16. Genetic code. Translation Flashcards
Genetic Code
- Function: Carries information from nucleic acids to proteins.
- TRIPLET genetic code - Each amino acid is encoded by a combination of 3 nucleotides (codons).
-> DNA has 4 types of nucleotides.
-> 1-nucleotide combinations can code for 4 amino acids. - Nucleotide triplets are called CODONS
- Genetic code is COLLINEAR, SEQUENTIAL - arrangement of amino acids in polypeptide depends on sequence of codons in the RNA.
- Genetic code is NON-OVERLAPPING - Insertion or deletion of 1 nucleotide pair shifts the frame and leads to mutation
- Genetic code is SYNONYMOUS - Some AA have more than 1 codon
- Genetic code is UNIVERSAL - For all organisms
Point Mutations
Definition: Changes of a single nucleotide.
Types of Point Mutations:
1. Substitution: Replacement of a nucleotide.
- Silent Mutations: Substitution results in a codon for the same amino acid, leaving protein unchanged due to synonymy of genetic code.
- Missense Mutations: Substitution results in a codon for a different amino acid. These can be harmful or create normal polymorphisms.
- Nonsense Mutations: Substitution converts a codon for an amino acid into a stop codon, prematurely terminating protein synthesis and often destroying the protein’s function.
Deletion: Loss of a nucleotide.
Insertion: Addition of a nucleotide.
Both deletions and insertions cause frameshift mutations, altering the reading frame. This results in a sequence of incorrect amino acids followed by a random stop codon, often rendering the protein nonfunctional, similar to nonsense mutations.
Translation - what is neccessary?
- mRNA
- tRNA
- aminoacyl-tRNA synthetases
- rybosomes
Translation - tRNA
- Function: match AA to codons
- Molecule has 4 loops, 3 of them give a secondary structure
- Tertiary structure is L-shaped
- 3’- end endes with CCA
- On opposite side, one of the large loops carries anticodon
Translation - Aminoacyl-tRNA synthetase
- tRNA recognizes the codon on mRNA, but not AA.
- All tRNAs have the same 3’-end.
- A second adapter, aminoacyl-tRNA synthetase (activating enzyme) is needed.
- Aminoacyl-tRNA synthetase is a cytosolic enzyme that binds AA to its corresponding tRNA.
- The enzyme catalyzes covalent attachment of the AA to tRNA, driven by ATP hydrolysis.
- The resulting aminoacyl tRNA is released from the enzyme.
- Aminoacyl tRNA delivers its amino acid to a growing polypeptide chain on a ribosome.
Ribosome
- R has 2 subunits linked by Mg2+
- at least one molceule of rRNA is in each subunit plus dozens of ribosomal proteins
- largest rRNA performs role of PEPTIDYL TRANSFERASE or RIBOZYME and forms peptide bond
Stages of translation
INITIATION
-small subunit recognizes mRNA by its cap at 5’ end. P factor is involved
- tRNA that carries 1st AA joins the complex
- mRNA binds with its anticodon - the peptidyl region of the small subunit
- Large subunit joins
- Proteins that are called initiation factors are needed
ELONGATION
A second aminoacyl-tRNA enters the aminoacyl (A)-region with help of an elongation factor called a
transport factor. If the anticodon matches the codon, the transport factor leaves the mRNA, which lodges in the A-site.
A and P Sections:
- anticodons of the two tRNAs are contiguous in the small subunit.
- at the catalytic center of the peptidyl transferase, the two amino acids are also contiguous.
Peptide Bond Formation:
- Ribozyme peptidyl transferase forms a peptide bond between the two amino acids.
- Cleaves the amino acid in the P-site from its tRNA.
- tRNA in the A-site now carries the growing peptide.
Translocation:
- An elongation factor called
translocase moves the ribosome one codon in the 5’-3’ direction.
- mRNA in the P-site is pushed out of the ribosome.
- mRNA with the peptide in the A-site moves to the P-site.
Cycle Continuation:
- A-site is free and ready to receive the next aminoacyl-tRNA.
- cycle continues, peptide chain grows, and the ribosome moves toward the 3’-end of the mRNA.
Energy Usage:
- Ribosome uses energy from GTP for its functions.
TERMINATION
- occurs when ribosome reaches a stop codon.
- no mRNA for it but it is recognized by proteins called releasing factors. - they change the activity of peptidyl transferase and cause it to hydrolyze the bond betw the last mRNA and the AA
- copleted olypeptide is releases
- whole complex falls apart
Summary - Enzymes of translation
- Aminoacyl-tRNA synthetase
- cytosolic enzyme
- binds AA to the corresponding tRNA - Peptidyl-transferase (ribozyme)
- forms the peptide bond
- it is part of the ribosome - Translocase
- causes the ribosome to move one codon in the 5’–3’ direction
