Chapter 15 AMF - Transcription and Translation PDF

Genes and How They Work Chapter 15 © McGraw Hill ©Dr. Gopal Murti/Science Source 1  Outline overall Central Dogma RNA and codons Transcription Landmarks Process in prokaryotes Differen...

Genes and How They Work Chapter 15 © McGraw Hill ©Dr. Gopal Murti/Science Source 1  Outline overall Central Dogma RNA and codons Transcription Landmarks Process in prokaryotes Differences in eukaryotes RNA processing in eukaryotes Translation tRNA and ribosomes Process in prokaryotes and eukaryotes Mutations © McGraw Hill 2  The Nature of Genes Early ideas to explain how genes work came from studying human diseases Early 1900s connected genes to enzymes Beadle and Tatum in 1941 created mutations in chromosomes and tracked inheritance, proposed one-gene/one- enzyme hypothesis Now one-gene/one-polypeptide hypothesis © McGraw Hill 3  The Central Dogma First described by Francis Crick, models information flow © McGraw Hill 4 Transcription uses DNA to code for RNA One of the two DNA strands called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript (DNA template is read 3’ to 5’, and RNA is synthesized 5’ to 3’) New nucleic acid strands are always generated 5’ to 3’ T (thymine) in DNA replaced by U (uracil) in RNA © McGraw Hill 5  Outline section 1b Central Dogma RNA and codons Transcription Landmarks Process in prokaryotes Differences in eukaryotes RNA processing in eukaryotes Translation tRNA and ribosomes Process in prokaryotes and eukaryotes Mutations © McGraw Hill 6  RNA All synthesized from DNA template by transcription Found in all domains of organisms: Messenger RNA (mRNA) – codes for proteins Ribosomal RNA (rRNA) – structural part of ribosomes Transfer RNA (tRNA) – carries amino acids for translation Only found in eukaryotes (not required to memorize): Small nuclear RNA (snRNA) Signal recognition particle RNA (srpRNA) MicroRNA (miRNA) Small interfering RNA (siRNA) © McGraw Hill 7  Genetic Code Codon – block of 3 mRNA nucleotides corresponding to an amino acid Indicates importance of reading frame © McGraw Hill 8  Codons Stop codons - 3 codons used to terminate translation Start codon (AUG) used to signify the start of translation Code is degenerate, meaning that some amino acids are specified by more than one codon Code is practically universal Very strong evidence that all living things share common ancestry © McGraw Hill 9  Outline section 2a Central Dogma RNA and codons Transcription Landmarks Process in prokaryotes Differences in eukaryotes RNA processing in eukaryotes Translation tRNA and ribosomes Process in prokaryotes and eukaryotes Mutations © McGraw Hill 10  Prokaryotic transcription Single RNA polymerase Does not require a primer Transcription unit on DNA includes: Promoter Transcription is initiated at a promoter sequence and Start site ends at a terminator sequence. The transcript is synthesized in a 5'-to-3' direction. Terminator Terminator Promoter DNA RNA TATA TATA Which strand is transcribed can differ from one gene to the next. © McGraw Hill 11  Promoter Forms a recognition and binding site for the RNA polymerase Found upstream of the start site Not transcribed © McGraw Hill 12 Initiation of transcription 5' 3' Enhancer sequence Mediator complex DNA Through looping of DNA, transcriptional activator proteins, mediator complex, RNA Pol II, and 5' 3' general transcription Promoter factors are brought into close proximity, allowing transcription to proceed. RNA polymerase complex (Pol II) Promoter region © McGraw Hill 13  Elongation of transcription RNA grows in the 5′-to-3′ direction as ribonucleotides are added Uses the template strand of DNA to add correct free nucleotides Transcription bubble – contains RNA polymerase, DNA template, and growing RNA transcript After the transcription RNA transcript RNA polymerase bubble passes, the Template RNA–DNA duplex complex (Pol II) DNA strand now-transcribed DNA is rewound as it leaves the bubble © McGraw Hill 14  Termination of transcription Marked by sequence that signals “stop” to polymerase Causes the formation of phosphodiester bonds to cease RNA–DNA hybrid within the transcription bubble dissociates RNA polymerase releases the DNA DNA rewinds © McGraw Hill 15  Prokaryotic transcription is coupled to translation mRNA begins to be translated before transcription is finished No nuclear membrane means both processes occur in nucleoid region of cytoplasm © McGraw Hill 16  Outline section 2b Central Dogma RNA and codons Transcription Landmarks Process in prokaryotes Differences in eukaryotes RNA processing in eukaryotes Translation tRNA and ribosomes Process in prokaryotes and eukaryotes Mutations © McGraw Hill 17 Eukaryotic transcription differences: proteins and process 3 different RNA polymerases RNA polymerase I transcribes rRNA RNA polymerase II transcribes mRNA (and some snRNA) RNA polymerase III transcribes tRNA (and other small RNAs) More proteins involved in initiation process Termination process somewhat different © McGraw Hill 18  Eukaryotic transcription differences: mRNA modifications In eukaryotes, the primary transcript must be modified to become mature mRNA Addition of a 5′ cap Involved in translation initiation Protects from degradation Addition of a 3′ poly-A tail Created by poly-A polymerase Protects from degradation Removal of non-coding sequences (introns) Pre-mRNA splicing done by spliceosome © McGraw Hill 19  Eukaryotic pre-mRNA splicing Introns – non- coding sequences Exons – sequences that will be translated EXons are EXpressed © McGraw Hill 20  Outline section 3 Central Dogma RNA and codons Transcription Landmarks Process in prokaryotes Differences in eukaryotes RNA processing in eukaryotes Translation tRNA and ribosomes Process in prokaryotes and eukaryotes Mutations © McGraw Hill 21  tRNA and Ribosomes tRNA molecules carry amino acids to the ribosome for incorporation into a polypeptide Anticodon loop contains 3 nucleotides complementary to mRNA codons Aminoacyl-tRNA synthetases add amino acids to the acceptor stem of tRNA, “charging” the tRNA © McGraw Hill 22  Aminoacyl tRNA synthetase Free amino acids Each aminoacyl tRNA synthetase binds to one uncharged tRNA Uncharged tRNA and its corresponding amino acid. There is a specific enzyme The enzyme attaches for each amino the amino acid to the 3' acid. end of the tRNA. Charged tRNA © McGraw Hill 23  Ribosomes Large subunit E P A Small subunit Exit Peptidyl Aminoacyl site site site The large subunit includes three binding sites for tRNAs. © McGraw Hill 24  Translation initiation part 1 In prokaryotes, initiation complex includes Initiator tRNA charged with N-formylmethionine Small ribosomal subunit mRNA strand © McGraw Hill 25  Translation initiation part 2 1. Ribosome binding sequence (RBS) of mRNA positions small subunit correctly 2. Large subunit now added 3. Initiator tRNA bound to P site with A site empty © McGraw Hill 26  Initiation in eukaryotes Initiation in eukaryotes similar except Initiating amino acid is methionine More complicated initiation complex Lack of an RBS – small subunit binds to 5′ cap of mRNA © McGraw Hill 27  Elongation part 1 Elongation adds amino acids 1. 2nd charged tRNA can bind to empty A site 2. Requires elongation factor called EF-Tu to bind to tRNA and GTP 3. Peptide bond can then form with the help of peptidyl transferase (part of ribosome) 4. Ribosome moves one codon (3 bases) downstream 5. Addition of successive amino acids occurs as a cycle © McGraw Hill 28  Elongation part 2 A reaction transfers the Met to the amino acid on the tRNA in the A site, forming a peptide bond. The ribosome moves down one codon, which puts the amino acid carrying the polypeptide into the P site and the now- uncharged tRNA into the E site, where it is ejected. A new tRNA complementary to the next codon binds to the A site. The polypeptide transfers to the amino acid on the tRNA in the A site. The polypeptide is elongated by repeating steps (d) and (e). © McGraw Hill 29  Termination Elongation continues until the ribosome encounters a stop codon Stop codons are recognized by release factors which release the polypeptide from the ribosome © McGraw Hill 30  Outline section 4 Central Dogma RNA and codons Transcription Landmarks Process in prokaryotes Differences in eukaryotes RNA processing in eukaryotes Translation tRNA and ribosomes Process in prokaryotes and eukaryotes Mutations © McGraw Hill 31  Figure 15.22 – review of transcription and translation together © McGraw Hill 32 Differences Between Prokaryotic and Eukaryotic Gene Expression Characteristic Prokaryotes Eukaryotes Introns No introns, although some archaeal Most genes contain introns. genes possess them. Number of genes in mRNA Several genes may be transcribed Only one gene per mRNA into a single mRNA molecule. Often molecule; regulation of pathways these have related functions and accomplished in other ways. form an operon, which helps coordinate regulation of biochemical pathways. Site of transcription and No membrane-bounded nucleus; Transcription in nucleus; mRNA translation transcription and translation are is transported to the cytoplasm coupled. for translation. Initiation of translation Begins at AUG codon preceded by Begins at AUG codon preceded special sequence that binds the by the 5’ cap(methylated GTP) ribosome. that binds the ribosome. Modification of mRNA after None; translation begins before A number of modifications while transcription transcription is completed. the mRNA is in the nucleus; Transcription and translation are introns are removed and exons coupled. are spliced together; a 5’ cap is added; a poly-A tail is added. © McGraw Hill 33  Point mutations Alter a single base Silent mutation – same amino acid inserted Missense mutation – changes amino acid inserted Nonsense mutations – changed to stop codon © McGraw Hill 34  Frameshift mutations Addition or deletion of one or a few bases (not multiples of three) Much more profound consequences Alter reading frame downstream Often introduces an early stop codon © McGraw Hill 35  Mutations are the original source of genetic variation Mutations are the starting point for evolution Too much change, however, is harmful to the individual Balance must exist between amount of new variation and health of species © McGraw Hill 36  Page 331 © McGraw Hill 37