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Gene Expresssion

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  • 1.MIC210 BASIC MOLECULAR BIOLOGYLecture 3 Gene Expression By SITI NORAZURA JAMAL (MISS AZURA) 03 006/06 483 2132

2. Outline 1. Gene expression in prokaryotic cells DNA to mRNA to protein. 2. Gene expression in eukaryotic cellsIntron splicing, 5 capping, 3-poly-A tail 3. DNA Replication 4. Reverse transcription 3. Every cell has the same DNA and therefore the same genes. But different genes need to be on and off in different types of cells. Therefore, gene expression must be regulated. 4. Gene expression must be regulated in several different dimensions In time:6 mos14 wks1 day12 mos10 wks18 mosAt different stages of the life cycle, different genes need to be on and off. 5. 1) Gene expression in prokaryotic cells DNA to mRNA to protein. 6. 1. Gene expression : DNA to mRNA to protein Gene expression process where the information in a gene is read and used to synthesize a protein Genetic information is linearly transferred from DNA to protein. What proteins you can make depends on what genes you have Gene expression in prokaryotes 7. Transcription a messenger RNA (mRNA) molecule is synthesize using the antisense strand as a template the genetic information is now transferred to the mRNA RNA is like DNA except : ribose sugar, single stranded, uracil 8. Molecular Components of Transcription RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine The DNA sequence where RNA polymerase attaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator The stretch of DNA that is transcribed is called a transcription unitCopyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 9. Translation the information in the mRNA is read in a set of 3 bases a codon each codon codes for an amino acid a chain of amino acids a polypeptide is built by reading the codons all these happen in the ribosome in the cytoplasm 10. 2) Gene expression in eukaryotic cells- Intron splicing, 5 capping, 3-poly-A tail 11. The story is much more complicated in Eukaryotes 12. Important differences A cap is added to the 5 end of the mRNA A polyA tail is added to the 3end Introns are removed by a process called splicing 13. Introns and mRNA splicing Most eukaryotic genes contain introns and exons Exons are DNA sequences that carry genetic information Introns do not carry genetic information the introns are removed from the mRNA by a process called splicing whereby the introns are cut out and the exons are rejoined this mature mRNA is then translated to make proteins 14. 3) DNA Replication 15. 3. DNA replication Every new cell must have a complete set of genes Before cell division occurs, the DNA is replicated so that each new cell has its own set of DNA 16. OverviewSynthesis of the leading strand during DNA replicationOrigin of replication Leading strand Lagging strandPrimer Lagging strand Leading strand Overall directions of replicationOrigin of replication3 5 RNA primer5Sliding clamp3 5Parental DNADNA poll III3 55 35 17. In general : the original DNA molecules to serve as a template the new DNA strand is synthesized by the enzyme DNA polymerase III Complementary base pairing ensures that the sequence of the template is copied accurately3TTT5 DNA polymerase IIINew DNA strand 18. Synthesis of new DNA strand requires a primer and can proceed only in a 5 3 direction (why?)5 PO4In the cell, the primer is a short RNA molecule In the test tube, a short piece of DNA will also work. 19. DNA replication step-by-step1) Double helix structure opened up by a helicase enzyme The single stranded regions are stabilized by SSBs (single stranded binding proteins) 20. DNA replication step-by-step2) Another enzyme, primase, makes a short RNA primer 21. DNA replication step-by-step3) Then DNA polymerase III begins to extend the new DNA strand 22. DNA replication step-by-stepDirection of replicasome4) All these enzymes work together in a complex known as a replicasome. The replicasome moves in one direction, following the replication fork 23. The two strands of a DNA are not equal (when it comes to replication)Replication can only happen in a 5 to 3 direction leading and lagging strands 24. DNA replication step-by-stepOn the leading strand, everythings OK- DNA synthesis occurs continuously in a 5 3 direction 25. DNA replication step-by-stepOn the lagging strand, we have a problem - DNA synthesis cannot happen in a 3 5 direction - thus, multiple primers are made - new DNA is synthesized as small Okazaki fragments (5 3) - the primers are then replaced with DNA by DNA polymerase I - the DNA fragments are then joined by DNA ligase 26. Fig. 16-17A summary of bacterial DNA replication Overview Origin of replication Lagging strand Leading strandLeading strand Lagging strand Overall directions of replicationSingle-strand binding protein Helicase5 3 Parental DNALeading strand3DNA pol III Primer 5Primase 3 DNA pol IIILagging strand5 4DNA pol I3 5 32DNA ligase 13 5 27. 3Synthesis of the lagging strand5 5Template strand33RNA primer3155315 35233 5Okazaki fragment3 515 3 5 215 3 1 2Overall direction of replication3 5 28. Proof reading minimized replication errorDNA polymerase III has a 3 5 exonuclease activity that can cut and repair mistakes Remember : DNA replication has to be very accurate (or else?) 29. DNA replication is semi conservativeReplication : From one DNA molecules to two Identical sequences 30. 4) Reverse Transcription 31. 4. Reverse transcription from RNA to DNA 32. The transfer of genetic information from RNA to DNA By the enzyme reverse transcriptase found in retrovirus This allows us to make cDNA (complementary DNA) from mRNA and obtain a gene sequence without the intronsReverse transcriptioncDNA

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