1. Purines (9 member ring) 2. Pyrimidines (6 member ring)
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Purines - 9 member ring
→ Adenine ; A
→ Guanine ; G
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Pyrimidines - 6 member ring
→ Cytosine ; C
→ Thymine ; T
→ Uracil ; U
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DNA contains ____
deoxyribose
* deoxy - without an oxygen
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RNA contains _____ sugar
ribose
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Bases of DNA and RNA
DNA bases ;
* A, C, T, G
RNA bases ;
* A, C, U, G
\ Only DNA contains T
Only RNA contains U
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Nucleoside
→ Contains nitrogenous base & pentose sugar
→ molecule is composed of purine or pyrimidine base and ribose or deoxyribose sugar
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Nucleotide
→ Nucleoside with phosphate group added
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Phosphodiester Bonds
→ Nucleotides are linked by phosphodiester bonds between phosphate group at C - 5’ position and O H group on C - 3’ position
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Nucleoside Monophosphates ; NMP
a nucleotide
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Nucleoside Diphosphate ; NDP
Nucleotide with addition of 2 phosphate groups
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Nucleoside Triphosphate ; NTP
Nucleotide with addition of 3 phosphate groups
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Triphosphate
→ Serve as precursor molecule during nucleic acid synthesis
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ATP and GTP
→ Adenosine triphosphate and guanosine triphosphate
→ large amount of energy involved in adding/removing terminal phosphate group
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Watson and Crick 1953
Proposed the structure of DNA as a double helix
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Chargaff 1949-1953
→ Proposed base composition
→ Amount of A is proportional to T
→ Amount of C is proportional to G
→ Percentage of C + G does not equal percentage of A + T
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Base composition analysis (Chargaff) and X-ray diffraction provided crucial data to _____ and Crick
Watson
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X-ray Diffraction
→ studies by Rosalind Franklin 50-53 showed DNA had a 3.4 angstrom periodicity, characteristic of helical structure
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Watson and Crick Model of DNA :
→ Double helix
→ 2 anti-parallel strands connected by base pairing
→ Stacked nitrogenous bases
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Base Pairing --- Hydrogen Bonds
→ Chemical affinity produces hydrogen bonds in pair of bases
* A-T and G-C base provides complementarity of 2 strands and chemical stability to the helix * A-T ; Double bond * G-C ; Triple bond
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Watson and Crick : Semiconservative Model
→ Storage of genetic information in sequence of bases
→ Mutations or genetic changes that could result in alteration of bases
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Nucleotide Bonding
→ @@Each nucleotide is bound to a nucleotide on the other chain by weak hydrogen bonds between specific pairs of bases@@
→ A pairs w/ T
→ G pairs w/ C
→ 2 chains are complementary w/ opposite polarities
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Genetic Code : Translation
→ involves the synthesis of proteins consisting of a chain of amino acids whose sequence id specified by the coding information in mRNA
* mRNA carries the “genetic code” = chemical info. originating in DNA which specifics the primary structure of proteins
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Translation of mRNA
→ Biological polymerization of amino acids into polypeptide chains
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Translation requires : (4)
1. Amino acids 2. mRNA 3. tRNA 4. Ribosomes
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tRNAs
→ adapt genetic information present as specific triplet codons in mRNA to corresponding amino acid
→ tRNA anticodons complement mRNAs
→ tRNAs carry corresponding amino acids
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Ribosomes
→ Essential role in expression of genetic information
→ Consist of ribosomal proteins and ribosomal RNAs
→ Consists of large and small subunits
\ * Prokaryote ribosomes are 70s * Eukaryote ribosome are 80s
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tRNAs characteristics
→ small in size and very stable
→ 75-90n nucleotides
→ transcribed from DNA
→ Contain posttranscriptional modified bases
* important for hydrogen bonding * confer structural stability
→ tRNAs have a cloverleaf structure
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Anticodon
→ tRNA has anticodon that complementarily base-pairs w/ codon in mRNA
→ Corresponding amino acid is covalently linked to CCA sequence at 3’ end of all tRNAs
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Translation
→ tRNAs are covalently attached to specific amino acids and contain anti-codon complementary to the mRNA codon
→ Base pairing between the tRNA anti-codon and the mRNA codon on the ribosome places amino acids in the correct linear sequence in translation
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Translation of mRNA divides into 3 steps :
1. Initiation 2. Elongation 3. Termination
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Initiation of Translation requires :
1. Small and large ribosomal subunits 2. mRNA molecule 3. GTP 4. Charged initiator tRNA 5. Mg^2+ 6. Initiation factors
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Elongation
→ Both ribosomal subunits assembled w/ mRNA
→ Forms P site and A site
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Termination
→ signaled by stop codons (UAG, UAA, UGA) in A site
→ Codons do not specify any amino acid
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GTP-dependent release factors
→ Stimulates hydrolysis of polypeptide from peptidyl tRNA - released from translation complex
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Charging tRNA
→ Aminoacylation : tRNA charging
* before translation can proceed, tRNA molecules must be chemically linked to respective amino acids * Aminoacyl tRNA synthetase * enzyme that catalyzes aminoacylation
→ 20 different synthetases, 1 for each amino acid
→ Highly specific; recognize only 1 amino acid
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Gene Expression Principles
→ Gene expression involves processes of transcription and translation which result in the production of proteins whose structure is determined by genes