protein synthesis Flashcards
3.18B
what 2 stages does protein synthesis occur in?
translation and transcription
what is transcription?
DNA is transcribed and an mRNA molecule is produced
where does transcription occur in?
the nucleus of the cell
what is the role of transcription?
produce an mRNA copy of a gene
what is the first stage of transcription?
1) the two strands of the DNA helix are unzipped by breaking the weak hydrogen bonds between complementary base pairs
what is the unwinding of the helix caused by?
an enzyme (helicase)
what does the unwinding of the helix cause? (the second stage)
this exposes the template strand of the gene that codes for the protein being synthesised
what does the enzyme RNA polymerase attach to? (second stage)
attaches to the DNA just before the gene
what do the mRNA nucleotides do (3rd stage)?
free mRNA nucleotides that are present in the nucleus bind to and form hydrogen bonds with exposed complementary nucleotides on the template strand by complementary base pairing
4th step of transcription?
RNA polymerase then catalyses the formation of covalent bonds between the RNA nucleotides to form a strand of mRNA
what bases do RNA nucleotides contain?
the same as DNA, except the T is replaced by U- U base pairs with A
why is mRNA a complementary copy?
because the opposite base bonds with the exposed DNA bases, the strand of mRNA is an opposite copy of the DNA strand (except that U replaces T)
where does the newly formed strand of mRNA leave via?
leaves the nucleus via a pore in the nuclear envelope and travels to the ribosomes
what does DNA contain?
the information to make a protein
what code for a specific amino acid?
three bases (a codon)
what is the sequence of bases?
a set of instructions for which order to join amino acids in to make a protein
where are proteins synthesised?
in the cytoplasm
what is tRNA?
carrier molecules
how many different naturally-occurring amino acids are there?
about 20
how many amino acids does each protein molecule have?
hundreds or even thousands of amino acids joined together in a unique sequence
where does translation occur?
in the cytoplasm of the cell
what is the end result of translation?
the production of a chain of amino acids that will go on to form a protein
what is the first stage of translation?
once the mRNA molecule leaves the nucleus, it travels through the cytoplasm and attaches to the ribosome
what is the second stage of translation?
in the cytoplasm, there are free molecules of tRNA- these tRNA molecules have a triplet of unpaired bases at one end known as the anticodon and the amino acid at the other end - each specific anticodon corresponds to a specific amino acid
what is a codon?
every three bases on mRNA
what is an anticodon?
the complementary 3 bases on the tRNA
for every 3 mRNA bases…
the ribosome lines up one complementary molecule of tRNA
what is the triplet code?
because there are 3 mRNA bases for each tRNA molecule
what is the 3rd stage of translation?
the anticodon on each tRNA molecule pairs with a codon on the mRNA molecule- bringing its specific amino acid with it
tRNA molecules transport specific amino acids to the ribosome
the ribosome allows two tRNA molecules to sit next to eachother
what is the 4th stage of translation?
the ribosome catalyses the formation of a covalent bond (peptide bond) between the two amino acids
what is the 5th stage of translation?
used tRNA molecules exit the ribosome and collect another specific amino acid
what is the 6th stage of translation?
this process continues until a stop codon on the mRNA molecule is reached- acts as a signal for translation to stop and the amino acid chain coded by the mRNA molecule is complete
how is a polypeptide formed (6th step in translation)?
a chain of several hundred amino acids forms in the correct order according to the original DNA is then made- a polypeptide
what happens to the polypeptide after translation?
it is finally folded into the correct shape and becomes a protein molecule e.g. an enzyme or an antibody
