From DNA to Protein
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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'
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
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)
- Micro RNAs (miRNAs)
- –Regulate gene expression
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
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
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)
What do general transcription factors do?
Assembles on the promoter
Position RNA polymerase
Pull apart the double helix
Launch RNA polymerase
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
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
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
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)
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
- 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)
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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