Protein Synthesis

Question 1

what contains the information to make a protein?

Answer 1

DNA

Question 2

how does protein synthesise?

Answer 2

• DNA remains in the nucleus, but a copy, called mRNA, moves from the nucleus to the ribosomes where proteins are synthesised in the cytoplasm
• the protein produced depends on the sequence of the bases in the mRNA, and if this sequence changes a different protein will be made
• tRNA (carrier molecules) bring specific amino acids to add to the growing protein in the correct order
• each protein molecule has hundreds, or even thousands, of amino acids joined together in a unique sequence
• it is then folded into the correct unique shape which allows the protein to do its job
• some proteins are enzymes, others are hormones, and others from structures within the body, such as collagen - each protein needs a different shape
• cells express their genes by converting the genetic message into protein

Question 3

what are the two stages of protein synthesis?

Answer 3

transcription and translation

Question 4

what happens in transcription?

Answer 4

when a gene is to be expressed, the sequence of bases in DNA is copied or transcribed into mRNA - this process takes place in the nucleus and occurs in a sequence of stages

1. the two strands of DNA helix are unzipped by breaking the weak hydrogen bonds between base pairs - the unwinding of the helix is caused by an enzyme (helicase)
2. the enzyme RNA polymerase attaches to the DNA just before the gene
3. RNA polymerase moves along the DNA strand - free RNA nucleotides are attracted to (and from hydrogen bonds with) the exposed DNA strand nucleotides by complementary base pairing
4. RNA polymerase then catalyses the formation of covalent bonds between the RNA nucleotides to form a strand of mRNA:

RNA nucleotides contain the same bases as DNA, except that T is replaced by U, U base pairs with A - 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) we call this a complimentary copy

Question 5

what happens in translation?

Answer 5

1. the mRNA strand travels through the cytoplasm and attaches to the ribosome - the strand passes through the ribosome
2. for every three mRNA bases the ribosome lines up one complimentary molecules of tRNA. we call every three bases on mRNA a codon - the complementary molecule of tRNA are an anticodon:

for example if CGG is the mRNA codon the tRNA anticodon is GCC - because there are three mRNA bases for each tRNA molecule, we call this the triplet code

3. tRNA molecules transport specific amino acids to the ribosome, the ribosome allows two tRNA molecules to sit next to each other, the ribosome catalyses the formation of a covalent bond (peptide bond) between the two amino acids
4. used tRNA molecules exit the ribosome and collect another specific amino acid
5. a chain of several hundred amino acids forms in the correct order according to the original DNA is made - this is called a polypeptide

after translation, the polypeptide is finally folded into the correct shape and becomes a protein