Unit 5 - Gene Expression Flashcards
Central Dogma of Molecular Biology
DNA is transcribed into mRNA, which is then translated to amino acids which synthesis to form a protein (polypeptide chain)
mRNA
codes for proteins
rRNA
forms core of ribosome and catalyzes protein synthesis
miRNA
regulates gene expression
tRNA
serves as adapter between mRNA and amino acids during protein synthesis
template strand
strand of DNA which is transcribed
coding strand
matches RNA, except U of RNA is T of DNA
RNApol (prokaryotic)
- large, globular enzyme with several channels running through it
- active site at intersection of channels
- holoenzyme with core enzyme (synthesizes RNA) and regulatory subunit (sigma factor)
Sigma Factor
- recognizes promoter sequence
- most bacteria have several types of sigma proteins
- each sigma binds to promoters with slightly different sequences
Initiation of Transcription in Prokaryotes
- initiated at promoters, regions on non-template strand:
1. -10 box 2. -35 box - transcription begins when sigma identifies and binds to promoters
Termination of Transcription in Prokaryotes
- RNApol reaches transcription termination signal in DNA template
- codes for RNA that folds back on itself, forms hairpin structure
- hairpin disrupts transcription complex
RNApol 1
transcribes most rRNA genes
RNApol 2
transcribes protein-coding genes, miRNA genes, genes of small RNAs
RNApol 3
transcribes tRNAs
Start of Transcription in Eukaryotes
- TATA binding protein (TBP) recognizes promoter sequence
- TBP+TFIID distort helix, allows other factors to pile and form the ‘transcription initiation complex’
mRNA processing
- 5’ cap
- 3’ poly (A) tail
- 5’ cap and 3’ tail are marked by proteins
- Exon junction complexes (EJC) bind to properly spliced mRNAs and transport them out of the nuclus
spliceosomes
- 5 small nuclear ribonucleic particles (snRNPs)
- catalytic activity provided by RNA component –> ribozymes
How are introns spliced out?
- Branch point A attacks 5’ splice cite and cuts the sugar-phosphate backbone
- cut end forms a covalent bond with the ribose sugar group
- lariat structure is degraded
Advantage of RNA splicing
-creates different proteins from same gene/same primary mRNA transcript depending on cell type
Disadvantages of RNA splicing
- more steps = more work
2. more opportunity for error
Genetic code exception
-assigning some of STOP codons to amino acids instead, usually in mitochondria
aminoacyl tRNA
-formed from tRNA, amino acid, and aminoacyl tRNA synthetase
tRNA
- carries specific amino acid
- CCA sequence at 3’ end which binds amino acids
Wobble Hypothesis
- proposed by Francis Crick
- anticodon of tRNAs can still bind successfully to codon whose 3rd position requires non-standard base pairing
A site
-acceptor site
P site
-peptidyl site, where peptide bond forms that adds amino acid to polypeptide chain
E site
-exit site
Translation (4 steps)
- aminoacyl tRNA diffuses into A site, anticodon binds to codon on MRA
- Peptide bond forms between amino acid on aminoacyl tRNA (A site) and growing polypeptide on tRNA (P site)
3/4. Ribosome moves along 3 bases and all 3 tRNAs move down one position - New aminoacyl tRNA moves in A site
What initiates translation?
-tRNA loaded with Met
What terminates translation?
- presence of one STOP codon, not recognizes by tRNA
- STOP codon binds to release factor which alters catalytic activity
Polyribosomes/Polysomes
- many ribosomes attach to one strand of mRNA
- makes protein quicker
Molecular Chaperones
- help proteins fold
- bind to ribosomes near ‘tunnel’ where the growing peptide exits
Post-Translational Modifications (PTMs)
- chemical modifications of protein structure
- generally involve addition of functional groups/small molecules to protein
- has major effect of charge, shape, & function
