ONPS2431 Week 1 Practice Exam - DNA Revision & Central Dogma

Question 1

Given the following DNA template strand sequence, write the corresponding mRNA sequence:

                        Template DNA strand (3'→5'): 3' TAC CCT GTG GAT GCG CCT 5'
                    

mRNA sequence (5'→3'):

Correct Answer

mRNA: 5' AUG GGA CAC CUA ACG GGA 3'

Explanation: mRNA is synthesized complementary and antiparallel to the template DNA strand using base pairing rules (A↔U, T↔A, G↔C, C↔G). The template is read 3'→5' and mRNA is synthesized 5'→3'.

Question 2

Using the genetic code table below, translate the following mRNA sequence into an amino acid sequence:

                        mRNA: 5' AUG GGA CAC CUA ACG GGA UAA 3'
                    

Genetic Code (Codon Table)

AUG → Met (M) START

GGA → Gly (G)

CAC → His (H)

CUA → Leu (L)

ACG → Thr (T)

GGA → Gly (G)

UAA → STOP

Amino acid sequence (use 3-letter or 1-letter codes):

Correct Answer

Amino Acid Sequence: Met-Gly-His-Leu-Thr-Gly (or M-G-H-L-T-G)

Explanation: Translation begins at the START codon (AUG, encoding Met) and proceeds in triplet codons until a STOP codon (UAA, UAG, or UGA) is reached. Each codon specifies one amino acid, joined by peptide bonds.

Question 3

Which of the following is NOT a component of a DNA nucleotide?

A deoxyribose sugar

A phosphate group

A nitrogenous base (A, T, G, or C)

A ribose sugar

Correct Answer: D

Explanation: DNA nucleotides contain deoxyribose sugar, not ribose. Ribose is found in RNA nucleotides. Each DNA nucleotide consists of: (1) a phosphate group, (2) a deoxyribose sugar, and (3) a nitrogenous base.

Question 4

How many hydrogen bonds form between complementary base pairs in DNA?

A-T pairs form 2 hydrogen bonds; G-C pairs form 3 hydrogen bonds

A-T pairs form 3 hydrogen bonds; G-C pairs form 2 hydrogen bonds

All base pairs form exactly 1 hydrogen bond

All base pairs form 4 hydrogen bonds

Correct Answer: A

Explanation: A-T base pairs form 2 hydrogen bonds, while G-C base pairs form 3 hydrogen bonds. This difference in hydrogen bonding affects DNA stability—DNA with more G-C content has a higher melting temperature (Tm).

Question 5

True or False: DNA strands in a double helix are antiparallel, meaning one strand runs 5'→3' while the other runs 3'→5'.

True

False

Correct Answer: True

Explanation: The two DNA strands are antiparallel. One strand has a 5'→3' orientation while the complementary strand has a 3'→5' orientation. This antiparallel arrangement is crucial for proper base pairing and DNA replication.

Question 6

Explain what "semi-conservative replication" means and describe what happens to the original DNA strands during this process:

Your answer:

Model Answer

Definition: Semi-conservative replication is the process by which DNA replicates such that each new DNA molecule contains one original (parental) strand and one newly synthesized strand.

Process: The DNA double helix unwinds, and the two strands separate. Each original strand serves as a template for the synthesis of a complementary new strand using DNA polymerase III. The result is two identical DNA molecules, each consisting of one parental strand and one new strand.

This mechanism was proven by Meselson and Stahl's experiments using nitrogen isotopes.

Question 7

Match each enzyme involved in DNA replication with its primary function:

Helicase

Primase

DNA Polymerase III

DNA Ligase

Correct Matches

Helicase: Unwinds the DNA double helix by breaking hydrogen bonds
Primase: Adds RNA primers to initiate DNA synthesis
DNA Polymerase III: Synthesizes new DNA strands in the 5'→3' direction
DNA Ligase: Joins Okazaki fragments by forming phosphodiester bonds

Question 8

Why is the lagging strand synthesized discontinuously (in fragments) while the leading strand is synthesized continuously?

Because DNA polymerase can only synthesize in the 5'→3' direction, and the lagging strand template runs 5'→3'

Because the lagging strand requires more primers than the leading strand

Because helicase unwinds the DNA faster on the lagging strand

Because the lagging strand contains more G-C base pairs

Correct Answer: A

Explanation: DNA polymerase synthesizes DNA only in the 5'→3' direction. The leading strand template runs 3'→5', allowing continuous 5'→3' synthesis. The lagging strand template runs 5'→3', so synthesis must occur in the opposite direction (toward the replication fork), creating short fragments called Okazaki fragments that are later joined by ligase.

Question 9

What are Okazaki fragments and in which direction are they synthesized?

Short DNA segments synthesized on the lagging strand; synthesized 5'→3' away from the replication fork

Short DNA segments synthesized on the leading strand; synthesized 3'→5' toward the replication fork

RNA primers used to start DNA synthesis

Regions where DNA polymerase stalls during replication

Correct Answer: A

Explanation: Okazaki fragments are short DNA segments (about 1000-2000 nucleotides in prokaryotes, 100-200 in eukaryotes) synthesized on the lagging strand. They are synthesized 5'→3' away from the replication fork. Each fragment begins with an RNA primer (added by primase) and is later joined to adjacent fragments by DNA ligase.

Question 10

Order the major steps of transcription from first to last:

Step 1:

Step 2:

Step 3:

Correct Order

Step 1: RNA polymerase recognizes and binds to the promoter
Step 2: RNA polymerase synthesizes mRNA in the 5'→3' direction
Step 3: RNA polymerase terminates at a stop signal and releases mRNA

Explanation: Transcription proceeds through initiation (promoter recognition), elongation (mRNA synthesis), and termination (release of completed mRNA). The template DNA strand is read 3'→5' to produce mRNA 5'→3'.

Question 11

Which of the following is a difference between DNA and RNA?

DNA contains uracil; RNA contains thymine

DNA contains ribose; RNA contains deoxyribose

DNA contains thymine; RNA contains uracil

DNA is typically single-stranded; RNA is typically double-stranded

Correct Answer: C

Explanation: DNA contains the base thymine (T) while RNA contains uracil (U). Additionally, DNA contains deoxyribose sugar while RNA contains ribose sugar. DNA is typically double-stranded, while RNA is typically single-stranded.

Question 12

True or False: The promoter is a region of DNA where mRNA synthesis begins and RNA polymerase binds to initiate transcription.

True

False

Correct Answer: True

Explanation: The promoter is a specific DNA sequence located upstream of a gene where RNA polymerase binds to initiate transcription. In prokaryotes, the promoter contains consensus sequences like the -10 box (TATAAT) and -35 box. In eukaryotes, common promoter elements include the TATA box and CAAT box.

Question 13

Which of the following is true about start and stop codons in translation?

The start codon is AUG and codes for methionine; stop codons are UAA, UAG, and UGA

The start codon is AUG and codes for alanine; stop codons are AAU, AAG, and AAA

Start codons are UAA, UAG, and UGA; stop codon is AUG

There are multiple start codons; there is only one stop codon (UAA)

Correct Answer: A

Explanation: The start codon is AUG, which codes for methionine (Met). This codon signals the ribosome to begin translation. Stop codons are UAA, UAG, and UGA, which do not code for amino acids but signal the ribosome to terminate translation and release the completed polypeptide chain.

Question 14

Match the ribosomal sites with their function during translation:

A-site

P-site

E-site

Correct Matches

A-site (Aminoacyl-tRNA site): Where the incoming aminoacyl-tRNA initially binds
P-site (Peptidyl-tRNA site): Where the peptidyl-tRNA holds the growing chain
E-site (Exit site): Where deacylated tRNA exits after releasing its amino acid

Question 15

The Central Dogma of molecular biology describes the flow of genetic information. Arrange these in the correct order:

Step 1:

Step 2:

Step 3:

Correct Order

Step 1: DNA (Replication and template for transcription)
Step 2: RNA (mRNA produced by transcription)
Step 3: Protein (Polypeptide synthesized by translation)

Central Dogma: DNA → RNA → Protein. This represents the unidirectional flow of genetic information from DNA (containing the genes) to RNA (messenger that carries instructions) to Protein (functional molecules).

Question 16

During transcription, which DNA strand serves as the template for mRNA synthesis?

The coding strand (same sequence as mRNA, except T instead of U)

The template strand (complementary to mRNA, read 3'→5')

Both strands equally, with one on each side of the mRNA

The strand that contains the most A-T base pairs

Correct Answer: B

Explanation: Only the template strand (also called antisense strand) is used as the template for mRNA synthesis. RNA polymerase reads this strand in the 3'→5' direction and synthesizes the complementary mRNA strand in the 5'→3' direction. The other strand (coding strand) has the same sequence as the mRNA, except with thymine instead of uracil.

Question 17

If a codon on mRNA is 5' GUA 3', what is the corresponding anticodon on the tRNA?

5' GUA 3'

3' CAT 5'

3' CAU 5'

5' CUA 3'

Correct Answer: C

Explanation: The anticodon on tRNA pairs with the codon on mRNA in an antiparallel fashion. The mRNA codon GUA (5'→3') pairs with the tRNA anticodon CAU (3'→5'). Base pairing follows complementary rules: G-C, U-A. The anticodon is written in the opposite direction because the two strands are antiparallel.

Question 18

In a DNA double helix, the major and minor grooves are formed between:

The sugar-phosphate backbones on opposite sides of the helix

The nitrogenous bases and the deoxyribose sugars

Adjacent phosphate groups on the same strand

The hydrogen bonds between complementary bases

Correct Answer: A

Explanation: The major and minor grooves are structural features of the DNA double helix formed between the two sugar-phosphate backbones. The wider major groove and narrower minor groove are formed as the two strands wind around each other. These grooves are important for protein-DNA interactions and are frequently recognized by regulatory proteins.

Question 19

True or False: Peptide bonds form between the amino group of one amino acid and the carboxyl group of an adjacent amino acid during translation.

True

False

Correct Answer: True

Explanation: Peptide bonds are covalent bonds that link adjacent amino acids in a protein chain. They form between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of the next amino acid, releasing a water molecule in the process (a condensation reaction). This occurs on the ribosome during translation.

Question 20

The genetic code is described as "degenerate." What does this mean?

Your answer:

Model Answer

Definition: The genetic code is degenerate because multiple codons can code for the same amino acid. There are 64 possible codons but only 20 amino acids (plus stop signals).

Example: The amino acid leucine is coded by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG.

Significance: This degeneracy provides a buffer against mutations, as some mutations (called "silent" or "synonymous" mutations) do not change the amino acid and therefore do not affect the protein's function. This is why the genetic code is also described as "redundant."