3.10 - Protein Synthesis

Question 1

Define transcription.

Answer 1

Conversion of genetic code to sequence of nucleotides in mRNA that leave the nucleus via nuclear pores

Question 2

Which is the template strand of DNA?

Answer 2

• 2 strand of DNA
• One codes for a protein (called sense strand) - runs 5’ to 3’ Other strand runs 3’ to 5’ (called antisense strand), doesn’t code for a protein
  
  • this is the template strand as you want the RNA to have the same bases as the sense strand.

Question 3

Describe the process of DNA transcription.

Answer 3

1. DNA helicase catalyses the breaking of hydrogen bonds between the bases
2. Free RNA nucleotides base pair to the template/antisense strand
3. Phosphodiester bonds from - catalysed by RNA polymerase
4. Transcription stops at the end of the gene - completed strand is mRNA
5. mRNA detaches from antisense and leaves via nuclear pores to ribosome

Question 4

Describe the structure of tRNA.

Answer 4

• One site of amino acid attachment at the top
• Anticodon - specific to an mRNA codon, attached amino acids are activated once the anticodon pairs with the mRNA codon - ATP used

Question 5

Describe the process of translation

Answer 5

1) mRNA binds to ribosomes small subunit at start codon AUG. 2) tRNA comes to access point in large subunit, tRNA has anticodon UAC to bind to mRNA - methionine is the first amino acid 3) Another tRNA enters ribosome, binding to the nect codon for another amino acid Max. 2 tRNA molecules on mRNA at a time

1. mRNA binds to small subunit of ribosome
   
   1. ribosome has space for 2 codons at once (only 2 tRNA molecules can bind at once)
2. tRNA with a complementary anticodon binds to the start codon (AUG) in the large subunit - the amino acid is activated (using ATP)
3. Another tRNA enters the ribosome, and binds to the next mRNA codon
4. The two amino acids form peptide bonds and is catalysed by peptidyl transferase (part of rRNA)
5. The ribosome moves along the RNA, the first tRNA is released, the second tRNA becomes the first and another tRNA binds to the large subunit

Question 6

What happens after translation?

Answer 6

• In translation a 1^o protein structure is formed
• After the ribosome reaches the end of the mRNA, the polynucleotide is released
• The polypeptide chain folds into 2^o and 3^o proteins
• Protein may undergo further modification in the Golgi Apparatus

Question 7

What is the structure of a ribosome?

Answer 7

• Comprises of a large subunit and a small subunit which come together and surround mRNA
• Once translation is finished, the subunits separate to be reused
• Made of proteins and rRNA
• rRNA maintains structure and function, protein supports rRNA in catalysing chemical reactions
  
  • (peptidyl transferase catalysing peptide bond formation between amino acids)

Question 8

Can ribosomes synthesise the same mRNA again?

Answer 8

Yes, it can produce several copies of the same polypeptide chain.