BIO111 Exam 3 Actual Exam V1 | BIO111 General Biology (BIO111
Exam 3) | UCLA
1. During which phase of the cell cycle does DNA replication occur?
A. S phase
B. G1 phase
C. G2 phase
D. M phase
Answer: A
Rationale: DNA replication occurs specifically during the S phase of interphase to ensure
that genetic material is doubled before division. The G1 and G2 phases are primarily for
growth and preparation, while the M phase involves the physical separation of
chromosomes. Proper timing of replication is critical to maintain genomic stability in
daughter cells.
2. Which enzyme is responsible for unwinding the DNA double helix at the replication fork?
A. DNA Polymerase III
B. DNA Ligase
C. Topoisomerase
D. Helicase
Answer: D
Rationale: Helicase functions by breaking the hydrogen bonds between the nitrogenous
bases of the two parental DNA strands. This action creates the replication fork, allowing
other enzymes access to the single-stranded templates. Without helicase, the double-
stranded DNA would remain coiled and inaccessible for replication.
3. In a Mendelian monohybrid cross between two heterozygotes (Aa x Aa), what is the
expected genotypic ratio of the offspring?
A. 1:2:1
B. 3:1
C. 9:3:3:1
D. 1:1
Answer: A
,Rationale: The genotypic ratio for a cross between two heterozygotes is 1 AA : 2 Aa : 1 aa.
While the phenotypic ratio is 3:1 for dominant to recessive, the underlying genotypes
follow the Mendelian law of segregation. This ratio demonstrates how alleles are
distributed into gametes and recombined during fertilization.
4. Which of the following processes occurs during Prophase I of meiosis but not during
mitosis?
A. Chromatin condensation
B. Spindle fiber formation
C. Nuclear envelope breakdown
D. Synapsis and crossing over
Answer: D
Rationale: Synapsis involves the pairing of homologous chromosomes to form tetrads,
which allows for the exchange of genetic material through crossing over. This process is
unique to meiosis and is a major source of genetic variation in sexually reproducing
organisms. In mitosis, homologous chromosomes do not pair up and instead behave
independently.
5. What is the function of the enzyme DNA Ligase during DNA replication?
A. To add new nucleotides to the 3’ end of the growing strand
B. To synthesize an RNA primer
C. To relieve torsional strain ahead of the replication fork
D. To join Okazaki fragments on the lagging strand
Answer: D
Rationale: DNA Ligase catalyzes the formation of phosphodiester bonds between adjacent
DNA fragments, specifically on the lagging strand. Because DNA polymerase can only
synthesize DNA in a 5’ to 3’ direction, the lagging strand is built in short segments called
Okazaki fragments. Ligase ensures the sugar-phosphate backbone is continuous by sealing
the nicks between these fragments.
6. If a cell has 24 chromosomes at the start of G1, how many sister chromatids will it have
during Metaphase of mitosis?
A. 12
B. 48
C. 24
D. 96
, Answer: B
Rationale: During the S phase, each of the 24 chromosomes is replicated to form two
identical sister chromatids held together at the centromere. By Metaphase, the cell contains
24 chromosomes, each consisting of two chromatids, totaling 48 sister chromatids. These
chromatids will later be separated into individual chromosomes during Anaphase.
7. Mendel’s Law of Independent Assortment is best illustrated by which stage of meiosis?
A. Prophase I
B. Telophase II
C. Anaphase II
D. Metaphase I
Answer: D
Rationale: During Metaphase I, homologous pairs align randomly at the metaphase plate
relative to other pairs. The orientation of one pair does not influence the orientation of
another, which results in different combinations of maternal and paternal chromosomes in
gametes. This random alignment provides the physical basis for independent assortment
observed in genetics.
8. Which component of the RNA polymerase complex is responsible for recognizing the
promoter region in prokaryotes?
A. Alpha subunit
B. Beta subunit
C. Sigma factor
D. Rho factor
Answer: C
Rationale: The sigma factor is a protein needed only for the initiation of transcription in
bacteria, as it enables specific binding of RNA polymerase to gene promoters. Once
transcription begins, the sigma factor is typically released from the core enzyme. This
mechanism ensures that transcription starts at the correct site to produce functional
mRNA.
9. A man with type AB blood marries a woman with type O blood. What are the possible
blood types of their children?
A. AB and O
B. A, B, and O
C. A and B
Exam 3) | UCLA
1. During which phase of the cell cycle does DNA replication occur?
A. S phase
B. G1 phase
C. G2 phase
D. M phase
Answer: A
Rationale: DNA replication occurs specifically during the S phase of interphase to ensure
that genetic material is doubled before division. The G1 and G2 phases are primarily for
growth and preparation, while the M phase involves the physical separation of
chromosomes. Proper timing of replication is critical to maintain genomic stability in
daughter cells.
2. Which enzyme is responsible for unwinding the DNA double helix at the replication fork?
A. DNA Polymerase III
B. DNA Ligase
C. Topoisomerase
D. Helicase
Answer: D
Rationale: Helicase functions by breaking the hydrogen bonds between the nitrogenous
bases of the two parental DNA strands. This action creates the replication fork, allowing
other enzymes access to the single-stranded templates. Without helicase, the double-
stranded DNA would remain coiled and inaccessible for replication.
3. In a Mendelian monohybrid cross between two heterozygotes (Aa x Aa), what is the
expected genotypic ratio of the offspring?
A. 1:2:1
B. 3:1
C. 9:3:3:1
D. 1:1
Answer: A
,Rationale: The genotypic ratio for a cross between two heterozygotes is 1 AA : 2 Aa : 1 aa.
While the phenotypic ratio is 3:1 for dominant to recessive, the underlying genotypes
follow the Mendelian law of segregation. This ratio demonstrates how alleles are
distributed into gametes and recombined during fertilization.
4. Which of the following processes occurs during Prophase I of meiosis but not during
mitosis?
A. Chromatin condensation
B. Spindle fiber formation
C. Nuclear envelope breakdown
D. Synapsis and crossing over
Answer: D
Rationale: Synapsis involves the pairing of homologous chromosomes to form tetrads,
which allows for the exchange of genetic material through crossing over. This process is
unique to meiosis and is a major source of genetic variation in sexually reproducing
organisms. In mitosis, homologous chromosomes do not pair up and instead behave
independently.
5. What is the function of the enzyme DNA Ligase during DNA replication?
A. To add new nucleotides to the 3’ end of the growing strand
B. To synthesize an RNA primer
C. To relieve torsional strain ahead of the replication fork
D. To join Okazaki fragments on the lagging strand
Answer: D
Rationale: DNA Ligase catalyzes the formation of phosphodiester bonds between adjacent
DNA fragments, specifically on the lagging strand. Because DNA polymerase can only
synthesize DNA in a 5’ to 3’ direction, the lagging strand is built in short segments called
Okazaki fragments. Ligase ensures the sugar-phosphate backbone is continuous by sealing
the nicks between these fragments.
6. If a cell has 24 chromosomes at the start of G1, how many sister chromatids will it have
during Metaphase of mitosis?
A. 12
B. 48
C. 24
D. 96
, Answer: B
Rationale: During the S phase, each of the 24 chromosomes is replicated to form two
identical sister chromatids held together at the centromere. By Metaphase, the cell contains
24 chromosomes, each consisting of two chromatids, totaling 48 sister chromatids. These
chromatids will later be separated into individual chromosomes during Anaphase.
7. Mendel’s Law of Independent Assortment is best illustrated by which stage of meiosis?
A. Prophase I
B. Telophase II
C. Anaphase II
D. Metaphase I
Answer: D
Rationale: During Metaphase I, homologous pairs align randomly at the metaphase plate
relative to other pairs. The orientation of one pair does not influence the orientation of
another, which results in different combinations of maternal and paternal chromosomes in
gametes. This random alignment provides the physical basis for independent assortment
observed in genetics.
8. Which component of the RNA polymerase complex is responsible for recognizing the
promoter region in prokaryotes?
A. Alpha subunit
B. Beta subunit
C. Sigma factor
D. Rho factor
Answer: C
Rationale: The sigma factor is a protein needed only for the initiation of transcription in
bacteria, as it enables specific binding of RNA polymerase to gene promoters. Once
transcription begins, the sigma factor is typically released from the core enzyme. This
mechanism ensures that transcription starts at the correct site to produce functional
mRNA.
9. A man with type AB blood marries a woman with type O blood. What are the possible
blood types of their children?
A. AB and O
B. A, B, and O
C. A and B
