DNA Double Helix: Videos & Practice Problems

The DNA double helix, discovered by Franklin, Watson, and Crick, consists of two antiparallel strands of nucleotides forming a twisted ladder structure. Each nucleotide comprises a phosphate group, a pentose sugar, and a nitrogenous base. The double helix has a width of 20 angstroms and typically contains 10 base pairs per turn, with a total length of 34 angstroms. The major groove (22 angstroms) and minor groove (12 angstroms) allow protein binding, crucial for regulating RNA transcription and DNA replication.

DNA Double Helix Concept 1

The structure of DNA, famously described by researchers Franklin, Watson, and Crick, is characterized as a double helix composed of two antiparallel strands of nucleotides. The term "antiparallel" indicates that while the strands run parallel to each other, they do so in opposite directions. This double helix can be visualized as a twisted ladder, where the phosphate-sugar backbone forms the sides and the nitrogenous bases are situated in the center.

A nucleotide, the fundamental building block of DNA, consists of three components: a phosphate group, a pentose sugar, and a nitrogenous base. The phosphate group is attached to the 5' carbon of the sugar, which is in turn connected to the 4' carbon of the pentose ring. The primary structure of DNA is formed by linking nucleotides together through phosphodiester bonds, creating a strand that runs from the 5' end to the 3' end. The antiparallel nature of the strands means that the 3' end of one strand aligns with the 5' end of the other strand, facilitating complementary base pairing.

In the double helix, nitrogenous bases pair through hydrogen bonds, forming the rungs of the twisted ladder. The sugar-phosphate backbones, represented in blue, provide structural support, while the bases, located in the interior, ensure the genetic information is preserved and transmitted. To visualize the twisting of the DNA, one can imagine rotating the top part of the ladder to the left and the bottom part to the right, resulting in the characteristic helical shape.

Understanding these concepts—nucleotides, phosphodiester linkages, primary structure, and complementary base pairing—is essential for grasping the overall structure and function of DNA. The double helix not only serves as a model for genetic information storage but also highlights the intricate relationships between its components, ultimately contributing to the complexity of biological systems.

DNA Double Helix Example 1

In DNA structure, understanding the concept of complementary base pairing is crucial. Each strand of DNA runs in opposite directions, a characteristic known as antiparallel orientation. This means that if one strand has a 5' end at the top and a 3' end at the bottom, the complementary strand will have its 3' end aligned with the 5' end of the first strand and vice versa.

To determine the sequence of bases in the complementary DNA strand, one must remember the specific base pairings: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). For example, if the original strand has a sequence of bases such as C, G, A, A, T, C, T, the complementary strand would be G, C, T, T, A, G, A, respectively. This pairing is facilitated by hydrogen bonds, where A and T form two hydrogen bonds, while G and C form three hydrogen bonds, contributing to the stability of the DNA structure.

In summary, when predicting the sequence of bases in a complementary DNA strand, it is essential to consider both the antiparallel nature of the strands and the specific base pairings. This foundational knowledge is vital for understanding DNA replication and the overall structure of genetic material.

Which of the following statements is false?

• A. The nitrogenous bases extend inward within the double helix. 
• B. The two polynucleotide strands in the DNA double helix are identical to one another. 
• C. Hydrogen bonding occurs between the two polynucleotide strands within DNA.
• D. The complementary strand of DNA segment of 5’ GCATTCAT 3’ would be 3’ CGTAAGTA 5’. 
• E. Phosphate groups and sugars form the backbone of the DNA double helix.

Answer: (B)

DNA Double Helix Concept 2

The DNA double helix is a fundamental structure in genetics, characterized by its specific dimensions and features. The width of the DNA double helix measures approximately 20 angstroms (Å), where an angstrom is a unit of length equal to 10^-10 meters. Each complete turn of the helix typically consists of 10 base pairs and spans a length of about 34 angstroms. This structural organization is crucial for the stability and functionality of DNA.

Within the helical structure, two distinct grooves are formed: the major groove and the minor groove. The major groove is the wider and deeper of the two, measuring around 22 angstroms in length, while the minor groove is narrower and shallower, accounting for the remaining 12 angstroms. These grooves play a significant role in molecular interactions, as proteins can bind to them to regulate essential biological processes such as RNA transcription and DNA replication.

Understanding the dimensions and characteristics of the DNA double helix, including the specific measurements of the major and minor grooves, is vital for comprehending how genetic information is accessed and manipulated within living organisms. This knowledge is particularly relevant in the context of drug design and therapeutic interventions that target these grooves to influence genetic activity.

DNA Double Helix Example 2

To calculate the approximate length of a DNA segment composed of 171 base pairs, we start by recognizing the relationship between base pairs and the physical structure of DNA. Each complete turn of the DNA helix contains approximately 10 base pairs. Therefore, to find the number of turns in 171 base pairs, we can use the formula:

Number of turns = \(\frac{171 \text{ base pairs}}{10 \text{ base pairs/turn}} = 17.1 \text{ turns}\)

Next, we need to convert the number of turns into a length measurement. Each turn of the DNA helix is approximately 34 angstroms in length. Thus, we can calculate the total length of the DNA segment using the following equation:

Length = Number of turns × Length per turn

Substituting the values we have:

Length = \(17.1 \text{ turns} \times 34 \text{ angstroms/turn} = 581.4 \text{ angstroms}\)

Therefore, the approximate length of a DNA segment with 171 base pairs is about 581 angstroms. This calculation illustrates the connection between the molecular structure of DNA and its physical dimensions, providing insight into the scale at which genetic information is organized.

Clinical trials include the use of DNA-binding drugs that facilitate the breaking of double-stranded DNA in their function. A hypothetical drug used in the treatment of leukemia requires interaction with the grooves of DNA. If the minor groove is A–T rich and the major groove is G–C rich, which of the following statements can be true?

• A. The drug would more readily bind to the minor groove of DNA. 
• B. The drug would more readily bind to the major groove of DNA. 
• C. The drug would equally bind to the minor and major grooves of DNA. 
• D. Not enough information is given. 

Answer: (A)

One turn of a DNA sequence is comprised of 30% guanine (G). Calculate the approximate number of hydrogens bonds likely to be found within this DNA sequence.  

• A. 15
• B. 26
• C. 38
• D. 24

Answer: (B)

One turn of a DNA sequence is comprised of 15% adenine (A). If the bonding energies of A–T and G–C are 24 kJ/mol and 18 kJ/mol respectively, calculate the approximate total bonding energy in this one turn. 

• A. 110 kJ/mol
• B. 432 kJ/mol
• C. 222 kJ/mol
• D. 198 kJ/mol

Answer: (D)

Here’s what students ask on this topic:

What is the structure of the DNA double helix?

The DNA double helix consists of two antiparallel strands of nucleotides forming a twisted ladder structure. Each nucleotide is composed of a phosphate group, a pentose sugar, and a nitrogenous base. The strands run in opposite directions, with the 5' end of one strand aligning with the 3' end of the other. The sugar-phosphate backbones form the sides of the ladder, while the nitrogenous bases pair in the center through hydrogen bonds. This structure was discovered by Franklin, Watson, and Crick.

What are the dimensions of the DNA double helix?

The DNA double helix has a width of 20 angstroms. Each full turn of the helix, which includes 10 base pairs, measures approximately 34 angstroms in length. The helical structure creates two grooves: the major groove, which is 22 angstroms wide, and the minor groove, which is 12 angstroms wide. These grooves are essential for protein binding, which regulates RNA transcription and DNA replication.

How do the major and minor grooves of the DNA double helix function?

The major and minor grooves of the DNA double helix are critical for protein binding. The major groove is wider and deeper, measuring about 22 angstroms, while the minor groove is narrower and shallower, measuring about 12 angstroms. Proteins that regulate RNA transcription and DNA replication often bind to these grooves, influencing genetic processes. The specific shape and size of these grooves allow for selective binding of regulatory proteins.

What is the significance of antiparallel strands in the DNA double helix?

The antiparallel nature of the DNA double helix means that the two strands run in opposite directions, with one strand oriented 5' to 3' and the other 3' to 5'. This arrangement is crucial for the complementary base pairing, where adenine pairs with thymine and cytosine pairs with guanine through hydrogen bonds. The antiparallel orientation also facilitates the replication process, as enzymes can read and synthesize the DNA strands in the correct direction.

Who discovered the structure of the DNA double helix?

The structure of the DNA double helix was discovered by Rosalind Franklin, James Watson, and Francis Crick. Franklin's X-ray diffraction images provided critical insights into the helical structure, while Watson and Crick built the first accurate model of DNA, describing it as a double helix with antiparallel strands and complementary base pairing.