Translation Flashcards
What is translation?
Process of synthesis of a protein from an m-RNA template
Where does translation take place?
Ribosomes in the cytoplasm
What are ribosomes made of?
rRNAs and >50 types of ribosomal proteins
What is the eukaryotic ribosome size?
80S
What is the prokaryotic ribosome size?
70S
What type of RNA is most abundant in the cell?
ribosomal-RNA (r-RNA) ~80%
Order these ribosomal RNAs from least abundant to most abundant.
Least abundant = messenger RNA (m-RNA) = 3.5%
Transfer RNA (t-RNA) = 15%
Most abundant = ribosomal RNA (r-RNA) = 80%
Some differences between m-RNA, r-RNA and t-RNA
What is the site of r-RNA processing and assembly into ribosomes in the cell?
Nucleolus
What is the main site of ribosome biogenesis in the cell?
Nucleolus
What is the structural role of rRNA?
Providing scaffold upon which ribosomal proteins can assemble
What is the catalytic role of rRNA?
Covalent peptide bond formation catalyzed by rRNA
What are responsible for the ribosomes overall structure, its ability to position tRNAs on the mRNA, and its catalytic activity in forming covalent peptide bonds?
RNAs
What is the genetic code?
The rules by which the nucleotide sequence of a gene, through the medium of mRNA, is translated into the amino acid sequence of a protein
What are the features of the genetic code?
- Degenerate/redundant
- Unambiguous
- Universal
What are bases organized into groups of three called?
Codons
What does each codon stand for?
an amino acid
What is the structure of tRNA?
Cloverleaf with an anticodon at one end and an amino acid at the other
How many different tRNAs are there roughly?
Cells contain around ~40 different tRNAs that serve as acceptors for the 20 amino acids
What is the name of the group of enzymes which catalyze the attachment of amino acids to specific tRNAs?
Aminoacyl tRNA synthetases
What are the two steps which involve attachment of amino acids to tRNAs?
1) Amino acid is activated by reaction with ATP to form intermediate = aminoacyl AMP synthetase
2) Activated amino acid is then joined to the 3’ terminus of the acceptor tRNA and AMP is released
What occurs between the mRNA codon and the ___ of the tRNA molecule?
Complementary base pairing occurs between the mRNA codon and the anticodon of the tRNA molecule
What is codon-anticodon base pairing wobble?
Some tRNAs are constructed so that they require accurate base-pairing only at the first two positions of the codon and can tolerate a mismatch (a ‘wobble’) at the third position
What does wobble allow?
Allows some tRNAs to bind to more than one codon
- Basis of the redundancy of the genetic code
How are sequences of nucleotides in mRNA read?
Sequences of nucleotides in mRNA are read from 5’ to 3’ end in consecutive sets of three nucleotides (codon)
What is a frameshift mutation?
Mutation that involves the insertion or deletion of nucleotide that shifts the grouping and changes the reading frame of the genetic message
What are examples of some diseases caused by frameshift mutations?
- Crohn’s disease
- Cystic Fibrosis
- Sickle cell anemia
- Tay-Sachs disease
- Certain types of cancer
What are the 5’ terminal portions of both eukaryotic and prokaryotic mRNAs referred to as?
5’ untranslated regions (5’UTR)
What is one difference between initiation of translation in prokaryotes vs eukaryotes?
In prokaryotes =
- Initiation codons in mRNA are preceded by Shine-Dalgarno sequence which aligns mRNA on ribosome for translation by base-pairing with complementary sequence near 3’ end.
In eukaryotes =
- Ribosomes recognize mRNAs by binding to the 7-methylguanosine cap (m7G) at their** 5’ terminus** - then scan downstream until they find an initiation codon (usually AUG)
In both prokaryotic and eukaryotic cells, what amino acid does translation always initiate with?
methionine which is usually encoded by AUG
What is the function of the small ribosome subunit in translation?
Provides framework on which tRNAs are accurately matched to codons of mRNA
What is the function of the large ribosome subunit in translation?
Catalyzes the formation of peptide bonds that link amino acids together to form a polypeptide chain
What are the three binding sites for tRNA on the ribosome?
A site (aminoacyl)
P site (peptidyl)
E site (exit)
What holds a tRNA molecule tightly at the A and P sites?
Its anticodon forms base pairs with complementary codon (allowing for a wobble) on the mRNA
What is the landing site for the next tRNA molecule on the ribosome?
A site (aminoacyl)
Which ribosome site does uncharged tRNA leave the ribosome?
E site (exit)
What enzyme catalyzes the formation of peptide bond during translation?
Peptidyl transferases (rRNA)
During translation, in which direction does the polypeptide ‘grow’?
from the amino terminus (N-term) to the carboxyl terminus
What are the stop codons or termination signals encountered at the ‘A’ site of the ribosome?
UAA, UAG, UGA
What factors recognize the stop codons/termination signals and terminate protein synthesis?
Release factors
What form are mRNA molecules being translated usually found in?
polyribosomes or polysomes = large cytoplasmic assemblies made up of several ribosomes spaced as close as 80 nucleotides apart along a single mRNA molecule
What are the differences between prokaryotes and eukaryotes in the translation process?
Prokaryotes and eukaryotes have different mechanisms and initiation processes for translation.
Inhibitors of prokaryotic protein synthesis are useful as what type of drug?
Antibiotics
What is an example of an antibiotic that acts only on bacteria?
Tetracycline
- Blocks the binding of aminoacyl tRNA (amino acid with trna) to A site of ribosome
What are some examples of antibiotics that act on bacteria only?
- Tetracycline
- Streptomycin
- Chloramphenicol
- Erythromycin
- Rifamycin
In which malignancies are K-Ras mutations common?
pancreatic, lung and colorectal malignancies
In which type of cancer are TERT gene promoter mutations common?
30% primary melanomas
