2.7 - 7.3 : Translation Flashcards
Translation
The protein-making machinery, called the ribosome, reads the mRNA sequence and translates it into the amino acid sequence of the protein. The ribosome starts at the sequence AUG, then reads three nucleotides at a time. Each three-nucleotide codon specifies a particular amino acid. The stop codons (UAA UAG UGA) tell the ribosome that the protein is complete.
what is translation
describe a ribosome
- Translation is the process of protien synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids in a polypeptide chain.
- A ribosome is composed of two halves, a large and a small subunit. During translation, ribosomal subunits assemble together like a sandwich on the strand of mRNA:
- Each subunit is composed of RNA molecules and proteins
- The small subunit binds to the mRNA
- The large subunit has binding sites for tRNA and also catalyzes the peptide bonds between amino acids
Ribosome structure
- aminoacyl tRNA binding site
- peptidyl tRNA binding site
- exit site
- small subunit
- large subunit
- mRNA binding site
Structure of tRNA
- The structure of tRNA matches its function
function: to bring amino acids from the cytoplasm to the growing polypeptide and attach them in the correct location - Clover leaf structure
- single chain of RNA
- Site attachment for the amino acids at the 3’ terminal end
- There are 3 loops, where base pairing doesnt occur
- Double stranded sections are base paired through H bonds
- Anticodon (3 bases) attaches to mRNA codons
tRNA activating enzyme
- tRNA is activated by a tRNA activating enzyme
- tRNA picks up new amino acids when activated by a specific tRNA activating enzyme
- This uses ATP
- There are 20 of these enzymes corresponding to the 20 amino acids, for which the tRNA molecule has the complementary anticodon
- The energy in the bond linking the tRNA molecule to the amino acid will be used in translation form a peptide bond between adjacent amino acids.
1) tRNA activating enzyme
2) Enzyme binds to ATP and a specific amino acid
3) Amino acid-AMP complex is formed and a specific tRNA molecule is recruited
4) The tRNA is bound to the amino acid and AMP is released
5) “charged” tRNA is produced
Codons of three bases on mRNA correspond to one amino acid in a polypeptide.
(what is genetic code and codons?)
The genetic code is the set of rules by which information encoded in mRNA sequences is converted into proteins (amino acid sequences) by living cells
- Codons are triplets of bases which encode a particular amino acid
- As there are 4 bases, there are 64 different codon combinations
- the codons translate for 20 amino acids
- different codons can translate for the same amino acid therefore the genetic code is said to be degenerate.
- The coding region always starts with a start codon AUG therefore the first amino acid in all polypeptides is Methionine
- The coding region of mRNA terminates with a stop codon - the stop codon does not add an amino acid, instead it causes the release of the polypeptide.
- amino acids are carried by transfer RNA . The anticodons on the tRNA are complementary to the codons on mRNA
Key components of translation that enable genetic code to synthesize polypeptides
1) mRNA has a sequence of codons the specifies the amino acid sequence of the polypeptide
2) tRNA molecules carry the amino acid corresponding to their codon
3) tRNA molecule shave an anticodon of three bases that binds to a complementary codon on mRNA
4) Ribosomes :
- Act as the binding site for mRNA and tRNA
- catalyze the peptide bonds of the polypeptide
3 stages of translation: whole description
Initiation:
1) mRNA binds to the small subunit of the ribosome
2) The small subunit of the ribosome moves along the mRNA molecules in a 5’ - 3’ direction, until it reaches a start codon (AUG)
3) A molecule of tRNA (with its amino acid, methionine attached) complementary to the start codon (UAC) binds to the P site of the ribosome
4) The large subunit of the ribosome binds to the tRNA and small subunit
Elongation:
5) A second tRNA (with amino acid attached) complementary to the second codon on the mRNA then binds to the A site of the ribosome
6) The amino acid carried by the tRNA in the P site is transferred to the amino acid in the A site as a consequence of the ribosome catalyzing a new peptide bond (condensation reaction). The growing polypeptide increases in length
7) The ribosome moves one codon along the mRNA in a 5’ to 3’ direction
- the tRNA in the P site is moved to the E site and then released
- the tRNa is the A site is moved onto the P site
8) Another tRNA binds, complementary to the next codon on the mRNA, binds to the A site.
9) steps 6,7, and 8 repeat untill the stop codon is reached.
Termination:
10) When a stop codon is reached translation is stopped:
- A release factor attaches to the A site
- the polypeptide chain is released
- the ribosome complex dissembles ready for reuse translating another mRNA molecule
Translation is prokaryotes vs eukaryotes
- translation can occur immediately after transcription prokaryotes due t the absence of a nuclear membrane
- In prokaryotes ribosomes can be adjacent to the chromosomes whereas in eukaryotes the mRNA needs to be relocated from the nucleus to the cytoplasm (through the nuclear membrane)
- In eukaryotes the mRNA is modified (spliced) after transcription before translation, this does not occur in prokaryotes
Polysomes
- a polysome is a structure that consists of multiple ribosomes attached to a single mRNA
- Multiple ribosomes translating mRNA simultaneously enables the cell to quickly create many copies of the required polypeptide
Amino acid propertiecs
- 20 different amino acids
- they can be combined in any order
types:
- polar, non-polar, + charge, - charge, contain sulfur
- The properties of amino acids determine how a polypeptide folds up into a protein
Proteins properties and types
Proteins are commonly described as either being fibrous or globular in nature. fibrous proteins have structural roles whereas globular proteins are functional.
Fibrous
- Long and narrow
- structural (strength and support)
- Generally insoluble in water
- repetitive amino acid sequence
- less sensitive to changes to heat and ph
- collagen, myosin, actin, fibrin
- Hydrophobic R groups are exposed
Globular:
- Rounded/spherical
- functional (catalytic, transport)
- generally insoluble in water
- irregular amino acid sequence
- more sensitive to change in heat, ph
- Catalase, heamoglobin
- Hydrophobic R groups are folded into the molecule
4 levels of protein structure:
Primary:
- Polypeptide
- The sequence of amino acids of which the protein is composed
- formed by covalent peptide bonds between adjacent amino acids
- Controls all subsequent levels of structure
Secondary:
- The chains of amino acids fold or turn upon themselves
- held together by hydrogen bonds between non-adjacent amine and carboxylic groups
- H-bonds provide structural stability
- Fibrous proteins
Tertiary:
- The polypeptide folds and coils to form a complex 3D shape
- Caused by interactions between R groups
- Important for function
- globular proteins
Quaternary:
- The interaction between multiple polypeptides or prosthetic groups
- fibrous/globular proteins
