5 Central Dogma I o II.ppt

Leading/lagging strand. 9 DNA ligase 5. Primase. DNA ligase 10. Termination . DNA PolymeraseDNA Polymerase Proofreading, removal of mismatched base fr...

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Central dogma I and II the flow of genetic information

1 Th 1. The Transforming T f i Principle 2. Overview of Central Dogma 3. Nucleic Acid Structure 4. The Organization of DNA iin C Cells ll 5. DNA Replication 6. Gene Structure and the Genetic Code 7. Transcription 8. Translation 9. Post-Translational Modification

DNA as Genetic Material Transformation – Transforming principle •

Griffith G iffith iin 1928 observed b d the th change h off non-virulent i l t organisms into virulent ones as a result of “transformation” – MacLeod and McCarty in 1944 showed that the transforming principle was DNA

Transforming principle

The Flow of Genetic Information – DNA stores genetic information



– Information is d li t d b duplicated by replication li ti and is passed on to next generation


Transcription RNA

– transcription yields a ribonucleic acid (RNA) copy of specific genes

Translation Polypeptide yp p

– translation uses information in messenger RNA (mRNA) to synthesize a polypeptide l tid • Also involves activities of transfer RNA (tRNA) and ribosomal ib l RNA (rRNA)

Posttranslational modification Protein Phenotype Cell

Nucleic Acid Structure •

The nucleic Th l i acids, id DNA and d RNA are polymers l off nucleotides l tid – linked together by phosphodiester bonds

DNA and RNA differ in – the nitrogenous bases they contain – the sugars they contain – whether they are single or double stranded

Deoxyribonucleic y Acid ((DNA)) • • • • •

Polymer of nucleotides Contains the bases adenine, guanine, cytosine and thymine S 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

Ribonucleic Acid (RNA) • Polymer P l off nucleotides l tid • Contains the bases adenine, guanine, cytosine and uracil • Sugar is ribose os RNA molecules o ecu es • Most are single stranded

• Th Three diff differentt types t which differ from each other in function, site of synthesis (in eucaryotic cells) and in structure – messenger RNA (mRNA) – ribosomal RNA (rRNA) – transfer RNA (tRNA)

The Organization of DNA in Cells • IIn allll Archaea A h and d 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

DNA Forms

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

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

In most procaryotes bidirectional from a single origin of replication

Rolling Circle Replication • some smallll circular genomes (e.g., viruses and plasmids – replicated by rolling-circle lli i l replication

Eucaryotic DNA Replication • eucaryotic DNA is ~1,400 times longer than procaryotic ti DNA and d iis lilinear • many y replication p forks are used simultaneously y with many replicons present

1. 2 2. 3. 4 4. 5.

Ori Helicase DNA Gyrase SSB Primase

6. RNA primer 7 DNA polymerase 7. 8. Leading/lagging strand 9 DNA ligase 9. 10. Termination

DNA Polymerase

Proofreading, removal of mismatched base from 3’ end of growing strand by exonuclease activity of enzyme

• Gene, defined as the nucleic acid sequence that codes for a • • • • •

polypeptide, tRNA or rRNA Template strand directs RNA synthesis (3’ (3 to 5 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

Genetic Code • code degeneracy – up to six different codons can code for a single amino acid

• sense codons d – the 61 codons that specify amino acids

• stop (nonsense) codons – the three codons used as translation termination signals – do not encode amino acids

Importance of reading frame

Transcription • RNA synthesis under the direction of DNA – RNA produced has complementary sequence to the template DNA – Three types off 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

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

Promoter - site where RNA polymerase binds to initiate transcription

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

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

Transcription in Eucaryotes •

Several RNA polymerases – Promotes P t differ diff from f 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 h exons ((expressed d sequences) and introns (intervening sequences) that code for RNA that is never translated into protein

Transcription in the Archaea • RNA p polymerase y 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

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

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

Aminoacyl tRNA Aminoacyl-tRNA • attachment tt h t off amino acid to tRNA • catalyzed by aminoacyl-tRNA synthetases th t – at least 20 • each specific p for single amino acid and for all the tRNAs to which each may be properly attached (cognate tRNAs)

Coupled Transcription and Translation in Procaryotes