Genetic code & translation Flashcards
What is the central dogma of molecular biology?
information passes from DNA to RNA and subsequently to proteins but proteins cannot pass the information back to DNA
Define the genetic code
A nucleotide base sequence on DNA and subsequently on mRNA by transcription which will be translated into a sequence of amino acids of the protein to be synthesised
what are the characteristics of the genetic code?
- Specific - specific codon always codes for same amino acid.
- Universal - same code is used on all living organisms (pro and eukaryotic)
- Degenerate - although each codon corresponds to a single amino acid, 1 amino acid could have >1 codon so 61 codons used to code for amino acids.
what is the start codon?
AUG initiates start of translation
what are stop codons?
UAA, UAG, UGA - termination of translation and the end of protein synthesis.
what is needed to translate genetic code into proteins?
mRNA, tRNA, ribosomes
what is the role of ribosomes in translation?
Translation takes place at ribosomes.
The small subunit reads the mRNA
The large subunit joins the amino acids to form polypeptide chains
- Each subunit contains rRNA and ribosomal proteins
what is the difference between bacterial and animal ribosomes?
Subunits differ in size:
Bacterial = 70S
Animal = 80S
Bacterial have less RNA in their small subunit and are target for antibiotics
what are the 3 phases of translation
initiation
elongation
termination
what happens in initiation?
Ribosome assembles around the mRNA to be read, and first tRNA (tRNA carries the amino acid methionine which matches start codon).
This forms initiation complex
What happens in elongation?
mRNA is read 1 codon (3 bases) at a time, and the amino acid matching this codon is added to a growing polypeptide chain
what happens as each new codon is exposed in elongation?
- a matching tRNA binds to the codon
- the existing amino acid chain is linked into the amino acid of the tRNA
- mRNA shifts one codon over in the ribosome, exposing a new codon
what happens at termination?
Begins with a stop codon entering the ribosome, triggering a series of events that separate the chain from its tRNA and allows it to drift out of the ribosome
what is the resultant polypeptide chain of amino acids?
It is polar and has an N terminus and a C terminus
what does mRNA have that allows it to take part in translation?
a poly A tail (3’) and a 5’ cap.
It also contains the start codons (AUG) and stop codons (UAA, UAG, UGA)
