Molecular Biology: DNA Structure and History

Comprehensive practice questions covering the historical discovery of DNA, its chemical components, structural variations (A, B, and Z forms), physical properties such as melting temperature and density, and genetic concepts like the C-value paradox.

In which year and by whom was the phosphorus-bearing substance 'Nuclein' first isolated from white blood cell nuclei?

In $1869$ by Miescher.

Who characterized the basic components of DNA and RNA at the end of the 19th and early 20th centuries?

Levene, Jacobs, et al.

What organism was used by Frederick Griffith in his $1928$ transformation experiments?

Streptococcus pneumoniae.

What was the key result of Frederick Griffith's experiments regarding the transformation of bacteria?

Heat-killed virulent bacteria could transform avirulent bacteria into virulent bacteria.

In the $1944$ Avery, Macleod, and McCarty experiment, which molecule was identified as the chemical agent of transformation?

DNA.

Which experiment in $1952$ provided evidence that bacteriophage infection comes from DNA, further confirming it as the genetic material?

Hershey and Chase.

In which year did Watson and Crick publish the double-helical model of DNA structure?

$1953$.

What are the three core components of a DNA nucleotide?

Nitrogenous bases (Adenine, Cytosine, Guanine, Thymine), Phosphoric acid, and Deoxyribose sugar.

How do the nitrogenous bases of RNA differ from those of DNA?

In RNA, Uracil (U) replaces Thymine (T).

What is the structural difference between a nucleotide and a nucleoside?

Nucleotides contain phosphoric acid, whereas nucleosides lack the phosphoric acid.

Which specific chemical group is missing in Deoxyribose compared to Ribose?

A hydroxyl group (OH) at the $2$-position.

Which nitrogenous bases are classified as purines?

Adenine (A) and Guanine (G).

Which nitrogenous bases are classified as pyrimidines?

Cytosine (C), Thymine (T), and Uracil (U).

By what type of bond are nucleotides linked in a DNA chain?

Phosphodiester bond.

What defines the $5'$ end and the $3'$ end of a trinucleotide?

The $5'$ end has a free $5'$-phosphate group, and the $3'$ end has a free $3'$-hydroxyl group.

Whose X-ray diffraction data suggested that DNA has a helical shape and a regular, repeating structure?

Rosalind Franklin.

What are Chargaff's rules regarding base pairing in DNA?

$A=T$ and $G=C$, meaning the amount of Adenine roughly equals Thymine, and Guanine roughly equals Cytosine.

How many base pairs are typically found per turn in the DNA double helix?

$10$ base pairs.

Which structural form of DNA exists at high humidity ($92\%$)?

B-form.

In what form is DNA found when the relative humidity is reduced to $75\%$ or when a DNA strand is hybridized with an RNA strand?

A-form.

What is the Z-form of DNA?

A left-handed helical DNA identifying in $1979$ that is required for the activation of at least one gene.

What is the range for total percentage of $G+C$ content across different organisms?

$22\%$ to $73\%$.

What is defined as the temperature at which DNA strands are $\frac{1}{2}$ denatured?

Melting temperature or $T_m$.

Why does higher GC content result in a higher melting temperature ($T_m$)?

GC pairs form three hydrogen bonds, whereas AT pairs form only two.

Which method is used to measure DNA density?

Density gradient centrifugation in $CsCl$ solution.

Besides heat, what are three other methods used to denature DNA?

Organic solvents (DMS and formamide), High pH, and Lowering salt concentration.

What is the best temperature for the process of annealing or renaturation?

About $250\,C$ below $T_m$.

What is the molecular weight of $1$ DNA base pair?

$660$.

What is the length per helical turn of $10.4$ base pairs?

$33.2\,Å$.

What are the common shapes in which DNA can appear across different genomes?

Circular (typically phage DNA), Linear, or Supercoiled.

Calculate the number of base pairs required to encode an average protein of $40,000\,D$, assuming the average amino acid mass is $110\,D$.

$1092$ base pairs ($40,000 / 110 = 364$ amino acids; $364 \times 3 = 1092$).

What is the C-value?

The DNA content per haploid cell.

What is the observation that more complex organisms do not always have more genes or more DNA than simple organisms?

C-value paradox.