3.4.2 DNA and protein synthesis

Question 1

transcription (basic)

Answer 1

• Complimentary mRNA copy of one gene from DNA is created in the nucleus
• mRNA is much shorter than DNA therefore it can fit through the nuclear pores and carry the genetic code to the ribosomes

Question 2

Transcription (mark scheme)

Answer 2

• The DNA helix unwinds to expose the bases to act as a template
• Only one strand of DNA acts as the template (coding strand)
• The unwinding of the helix is catalysed by DNA helicase
• DNA helicase breaks the hydrogen bonds between the base pairs
• Free mRNA nucleotides line up the exposed base pairs in complimentary order
• RNA polymerase bond together the RNA nucleotides to create a new pre-mRNA polymer chain
• pre-mRNA undergoes splicing
• The mRNA leaves the nucleus via nuclear pores

Question 3

Splicing

Answer 3

• Introns are spliced out by a protein called splicesome
• Leaving only the exons
• Creating the mRNA chain that is now ready to leave the nucleus

Question 4

translation (basic)

Answer 4

stage in which the polypetide chain is created using both the mRNA base sequence and tRNA molecules

Question 5

Translation (mark scheme)

Answer 5

• Once the modified mRNA enters the cytoplasm a ribosome attaches to the start codon
• The tRNA molecule with the complimentary anticodon, aligns opposite the start codon of the mRNA which is held in place by the ribosome
• The ribosome moves along the mRNA strand to allow another tRNA molecule to attach to the next codon
• The two amino acids that have been delivered by the tRNA are bonded by peptide bond this requires an enzyme and ATP
• This continues until the ribosome reaches the stop codon which will not code for an amino acid and the ribosome will detach
• The polypetide chain will now enter the golgi for folding and modification