Lesson 21: Gene Expression: Translation Flashcards
What is translation?
Synthesis of a polypeptide using the genetic information encoded in an mRNA molecule. Change of of language from nucleotides to amino acids
What is the genetic code for DNA and RNA?
4 bases in DNA and RNA. 20 different amino acids. Multiple nucleotides must combine together to specify the different amino acids
What is the triplet code in the genetic code?
Set of three-nucleotide-long words specify the amino acids for polypeptide chains
What are codons?
mRNA nucleotide triplets
How was the genetic code determined?
The genetic code was determined by synthetic mRNAs and combining them with ribosomes, amino acids in a test tube
What would an mRNA molecule with only U make?
(UUU-UUU-UUU-UUU) produces a polypeptide with only phenylalanine (Phe-Phe-Phe-Phe)
What would an mRNA molecule with only A make?
Produces a polypeptide with only lysine (Lys-Lys-Lys-Lys)
What are two ways to describe the genetic code?
Redundant (more than one codon is used for most amino acids) and not ambiguous (one codon codes for only one amino acid)
What does AUG code for?
Methionine it is also the start codon for translation
What do UAA, UAG, and UGA have in common?
All do not code for an amino acid. Stop codons for translation
What does each mRNA have?
Three possible frames that can be translated into amino acids
What are reading frames?
Only one frame is used and the reading frame generally begins with the first AUG in the mRNA sequence
What are some facts on genetic code?
The genetic code applies to all organisms
The same code is used in bacteria, plants, and people
Implies that all life on earth had a common ancestor
A useful feature for molecular biologists
What is the goal of translation?
Translation brings together mRNA, rRNA, and tRNA in order to make a protein
What is mRNA?
Carries the genetic information from the DNA
What is rRNA?
Integral parts of ribosomes where translation occurs
What does tRNA do?
Converts the codons in the mRNA to the proper amino acid sequence
What is tRNA?
Short, single strand of RNA that adopts a specific shape, containing three loop domains
One loop contains a triplet anticodon that is complementary to the codon mRNA
What does the anticodon do?
The anticodon dictates a specific amino acid
What is aminoacyl-tRNA synthetases in tRNA?
Links the correct amino acid to the correct tRNA
There are 20 different aminoacyl-tRNA synthetases, one for each amino acid
What do ribosomes do in rRNA and protein?
Each ribosomes is made of two subunits, a large subunit and a small subunit
Each subunit contains one or more rRNA molecules
What are ribozymes?
The rRNA molecules are ribozymes and carry out the main functions of the ribosomes. The proteins support the function of the ribozymes
What is the most abundant type of RNA in the cell?
RRNA
What are the three locations that ribosomes contain for tRNAs to bind to mRNA?
A site, P site, and E site
What is the A site in ribosomes?
Holds the tRNA bringing the next amino acid to be added to the polypeptide
What is the P site in ribosomes?
Holds the tRNA attached to the growing polypeptide
What is the E site in ribosomes?
The exit site for the prior tRNA that has already incorporated its amino acid
What are the three phases of translation?
Initiation, elongation, and termination
What needs to be gathered in initiation phase for translation?
We need to gather small ribosomal subunits, mRNA, and initiator tRNA, carrying met
What happens in initiation of translation in prokaryotes?
In bacteria, small ribosomal subunits can bind the mRNA and tRNA in any order
What happens during initiation of translation in eukaryotes?
In eukaryotes, small ribosomal subunit binds the 5’ cap of mRNA and scans until it finds the start codon (AUG). Then the initiator tRNA hydrogen bonds the anticodon (UAC) to the start codon (AUG)
What is the translation initiation complex?
Once the mRNA and tRNA are in place on the small ribosomal subunit, the large ribosomal subunit binds to create the translation initiation complex
What does the translation initiation complex need?
Requires an input of energy, which comes from the hydrolysis of GTP. Other proteins called initiation factors help to assemble the complex. The initiator tRNA sits in the P site of the ribosome
How does elongation occur in translation?
Occurs with the help of elongation factors which are other proteins necessary for elongation
What happens during elongation phase of translation?
The codon in the A site of the ribosome base pairs with the appropriate tRNA. A covalent bond (peptide bond) forms between the two adjacent amino acids and the growing polypeptide is attached to the tRNA in the A site. Ribozymes catalyze the peptide bond. Energy input from GTP moves the mRNA through the ribosome
Where does the peptide bond form during elongation?
A peptide bond forms between the carbonyl group on amino acid one (in the P site) and the amino group on the next amino acid (in the A site)
Where is the Met amino acid during elongation of transcription?
At the N-terminus of the polypeptide
What is the last amino acid added to the polypeptide chain during elongation?
C-terminus
What are facts about polypeptides during elongation?
Polypeptides are always listed in order from N-terminus to the C-terminus.
The codons for the amino acids at the N-terminus of a polypeptide are found at the 5’ end of an mRNA molecule (mRNA built 5’ to 3’)
What is the release factor and what does it do in the termination phase of translation?
When a stop codon enters the A site of the ribosome, a release factor binds to the codon. The release factor is a protein, shaped like a tRNA
What happens during the termination stage of translation?
The release factor promotes the hydrolysis of the covalent bond between the amino acid and the tRNA sitting in the P site of the ribosome, freeing the polypeptide. The hydrolysis of two additional GTP molecules breaks apart the ribosomal complex
Where can many ribosomes be found on one mRNA?
Many proteins from on mRNA or the whole complex is called poly some or polyribosomes
What are post-translational modifications?
The polypeptide begins to fold into its secondary and tertiary shapes as it is being synthesized.
After synthesis, a polypeptide may undergo modification
What are signal peptides for targeting polypeptides to ER?
Sequence of about 20 amino acids at or near the N-terminus of a polypeptide
What is a signal recognition particle (SRP) in targeting polypeptides to ER?
Binds to the signal peptide
What are mutations in DNA?
Changes to the nucleotide sequence of DNA. Mutations may affect the structure and function of the target protein.
What is point mutation?
A change in a single nucleotide pair in a gene. Point mutations can result in three different classes of mutations, silent mutation, missense mutation, and nonsense mutations
What are silent mutations?
Occur when a nucleotide-pair substitution results in a codon that codes for the same amino acid in the polypeptide. There is no functional change in the polypeptide
What are missense mutations?
Occurs when a nucleotide-pair substitution results in a codon that codes for a different amino acid in the polypeptide. This kind of mutation could impair, not effect, or improve the protein function
What are nonsense mutations?
Occurs when a nucleotide-pair substitution creates a stop codon and terminates a polypeptide prematurely. Almost always produce nonfunctional proteins
What are some other kinds of mutations?
Insertions and deletions which are additions or loses of nucleotide pairs in a gene
What can happen when insertions and deletions occur?
Can alter the reading frame, which will alter all of the amino acids that follow the insertion or deletion. This is referred to as a frameshift mutation
What are mutagens?
Physical and chemical agents that interact with DNA in ways that cause mutations (ex: X rays, UV light, chemicals that mimic nucleotides, and chemicals that insert into DNA and distort the DNA structure
