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RNA and Transcription Lecture #24 Honors Biology Ms. Day Gene expression DNA contains… information in a specific sequence (order) of nucleotides along 2 DNA strands Leads to specific traits Gene expression includes two stages Transcription: DNA RNA “transcribe” = to copy into another form Translation: RNA polypeptide “translate” = to change into another language THINK: Translation happens “LATER” What are the characteristics of RNA? Ribose Nucleic Acid Single stranded Made of sugars (called ribose), phosphate groups and nitrogen bases Backbone= alternating ribose sugar/phosphates held together by PHOSPHODIESTER BONDS Made of RNA nucleotides Contains bases: (A) Adenine (G)Guanine (C) Cytosine (U) Uracil (replaces Thymine) Type of RNA Function Job mRNA Brings message from DNA (DNA instructions) to ribosome to make protein (messenger RNA) tRNA (transfer RNA) Transfers/moves amino acids to ribosomes Makes up rRNA ribosomes along (ribosomal RNA) with proteins Picture The Ribosome Part of cell where translation (protein synthesis) occurs Where proteins are actually made Basic Principles of Transcription and Translation Transcription (uses mRNA) the synthesis of RNA from DNA Produces messenger RNA (mRNA) Occurs in the nucleus of eukaryotes and nucleid region of prokaryotes Translation (uses mRNA, tRNA, rRNA) actual synthesis of a polypeptide mRNA polypeptide protein Occurs on ribosomes In prokaryotes, transcription and translation occur together TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell. In a cell lacking a nucleus, mRNA produced by transcription is immediately translated without additional processing. In eukaryotes, pre mRNA transcripts are modified (changed) before becoming true “mature” mRNA Nuclear envelope “Transcript” is a fancy word for “message” DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION (b) Polypeptide Eukaryotic cell. The nucleus provides a Separate compartment for transcription. The original RNA transcript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA. How is Protein Made? Cells are controlled by a cellular chain of command DNA RNA protein Called the “Central dogma of biology” What are proteins made out of? Amino acids There are 20 different amino acids building blocks of proteins All living things use the same 20 amino acids to make proteins!!! The Genetic Code It’s a table used that TRANSLATES RNA nucleotides (or mRNA “words”) into one of the 20 amino acids 3 letter mRNA “word” = 1 amino acid There are 4 different RNA “letters” that can be used to make “words” A, U, C, and G Codons: Triplets of Bases nonoverlapping base triplet is a codon 3 letter mRNA “words” = codon FOUND ONLY ON mRNA Codons must be read in the correct order For specified polypeptide to be produced Always read in the 5’ 3’ direction THE GENETIC CODE 64 CODONS TOTAL During transcription, a gene determines the sequence of bases along length of mRNA. Gene 2 DNA molecule Gene 1 Gene 3 DNA strand (template) 3 A C C A A A C C G A G T U G G U U U G G C U C A 5 TRANSCRIPTION mRNA 5 3 Codon TRANSLATION Protein Trp Amino acid Phe Gly Ser Transcription DNA RNA mRNA is made by RNA polymerase breaks DNA strands apart (breaks H bonds btw bases) and hooks together RNA nucleotides Transcription Follows same DNA base- pairing rules, except in RNA, uracil substitutes for thymine A = U (T on DNA = A in RNA) C = G Synthesis of an RNA Transcript Initiation DNA strands unwind RNA polymerase binds DNA at a special place called a promoter “TATA box” = start signal on DNA promoter Determines which strand is used as template only 1 side is used at a time! Elongation RNA polymerase makes mRNA by reading DNA’s message Termination mRNA transcript is released at terminator signal RNA polymerase detaches from the DNA Transcription Animation http://www.stolaf.edu/people/giannini/flashani mat/molgenetics/transcription.swf http://wwwclass.unl.edu/biochem/gp2/m_biology/animati on/gene/gene_a2.html Another good animation http://www.biostudio.com/d_%20Transcription .htm Pre-mRNA Modification: pre-mRNA mature mRNA Eukaryotic cells modify (process) mRNA after transcription First mRNA made is called premRNA Pre-mRNA made into mRNA 2 things have to happen… NOT IN PROKARYOTES 1. Alteration of mRNA Ends Each end of a pre-mRNA is capped 5 end gets a GTP cap 3 end gets a poly-A tail LOTS of adenine nucleotides added to the 3 end GTP added to the 5 end SCRIPTION OCESSING DNA Pre-mRNA 5 mRNA Ribosome TION Polypeptide17.9 Figure Protein-coding segment G P P P 5 Cap AAA…AAA Start codon Stop codon Poly-A tail The function of the 5’ cap is: prevents mRNA degradation helps attach to the ribosome Function of the 3’ tail (Poly A cap): prevents mRNA degradation Exports mRNA from nucleus into cytoplasm 2. RNA Splicing RNA splicing Removes Introns = non-coding regions Exons = coding regions that EXIT nucleus Pre-mRNA TRANSCRIPTION RNA PROCESSING introns and joins exons DNA 5 Intron Intron 3 5 Cap Poly-A tail Exon Exon Exon Pre-mRNA Coding segment mRNA Ribosome Mature mRNA Introns cut out and exons spliced together TRANSLATION Polypeptide Figure 17.10 5 Cap Poly-A tail RNA Animations #1 www.geneticsplace.com CHAPTER 14: RNA Animations RNA Biosynthesism RNA Production in Eukaryotes RNA Splicing RNA Splicing Animations #2 http://highered.mcgraw- hill.com/sites/0072437316/student_view0/cha pter15/animations.html http://highered.mcgrawhill.com/sites/0072437316/student_view0/cha pter15/animations.html# http://media.pearsoncmg.com/bc/bc_campbel l_biology_7/media/interactivemedia/activities/l oad.html?19&B