucleotides that are translated into amino acids of proteins. Exons are separated from one another by intervening segments of junk DNA called introns. Introns do not code for protein. They are removed when eukaryotic mRNA is processed. Exons make up those segments of mRNA that are spliced back together after the introns are removed; the intron-free mRNA is used as a template to make proteins. Splicing • Exons are sequences of DNA that are expressed3into protein. • Introns are intervening sequences that are not translated into protein C DNA 1 2 Pre-mRNA 1 2 Spliced mRNA 1 2 C C 3 3 3 Exons and Coding What’s the difference between exons and coding sequence? Exons often are described as short segments of protein coding sequence. This is a bit of an oversimplification. Exons are those segments of sequence that are spliced together after the introns have been removed from the pre-mRNA. Yes, the coding sequence is contained in exons, but it is possible for some exons to contain no coding sequence. Portions of exons or even entire exons may contain sequence that is not translated into amino acids. These are the untranslated regions or UTRs. UTRs are found upstream and downstream of the protein-coding sequence. Genetic Mutation A mutation is a change in the DNA sequence or arrangement of DNA. Yassine Mrabet . Source:Wikipedia Dr./Salwa Hassan Teama 2012 Mutations can change the meaning of genes • Mutations are changes in the DNA base sequence – caused by errors in DNA replication or by mutagens – change of a single DNA nucleotide causes sickle-cell disease RNA TACTCT ATGAGA TACTCT ATGAGA 3’ 5’ PROMOTER DNA exon 1 template 5’ 3’ exon 2 exon 3 ATG TAA AATAAA TRANSCRIPTION intron Pre-mRNA intron AUG UAA AAUAAA RNA SPLICING mRNA AUG UAA AAUAAA coding sequence TRANSLATION protein MAPSSRGG….. Normal hemoglobin DNA mRNA Mutant hemoglobin DNA mRNA Normal hemoglobin Sickle-cell hemoglobin Glu Val Figure 10.16A • Types of mutations NORMAL GENE mRNA Protein Met Lys Phe Gly Ala Lys Phe Ser Ala BASE SUBSTITUTION Met Missing BASE DELETION Met Lys Leu Ala His Figure 10.16B