Chapter 8 Flashcards
(31 cards)
transcription
- Reading a DNA template to make an RNA copy
- RNA transcripts complementary to the DNA template strand
- carried outa by a complex enzyme, RNA polymerase
translation
Decoding mRNA to assemble protein
Gene expression
After translation, each polypeptide must be properly folded and placed at the correct cellular or extracellular location
RNA polymerase holoenzyme is composed of
Core polymerase
-Required for the elongation phase
Sigma factor
- Required for the initiation phase
- helps the core enzyme detect promoters
“housekeeping” sigma factor
- In E. coli, it is sigma-70.
- Recognizes consensus sequences at the –10 and –35 positions, relative to the start of the RNA transcript (+1)
Transcription occurs in three phases
- Initiation
- RNA polymerase holoenzyme binds to the promoter
- This is followed by melting of the helix and synthesis of the first nucleotide of the RNA - Elongation
- the RNA chain is extended - Termination
- RNA polymerase detaches from the DNA and releases the transcript
two transcription termination signals
- Rho-dependent
Relies on a protein called Rho and a strong pause site at the 3′ end of the gene after the stop codon - Rho-independent
Requires a GC-rich region of RNA that forms a hairpin structure, as well as 4–8 consecutive U residues
Different Classes of RNA
- Messenger RNA (mRNA): encodes proteins
- Ribosomal RNA (rRNA): forms ribosomes
- Transfer RNA (tRNA): shuttles amino acids
- Small RNA (sRNA): regulates transcription or translation
- tmRNA: frees ribosomes stuck on damaged mRNA
- Catalytic RNA: carries out enzymatic reactions, e.g., rRNA
nucleotide triplets
-64 possible codons
-61 specify amino acids
Include the start codon(s)
-3 are stop codons
-Multiple codons can encode the same amino acid
-operates universally across species
tRNA Molecules
- convert the language of RNA into that of proteins
- Anticodon: hydrogen bonds with the mRNA codon specifying an amino acid
- 3′ (acceptor) end: binds the amino acid
Attaching Amino Acids to tRNA
- Each tRNA must be charged with the proper amino acid before it encounters the ribosome
- carried out by a set of enzymes called aminoacyl-tRNA synthetases
Ribosome
- composed of two subunits, each of which includes rRNA and proteins
- subunits are 30S and 50S and combine to form the 70S ribosome
70S ribosome harbors three binding sites for tRNA
- A (acceptor) site: binds incoming aminoacyl-tRNA
- P (peptidyl-tRNA) site: harbors the tRNA with the growing polypeptide chain
- E (exit) site: binds a tRNA recently stripped of its polypeptide
peptidyltransferase
- actually a ribozyme
- makes the peptide bonds that stitch amino acids together
Shine-Dalgarno sequence
ribosome-binding site
-is complementary to a sequence at the 3′ end of 16S rRNA of the 30S subunit
Three Stages of Protein Synthesis
- Initiation: brings the two ribosomal subunits together, placing the first amino acid in position
- Elongation: sequentially adds amino acids as directed by the mRNA transcript
- Termination: releases the completed polypeptide and the ribosomal subunits
Antibiotics That Affect Translation
- Streptomycin
- Tetracycline
coupled transcription and translation
- Different ribosomes can be translating each gene within a polycistronic mRNA.
- Before RNA polymerase has even finished making an mRNA molecule, ribosomes will bind to the 5′ end of the mRNA and begin translating protein
Protein structure may be modified after translation
- Phosphate and other groups can be attached
- Sugars can be attached to make glycoproteins
- Lipids can be attached to make lipoproteins
- Proteolytic cleavages may occur
Protein Folding
-Folding of many proteins requires assistance from other proteins called chaperones
Proteasomes
-protein-degrading machines found in eukaryotes and archaea
Protein Degradation
- proteins have a half-life
- Bacteria contain ATP-dependent proteases
Proteins destined for the bacterial cell membrane
- require special export systems.
- tagged with hydrophobic N-terminal signal sequences of 15–30 amino acids.
- sequences are bound by the signal recognition particle (SRP)
- Proteins then undergo cotranslational export
periplasmic proteins
- in gram negative cells
- maltose-binding protein
- delivered to the periplasm by a common pathway called the SecA-dependent general secretion pathway
