Unit 6: Topic 4 - Translation

Question 1

Where does translation of mRNA into polypeptides occur for eukaryotic and prokaryotic cells?

Answer 1

Eukaryotic cells - ribosomes in the cytoplasm and on the rough endoplasmic reticulum

Prokaryotic cells - ribosomes in the cytoplasm

Translation takes place inside ribosomes (consisting of rRNA and protein). Ribosomes in the cytoplasm synthesize proteins that function within the cell. Ribosomes embedded into the membrane of rough ER synthesize proteins that are exported out of the cell or embedded into the cell membrane.

Question 2

How does the location of transcription and translation affect the timing of when they occur in both prokaryotic and eukaryotic cells?

Answer 2

Eukaryotic translation and transcription are asynchronous
- Transcription occurs in the nucleus, whereas translation occurs in the cytoplasm.
Prokaryotic translation and transcription are synchronous
- Transcription and translation occur simultaneously in the cytoplasm.

Question 3

Explain the first step of translation for eukaryotic cells: initiation.

Answer 3

Initiation -
- The initiator tRNA carrying methionine attaches to the rRNA of the small ribosomal subunit, forming a complex. The complex bind to the 5’ GTP cap of the mRNA and scans the mRNA in the 5’ to 3’ direction until reaching the start codon on the mRNA (usually AUG). The initiator tRNA binds to start codon and the large ribosomal subunit joins to form initiation complex. The initiator tRNA sits in the P site (middle slot) of the ribosome.
https://cdn.kastatic.org/ka-perseus-images/ae16d87eace538e032b1cf5ac3adbb7335a86fb2.png

Question 4

Explain the second step of translation for eukaryotic cells: elongation.

Answer 4

Elongation -
- The initiator tRNA begins elongation in P site. A tRNA molecule with an anticodon complementary to the unpaired codon in the A site enters A site. The codon and anticodon bind with additional energy from GTP. The ribosome catalyzes a peptide bond between the amino acid carried by P site tRNA and the amino acid carried by A site tRNA. The ribosome translocates along the mRNA to the next codon, P site tRNA exits through E site and a new codon is now at the A site, the process repeats.
https://teachmephysiology.com/wp-content/uploads/2020/06/elongation.jpeg
- The sequences of nucleotides on the mRNA is read in codons (triplets encoding an amino acid). The appropriate amino acids are delivered as specified by the codons on mRNA.

Question 5

Explain the third step of translation for eukaryotic cells in order: termination.

Answer 5

Termination -
- A stop codon (UAA, UAG, or UGA) on the mRNA enters the A site of the ribosome. This stop codon does not bind to any tRNA but binds to a protein release factor, which breaks the bond between the polypeptide chain and the tRNA in the P site. The ribosome releases the polypeptide. The large and small ribosomal subunits are released from mRNA.
https://ib.bioninja.com.au/_Media/translation-termination_med.jpeg

Question 6

Define codon and explain its role in translation. What is the genetic code?

Answer 6

• A codon is a set of three mRNA nucleotides (triplets) that codes for a specific amino acid or specify stop/start signals. The four nucleotides (A, U, C, and G) can be combined into 64 different codons. Each codon base pair with the three bases of the corresponding anticodon of a tRNA molecule that carries an amino acid. mRNA codons are read from 5’ to 3’, beginning with a start codon and continuing until a stop codon is reached.
• 61 codons code for amino acids. This includes the start codon (AUG) that codes for the amino acid methionine. The start codon signals the beginning of protein synthesis.
• 3 codons (UAA, UAG, and UGA) act as stop codons. They do not code for amino acids but signal the end of translation.
• The genetic code links mRNA codons to amino acids (deduced by using a genetic code table). Many amino acids are encoded by more than one codon. Genetic code is used by nearly all species, which is evidence for the common ancestry of all living organisms.

Question 7

What are retroviruses, and how is the genetic information in retroviruses a special case?

Answer 7

A retrovirus is a virus that uses RNA instead of DNA as its genetic material. The retroviruses (such as HIV, influenza, and hepatitis B) transcribe RNA into DNA through a special enzyme called reverse transcriptase. The DNA generated is then integrated into the host cell’s DNA and is transcribed and translated for the assembly of new viral progeny. The cell then produces more retroviruses, infecting other cells.