Principle 2. Overview of Central. 3. Nucleic Acid Structure 4. The Organization of

Transcription

1 Central dogma I and II the flow of genetic information 1. The Transforming Principle 2. Overview of Central Dogma 3. Nucleic Acid Structure 4. The Organization of DNA in Cells 5. DNA Replication 6. Gene Structure and the Genetic Code 7. Transcription 8. Translation 9. Post-Translational Modification

2 DNA as Genetic Material Transformation Transforming principle Griffith in 1928 observed the change of non-virulent organisms into virulent ones as a result of transformation MacLeod and McCarty in 1944 showed that the transforming principle was DNA

3 Transforming principle

4 The Flow of Genetic Information DNA stores genetic information Information is duplicated d by replication and is passed on to next generation transcription yields a ribonucleic acid (RNA) copy of specific genes translation uses information in messenger RNA (mrna) to synthesize a polypeptide Also involves activities of transfer RNA (trna) and ribosomal RNA (rrna) Replication Transcription Translation Posttranslational modification DNA RNA Polypeptide p Protein ib l RNA Cell Genotype Phenotype

5 Nucleic Acid Structure The nucleic acids, DNA and RNA are polymers of nucleotides linked together by phosphodiester bonds DNA and RNA differ in DNA and RNA differ in the nitrogenous bases they contain the sugars they contain whether they are single or double stranded

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7 Deoxyribonucleic Acid (DNA) Polymer of nucleotides Contains the bases adenine, guanine, cytosine and thymine Sugar is deoxyribose Molecule is usually double stranded Base pairing Adenine (purine) and thymine (pyrimidine) pair by 2 hydrogen bonds Guanine (purine) and cytosine (pyrimidine) pair by 3 hydrogen bonds

8 Ribonucleic Acid (RNA) Polymer of nucleotides Three different types which differ from each Contains the bases other in function, site of adenine, guanine, synthesis (in eucaryotic cytosine and uracil cells) and in structure messenger RNA Sugar is ribose (mrna) ribosomal RNA Most RNA molecules es (rrna) are single stranded transfer RNA (trna)

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11 The Organization of DNA in Cells In all Archaea and most bacteria DNA is a circular double helix Further twisting results in supercoiled DNA In bacteria the DNA is associated with basic proteins Help organize the DNA into a coiled chromatin like structure

12 DNA Forms

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16 Eucaryotic DNA Organization DNA is more highly organized in eucaryotic chromatin where it is associated with histones, small basic proteins The combination of DNA and proteins is called a nucleosome

17 DNA Replication Complex process involving numerous proteins which help ensure accuracy The 2 strands separate, each serving as a template t for synthesis of a complementary strand Synthesis is semiconservative; each daughter cell obtains one old and one new strand

18 In most procaryotes bidirectional from a single origin of replication

19 Rolling Circle Replication some small circular genomes (e.g., viruses and plasmids replicated by rolling-circle i l replication

20 Eucaryotic DNA Replication eucaryotic DNA is ~1,400 times longer than procaryotic DNA and is linear many replication forks are used simultaneously with y p y many replicons present

21 1. Ori 2. Helicase 3. DNA Gyrase 4. SSB 5. Primase 6. RNA primer 7. DNA polymerase 8. Leading/lagging strand 9. DNA ligase 10. Termination

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23 DNA Polymerase Proofreading, removal of mismatched base from 3 end of growing strand by exonuclease activity of enzyme

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26 Gene, defined as the nucleic acid sequence that codes for a polypeptide, trna or rrna Template strand directs RNA synthesis (3 to 5 direction) Promoter is located at the start of the gene and the binding site for RNA polymerase Leader sequence is transcribed into mrna but is not translated into amino acids Shine-Delgarno sequence important for initiation of translation reading frame, organization of codons such that they can be read to give rise to a gene product

27 Genetic Code code degeneracy up to six different codons can code for a single amino acid sense codons the 61 codons that specify amino acids stop (nonsense) codons the three codons used as translation termination signals do not encode amino acids

28 Importance of reading frame

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30 Transcription RNA synthesis under the direction of DNA RNA produced has complementary sequence to the template DNA Three types of RNA are produced mrna carries the message for protein synthesis trna carries amino acids during protein synthesis rrna molecules are components of ribosomes Polygenic mrna often found in bacteria and archaea contains directions for > 1 polypeptide Catalyzed by a single RNA polymerase Reaction similar to that catalyzed by DNA polymerase ATP,GTP,CTP and UTP are used to produce a complementary RNA copy of the template DNA sequence

31 Transcription in procaryotes Initiation Elongation Termination - the sigma factor has no catalytic activity but helps the core enzyme recognize the start of genes holoenzyme = core enzyme + sigma factor Only the holoenzyme can begin transcription

32 Promoter - site where RNA polymerase binds to initiate transcription

33 The Transcription Bubble After binding, RNA polymerase unwinds the DNA (elongation) Transcription bubble produced Moves with the polymerase as it transcribes mrna from template strand Within the bubble a temporary RNA:DNA hybrid is formed

34 Transcription Termination Occurs when core RNA polymerase dissociates from template DNA DNA sequences mark the end of gene in the trailer and the terminator Some terminators require the aid of the rho factor for termination

35 Transcription in Eucaryotes Several RNA polymerases Promotes differ from those in bacteria by having combinations of many elements Catalyzes production of heterogeneous nuclear RNA (hnrna) which undergoes posttranscriptional modification to generate mrna eucaryotic genes have exons (expressed sequences) and introns (intervening sequences) that code for RNA that is never translated into protein

36 Transcription in the Archaea RNA polymerase has similarities to both bacteria and eucaryotic enzyme similarities with eucaryotes archaeal gene promoters and binding of the RNA polymerase introns present in some archaeal genes similarities with procaryotes mrna is polygenic

37 Translation synthesis of polypeptide directed by sequence of nucleotides in mrna direction of synthesis N terminal C-terminal ribosome site of translation polyribosome complex of mrna with several ribosomes

38 The Ribosome Procaryotes, 70S ribosomes = 30S + 50S subunits Eucaryotes, 80S ribosomes = 40S + 60S subunits mitochondrial and chloroplast ribosomes resemble procaryotic ribosomes peptidyl (donor; P) site, binds initiator trna or trna attached to growing polypeptide (peptidyl-trna) aminoacyl (acceptor; A) site, binds incoming aminoacyl-trna exit (E) site, briefly binds empty trna before it leaves ribosome

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40 Aminoacyl-tRNA attachment t of amino acid to trna catalyzed by aminoacyl-trna synthetases t at least 20 each specific for single amino acid and for all the trnas to which each may be properly attached (cognate trnas)

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46 Coupled Transcription and Translation in Procaryotes