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What is transcription?
- Uses ONE strand of DNA as a template to make complementary strand of RNA.
- RNA Polymerase - Synthesizes 5' to 3'
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How is transcription similar and different to DNA replication?
Similar - Polymerase proteins, produces linear polymer of nucleotides connected by phosphodiester bonds, synthesizes 5'-3'
Different - Uses Uracil instead of Thymine, only short DNA is transcribed
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What are the different types of RNA?
- Messenger RNA (mRNA)
- –Copy of a STRUCTURAL gene. mRNA is TRANSLATED
- Ribosomal RNA (rRNA)
- –Makes up ribosomes (in combination with protein)
- Transfer RNA (tRNA)
- –Carries amino acids to ribosome
- during translation
- Small Nuclear RNA (snRNAs)
- –Splicing
- Micro RNAs (miRNAs)
- –Regulate gene expression
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What genes do the three RNA Polymerases transcribe?
- RNA Polymerase I - most rRNA genes
- II - protein-coding genes, miRNA genes
- III - tRNA genes, 5S rRNA gene
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What are the two DNA strands called when transcribed?
- Template strand
- –Strand that is used to make RNA
- Coding strand
- –Non-template strand.
- Has same sequence as mRNA
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What are few of the parts of a gene?
Promoter - sequence of DNA that specifies starting point and direction (which strand), place where polymerase attaches
Terminator - stop signal (sequence)
Coding region - sequence that actually encodes the protein (translated portion)
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What do general transcription factors do?
Assembles on the promoter
Position RNA polymerase
Pull apart the double helix
Launch RNA polymerase
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What are the 2 main transcription factors of Polymerase II?
TFIID - Binds to TATA box, distorts DNA helix to attract other TFs to site
TFIIH - Pulls apart helix (using ATP), phosphorylation releases it so the polymerase can transcribe
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mRNA is processed through what 2 processes?
5' capping - the addition of a methyl guanine nucleotide to the 5' end; for export, stability, interaction with ribosome
Polyadenylation - addition of adenine nucleotides to the 3' end = poly-A tail; for export, stability, interaction with ribosome
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What are the interrupted 2 coding sequences and the process?
- Introns: Areas of non-coding sequence in genes (also
- end up in mRNAs)
Exons: Coding sequence
Splicing: removal of intron sequences from mRNA
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What is the splicing process?
Introns are cut out and exons are re-joined
Carried out by spliceosomes (enyzmes made of RNA and protein)
- Spliceosomes are made of
- –Proteins and small nuclear ribonucleoproteins (snRNPs)
- –snRNPs = small nuclear RNA + some proteins
- Intron-Exon junctions are marked by specific sequences that are recognized by splicing machinery.
- –Sequences present in BOTH the intron and the exon are needed.
- - snRNPs recognize them by complementary base pairing
Lariat = Loop-like structure produced by splicing (it’s the intron after being removed)
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The Genetic code
Read in groups of 3 : 64 possible codons, 20 amino acids
- More than 1 codon per amino acid
- 3 possible reading frames
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tRNA
- transfer RNA
- ~80 nucleotide RNA
contains an anticodon that base-pairs with mRNA
“charged” with a specific amino acid
interact with mRNA via complementary base pairing
some tRNAs recognize more than 1 codon (wobbles)
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Translation in 3 steps
Initiation - small ribosomal subunit binds to P-site (methionine to AUG)
Elongation - growing polypeptide is in P-site, spent tRNA is in E-site (exit), with A-site empty where a charged tRNA binds via complementary base pairing with next codon)
Termination - specified by stop codons (UAA, UGA, UAG), release factor binds to A site and complex is released
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