Microbial genetics lectures 7, 8, and 9
4 likes918 views
This document discusses the relationship between genotype and phenotype. It provides examples of how gene expression in E. coli and Serratia marcescens is dependent on environmental conditions. It also summarizes the key steps in transcription, including initiation at the promoter, elongation as RNA polymerase copies DNA into mRNA, and termination. Transcription differs between prokaryotes and eukaryotes, with eukaryotic mRNA undergoing additional processing before translation.
Microbial genetics lectures 7, 8, and 9
- 1. Microbial genetics Bio 433 By Dr. Mona Othman Albureikan
- 2. The relationship between genotype and phenotype • Genotype refers to the complete genetic makeup of an organism. • Phenotype refers to the expression of a gene or genes carried by an organism. • Genotype directs phenotypic expression, but since all genes are not expressed all of the time, genotype is not always expressed as phenotype.
- 3. • Two examples are lactose catabolism in E. coli and ecophenotypic expression of pigment in Serratia marcescens. The relationship between genotype and phenotype
- 4. • E. coli only produces the enzyme b-galactosidase (lactase) when lactose is present. • When lactose is depleted or not present, the sequence of genes controlling enzyme production is shut down. The relationship between genotype and phenotype
- 5. • S. marscescens produces the red pigment (Prodigiosin) when incubated at a temperature range between 25 and 30 oC. • When incubated at 37 oC, the pigment is not produced or produced very little. The relationship between genotype and phenotype
- 6. Transcription (Transcription unit) The structure of gene
- 7. • A promoter is a region of DNA that initiates transcription of a particular gene. • Promoters are located near the transcription start sites of genes, on the same strand and upstream on the DNA (towards the 5' region of the sense strand). Transcription (a promoter)
- 8. • In eukaryotes, they are found at -30, -75, and -90 base pair. • In prokaryotic, approximately -10 and -35 nucleotides upstream from the transcription start site. Transcription (a promoter)
- 9. • A transcription terminator is a section of nucleic acid sequence that marks the end of a gene or operon in genomic DNA during transcription. Transcription (a promoter)
- 10. Transcription • Transcription is the first step of gene xpression. • The template for transcription is DNA. • The product of this process is messenger RNA (mRNA). • The enzymes responsible for transcription in both prokaryotic and eukaryotic cells are called DNA-dependent RNA polymerases, or simply RNA polymerases.
- 11. • Transcription proceeds in the nucleus in eukaryotics ; in the cytoplasm in procaryotes. • The mechanism of transcription has parallels in that of DNA replication. • Unlike DNA replication, in which both strands are copied, only one strand is transcribed. Transcription
- 12. • The strand that contains the gene is called the sense strand, while the complementary strand is the antisense strand. • The mRNA produced in transcription is a copy of the sense strand, but it is the antisense strand that is transcribed. Transcription
- 13. Four different type of RNA can be transcribed from DNA: 1. Primase - primer RNA used to initiate DNA replication. 2. Messenger RNA (mRNA) - used to carry the instructions about polypeptide structure to the site of translation. 3. Ribosomal RNA (rRNA) - part of the ribosome, used in the construction of polypeptides. 4. Transfer RNA (tRNA) - carry amino acids to the site of translation.
- 14. The three steps of transcription
- 15. Steps of Transcription • Transcription begins when an RNA polymerase binds to a promoter site on the 5' to 3' DNA.
- 16. • A small polypeptide portion of the RNA polymerase molecule called the sigma factor recognizes the promotor site sequence. • A short leader sequence of about 10 DNA nucleotides separates the promoter from the actual gene sequence. Steps of Transcription
- 17. • The RNA polymerase molecule untwists and separates the hydrogen bonds between the two DNA nucleosides. • Triphosphate ribonucleotides (rATP, rUTP, rGTP and rCTP) are joined to their complementary bases on the DNA (A-U, G-C). Steps of Transcription
- 18. • As the growing mRNA strand is synthesized, it detaches from the DNA. • This process continues until RNA polymerase reaches a terminator sequence of guanines and cytosines, followed by a repeating sequence of adenines. Steps of Transcription
- 19. Steps of Transcription • Since guanine forms three hydrogen bonds with the cytosine on the DNA template, it takes longer for the RNA polymerase to join these.
- 20. • A loop forms in the guanine-rich portion of mRNA, putting strain on the adenine-rich portion behind it, causing the molecule to separate from the DNA strand and releasing the RNA polymerase. Steps of Transcription
- 21. • Another way transcription is terminated is called Rho-dependent, since it depends on a protein called Rho, located near the end of the mRNA transcript. • The Rho protein moves along the mRNA until the termination sequence is reached, separating the DNA strand from the mRNA transcript and the RNA polymerase enzyme. Steps of Transcription
- 22. Transcription (termination) Termination (a hairpin loop terminator) - type 1 terminator; • RNA moves pass the inverted repeats and transcribes the termination sequence. • Because of the inverted repeat arrangement ➔ RNA synthesized forms a hairpin loop structure. • Hairpin loop makes the RNA polymerase slow down and eventually stops.
- 23. Termination (a hairpin loop terminator) - type 1 terminator • RNA polymerase can not continue attached to DNA. • RNA polymerase dissociate. Transcription (termination)
- 24. Transcription (termination) Termination (Rho-dependent terminator) – type 2 terminator • Terminator is a sequence rich in C (C-rich) and poor in G (G-poor). • Terminator DOES NOT form a hairpin loop. • Rho (protein) binds to C-rich sequence upstream of the termination site.
- 25. Eukaryotic transcription differs from that of bacteria in several ways: 1. RNA synthesis in eukaryotes occurs primarily in the nucleus, with the exception of RNA produced in the mitochondrion or chloroplast. 2. Eukaryotes have three different types of nuclear RNA polymerase, one for the synthesis of mRNA, one for the major rRNA gene and one for tRNA and smaller rRNA molecules.
- 26. 3. At least five different transcription factor proteins assist the binding of eukaryote RNA polymerase to promoter sequences (bacteria only use a single sigma factor). - These are removed following initiation of transcription and replaced by proteins called elongation factors (not found in bacteria). Eukaryotic transcription differs from that of bacteria in several ways:
- 27. A sigma factor (σ factor) • A sigma factor (σ factor) is a protein needed only for initiation of RNA synthesis. It is a bacterial transcription initiation factor that enables specific binding of RNA polymerase to gene promoters. The specific sigma factor used to initiate transcription of a given gene will vary, depending on the gene and on the environmental signals needed to initiate transcription of that gene.
- 28. Eukaryote mRNA undergoes RNA processing (pre- translational modification) prior to translation: a. The 5' end of the mRNA transcript has a guanine nucleotide added during elongation. This is called capping. b. At termination, proteins remove the mRNA transcript and adds 200 adenines to the 3' end. This is called polyadenylation.
- 29. c. Capped and polyadenylated mRNA is called premessenger RNA, since it contains intervening sequences of RNA that do not code for the placement of amino acids. These "junk" sequences are called introns and they separate usable portions called "expressed" sequences or exons. Introns are removed from the mRNA by small rRNA molecules called spliceosomes or "ribozymes." Eukaryote mRNA undergoes RNA processing (pre- translational modification) prior to translation:
- 30. - Prokaryotes often exhibit concurrent RNA transcription. - In this process several RNA polymerase molecules attach to the initiation sequence one after another forming a polyribosome complex. - In this way, multiple copies of the same mRNA molecule can be made from the same gene or set of genes. Transcription (Prokaryotes )
- 31. References Molecular Genetics of Bacteria ( 4th Edition ) (2013), Larry Snyder , Joseph E. Peters , Tina M. Henkin , Wendy Champness ISBN 10: 1555816274 ISBN 13: 9781555816278. Molecular Genetics of Bacteria, 5th Edition, by Jeremy W. Dale, Simon F. Park ,April 2010, ©2010. Genetics of Bacteria, Sheela Srivastava,(2013) ISBN: 978-81-322-1089-4 Microbial Genetics. (1994). Jones and Bartlett Series in Biology. Jones and Bartlett Publishers, Inc.; 2nd edition, ISBN-10: 0867202483, ISBN-13: 978-0867202489, 484 pages. Microbial genetics. (2008). Jones and Bartlett series in biology Series of books in biology. David Freifelder, publisher, Jones and Bartlett, 1987. 601 pages. Molecular Biology: Genes to Proteins Hardcover . (2007). Burton E. Tropp, Publisher: Jones & Bartlett Publishers; 3 edition, ISBN-10: 0763709166, ISBN-13: 978-0763709167, 1000 pages .