Lecture 14 Flashcards
Translation
- the synthesis of proteins from mRNA
- translation takes information from the language of nucleic acid (ribonucleotides) and translates it into the language of proteins (amino acids)
- takes place in cytosol
Genetic code:
- a set of triplet code codons of mRNA or DNA coding for amino acids
- mRNA is read in a non-overlapping, consecutive groups of three nucleotides known as codons
How many codons are there:
- there are 64 codons that encode for 20 amino acids
- the code is redundant because several different codons encode for the same amino acid
What is the initiation codon and what does it signal?
- it is AUG
- it signals the beginning of a polypeptide (and encodes for methionine)
How many stop codons are there and what do they do?
- There are three: UAA, UAG, UGA
- They terminate translation
How many possible reading frames are there in mRNA
- There are 3 possible nonoverlapping reading frames in mRNA going in one direction (5’ to 3’)
What is an open reading frame and why are they important?
- ORF is a reading frame that has the potential to code for a protein or polypeptide
- The first base of the ORF is a start codon and it ends with a stop codon
- Usually greater than or equal to 50 codons
Types of mutations:
Missense mutation is a mutation in which a single nucleotide changes that results in a codon that codes a different amino acid
Nonsense mutation is a mutation that results in a premature stop codon
Silent mutation is a mutation that takes place in the gene but has no negligible effect on the gene product
Frameshift mutation is a mutation that causes a shift in the reading frame. This can be caused by deletions or insertions of nucleotides
What are transfer RNAs (tRNA)?
- tRNAs act as a bridge between RNA and protein
- tRNAs are composed of a single strand of RNA that forms a double helical structure
- tRNA contains a 3-nucleotide anticodon that recognizes the codons on the RNA. mRNA and tRNA are oriented antiparallel
- each tRNA can be linked to a specific amino acid
What is the aminoacyl-tRNA synthetases
- the aminoacyl-tRNA synthetases couples the correct amino acid to the correct tRNA molecule using ATP
- when coupled to an amino acid, tRNA is said to be charged
- there is a different synthetases for every amino acid
Ribosomes:
- translation is conducted by ribosomes in the cytoplasm
- ribosomes are very large complexes of ribosomal RNA (2/3) and protein (1/3)
- each ribosome has two subunits, one large and one small
What is a ribozyme:
- RNA molecules with catalytic activity
- rRNA allows for the catalytic activity and determines overall structure of ribozyme
What are the distinct sites on a ribosome
- A= aminoacyl-tRNA site
- P= peptidyl-tRNA site
- E= exit site
How does mRNA interact with the ribosome?
- It binds to the small subunit
- The ribosome moves along the mRNA in a 5’ to 3’ direction
Initiater tRNA is the only one that can bind directly to the P site of the small subunit. All other bind to the A site
Initiation of translation (eukaryotes)
- The initiator tRNA binds to the P site of the small subunit along with initiation factors
- The mRNA binds to the small subunit, recognizing 5’ cap
- This moves until it encounters the first AUG
- Initiation factors dissociate and the large ribosomal subunit binds
- A charges tRNA molecule binds to the A-site
- The methionine in the P-site is linked to the amino acid in the A-site through a peptide bond
Translation (continuation in eukaryotes)
- Step 1 : Aminoacyl-tRNA bind to the A site by base pairing with the codon
- Step 2 : The carboxyl end of the polypeptide chain is uncoupled from the tRNA at the P site and joined by a peptide bond to the free amino group of the amino-acid linked to the tRNA at the A site by a peptidyl transferase
- Step 3 : Translocation of the large subunit relative to the small subunit the two RNA are moved to E and P sites of the large subunit
- Step 4 : The small subunit translocate moving three nucleotides along mRNA bringing it back to the original position relative to the large subunit. It reset the ribosome on an empty A site allowing the binding of the next amino-acyl tRNA
Termination: translation in eukaryotes
- When a stop codon is encountered, a protein known as release factors bind
- The peptide is transferred into a water molecule (instead of another amino acid) causing the peptide chain to be released
- The ribosomes dissociate
Translation in bacteria
- Very similar, but some differences
- The mRNA has no 5’ cap so instead the small subunit recognizes the Shine-Dalgarno sequence which lies upstream of the start codon
Compare polycistronic and monocistronic mRNA
- Bacteria has polycistronic mRNA: several genes are encoded on the same mRNA transcript
- Each individual gene has its own Shine-Dalgarno sequence, so that each product can be individually translated
- Eukaryote mRNA is usually monocistronic
What are a polyribosome:
- A single mRNA transcript can be translated by several ribosomes simultaneously
- This increases overall rate in protein synthesis
- Occurs both in prokaryotes and eukaryotes
Differences between prokaryotic and eukaryotic translation have be used to create
- Antibiotics
What antibiotic blocks the binding aminoacyl-tRNA to A site of ribosome
- Tetracyline
What antibiotic blocks the peptidyl transferase reaction on ribosomes (step 2)
- Chloramphenicol
What antibiotic blocks the translocation reaction on ribosomes (step 3)
- Cycloheximide
What are proteases?
- Proteases are enzymes that degrade proteins
What are proteasome?
- Proteasome is a large protein machine that degrades proteins in eukaryotes. Unfolds proteins and degrades them in the interior of a chamber
List a couple of post translational modifications
- Phosphorylation
- Glycosylation
- Proteolytic cleavage
Autocatalysis:
the ability to catalyze reactions directly or indirectly to the production of molecules like itself (RNA can do this)
RNA can…
- Store information and catalyze reactions
- Formation of peptide bonds, splicing, RNA polymerization and DNA logation
Provide evidence that supports the idea of an RNA world
- Ribose can be formed from formaldehyde under conditions that stiumulate primitive earth
- Ribose is easier to form than deoxyribose
- Spontaneous deamination of cytosine to form uracil
