[8.4-5] polypeptide synthesis: transcription, splicing, translation

Question 1

what is transcription?

Answer 1

the process of making pre-mRNA using part of the DNA as a template

Question 2

how is transcription different in eukaryotic and prokaryotic cells?

Answer 2

• in prokaryotic, transcription results directly in the production of mRNA from DNA
• in eukaryotic, transcription results in the production of pre-mRNA, which is then spliced to form mRNA

Question 3

what is splicing?

Answer 3

when base sequences corresponding to the introns are removed and the functional exons are joined together

Question 4

describe how mRNA is produced in the nucleus of a cell by transcription [6]

Answer 4

1. DNA helicase unwinds the two strands by breaking the hydrogen bonds between bases
2. only one DNA strand acts as a template
3. RNA polymerase binds to the exposed DNA strand
4. RNA nucleotides are attracted to exposed bases according to the complementary base pairing rule
5. RNA polymerase joins the RNA nucleotides by forming phosphodiester bonds which creates a strand of pre-mRNA
6. pre-mRNA is spliced to remove introns

Question 5

where does translation take place and fun facts about where it takes place?

Answer 5

• translation takes place in ribosomes in prokaryotes and eukaryotes
• ribosomes are made of rRNA and protein
• many ribosomes can translate the same mRNA, giving many copies of the same protein
• there are lots of ribosomes on the RER

Question 6

about the start codon

Answer 6

• the start codon is where translation starts
  > it has a code AUG, so it’s anti-codon is UAC
• a tRNA with the complementary anti-codon forms hydrogen bonds with it
• this codes for the AA called methionine
• all proteins therefore start with methionine

Question 7

about the stop codon and extra bases

Answer 7

• the stop codon is where translation stops
• it does not code for an AA
• therefore, you always have more bases than you would expect
  > 100 AA = 300 bases + 3 bases for stop codon
• you also have extra bases in the non-coding introns inside the gene and the non-coding regions outside the gene

Question 8

describe how the process of translation leads to the production of a polypeptide [10]

Answer 8

1. mRNA leaves the nucleus via the nuclear pore
2. mRNA attaches to a ribosome
3. ribosome moves along to the start codon on the mRNA
4. start codon on mRNA binds to the complementary anti-codon on tRNA
5. each tRNA brings a specific amino acid
6. sequence of codons on the mRNA determines the order of amino acids that arrive at the ribosome
7. ribosome moves along to the next codon and a new tRNA molecule arrives with a new amino acid
8. peptide bonds are formed between each amino acid joined by condensation reactions
9. this requires energy from ATP hydrolysis and releases a molecule of water
10. tRNA detaches and collects another amino acid

Question 9

how is transcription and translation different in prokaryotes and eukaryotes?

Answer 9

• no nucleus in prokaryote so does not leave nuclear pore
• transcription occurs in cytoplasm in prokaryote but nucleus in eukaryote
• faster in prokaryote
• processes happen at the same time in prokaryote, cannot happen at the same time in eukaryote
• no splicing in prokaryote