2: DNA Structure

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

Statements about DNA Structure

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

• purine bases pair up with like purine bases
• the DNA bases for each strand face the outside of the helix
• the backbone contains only phosphate and nucleobase
• the phosphate groups are on the inside of the DNA double helix
• DNA exists as an anti-parallel double helix

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Statements about Nucleotides 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?

• nucleobase
• phosphate group
• sugar molecule
• glycerin

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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 thymine.
What are the percentages of the other three bases?

• adenine (A): 4% — cytosine (C): 46% — guanine (G): 46%
• adenine (A): 46% — cytosine (C): 4% — guanine (G): 46%
• adenine (A): 46% — cytosine (C): 25% — guanine (G): 25%
• adenine (A): 46% — cytosine (C): 46% — guanine (G): 4%
• adenine (A): 25% — cytosine (C): 25% — guanine (G): 46%

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Directionless Complementary DNA Sequences

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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.

 

T

G

T

,

C

A

C

,

T

A

C

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

•  

  A

  C

  A

  ,

  G

  T

  G

  ,

  A

  T

  G

•  

  C

  T

  G

  ,

  T

  A

  C

  ,

  T

  A

  C

•  

  C

  A

  T

  ,

  C

  A

  C

  ,

  T

  G

  T

•  

  A

  C

  A

  ,

  G

  G

  T

  ,

  A

  G

  T

•  

  A

  C

  A

  ,

  G

  A

  C

  ,

  T

  A

  C

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5′ and 3′ Complementary DNA Sequences

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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.

 

5′–

C

T

C

,

C

A

A

,

C

T

A

–3′

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′–

  C

  T

  C

  ,

  C

  A

  A

  ,

  C

  T

  A

  –5′

•  

  3′–

  C

  T

  C

  ,

  C

  T

  T

  ,

  G

  A

  T

  –5′

•  

  3′–

  T

  A

  G

  ,

  T

  T

  G

  ,

  G

  A

  G

  –5′

•  

  3′–

  G

  A

  G

  ,

  G

  T

  T

  ,

  G

  A

  T

  –5′

•  

  3′–

  A

  T

  C

  ,

  A

  A

  C

  ,

  C

  T

  C

  –5′

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