Question 1

Match the missing words in the following phrase. Darwin's principle of 'descent with modification' describes the balance between fitness and [A]. Natural [B] are acted on by [C], favoring individuals with the best [D] for that particular environment.

Accepted Answer

a. A = diversity
b. B = variations
c. C = selection 
d. D = fitness

Question 2

The Meselson-Stahl experiment used DNA containing ¹⁴N and ¹⁵N to show that DNA replication is semi-conservative. If replication had been conservative (i.e., one double helix formed of 'old' DNA, and one completely new double helix), what would have been the expected outcome of the experiment after one round of replication?

Accepted Answer

In a conservative model of DNA replication, after one round of replication, one would expect to see two distinct populations of DNA molecules: one with the original density (¹⁴N) and one with the new density (¹⁵N), with no intermediate densities.

Question 3

What holds the two strands of the DNA double helix together?

Accepted Answer

Hydrogen bonds hold the two strands of the DNA double helix together. Specifically, hydrogen bonds form between complementary base pairs on the opposite strands - for example, between an adenine (A) base on one strand and a thymine (T) base on the other strand.

Question 4

Using the following DNA as template, the sequence of the newly-synthesized strand is 5'______________3'
Template DNA: 5'ACGTAGCACATGACT3'

Accepted Answer

The answer of Using the following DNA as template, the...

Question 5

Which enzyme separates the strands of the DNA double helix during replication?

Accepted Answer

DNA helicase is responsible for unwinding the double-stranded DNA molecule by breaking the hydrogen bonds between the complementary base pairs. This creates a replication fork, which allows DNA polymerase to synthesize new strands of DNA.

Question 6

Match the missing words in the following phrase. Single nucleotides are added to a growing nucleotide chain during DNA synthesis. A covalent [A] bond is formed between the [B] carbon on [C] of the nucleotide at the end of a DNA chain and [D] on the [E] carbon of an incoming nucleotide.

Accepted Answer

The answer of Match the missing words in the following...

Question 7

Which of the statements about replication and replication origins in eukaryotes and bacteria are true? Please select all that apply.

Accepted Answer

Replication origins tend to be A-T rich in both bacteria and eukaryotes. A-T base pairs have two hydrogen bonds, as opposed to C-G pairs, which have three bonds. This means it is easier to separate the strands of the double helix in A-T rich regions than in regions with higher G-C content. Bacterial chromosomes are small, and only a single origin of replication is needed. Eukaryotic chromosomes are much larger, and there would not be enough time to complete replication if there were only one origin. Bacterial replication tends to be faster than eukaryotic replication (~45,000 bp/min vs. 2000-3000 bp/min).

Question 8

Which of these are found at a replication fork? Please select all that apply.

Accepted Answer

All the options are found at a replication fork.

Question 9

What is the function of primase?

Accepted Answer

Primase is an enzyme responsible for synthesizing short RNA primers during DNA replication. These primers are essential for the subsequent action of DNA polymerase, which extends the primers by adding nucleotides to form new DNA strands. Primase plays a crucial role in ensuring the accuracy and efficiency of DNA replication.

Question 10

During DNA replication, the leading strand of the separated double helix is synthesized using continuous replication. The lagging strand is synthesized using discontinuous replication.

Accepted Answer

DNA synthesis proceeds 5' to 3' along the single stranded DNA. The leading strand is in the correct orientation for continuous synthesis to be possible. The lagging strand is in the wrong orientation for this to be possible, so short pieces of DNA are synthesized, then DNA polymerase releases, reattaches, then synthesizes the next section - this is discontinuous replication.

Question 11

Where is the next nucleotide added to the growing chain during DNA replication?

Accepted Answer

The next nucleotide is added to the 3' hydroxyl group of the previous nucleotide, so choice A is correct. Choice B is incorrect because a phosphate group is not involved in forming the bond between nucleotides during DNA replication. Choices C and D are incorrect because they refer to the opposite end of the nucleotide and are not involved in forming the phosphodiester bonds that connect nucleotides in a growing DNA chain.

Question 12

What are Okasaki fragments?

Accepted Answer

Okasaki fragments are short fragments of DNA that are created on the lagging strand during DNA replication in cells. These fragments are produced because the lagging strand can only be replicated in small fragments, which are then later joined together by DNA ligase.

Question 13

Match the missing words in the following phrase. [A] relieves supercoils ahead of the replication fork, then [B] unwinds the double helix. [C] synthesizes DNA, and [D] joins discontinuous pieces of DNA together.

Accepted Answer

The answer of Match the missing words in the following...

Question 14

The leading strand is more severely affected by the end-replication problem than the lagging strand.

Accepted Answer

The lagging strand is more severely affected. DNA replication on the leading strand can continue to the end of the linear chromosome molecule in the 3' to 3' direction. After removal of the RNA primer at the end of the lagging strand, a gap remains that cannot be filled by normal DNA synthesis primed from RNA.

Question 15

What do reverse transcriptases do?

Accepted Answer

Reverse transcriptases are enzymes that can copy RNA into DNA. This process is called reverse transcription and is important in the replication of retroviruses, such as HIV. Reverse transcriptases are also used in laboratory techniques, such as gene cloning and gene expression analysis.

Question 16

Which of the statements about replication error and repair during DNA synthesis are true? Please select all that apply.

Accepted Answer

Bases can exist as tautomers, which increases the likelihood that they will pair with the incorrect base, i.e., non-Watson-Crick base pairing. The replication error rate is low, at 0.0001-0.001% of bases, but this error rate is lowered approximately 1000-fold with the addition of proofreading. DNA polymerases with proofreading activity can remove the last nucleotide at the 3' end of the DNA strand - this is 3' exonuclease activity (endonucleases are enzymes that cut DNA in the middle of the strand)

Question 17

How do bacteria determine which is the newly-synthesized DNA strand during mismatch repair?

Accepted Answer

The new DNA strand is not methylated immediately after synthesis, allowing the mismatch repair system to distinguish it from the old, methylated strand. This difference is crucial for the repair system to correct errors on the newly synthesized strand.

Question 18

Match the DNA alteration to its description.
-pyrimidine replacement with a purine

Accepted Answer

Transversion is the substitution of a purine for a pyrimidine or vice versa, which can lead to changes in the amino acid sequence of a protein.

Question 19

Match the DNA alteration to its description.
-sequence change involving several bases

Accepted Answer

An indel is a sequence change involving several bases, either the insertion or deletion of one or more nucleotides. The other options refer to single nucleotide changes.

Question 20

Match the DNA alteration to its description.
-purine replacement with a purine

Accepted Answer

A purine replacement with another purine is a specific type of substitution called a transition.

Quiz 4: Descent With Modification: Dna Replication and Mutation