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Transcript
Basic Genetics DNA Transfer & Genetic Variation Ramy K. Aziz, PhD 5 May 2015 With slides from: Dr. Aymen Yassin and Dr. Marwa Elrakaiby “Variety is the spice of life” No varia4on = No life as we know it! Announcements • Remember: Website – h-p://egybio.net/courses/FOPCU/pm401 (quesAons/ videos/ assignments/ future: exam info) • Study tool: Quizlet (web & cell phone App) – h-ps://quizlet.com/ramykaram • Assignment: – UnAl midnight: only ONE student: Reem Yasser (PCR animaAon) • Next week: No lecture, but I CAN be here for an hour of quesAons if you want. 5 May 2015 PM401-‐GeneAcs Genotypic varia4on Transfer of DNA Transforma4on Muta4on Transduc4on Spontaneous Induced -‐SubsAtuAon (Point mutaAon) Conjuga4on Transposable gene4c elements -‐DeleAon, inserAon (Frame shi[) 5 May 2015 PM401-‐GeneAcs Intercellular gene4c transfer 5 May 2015 PM401-‐GeneAcs Gene4c transfer & recombina4on • GeneAc RecombinaAon is the exchange of genes between two DNA molecules to form new combina4ons or variants of genes • In prokaryotes, recombinaAon occurs via gene transfer between cells or within cell by Conjuga4on Transposi4on Transforma4on Transduc4on 5 May 2015 PM401-‐GeneAcs Gene4c recombina4on 5 May 2015 “Crossing Over” PM401-‐GeneAcs DO NOT MEMORIZE Ver4cal vs. horizontal gene transfer HGT: Transmission of genes to neighboring cells of the same generaAon VGT : Transmission of genes from the parental generaAon to offspring 5 May 2015 PM401-‐GeneAcs HOW DO CELLS PROTECT THEIR DNA (IDENTITY)? 5 May 2015 PM401-‐GeneAcs Image source: h-p://www.howstuffworks.com/ali-‐baba-‐story.htm Ali Baba has the answer! 5 May 2015 PM401-‐GeneAcs Restric4on/modifica4on systems Two enzymes, closely related in their specificity, modify and protect DNA of a given bacterial species against foreign DNA (which they will degrade): A) Methyl transferase: adds methyl groups to Adenine and Cytosine residues in the same target sequence that consAtutes the restricAon enzyme binding site. (MethylaAon renders the target site resistant to restricAon) B) Restric4on Endonuclease: degrade non-‐methylated DNA at specific posiAons. The specific posiAon is called palindrome which consist of 6 -‐ 8 bases pairs having the same sequence on both strands. 5 May 2015 PM401-‐GeneAcs Palindrome? • Words: – radar, level, madam • Phrases: – Nurses run • n u r s E s r u n – Madam In Eden, I’m Adam • m a d a m i n e d e n i m a d a m – Was it a rat I saw • w a s i t a r a t i s a w 5 May 2015 PM401-‐GeneAcs Restric4on/modifica4on systems Modifying enzyme Restric4on enzyme Me • GAATTC CTTAAG Me 5 May 2015 PM401-‐GeneAcs • GAATTC CTTAAG Palindrome 5 May 2015 PM401-‐GeneAcs a. Transforma4on • Gene transfer by uptake of naked/soluble DNA fragments from the surrounding environment and the expression of the encoded geneAc informaAon in the recipient cell. • It works best with DNA from closely related species • Competent cells are those able to pick naked DNA and incorporate it into its genome by recombinaAon 5 May 2015 PM401-‐GeneAcs Griffith Experiment (1928) The transforming principle: virulent avirulent DO NOT MEMORIZE 5 May 2015 PM401-‐GeneAcs Transforma4on Mechanism Gram posi4ve Gram nega4ve • In general, Gram-‐posiAve cells degrade one strand of the DNA as it is being transferred, the other strand enters and can associate with a similar region in the chromosome and replace a similar sequence. 5 May 2015 • In Gram-‐negaAve bacteria, both strands enter the cell then one strand is digested before incorporaAon into the bacterial chromosome. PM401-‐GeneAcs 5 May 2015 PM401-‐GeneAcs How to make cells competent? • E.coli does not develop competence in normal growth; however, competence can be induced in the lab: – Chemically: by chilling the cells at 4°C a[er treaAng with CaCl2 then heaAng the cells at 42°C for ~ one minute. – Electrically: by growing cells to mid-‐log phase, chilling them at 4˚C, then treaAng them with an electric pulse (high voltage for very short Ame), a process known as electropora4on. • This treatment apparently alters the membrane and allow the passage of DNA. • In lab and in Industry, competent cells play an important role in geneAc engineering and cloning. 5 May 2015 PM401-‐GeneAcs Genotypic varia4on Transfer of DNA Transforma4on Muta4on Transduc4on Spontaneous Induced -‐SubsAtuAon (Point mutaAon) Conjuga4on Transposable gene4c elements -‐DeleAon, inserAon (Frame shi[) 5 May 2015 PM401-‐GeneAcs b. Conjuga4on • Gene transfer by direct contact between cells via sex pilus (G-‐) or ma4ng bridge (G+) • Transfer mediated by a plasmid which has genes for its own transfer • ConjugaAon requires cell to cell contact between two cells of opposite ma4ng type (donor: F+ and recipient: F-‐) 5 May 2015 PM401-‐GeneAcs Plasmids • Plasmids are small circular double stranded DNA extrachromosomal geneAc element. They contain few genes controlling non essenAal funcAons. They are capable of autonomous replicaAon (Self replica4on) • Copy number: RaAo of plasmid copies to the chromosome copies in a cell. It is higher for small non conjugaAve plasmids and low approaching one for large conjugaAve plasmids. • Plasmid curing: Acridine dye and UV inhibit plasmid replicaAon and at certain doses could get rid of it from the cell 5 May 2015 PM401-‐GeneAcs Plasmids 5 May 2015 PM401-‐GeneAcs Examples of plasmids 1-‐ Sex Factor: (F plasmid) FerAlity plasmid mediaAng transfer of gene from one cell to the other through sex pilus (conjuga4ve plasmid). 2-‐ Resistance Factor: (R plasmid) plasmid mediaAng resistance to anAmicrobial agents. In G-‐, R plasmids are conjugaAve and called resistance transfer factor (RTF) i.e. : carry F and R genes. Many of the "R" or "RTF" plasmids could be integrated into the bacterial chromosome at different posiAons acAng as transposons or jumping genes 5 May 2015 PM401-‐GeneAcs Conjuga4on 5 May 2015 PM401-‐GeneAcs High frequency of recombina4on conjuga4on DO NOT MEMORIZE 5 May 2015 PM401-‐GeneAcs Genotypic varia4on Transfer of DNA Transforma4on Muta4on Transduc4on Spontaneous Induced -‐SubsAtuAon (Point mutaAon) Conjuga4on Transposable gene4c elements -‐DeleAon, inserAon (Frame shi[) 5 May 2015 PM401-‐GeneAcs c. Transduc4on Gene transfer by bacteriophage 5 May 2015 PM401-‐GeneAcs Ly4c Cycle vs. Lysogenic Cycle 5 May 2015 PM401-‐GeneAcs Prophage Rare: 10-‐5-‐10-‐6 5 May 2015 Specialized Transduc4on Generalized Transduc4on DO NOT MEMORIZE Rare too: 10-‐5-‐10-‐6 PM401-‐GeneAcs Transduc4on 1. Generalized 2. Restricted 3. Co-‐transducAon: – Used for geneAc mapping (no longer common with DNA sequencing made easy) 4. AborAve – A special case of generalized transducAon, where the exogenote fails to be integrated for several generaAons. 5 May 2015 PM401-‐GeneAcs Genotypic varia4on Transfer of DNA Transforma4on Muta4on Transduc4on Spontaneous Induced -‐SubsAtuAon (Point mutaAon) Conjuga4on Transposable gene4c elements -‐DeleAon, inserAon (Frame shi[) 5 May 2015 PM401-‐GeneAcs Intracellular gene4c transfer 5 May 2015 PM401-‐GeneAcs Transposable gene4c elements Inser4on sequences • Found at one or more site at the bacterial chromosome. • No geneAc informaAon other than the ability to insert copies of themselves into the bacterial chromosome. • IS form copies of themselves and the copies move into other areas of the chromosome. • They can interrupt the coding sequence of a gene resulAng in the producAon of a wrong protein or no protein at all. 5 May 2015 Transposons • • Are pieces of DNA that move readily from one site to another, either within or between the DNAs of bacteria, plasmids or bacteriophages. They are named “jumping genes”. • They can code for drug resistance, enzymes or toxins and are larger than inserAon sequences. • When they “move” or “jump” they leave their original locaAon empty (in IS the copies move) • They can either cause mutaAons in the gene in which they insert or alter the expression of the nearby genes. PM401-‐GeneAcs Source: Mobile gene/c elements: the agents of open source evolu/on. Nat Rev Microbiol. 2005 Sep;3(9):722-‐32. Summary: Mobile gene4c elements 5 May 2015 PM401-‐GeneAcs What’s taking part of the exam? • The geneAc code • Possible diagrams • PCR principle and simple steps • The 3 methods of intercellular gene transfer + intracellular gene transfer (transposiAon) 5 May 2015 PM401-‐GeneAcs [email protected] 5 May 2015 PM401-‐GeneAcs
