Genetic Code & Protein Synthesis Flashcards
What is a gene?
A specific length of DNA on a chromosome that contains a code which determines the sequence of amino acids in a protein / polypeptide.
What is a codon?
A triplet code - whereby 3 bases code for an amino acid.
How many triplet code combinations are there?
64 (4^3)
There are 20 amino acids, meaning some triplets code for the same amino acid.
What is the benefit of multiple triplets coding for the same amino acid?
A mutation may have no effect on the amino acid being coded for.
What is meant by the genetic code being “universal”?
In almost all living organisms, the same triplet of DNA bases code for the same amino acid.
What is meant by degenerate?
In almost all amino acids, there is more than one base triplet.
Which amino acids do not have a degenerate genetic code?
Methionine, Tryptophan
What is a frame shift in the genetic code?
The genetic code is non overlapping, and read starting from a fixed point in groups of three bases.
If a base is added / deleted, it causes a frame shift as every base triplet after than is changed.
What are the two stages of protein synthesis?
Transcription and translation
Describe the process of transcription (1):
- RNA polymerase attaches to the DNA double helix at the beginning of a gene.
- Hydrogen bonds between the two DNA strands break and the DNA molecule begins to uncoil.
- One strand is used as a template to make the mRNA copy.
- RNA polymerase lines up free RNA nucleotides alongside the template strand.
- Complementary base pairings means the mRNA is a complementary copy of the DNA template strand (Thymine replaced with Uracil).
Describe the process of transcription (2):
- Once the bases are paired up, RNA polymerase joins them together to form an mRNA strand.
- RNA polymerase moves along the DNA and assembles the mRNA strand.
- Hydrogen bonds reform between the strands once RNA has passed and the strands coil back into a double helix.
- mRNA leaves the nucleus via the nuclear pores.
Why is a-amanitin a problem?
It is a deadly toxin which inhibits RNA polymerase, which prevents the transcription of mRNA, hence preventing protein synthesis.
Describe the process of translation (1):
- mRNA attaches itself to a ribosome at it’s start codon (AUG) and transfer RNA (tRNA) molecules carry amino acids to the ribosome.
- A tRNA molecule, with an anticodon that’s complementary to the start codon on the mRNA attaches itself to the mRNA by complementary base pairing.
- A second tRNA molecule attaches itself to the next codon in the same way.
- Ribosomal RNA (rRNA) in the ribosome catalyses the formation of a peptide bond between the two amino acids attached to the tRNA molecules.
Describe the process of translation (2):
- The two amino acids join together and the first tRNA molecule moves away, leaving the amino acid behind.
- A third tRNA molecule binds to the next codon on the mRNA and the second tRNA moves away.
- This repeats continuously until there is a stop codon on the mRNA molecule.
- Polypeptide chains move away from the ribosome and translation is complete. Protein = synthesised.
What is mRNA?
mRNA = messenger RNA
mRNA is made in the nucleus during transcription and contains uracil, not thymine. In mRNA, groups of 3 adjacent bases are called codons.
