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notes 17

Uploaded: 7 years ago
Contributor: halmitch96
Category: Biology
Type: Lecture Notes
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Filename:   Molecular Biology Chapter 17 Notes.docx (17.21 kB)
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Molecular Biology Chapter 17 Central Dogma DNARNAProteins Transcription of DNA to mRNA RNA processing if you’re a eukaryote Translation of mRNA to polypeptides Relationship[ between nucleus and the rest of the cell Acetbularia acetabulum Nucleus is located down in the “foot”, can be removed without killing the cell How will the cell be affected? If the nucleus is where proteins are made, it should die quickly Without a nucleus, cells can live for a few months Proteins are still made for about 2 weeks Transcription Taking the information in DNA out to the rest of the cell Controls which proteins get made Similar rules to DNA synthesis Bubble need to open up to access the information NTPs (the basic building blocks) use complimentary base pairing Only one strand is “read” Strand read by RNA polymerase= template Strand not read by RNA polymerase= non-template or coding strand DNA and RNA 5’ - CGGCTCGAACTGCTA - 3’ 3’ - GCCGAGCTTGACGAT - 5’ (template strand) AFTER TRANSCRIPTION 3’ – GCCGAGCTTGACGAT – 5’ 5’ – CGGCUCGAACUGCUA – 3’ RNA polymerase Synthesizes in the 5’ – 3’ direction Unlike DNA polymerase, does not need a primer to start 2 subunits: RNA polymerase (core enzyme) Sigma RNA polymerase contains the active site RNA polymerase won’t work without sigma Sigma recognizes specific region in the DNA called promoter In prokaryotes, promoters are upstream of the gene 10 bases upstream (-10) = TATAAT Prokaryotes also have “TTGACA” at -35 Sequence can vary outside those boxes Prokaryotic transcription- INITIATION Sigma binds to the DNA at the promoter More than 1 type of sigma E.coli has 7 Streptomyces coelicolor has 60 Why have different versions of sigma? Prokaryotic transcription- INITIATION and ELONGATION Structure of RNA polymerase has multiple channels DNA passage NTP’s entrance mRNA’s exit Prokaryotic transcription- INITIATION Double helix is opened up Template is fed to the active site NTPs enter via a channel Prokaryotic transcription- INITIATION and ELONGATION DNA is read 3’ – 5’. mRNA is synthesized 5’- 3’ Growing strand extends out of the RNA polymerase About 5 bases/ second Prokaryotic transcription- TERMINATION Transcription ends when RNA polymerase hits a signal in the DNA Termination sequence Termination sequence forms a hairpin loop RNA now ready as template for proteins Eukaryotic transcription Eukaryotic promoters are more varied Generally, “TATAAA” between -25 and -35 TATA box Also, not just 1 protein to start transcription Lots of basal transcription factors More than 1 RNA polymerase RNA pol II makes mRNA Termination occurs at a consensus sequence Can be s f bases past the gene Eukaryotic transcription- TERMINATION In eukaryotes, mRNA is “immature”, a primary transcript Eukaryotic RNAs Eukaryotes have lots of non-coding DNA What would be the sensible/efficient way to organize coding and non-coding DNA? RNA processing Regions in genes that become part of the final mRNA = exons (expressed information) Regions in genes that don’t become part of the final mRNA = introns (intervening sequence) Discovering introns mRNA should bind with its template DNA due to complimentary base pairing Test: denature double stranded DNA, allow to anneal with mRNA In eukaryotes, mRNA matches the template, but not continuously Genes occur in pieces!!! Information occurs in fragments ISRB IVZVT SPVZRING BRZCXZXVEAK YVVZET? ISRB IVZVT SPVZRING BRZCXZXVEAK YVVZET? Post editing IS IT SPRING BREAK YET? RNA processing- splicing Have to get the introns out! Facilitated by small nuclear ribonucleoproteins (snRNPs) Complex of proteins and small RNA molecules RNA acting as an enzyme snRNPs bind to the intron 5’ end is GU, 4; end is an A Spliceosome is formed Intron is looped on itself, U is connected to the A Exon fragments are ligated together Post transcription modification 5’ cap A molecule of 7- methyl- guanylate with 3 phosphate groups is attached to the 5’ end of the mRNA Poly- A tail The 3’ end of the mRNA is chopped off Why? Another enzyme adds on a long run of A nucleotides About 100- 250 total Why modify? mRNA is more stable End up producing more proteins Recognition signals Translation mRNA to proteins occurs at the ribosomes controls rate of protein synthesis in bacteria, translation starts before the mRNA is fully synthesized Polyribosome = multiple ribosomes on a single mRNA Polyribosomes What traits do prokaryotes have (or don’t have) that allow them to do this? Translation- Eukaryotes Transcription and translation are separated by space and time Information in the genetic code is organized into triplet codons Translation How does mRNA interact with amino acids? Interact directly Difficult to accomplish Too much chemical variation in amino acids Intermediary molecule “Adapter” mole holes amino acids while interacting with the individual codons in the mRNA (Crick) Transfer RNAs (tRNA) Discovered by accident “Extra” RNA was found to be required for protein synthesis in vitro When radioactivity labeled amino acids were in the mixture, they would end up attached to this “extra” RNA, then transferred to new polypeptides tRNAs match amino acids with codons key properties energy is required to attach the amino acid (charge the tRNA) Enzymes= aminoacyl tRNA synthetases Each amino acid has its own aminoacyl tRNA synthetase tRNA structure small molecules 75-85 bases long Several stems and loops 3’ end: site of amino acid binding Anticodon on the loop furthest from the 3’ end Anticodon matches the codon on the mRNA Anticodon is antiparallel to the mRNA Tertiary structure Structure is highly conserved between the different tRNA sequences Distance between anticodon and the amino acid Translation 61 different codons But most cells only have about 30-40 different tRNAs Wobble hypothesis (Crick) Most amino acids have more than 1 codon Does it matter that CCG is read as if it were CCA? No it does not!!! 3rd position bases may not follow strict base pairing rules Non-standard pairing are ok as long as it doesn’t change the amino acid that is specified by the codon Ribosome (bacterial) Ribosomes are combinations of proteins and RNAs 2 subunits Small (30S) subunit 21 different proteins 1 16S rRNA (ribosomal RNA) molecules Large (50S) subunit 34 different proteins 2 rRNA moles, 5S and 23S Active site is made of RNA- ribozyme Small subunit hold the mRNA Large subunit synthesizes polypeptides Suring translation, 3 tRNA molecules are held in the ribosome A site- tRNA with amino acid P site- tRNA holding polypeptide chain E site- tRNA with nothing attached, ready to exit Proteins have direction too Synthesized from N-terminus to C-terminus 20 amino acids / second in bacteria 2/second in eukaryotes 3 phases Initiation Elongation Termination Translation- Initiation Upstream of the start codon on the mRNA is the ribosome binding site (Shine-Dalgarno sequence) 5’ – AGGAGGU – 3’ Binds to rRNA in the small subunit with complementary sequences tRNA with methionine binds to the start codon Proteins called initiation factors help out Large subunits attaches tRNA with methionine is in the P-site Translation begins Translation- Elongation Charged tRNA enters the ribosome at the A site Which amino acid? Peptide bond forms Amino acid in the P site is transferred to the amino acid in the A site Translocation Amino acid in the P site shifts to the E site Amino acid in the A site shifts to the P site New charged tRNA enters the ribosome at the A site Which amino acid? Peptide bond forms Amino acid in the P site is transferred to the amino acid in the A site Translocation Amino acid in the P site shifts to the E site Amino acid in the A site shifts to the P site Translation- Termination Proceeds down the length of the mRNA until the stop codon is reached No tRNA that matches Instead, a release factor enters the A site Release factor is a protein that mimics the structure of a tRNA Does not carry an amino acid Growing polypeptide is cut from the tRNA in the P site, but there is no attachment point in the A site Polypeptide is released 2 remaining tRNAs are released Ribosome separates into subunits Can reform around the new mRNA Translation Are the proteins finished at this point? Proper folding (molecular chaperones) Chemical modifications Where is this happening?

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