Ch 9

  1. What holds one DNA strand against the other in the double helix?
    Hydrogen bonds hold the two strands of DNA together in the double helix.

  2. What is a "codon"?
    A codon is a three-nucleotide sequence on mRNA that codes for a specific amino acid.

  3. In a DNA double helix, why doesn't an A or T form two hydrogen bonds (out of the three possible) with G or C?
    Adenine (A) pairs with Thymine (T) via two hydrogen bonds, while Guanine (G) pairs with Cytosine (C) via three hydrogen bonds. The molecular structure does not allow alternative bonding.

  4. You and your lab partner are conducting a DNA replication experiment. She reads one DNA strand as “AAAGGG” and says, “The complementary strand must be TTTAAA”. Is she correct? Why or why not?
    Yes, she is correct. DNA replication follows the complementary base-pairing rule where A pairs with T, and G pairs with C.

  5. If the DNA sequence is (TAGGCAT), what would the complementary messenger RNA code be?
    The complementary mRNA sequence would be AUCCGUA.

  6. If an mRNA is missing an “AUG” codon, will the mRNA ultimately be translated? Why or why not?
    No, because AUG is the start codon that signals the beginning of translation.

  7. True or False. The anticodon is found on a complementary DNA strand.
    False. The anticodon is found on tRNA, not DNA.

  8. The region of DNA that directs the formation of a polypeptide (protein) is called a ____________.
    Gene.

  9. Non-coding portions of the DNA molecule are called ____________.
    Introns.

  10. If the code for phenylalanine in humans is UUU, then the code for phenylalanine (Phe) in Escherichia coli would be ___________.
    UUU. The genetic code is universal across organisms.

  11. The enzyme that causes DNA molecules to split apart during replication and transcription is called ______________.
    Helicase.

  12. What are the three types of RNA?
    Messenger RNA (mRNA), Ribosomal RNA (rRNA), and Transfer RNA (tRNA).

  13. Name and describe one of the three types of RNA:
    mRNA carries genetic information from DNA to the ribosome for protein synthesis.

  14. Why is DNA replication described as a semiconservative process?
    Each new DNA molecule consists of one original strand and one newly synthesized strand.

  15. If the helicases were missing during replication, what would happen to the replication process?
    The DNA strands would not separate, preventing replication from occurring.

  16. If thymine makes up 15 percent of the bases in a specific DNA molecule, what percentage of the bases is cytosine?
    35%. Since DNA follows Chargaff's rule, if thymine is 15%, adenine must also be 15%, leaving 70% for cytosine and guanine, which are equal.

  17. During DNA replication, the __________ strand is replicated in a discontinuous fashion, containing gaps in the sequence called ____________, that will be completed by DNA polymerase I.
    Lagging; Okazaki fragments.

  18. The enzyme that catalyzes the synthesis of DNA is called:
    a. DNA polymerase.

  19. Name and state the function of each enzyme involved with DNA replication.

    • Helicase: Unwinds DNA strands.

    • DNA polymerase: Synthesizes new DNA strands.

    • Ligase: Joins Okazaki fragments.

  20. Why does helicase have to “unwind” the DNA molecule before replication can begin?
    The two DNA strands must be separated to allow replication machinery access to the templates.

  21. True or False. Replication can happen in the absence of a DNA strand.
    False. A template DNA strand is required for replication.

  22. What is a gene? Where is it found?
    A gene is a sequence of DNA that codes for a protein. It is found in the chromosome.

  23. How are replication and transcription similar?
    Both involve the synthesis of a new nucleic acid strand using a DNA template.

  24. How are replication and transcription different?
    Replication produces an identical DNA copy, while transcription produces RNA.

  25. What is the function of the ribosome during translation?
    The ribosome reads mRNA and assembles amino acids into a protein.

  26. What is the function of tRNA during translation?
    tRNA carries amino acids to the ribosome and matches them with the correct codons.

  27. The codon for methionine is AUG. What is the anticodon for the tRNA that carries methionine to the ribosome?
    UAC.

  28. True or False. The first amino acid is always methionine.
    True. Translation starts with methionine (AUG start codon).

  29. What are the letters in DNA replication and transcription/translation?
    A, T, C, G (DNA); A, U, C, G (RNA).

  30. What are the words in DNA replication and transcription/translation?
    Codons (three-base sequences that specify amino acids).

  31. What are the instructions in DNA replication and transcription/translation?
    Genes provide the instructions to assemble proteins.

  32. Will a protein still function if a nucleotide is changed? Why or why not?
    It depends. Some mutations are silent, while others change amino acids and alter function.

  33. How does the mRNA copy differ from the DNA template from which it is made?
    mRNA contains uracil (U) instead of thymine (T).

  34. How is mRNA made from DNA? What enzyme catalyzes this reaction?
    Transcription, catalyzed by RNA polymerase.

  35. Why can transcription and translation be simultaneous in prokaryotes but not in eukaryotes?
    Prokaryotes lack a nucleus, so ribosomes can immediately translate mRNA.

What holds one DNA strand against the other in the double helix?

Hydrogen bonds hold the two strands of DNA together in the double helix

What is the function of the 3' poly A tail?
The 3' poly A tail stabilizes mRNA and regulates its translation efficiency.

A protein contains 30 amino acids. How many nitrogen bases code for this protein?
Since each amino acid is coded for by three nitrogen bases (a codon), a protein with 30 amino acids requires 90 nitrogen bases.

39. What tells the ribosome where to start translation and stop translation?
The start codon (AUG) and stop codons (UAA, UAG, UGA) signal translation initiation and termination.

  1. What is an intron?
    An intron is a non-coding sequence of DNA that is removed from pre-mRNA during RNA processing.

What is an exon?
An exon is a coding sequence of DNA that remains in mature mRNA and is translated into protein.

Some have previously described introns as “trash” DNA. We discussed in

lecture why having introns may actually be beneficial “treasure”. Why are introns beneficial?

Introns are beneficial because they protect the Mrna from UV damage. 

Q: Is the repressor bound to the operator?
A: Yes. The repressor (active) binds to the operator, blocking RNA polymerase.

Q: Can RNA polymerase bind the Lac promoter?
A: No. The repressor blocks access to the operator/promoter region.

Q: Are structural genes transcribed?
A: No. The operon is "off" without lactose to inhibit the repressor.

 

Q: Is the repressor bound to the operator?
A: No. Lactose (inducer) binds the repressor, inactivating it.

Q: Can RNA polymerase bind the Lac promoter?
A: Yes. CAP-cAMP activates RNA polymerase binding.

Q: Are structural genes transcribed?
A: Yes. Operon is "on" (lactose enzymes made).



Q50: In the genetic code, there are:
A: b. More codons (64) than amino acids (20).

Q51: Which DNA strand takes longer to synthesize?
A: Lagging strand (discontinuous Okazaki fragments).

Q53: The genetic code is "universal" means:
A: Same codons encode same amino acids in most organisms.

Q54: The code is "degenerate" means:
A: Multiple codons can code for the same amino acid.

Q69: Define mutation.
A: A change in DNA sequence (e.g., substitution, insertion).

Q71: Point vs. frameshift mutati

A: Point: 1 base change. Frameshift: Insertion/deletion shifts reading frame.

Q73: UAU (Tyr) → UAA (STOP):
A: No protein. Premature STOP codon truncates translation.



Q77: Horizontal vs. vertical gene transfer?
A: Horizontal: Between same-gen organisms. Vertical: Parent to offspring.

Q79: What is transformation?
A: Uptake of free DNA from the environment (e.g., Griffith’s experiment).

Q82: What is transduction?
A: Virus (bacteriophage) transfers DNA between bacteria.

Q84: What is conjugation?
A: DNA transfer via pilus (e.g., plasmid transfer).

Q61: What is an operon?
A: Gene cluster controlled by a single promoter/operator (e.g., lac operon).

Q62: "Inducible" means:
A: Usually off; turned on by inducer (e.g., lactose).

Q66: What turns the lac operon off?
A: Repressor binds operator (lactose absent → active repressor).

Q86: Role of cAMP in lac operon?
A: Binds CAP to activate transcription when glucose is low.

Translation & Ribosomes

Q91: Large subunit function?
A: Catalyzes peptide bond formation.

Q93: Initiation factors do?
A: Help assemble ribosome on mRNA start codon.

Q94: Aminoacyl-tRNA synthetase?
A: Attaches correct amino acid to tRNA.



True/False

Q52: mRNA is double-stranded? False. (Single-stranded.)
Q64: RNA polymerase makes repressor first? True. (Regulatory gene transcribed separately.)
Q83: Transduction requires a virus? True.

What is conjugation?
 Conjugation is a form of horizontal gene transfer where one bacterium passes DNA (typically a plasmid) to another bacterium through a pilus​.



85. Name one cell structure a bacterium must have in order to conjugate.
 A bacterium must have a pilus to conjugate, as the pilus connects donor and recipient cells to transfer DNA​.



86. What role does cAMP play in the positive inducible lactose operon?
 cAMP binds to the CAP protein to form a complex that activates the lac operon, allowing transcription of lactose-digesting enzymes. It acts as an “On” switch when glucose is low and lactose is present​.



87. Which carbohydrate is fermented in the presence of cAMP?
 Lactose is fermented when cAMP is present because the CAP-cAMP complex activates transcription of lactase enzymes​.



88. Which carbohydrate is fermented in the absence of cAMP?
 Glucose is fermented in the absence of cAMP because the lac operon remains inactive, favoring glucose metabolism​.



89. DNA molecules always results in pyrimidine-to-purine base pairing. Which of the following represents the correct pairing AND why do they bond with each other?
 Correct pairing: pyrimidine-to-purine because they make a stable molecule. This pairing forms stable hydrogen bonds and maintains uniform DNA helix width​.



90. During transcription, a positive sense single-stranded mRNA molecule is made. What makes this mRNA positive sense?
 It’s called positive sense because it has the same sequence as the DNA coding strand (except U for T) and can be directly translated into protein​.



91. What specifically is the function of the ribosomal large subunit?
 The large subunit of the ribosome forms peptide bonds between amino acids during translation​.



92. What specifically is the function of the ribosomal small subunit?
 The small subunit binds to the mRNA and helps initiate translation by positioning the start codon correctly​.



93. What do initiation factors do?
 Initiation factors assist in the assembly of the ribosomal subunits and help bind the initiator tRNA to the start codon on mRNA【inferred from slide context – standard molecular bio function】.



94. What do aminoacyl-tRNA synthase enzymes do?
 These enzymes attach the correct amino acid to its corresponding tRNA, ensuring accurate translation of the genetic code​.