4-2 DNA and protein synthesis

Question 1

What are the two stages of protein synthesis?

Answer 1

• Transcription, which occurs in the nucleus and involves DNA and mRNA.
• Translation, which involves mRNA, tRNA and ribosomes, during transcription a section of a DNA strand is transcribed into mRNA which is then translated into a polypeptide chain formed of amino acids.

Question 2

Describe the structure of mRNA?

Answer 2

• Long single strand created during transcription in which the base sequence is complementary to DNA.
• Each set of three bases, called the codon, match a triplet on the DNA as well as the tRNA.

Question 3

Describe the structure of tRNA?

Answer 3

• tRNA is a small molecule made up of around 80 nucleotides.
• It is a single strand that is folded into a clover leaf shape in which one end extends from the others.
• Here an amino acid attaches.
• At the opposite end is an anti-codon which is specific to the amino acid that the tRNA attaches to.

Question 4

Describe the stages of transcription?

Answer 4

1. The hydrogen bonds between the complementary basses break due to the action of an enzyme and the DNA uncoils thus separating the two strands exposing the organic bases.
2. One of the DNA strands is used as a template to make the mRNA molecule, the template is called the antisense strand.
3. Free nucleotides line up by complementary base pairing and adjacent nucleotides are joined by phosphodiester bonds thus forming a molecule of mRNA, the enzyme RNA polymerase catalyses the formation of phosphodiester bonds, when a stop codon is reached this stops, as the RNA polymerase moves away the DNA re-joins, with only 12 bases being exposed at a time to reduce the chance of damage to the DNA.
4. In eukaryotic cells the pre-mRNA is then spliced to remove the introns leaving. Just a strand of exons, the mRNA then moves out of the nucleus through a proe and attaches to a ribosome in the cytoplasm which is the site of the next stage of protein synthesis called translation.

Question 5

Describe the stages of translation?

Answer 5

1. mRNA attaches to a ribosome and tRNA collects amino acids from the cytoplasm and carries them to the ribosome.
2. tRNA attaches itself to mRNA by complementary base pairing, two molecules attach to mRNA at a time.
3. The amino acids attached to two tRNA molecules join by a peptide bond and then tRNA molecules detach themselves from the amino acids, leaving them behind.
4. This process is repeated thus leading to the formation of a polypeptide chain until a stop codon is reached on mRNA and ends the process of protein synthesis.