Lecture 11

In eukaryotic cells, transcription cannot begin until

• the two DNA strands have completely separated

  and exposed the promoter.

• several transcription factors are bound to the

  promoter

• the 5' caps are removed from the mRNA.

• the DNA introns are removed from the template.

Answer explanation

In eukaryotic cells, transcription requires the assembly of a transcription initiation complex at the promoter region of a gene. This complex includes RNA polymerase II and several transcription factors, which help in recognizing and binding to the TATA box or other promoter elements.

Which of the following is not true of a codon?

• It may code for the same amino acid as another codon.

• It never codes for more than one amino acid.

• It extends from one end of a tRNA molecule.

• It is the basic unit of the genetic code.

Answer explanation

A codon is found on mRNA, not on tRNA. The anticodon, a complementary sequence to the codon, is located on tRNA.

Which of the following mutations would be most likely to have a harmful effect on an organism?

• a deletion of three nucleotides near the middle of a gene

• a single nucleotide deletion in the middle of an intron

• a single nucleotide deletion near the end of the coding sequence

• a single nucleotide insertion downstream of, and close to, the start of the coding sequence

Answer explanation

A single nucleotide insertion early in the coding sequence can cause a frameshift mutation, which is highly detrimental because it alters the reading frame of the mRNA. This typically results in:

• - Completely different amino acids being incorporated into the protein.

• - A premature stop codon that truncates the protein, making it nonfunctional.

Generally speaking, how many genetic codes are there?

• one

• one for each organism

• 20

• 3

Answer explanation

Although slight variations do exist, the genetic code is essentially universal. Nearly all organisms use the same genetic code to translate mRNA to protein

What catalyzes the linkage between ribonucleotides to form RNA during gene expression?

• rRNA

• a ribozyme

• RNA polymerase

• aminoacyl-tRNA synthetase

Answer explanation

The enzyme responsible for transcription is RNA polymerase.

Two different proteins with mostly different structures are translated from two different mRNAs. These mRNAs, however, were transcribed from the same gene. Which mechanism could best account for this?

• Exons from the same gene could be spliced in different ways to make different mRNAs.

• Different transcription factors were involved in the transcription of the two mRNAs.

• A point mutation might have altered the gene.

• Different systems of DNA unpacking could result in two different mRNAs.

What is their proper sequence for these steps?

1. translation
2. RNA processing
3. transcription
4. modification of protein

• 1, 2, 3, 4

• 2, 3, 4, 1

• 4, 2, 3, 1

• 3, 2, 1, 4

Answer explanation

In eukaryotic cells, the transcription of a pre-mRNA is followed by processing. The mRNA then moves to the cytoplasm, where ribosomes construct a polypeptide based on the codons in the RNA. The polypeptide is then modified, if necessary.