Molecular Biology 9 : Translation Flashcards
What is the genetic code ?
The genetic code consists of 64 triplets of nucleotides. These triplets are called codons. With three exceptions, each codon encodes for one of the 20 AAs used in the synthesis of proteins. That produces some redundancy in the code: most of the amino acids being encoded by more than one codon.
Why is the genetic code degenerate ?
Because 61 codons specify AAs, but there are only 22 known amino acids.
How many reading frames exist ?
Three (because the genetic code s a triplet code).
What are the 3 types of single nucleotide mutations (point mutations) ?
- Missense mutation-substitution of one nucleotide so that an alternative amino acid is incorporated into the polypeptide.
- Nonsense mutation-substitution of one nucleotide so that a stop codon is prematurely introduced, and this results in early termination of the polypeptide.
- Frameshift mutation-a point mutation which results in the ribosome either reading four nucleotides as three, or backing up one nucleotide and reading from a different reading frame.
The genetic code is universal, but does it’s usage ever vary ?
Yes, for examples there are distinctive codons in the human mitochondria. Standard code : UGA = Stop AUA = Ile AGA = Arg AGG = Arg Mitochondrial code : UGA = Trp AUA = Met AGA = Stop AGG = Stop
What are the three forms (and thus roles) of RNA in translation ?
- Messenger RNA (mRNA) carries the genetic information in the form of three-nucleotide sequences, called codons.
- Transfer RNA (tRNA) are adaptors between codons and amino acids: keys to deciphering the codons.
- Ribosomal RNA (rRNA) associates with a set of proteins to form ribosomes.
Give a general overview of the events that occur during translation.
- the initiator tRNA and the small ribosomal subunit binds thestart sequence of translation
- the large ribodsomal subunit binds
- aminoacyl-tRNAs bring in the AAs
- both subunits dissociate and the polypeptide is released
To what ends of the growing polypeptide chain are new AAs attached to ?
The C-terminus
What is the structure of tRNA ?
A clover leaf structure :
- 3’-ACC end (amino accepting arm)
- a D loop (left)
- an anti-codon loop
- a T loop (right)
What are the characteristics of tRNA ?
- Up to 100 different species of tRNA.
- Each tRNA is encoded by multiple genes organized in tandem arrays.
- Clusters of tRNA genes are scattered around the genome.
- Transcribed by RNA Pol III from an internal promoter. •All mature tRNA have the triplet sequence CCA at their 3’ end (added after tRNA is processed).
WHat is the function of aa-tRNA synthetases ?
- Each of the 20 amino acids is attached to the appropriate tRNA by a single, dedicated tRNA synthetase.
- Aminoacyl-tRNA formation is very accurate.
- The ribosome is unable to discriminate between correctly or incorrectly charged tRNAs.
What is the “2 step” decoding process of translation ?
- AA is loaded in tRNA by aa-tRNAs (ATP ==> AMP + 2Pi)
- tRNA binds to its codon in RNA
What is wobble ?
Wobble = position of mRNA (3rd position) or on tRNA (1st position) where non-standard base-paiing can pccur
Give examples of wobble.
If G, U or I are in the first position of the anticodon, then they can pair w/ the following : --> G-C or G-U --> U-A or U-G --> I-C. I-A or I-U If C, a< G or U are in the 3rd position of the codon of mRNA, then they can pair w/ the following : --> C-G or C-I --> A-U or A-I --> G-C or G-U --> U-A, U-G or U-I
What are the 3 steps of proteins synthesis ?
- Initiation : assembly of ribosome and recognition of start site AUG.
- Elongation.
- Termination : STOP codon, release factors.
