2: DNA Structure

DNA composition, base pairing, nucleotide components, and double-helix complementarity.

LibreTexts reference: DNA Structure and Sequences

Determining True/False Statements About DNA Structure

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Which one of the following statements is TRUE concerning the structure of DNA?

• the two strands of DNA are anti-parallel
• pyrimidine bases pair up with like pyrimidine bases
• the backbone contains only nucleobase and sugar
• the phosphate groups are on the inside of the DNA double helix
• A-T base pairs have three hydrogen bonds, whereas G-C base pairs only have two hydrogen bonds

Check Answer

 

Determining True/False Statements About Nucleotide Components

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Which one of the following is NOT one of the three (3) major components of a DNA or RNA nucleotide?

• phosphate group
• nitrogenous base
• amino acid
• deoxyribose

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Identifying Chargaff's Base Pairing Composition

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According to Chargaff's rules concerning the base pairing composition in double-stranded DNA, consider a sample where the percentage composition of 4% is guanine.
What are the percentages of the other three bases?

• adenine (A): 46% — cytosine (C): 25% — thymine (T): 25%
• adenine (A): 46% — cytosine (C): 46% — thymine (T): 4%
• adenine (A): 4% — cytosine (C): 46% — thymine (T): 46%
• adenine (A): 46% — cytosine (C): 4% — thymine (T): 46%
• adenine (A): 25% — cytosine (C): 25% — thymine (T): 46%

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Identifying Complementary DNA Sequences (Directionless)

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Click to show Identifying Complementary DNA Sequences (Directionless) example problem

DNA is organized as a double helix where two strands run in opposite, or anti-parallel, directions. The ends of these strands are referred to as 5′ (five prime) and 3′ (three prime) ends. The strands are held together by hydrogen bonds between complementary base pairs. Specifically, adenine (A) forms a pair with thymine (T), and guanine (G) pairs with cytosine (C). This base pairing is critical for the stability of the DNA molecule and is central to processes like DNA replication and transcription.

 

G

A

C

,

A

T

C

,

G

C

T

Which one of the following DNA sequences is complementary to the direction-less DNA sequence shown above?

•  

  T

  G

  T

  ,

  C

  A

  G

  ,

  C

  G

  A

•  

  T

  C

  G

  ,

  C

  T

  A

  ,

  C

  A

  G

•  

  C

  T

  G

  ,

  T

  T

  C

  ,

  G

  C

  T

•  

  G

  A

  C

  ,

  A

  T

  C

  ,

  G

  C

  T

•  

  C

  T

  G

  ,

  T

  A

  G

  ,

  C

  G

  A

Check Answer

 

Identifying 5' and 3' Complementary DNA Sequences

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Click to show Identifying 5' and 3' Complementary DNA Sequences example problem

DNA is organized as a double helix where two strands run in opposite, or anti-parallel, directions. The ends of these strands are referred to as 5′ (five prime) and 3′ (three prime) ends. The strands are held together by hydrogen bonds between complementary base pairs. Specifically, adenine (A) forms a pair with thymine (T), and guanine (G) pairs with cytosine (C). This base pairing is critical for the stability of the DNA molecule and is central to processes like DNA replication and transcription.

 

3′–

T

C

A

,

A

G

T

,

G

A

C

–5′

Which one of the following sequences below is complementary to the DNA sequence shown above?

Hint: pay close attention to the 5′ and 3′ directions of the strand.

•  

  3′–

  T

  C

  A

  ,

  A

  G

  T

  ,

  G

  A

  C

  –5′

•  

  3′–

  G

  T

  C

  ,

  A

  C

  T

  ,

  T

  G

  A

  –5′

•  

  3′–

  A

  G

  T

  ,

  T

  C

  A

  ,

  C

  T

  G

  –5′

•  

  3′–

  C

  A

  G

  ,

  T

  C

  T

  ,

  T

  G

  A

  –5′

•  

  3′–

  C

  A

  G

  ,

  T

  G

  A

  ,

  A

  C

  T

  –5′

Check Answer