Chapter 17: From Gene to ProteinGenes Specify Proteins using Transcription & Translation(DNA --> RNA --> Protein)*Gene Expression- the process where DNA directs the synthesis of proteins (in some cases just RNAs).! Gene --> Protein requires 2 stages:- transcription = synthesis of RNA under the direction of DNA (DNA read 3ʼ to 5ʼ)- DNA serves as a template for the newly synthesized strand of mRNA. DNA and RNA use the “same language.”- mRNA carries a genetic message from the DNA to the “protein-synthesizing machinery” of the cell.- acts like a transcript for a protein-coding gene, sending out multiple copies (which is why RNA is preferred over DNA; RNA can make multiple copies of the “original” or (DNA).BIO1107 Test 3: Spring 2012- translation = synthesis of a polypeptide, which occurs under the direction of mRNA.- “Change of language”- Base sequence of of mRNA molecule is translated to an amino acid sequence of a polypeptide.- Ribosomes are the site of translation.- orderly linkage of amino acids --> polypeptide chains- Transcription & Translation (Eukaryotes vs. Bacteria)- Eukaryotes: contains nuclear envelope to separate the two stages.- Transcription occurs in the nucleus, first resulting in pre mRNA, eventually making mRNA which exits via nuclear pores.- Translation is a separate process in the cytoplasm, assisted by ribosomes. - Bacteria: no nucleus or membrane bound organelles.- DNA not segregated from ribosomes or other protein-synthesizing equipment.- Allows for translation to occur while transcription is still in progress. *The Genetic Code- Redundancy, but no ambiguity. - Only 4 nucleotides to specify 20 amino acids.- Triplets are the smallest units of uniform length that can code for amino acids.- doublets or single nucleotides would not be enough to code for ALL 20 amino acids.- still somewhat redundant (more than one codon specified per each amino acid)- “Triplet Code” = example: base triplet AGT along a particular position of DNA strand codes for amino acid Serine. - Template strand- the one strand of a DNA double stranded molecule that is being transcribed.BIO1107 Test 3: Spring 2012- Acts as a template, or sets pattern, for developing strand.- mRNA strand is complementary to its DNA template, rather than identical.- because Uracil instead of Thymine.- base triplets of mRNA called codons.- because codons are base triplets, it takes 3 times the amount of nucleotides than amino acids in protein synthesis.Start codon: AUG (codes for Methionine)! - therefore all protein polypeptide chains start with (Met). Stop codons: UGA, UAA, UAGTranscription is the DNA-directed synthesis of RNA* RNA polymerase pries open double stranded DNA molecule. - also joins together RNA nucleotides as they base pair along the DNA template strand.- can only assemble in 5ʼ to 3ʼ direction.- DONʼT NEED A PRIMER like DNA polymerase- Instead, contains regions on DNA strand acting as- Promoter: sequence initiating transcription- Eukaryotic promoter commonly includes “TATA box” (pg. 333)- Terminator: sequence signaling for the end of transcription. ! - Transcription unit = the stretch of DNA that is transcribed into RNA molecule.BIO1107 Test 3: Spring 2012- RNA polymerase in Prokaryotes vs Eukaryotes- Prokaryotes: contain one type- Eukaryotes: contain 3+ types- (RNA polymerase II) is the type used in mRNA synthesis.* 3 Stages of Transcription- Initiation- after RNA polymerase binds to promoter, the DNA strands unwind. Polymerase initiates RNA synthesis at the start point of the template strand.- In addition to serving as binding site for RNA pol and determining where transcription starts...- the promoter determines which of 2 DNA strands will serve as template.- Elongation- The polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5ʼ to 3ʼ. (In the end of transcription, the DNA strands reform a double helix).- Exposes about 10-20 nucleotides at a time- Termination of RNA chain- RNA transcript is released, and the polymerase detaches from the DNA.- Differs in BACTERIA and EUKARYOTES- Bacteria: terminal signal = transcribed terminator (an RNA sequence)- causes polymerase to detach from DNA and release the transcript. (available for immediate use as mRNA)- Eukaryotes: “polyadenylation signal sequence” that codes for a polyadenylation signal (AAUAAA) in the pre-mRNA.- pre-mRNA then gets processed to form product, mRNA.Eukaryotic cells modify RNA after transcription*RNA processing- modification of pre-mRNA in specific ways before the genetic messages are sent to the cytoplasm.- most of the time sections of RNA molecule are cut out while the remaining pieces are “spliced” together.- during process, BOTH ends of the primary transcript are altered.http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swfBIO1107 Test 3: Spring 2012*Split Genes and RNA splicing- (in nucleus of eukaryotes)- RNA splicing: the removal of large portions of the RNA molecule that is initially synthesized (pre-mRNA).- Most eukaryotic genes (DNA) and their RNA transcripts have long noncoding stretches of nucleotides, regions that are not translated. - Introns (or intervening sequences) are the noncoding segments located in between coding regions.- Functional importance of introns: the presence of introns in genes is that a single gene can encode more than one kind of polypeptide. - Alternative RNA splicing is when many genes are known to give rise to multiple kinds of polypeptides, depending on which segments are treated as exons during RNA processing.- one reason humans can get by with such a small number of genes but with so much variation.* Allows for higher number of different protein products an organism produces than its number of genes.- “Exon shuffling:” introns increase the probability of beneficially crossing over between the exons of alleles. [fig 17.12]- Exons are the other regions, and are eventually “expressed” by being translated to amino acid sequences.- “exit” the cell-[Both terms intron and exon are used for both DNA and RNA].BIO1107 Test 3: Spring 2012RNA pol II transcribes both the introns and exons of the DNA, but the mRNA that exits to the cytoplasm is the “abridged version” or introns have been taken out.- Spliceosome: (how pre-mRNA splicing is carried out)- made of up many snRNPs- almost as big as a ribosome- recognize signal for RNA splicing = snRNPs (small nuclear ribonucleoproteins)- pronounced