Q: What is the probability of the sequence 'ACGT' occurring at a specific site in a genome, assuming each nucleotide has a probability of 0.25?

$$0.25 \times 0.25 \times 0.25 \times 0.25 = 0.0039$$

Q: Phage λ has a genome size of 48,500 bp. Using the probability calculated for 'ACGT', how many times is this sequence expected to occur?

$$0.0039 \times 48,500 = 189.5$$

Q: A genome has 100,000 bp. What is the expected number of times a 5-nucleotide sequence occurs, assuming each nucleotide is equally likely and independent?

$$0.25^5$$ \times 100,000 = 97.7$$

Question 1

A 600 bp linear DNA is cut with EcoRI, producing two fragments: one 200 bp and one 400 bp. When cut with BamHI, a 100 bp fragment is produced. If both EcoRI and BamHI are used, the 400 bp EcoRI fragment is replaced by two fragments, one of which is 100 bp. Where is the BamHI site located?

Accepted Answer

500 bp from the left end

Question 2

If a circular DNA molecule is cut with EcoRI at a single site, what would you expect to see on an agarose gel?

Accepted Answer

A single band at 600 bp

Question 3

In a promoter deletion experiment, removal of segment A does not change reporter gene expression. What does this suggest about segment A?

Accepted Answer

Segment A does not contain a functional regulatory element

Question 4

Removal of segment B from a promoter decreases reporter gene activity from 100 units to 20 units. What does this indicate about segment B?

Accepted Answer

Segment B contains a positive regulatory element

Question 5

If segment C is removed from a promoter and reporter gene activity increases from 20 units to 30 units, what is the most likely function of segment C?

Accepted Answer

Segment C contains a negative regulatory element

Question 6

Based on the promoter mapping experiment, which segment(s) contain functional elements and what is their effect?

Accepted Answer

Segment B (positive effect), Segment C (negative effect)

Question 7

What is the probability of the sequence 'ACGT' occurring at a specific site in a genome, assuming each nucleotide has a probability of 0.25?

Accepted Answer

$$0.25 	imes 0.25 	imes 0.25 	imes 0.25 = 0.0039$$

Question 8

Phage λ has a genome size of 48,500 bp. Using the probability calculated for 'ACGT', how many times is this sequence expected to occur?

Accepted Answer

$$0.0039 	imes 48,500 = 189.5$$

Question 9

How does the length of a specific DNA sequence affect its probability of occurring, assuming each nucleotide is equally likely?

Accepted Answer

The probability decreases by a factor of 0.25 for each additional nucleotide

Question 10

A DNA fragment is cut with EcoRI and BamHI. The EcoRI site is 200 bp from the left end, and the BamHI site is 500 bp from the left end. What are the sizes of the fragments produced when both enzymes are used?

Accepted Answer

200 bp, 300 bp, and 100 bp

Question 11

If a restriction enzyme cuts a linear DNA at two sites, what is the minimum number of fragments produced?

Accepted Answer

Three

Question 12

A student claims that removing segment B from a promoter will always decrease gene expression. What experimental evidence would contradict this claim?

Accepted Answer

Gene expression increases after segment B removal

Question 13

If the probability of a nucleotide at each position is 0.25, what is the probability of a 6-nucleotide sequence occurring at a specific site?

Accepted Answer

$$0.25^6 = 0.000244$

Question 14

Which scenario best illustrates a negative regulatory element in a promoter region?

Accepted Answer

Removal of a segment increases gene expression

Question 15

A DNA sequence is analyzed for restriction sites. If a site is 100 bp from the right end and another is 200 bp from the left end, what is the distance between the two sites?

Accepted Answer

300 bp

Question 16

If a DNA fragment is circular and cut at two sites, how many fragments will be produced?

Accepted Answer

Two

Question 17

Which of the following best describes the effect of removing a positive regulatory element from a promoter?

Accepted Answer

Gene expression decreases

Question 18

A genome has 100,000 bp. What is the expected number of times a 5-nucleotide sequence occurs, assuming each nucleotide is equally likely and independent?

Accepted Answer

$$0.25^5$$ 	imes 100,000 = 97.7$$

Practice Exam Guidance: DNA Mapping, Promoter Analysis, and Probability in Genomics

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