7.3 Translation (HL) Flashcards
What does the ribosome structure consist of?
- Large subunit
- Small subunit
- 3 tRNA binding sites (Large subunit)
- mRNA + mRNA binding site (Small subunit)
- Growing polypeptide chain
tRNA molecule: What is the function of the acceptor stem?
Carries amino acid to combine with the growing polypeptide
tRNA molecule: Function of hydrogen bonds
Holds the bases and molecule together into an L shaped tertiary structure
tRNA molecule: Function of anticodon
Associates with mRNA codon via complementary base pairing
tRNA molecule: Function of T arm + which side is it on?
- Associates with ribosome via E, P, A binding sites
- On the right side
tRNA molecule: Function of D arm + side it is on
- Left side
- Associates with the tRNA activating enzyme
Function of tRNA activating enzyme
It adds the amino acid to the acceptor stem
Why is there more than one type of tRNA activating enzyme
- Each tRNA molecule has different chemical properties
- Each amino acid is recognized by a specific enzyme although one enzyme may recognize multiple due to degeneracy
How is tRNA activated?
- Enzyme binds ATP to amino acid to form amino acid-AMP complex linked by high energy bond
- Amino acid is coupled to tRNA and AMP is released
- tRNA is now charged and ready for use
Use of the energy transferred to charged tRNA molecule by ATP
Energy is transferred and stored energy will provide effort required for peptide bond formation during translation
3 stages of translation
Initiation
Elongation
Termination
What happens during initiation?
Mature mRNA binds to small ribosomal subunit and large subunit joins to complete assembly
What is the first tRNA called and what does mRNA have to combine with it?
It’s called initiator tRNA and mRNA has a start codon (AUG) to link with it
What happens during elongation?
A repeated cycle to create a growing polypeptide chain
What is the A site and what happens?
- Aminoacyl tRNA binding site
- Incoming tRNA with attached amino acid binds to chain
What is the P site and what happens?
- Peptidyl-tRNA binding site
- Where tRNA from A site moves after amino acid forms peptide bond with chain.
- Where the tRNA holding the chain is found
What is the E site and what happens?
- Exit site
- Where tRNA moves after transferring amino acid
- Where it readies to detach from ribosome
What happens in termination?
Once termination codon is reached, release factor binds in A site and causes disassembly of components of translation complex. . i.e. ribosomal subunits, tRNA can all be reused
Why does translation occur immediately after transcription in prokaryotes?
- Prokaryotes don’t have nuclear membrane so ribosomes directly bind to mRNA rather than wait for relocation to cytoplasm
- In eukaryotes, mRNA is spliced and processed to create mature mRNA
Polysome
Structure that consists of multiple ribosomes attached to single mRNA
Purpose of polysome
Translates mRNA simultaneously to quickly create multiple copies of required protein
How do polysomes in prokaryotes differ from eukaryotes?
- Prokaryotes: Chromosome may have numerous polysomes attached directly
- Eukaryotes: Polysomes occur separately in cytoplasm or on ER as ribosomes attach to mRNA as it is being translated
What are bound ribosomes attached to?
Endoplasmic reticulum
Where do proteins synthesized by bound ribosomes go?
- Outside the cell
- ER
- Golgi apparatus
- Lysosomes
- Plasma membranes
Bound ribosomes: What does beginning of the polypeptide contain?
Signal sequence that is recognized by SRP (Signal recognition particle)
Bound ribosomes: What happens after SRP binds to recognition protein on ER membrane?
After binding on ER, polypeptide enters rER as it grows
Bound ribosomes: What happens when translation ends?
- Complex disassembles
- Signal peptide is removed and whole protein is taken into rER
Bound ribosomes: What happens after protein is taken into rER?
Post-translational modifications and sorting of protein takes place in lumen of ER
What type of proteins do free ribosomes synthesize for?
- Proteins that are destined for mitochondria, chloroplasts, cytoplasm or nucleus of the cell
Difference between bound vs. free proteins
Bound proteins are meant for secretion or use in lysosomes while free proteins are mostly for intracellular use
Primary structure of protein
Protein is maintained by peptide bonds between subunits
Secondary structure of protein
- Folds back onto itself in beta-pleated sheets or alpha helices
Bonds in secondary structure
Stabilized by hydrogen bonds between -NH groups and C=O groups on different peptides in the same chain
Tertiary structure of protein
Further folding such that hydrophilic amino acids are outside and hydrophobic in the center
Bonding in tertiary structure
- Stabilized by interaction between R groups
- Polar R groups have ionic interactions between acidic/basic groups or H bonding
- Covalent bonds form between R groups with sulfur
Quaternary structure in proteins
More than one polypeptide chain where each chain is a subunit. Joined by similar bonds to tertiary
Conjugated protein
Quaternary structure which may have a prosthetic group that attaches to polypeptides
Properties of R groups
- Polar/hydrophilic
- Non-polar/ hydrophobic
- Positively/negatively charged
- Some may contain sulfur
