Transcription Ms. Day AP Biology

The Flow of Genetic Information DNA information in a specific sequence (order) of nucleotides along 2 DNA strands Leads to specific traits by controlling the synthesis of proteins Gene expression includes two stages Transcription: DNA  RNA “transcribe” = to copy into another form Translation: RNA  polypeptide “translate” = to change into another language

What are the characteristics of RNA? Ribose Nucleic Acid Single stranded Pentose sugar = ribose 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)

Brings message from DNA to ribosome to make protein Type of RNA Function Job Picture mRNA (messenger RNA) Brings message from DNA to ribosome to make protein tRNA (transfer RNA) Transfers/moves amino acids to ribosomes rRNA (ribosomal RNA) Makes up ribosomes along with proteins snRNA (small nuclear RNA) Helps makes splicesomes  modifies mRNA **ONLY in eukaryotes

Basic Principles of Transcription and Translation Makes RNA from DNA Produces messenger RNA (mRNA) Translation (happens later) actual synthesis of a polypeptide mRNA  polypeptide  protein Occurs in ribosomes

In prokaryotes, transcription and translation occur together Prokaryotic cell. In a cell lacking a nucleus, mRNA produced by transcription is immediately translated without additional processing. TRANSLATION TRANSCRIPTION DNA mRNA Ribosome Polypeptide

“Transcript” is a fancy word for “message” In eukaryotes, pre mRNA transcripts are modified (changed) before becoming true “mature” mRNA 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. (b) TRANSCRIPTION RNA PROCESSING TRANSLATION mRNA DNA Pre-mRNA Polypeptide Ribosome Nuclear envelope “Transcript” is a fancy word for “message”

Transcription: DNA  mRNA RNA polymerase = enzyme used breaks H bonds btw bases and hooks together RNA nucleotides Follows RNA base pairing rules A = U (T on DNA = A in RNA) C = G

Transcription makes a coded mRNA message… mRNA is created by RNA polymerase mRNA contains codons 3 letter mRNA “word” = codon 1 codon = 1 amino acid in protein There are 4 different RNA “letters” that can be used in codons A, U, C, and G

Codons: Triplets of Bases mRNA message is coded as a sequence of nonoverlapping codons FOUND ONLY ON mRNA Codons must be read in the correct order Always read in 5’  3’ direction **Need to use the Genetic Code table to then convert the codon to amino acids

THE GENETIC CODE

During transcription, a gene determines the sequence of bases along length of mRNA. Figure 17.4 DNA molecule Gene 1 Gene 2 Gene 3 DNA strand (template) TRANSCRIPTION mRNA Protein TRANSLATION Amino acid A C G T U Trp Phe Gly Ser Codon 3 5

Making an mRNA Transcript Initiation DNA strands unwind RNA polymerase initiates mRNA synthesis at start point on templates called promoters RNA polymerase binds to promoter Elongation RNA polymerase makes mRNA in 5  3 direction In wake of transcription, DNA strands re-form a double helix. Termination mRNA transcript is released at terminator signal RNA polymerase detaches from the DNA

Promoters and Transcription Promoters (on DNA) RNA polymerase binds here to make mRNA (BOTH prokaryotes & eukaryotes) “TATA box” = start signal on DNA promoter Determines which strand is used as template  only 1 side is used at a time!

Transcription Animation http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf http://highered.mheducation.com/sites/0072507470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html

Elongation of the RNA Strand RNA polymerase moves along DNA continues to untwist helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides

Termination of Transcription Different in prokaryotes and eukaryotes Prokaryotes RNA polymerase falls off DNA when it hits “termination signal”  transcription ends & transcript is released

Termination of Transcription (con’t) Eukaryotes RNA polymerase transcribes until it hits a polyadenylation signal (TTATTT) Makes “AAUAAA” on mRNA Then it falls off mRNA!!!

Pre-mRNA Modification: pre-mRNA  mature mRNA ONLY in eukaryotic cells Occurs AFTER 1st mRNA made  called pre-mRNA Pre-mRNA  made into mRNA 2 things have to happen… NOT IN PROKARYOTES

1. Alteration of mRNA Ends Each end of pre-mRNA is modified 5 end gets a modified nucleotide cap The 3 end gets a poly-A tail GTP cap added to the 5 end LOTS of adenine nucleotides (~200) added to the 3 end Protein-coding segment Polyadenylation signal Poly-A tail 3 UTR Stop codon Start codon 5 Cap 5 UTR AAUAAA AAA…AAA TRANSCRIPTION RNA PROCESSING DNA Pre-mRNA mRNA TRANSLATION Ribosome Polypeptide G P 5 3 Figure 17.9

Function of 3’ tail (Poly A cap): The function of 5’ cap (GTP) is: helps attach to the ribosome prevent mRNA degredation Function of 3’ tail (Poly A cap): helps export of mRNA from nucleus

2. Split Genes and RNA Splicing RNA splicing and RNA Modification Removes introns and joins exons Introns = non-coding regions (stay “in”side nucleus) Exons = coding regions that EXIT nucleus TRANSCRIPTION RNA PROCESSING DNA Pre-mRNA mRNA TRANSLATION Ribosome Polypeptide 5 Cap Exon Intron 5 3 Poly-A tail Introns cut out and exons spliced together Coding segment 1 146 3 UTR 5 UTR Intron Pre-mRNA Exon Exon Mature mRNA Figure 17.10

Called small nuclear RNA + proteins (ribonucleoproteins) RNA splicing is carried out by spliceosomes RNA transcript (pre-mRNA) Exon 1 Intron Exon 2 Other proteins Protein snRNA snRNPs Spliceosome components Cut-out intron Mature mRNA 5 1 2 3 Called small nuclear RNA + proteins (ribonucleoproteins)

RNA Splicing Animation http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html# http://www.sumanasinc.com/webcontent/animations/content/mRNAsplicing.html

The Functional and Evolutionary Importance of Introns Presence of introns allows for alternative RNA splicing REASON HUMANS HAVE SO FEW GENES?? MORE TERRAIN FOR CROSSING OVER Exon Shuffling Proteins often have shapes consisting of discrete functional regions called domains In many cases… Different exons code for the different domains in a protein

Gene Transcription RNA processing Translation Polypeptide DNA Exon 1 Intron Exon 2 Exon 3 Transcription RNA processing Translation Domain 3 Domain 1 Domain 2 Polypeptide https://highered.mheducation.com/sites/9834092339/student_view0/chapter15/animation_-_exon_shuffling.html

RNA Splicing Animations #2 http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/interactivemedia/activities/load.html?19&B