translation Flashcards
what direction does translation occur in
what direction does the ribosome move along the mRNA strand
occurs in a 5’ to 3’ direction, start codon is nearer the 5’ end than the stop
codon
- moves along the mRNA to 3’ end
describe the 3 structures of a ribosome
A site= aminoacyl site
P site = peptidyl site
E site = exit site
describe the structure of a tRNA
- OH attaches an amino acid
- anticodon loops binds to the codon of an mRNA
- base sequences of tRNA anticodons are complementary to the sequences of mRNA and relate to the same amino acid
Step 1 initiation ( 5 points)
where does it also occur
the small ribosomal subunit binds to the 5’ cap of the mRNA transcript with the help of an initiation factor (enzyme)
the large subunit joins the complex and the initiation factor dissociates
the first amino acid which initiates translation is always the same, an ordinary methionine amino acid
the codon for this initiation amino acid is AUG (start signal), it only causes initiation when it occurs near the beginning of the mRNA
the initiation tRNA (carrying methionine) binds to the P site by
complementary pairing its anticodon with the mRNA codon
cytoplasm outside nucleus
Step 2 Elongation
6
a second amino acid on the tRNA is controlled by the second codon on the mRNA strand
binding of amino acids requires an enzyme
methionine disassociates from its tRNA, and then forms a peptide bond with the second amino acid (which is still attached to its tRNA)
the ribosome moves down the mRNA strand (called translocation) one codon length to the right
the tRNA which has lost its amino acid moves into the E site and then leaves the ribosome, the tRNA with two amino acids moves over to the P site and the next tRNA complimenting the new codon moves into the A site
the process continues until the ribosome reaches the last codon in the
message
Step 3: Termination
Step 3: Termination
three codons cause the termination of protein synthesis UAG, UAA andUGA
known as the stop signals
the stop signals do not code for an amino acid, also called nonsense
codons
every gene has at least one and frequently two of these stop codons at the
end after the last codon for an amino acid
termination is triggered by a release factor which causes the ribosomes to
fall off the mRNA and release the polypeptide chain from the last tRNA
read over and understand
cells which must rapidly synthesize many copies of different proteins will stockpile mRNA transcripts in their cytoplasm
a polysome is a cluster of many ribosomes translating the same mRNA transcript at the same time
free-floating ribosomes synthesize proteins for use primarily within the cell
ribosomes associated with ER synthesize proteins primarily for
secretion or for lysosomes
what is another name for UAA UGA and UAG stop signals
nonsense codons are the stop signals do not code for an amino acid during third step of translation
what is translocation!
the ribosome moving down the mRNA strand is translocation, one codon length to the right
how many amino acids found in protein
20
how many nucleotides are in 20 amino acids
20x3= 60 therefore 60 as there are 20 amino acids in a protein
how many possible combinations are there in 4 diff bases
64
what are the stop codons
UAG UAA UGA
explain why the genetic code is redundant (exceeding what is necessary or normal) and how one specific amino acids may have more than one ___ which codes for it ex (GUU, GUA)
There are many variations to genetic code and for a specific amino acid to minimize errors which lead to mutation
what is a gene mutation
A change in the nucleotide sequence of a section of DNA coding for a specific trait.
