Molecular Genetics DNA Structure And Replication Quiz

Question 1

Nucleotide Structure:

Answer 1

Each nucleotide consists of:
- A phosphate group
- A deoxyribose sugar
- A nitrogenous base (adenine, thymine, cytosine, guanine)

Question 2

Pyrimidines and Purines:

Answer 2

Pyrimidines: Cytosine (C) and Thymine (T) (one-ring structure)

Purines: Adenine (A) and Guanine (G) (two-ring structure)

Question 3

Complementary Base Pairing:

Answer 3

A pairs with T (two hydrogen bonds)
C pairs with G (three hydrogen bonds)

Question 4

Chargaff’s Rule:

Answer 4

States that in any given DNA molecule, the amount of adenine equals thymine, and the amount of cytosine equals guanine (A=T, C=G).

Question 5

5’ and 3’ Ends: IS this enough

Answer 5

The DNA strand has a directionality:

5’ end: Phosphate group attached to the 5’ carbon of the sugar.

3’ end: Hydroxyl group (-OH) attached to the 3’ carbon of the sugar.

The two strands of DNA run in opposite directions (anti-parallel).

Question 6

Anti-Parallel Double Helix Structure:

Answer 6

DNA Structure
Double helix structure (developed by Watson and Crick)

Anti-parallel strands that twist together

2 complementary strands of
nucleotides held together by
hydrogen bonds between C-G and A-T base pairs

Phosphate and sugar
backbone

Question 7

Enzymes and Their Functions:

Answer 7

DNA helicase - unwinds and separates double stranded DNA as it moves along the DNA. It forms the replication fork by breaking hydrogen bonds between nucleotide pairs in DNA.

DNA primase - a type of RNA polymerase that generates RNA primers. Primers are short RNA molecules that act as templates for the starting point of DNA replication.

Topoisomerase or DNA Gyrase - relieves tension in DNA strands to prevent the DNA from becoming tangled or supercoiled. (Stabilizes DNA Helix)

DNA polymerases - synthesize new DNA molecules by adding nucleotides to leading and lagging DNA strands.

-DPI → Replaces RNA primers with DNA nucleotides and proofreads and corrects typos in the DNA

-DPIII → adds DNA nucleotides in the 5’ to 3’ direction

DNA ligase - joins DNA fragments together by forming phosphodiester bonds between nucleotides. (Joins Okazaki fragments on lagging stan)

Question 8

Stages of DNA Replication

Answer 8

• Initiation
  -Elongation
  -Termination
  -Proofreading and Repair

Question 9

1. Intiation

Answer 9

Unwinding the DNA: The process begins at specific locations called origins of replication. The enzyme helicase unwinds the double helix, creating a replication fork. Single-stranded binding proteins (SSBPs) stabilize the unwound DNA strands.

Relieving Tension: As DNA unwinds, topoisomerase alleviates the tension ahead of the fork, preventing supercoiling.

Primer Synthesis: The enzyme primase synthesizes short RNA primers complementary to the template strands, providing starting points for DNA synthesis.

Question 10

2. Elongation

Answer 10

Nucleotide Addition: DNA polymerase III attaches to the RNA primer and begins adding nucleotides in the 5’ to 3’ direction. On the leading strand, this occurs continuously.

Lagging Strand Synthesis: On the lagging strand, DNA is synthesized in short segments called Okazaki fragments. Each fragment is initiated by a new RNA primer.

Primer Replacement: Once DNA synthesis is complete, DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides.

Sealing Gaps: The enzyme ligase then seals the gaps between Okazaki fragments (using phosphodiester bonds), ensuring a continuous DNA strand.

Question 11

3. Termination

Answer 11

-Occurs upon completion of the 2 new strands of DNA are synthesized, 2 double strand DNA molecules are produced that will recoil
into a helix.

• replication machine is dismantled

Question 12

Meselson-Stahl Experiment:

Answer 12

The Meselson-Stahl experiment demonstrated that DNA replication is semi-conservative.

Setup: E. coli bacteria were grown in a medium containing the heavy nitrogen isotope N-15, labeling their DNA.

Transfer: The bacteria were then moved to a medium with the lighter isotope N-14.

Sampling: After one and two rounds of replication, samples of DNA were taken.

Analysis: The DNA was separated by density using centrifugation.

Results:
- After one replication, the DNA had an intermediate density (one old strand and one new).
- After two replications, there were both light DNA (two N-14 strands) and intermediate DNA (one N-15 and one N-14 strand).

This proved that the Semi-conservative model was correct

Question 13

Replication Models

Answer 13

1) Conservative: The original DNA molecule remains intact, and a completely new molecule is synthesized.

2) Semi-conservative: the original molecule is split in half, and the other side is filled-in

3) Dispersive: Each new molecule is comprised of bits and pieces of both new DNA and the original strand

Question 14

Telomeres

Answer 14

Telomeres: Repetitive sequences at the ends of chromosomes that protect them from degradation and prevent loss of genetic information during replication.

Telomeres are repeating sequences of nucleotides

• If chromosomes were not capped by telomeres, a small portion of a gene near the end of the chromosome could be lost every time DNA replication occurred
• Instead, only portions of telomeres are lost

Question 15

Telomerase

Answer 15

Telomerase: An enzyme that extends telomeres, particularly active in germ cells and some stem cells, helping maintain chromosome integrity.

Germ line cells are unique because they must be able to continue replicating-

• Must maintain genetic integrity from parent to offspring

-Enzyme called telomerase adds more DNA to shortening telomeres, restoring their length

-Stem cells and some white blood cells also show presence of telomerase