9.E: Molecular Biology (Exercises)

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9.1: The Structure of DNA

Multiple Choice

Which of the following does cytosine pair with?

A. guanine
B. thymine
C. adenine
D. a pyrimidine

Answer: A

Prokaryotes contain a ________chromosome, and eukaryotes contain ________ chromosomes.

A. single-stranded circular; single-stranded linear
B. single-stranded linear; single-stranded circular
C. double-stranded circular; double-stranded linear
D. double-stranded linear; double-stranded circular

Answer: C

Free Response

Describe the organization of the eukaryotic chromosome.

Answer: The DNA is wound around proteins called histones. The histones then stack together in a compact form that creates a fiber that is 30-nm thick. The fiber is further coiled for greater compactness. During metaphase of mitosis, the chromosome is at its most compact to facilitate chromosome movement. During interphase, there are denser areas of chromatin, called heterochromatin, that contain DNA that is not expressed, and less dense euchromatin that contains DNA that is expressed.

Describe the structure and complementary base pairing of DNA.

Answer: A single strand of DNA is a polymer of nucleic acids joined covalently between the phosphate group of one and the deoxyribose sugar of the next to form a “backbone” from which the nitrogenous bases stick out. In its natural state, DNA has two strands wound around each other in a double helix. The bases on each strand are bonded to each other with hydrogen bonds. Only specific bases bond with each other; adenine bonds with thymine, and cytosine bonds with guanine.

9.2: DNA Replication

Multiple Choice

DNA replicates by which of the following models?

A. conservative
B. semiconservative
C. dispersive
D. none of the above

Answer: B

The initial mechanism for repairing nucleotide errors in DNA is ________.

A. mismatch repair
B. DNA polymerase proofreading
C. nucleotide excision repair
D. thymine dimers

Answer: B

Free Response

How do the linear chromosomes in eukaryotes ensure that its ends are replicated completely?

Answer: Telomerase has an inbuilt RNA template that extends the 3' end, so a primer is synthesized and extended. Thus, the ends are protected.

9.3: Transcription

Multiple Choice

A promoter is ________.

A. a specific sequence of DNA nucleotides
B. a specific sequence of RNA nucleotides
C. a protein that binds to DNA
D. an enzyme that synthesizes RNA

Answer: A

Portions of eukaryotic mRNA sequence that are removed during RNA processing are ________.

A. exons
B. caps
C. poly-A tails
D. introns

Answer: D

9.4: Translation

Multiple Choice

The RNA components of ribosomes are synthesized in the ________.

A. cytoplasm
B. nucleus
C. nucleolus
D. endoplasmic reticulum

Answer: C

How long would the peptide be that is translated from this MRNA sequence: 5'-AUGGGCUACCGA-3'?

A. 0
B. 2
C. 3
D. 4

Answer: D

Free Response

Transcribe and translate the following DNA sequence (nontemplate strand): 5'-ATGGCCGGTTATTAAGCA-3'

Answer: The mRNA would be: 5'-AUGGCCGGUUAUUAAGCA-3'. The protein would be: MAGY. Even though there are six codons, the fifth codon corresponds to a stop, so the sixth codon would not be translated.

9.5: How Genes Are Regulated

Multiple Choice

Control of gene expression in eukaryotic cells occurs at which level(s)?

A. only the transcriptional level
B. epigenetic and transcriptional levels
C. epigenetic, transcriptional, and translational levels
D. epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels

Answer: D

Post-translational control refers to:

A. regulation of gene expression after transcription
B. regulation of gene expression after translation
C. control of epigenetic activation
D. period between transcription and translation

Answer: B

Free Response

Describe how controlling gene expression will alter the overall protein levels in the cell.

Answer: The cell controls which protein is expressed, and to what level that protein is expressed, in the cell. Prokaryotic cells alter the transcription rate to turn genes on or off. This method will increase or decrease protein levels in response to what is needed by the cell. Eukaryotic cells change the accessibility (epigenetic), transcription, or translation of a gene. This will alter the amount of RNA, and the lifespan of the RNA, to alter the amount of protein that exists. Eukaryotic cells also change the protein’s translation to increase or decrease its overall levels. Eukaryotic organisms are much more complex and can manipulate protein levels by changing many stages in the process.