AP Biology Unit 1 Topic 1.6: Nucleic Acids — DNA, RNA, Nucleotide Structure and the Double Helix

Question 1

What are nucleic acids and what are their functions?

Accepted Answer

Nucleic acids are macromolecules that store and transmit hereditary information. The two main types are DNA, which stores the genetic information of all known organisms, and RNA, which serves multiple roles including translating that information into proteins. Both are built from monomers called nucleotides.

Question 2

What are the three components of a nucleotide?

Accepted Answer

Every nucleotide has three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. In DNA the sugar is deoxyribose, while in RNA it is ribose. The base can be one of five types: adenine, guanine, cytosine, thymine, or uracil.

Question 3

What is the difference between DNA and RNA?

Accepted Answer

DNA contains deoxyribose sugar, is double-stranded, and uses the bases adenine, thymine, guanine, and cytosine. RNA contains ribose sugar, is usually single-stranded, and uses uracil instead of thymine. DNA stores information long-term while RNA mostly transmits and translates that information.

Question 4

What are purines and pyrimidines?

Accepted Answer

Purines and pyrimidines are the two structural classes of nitrogenous bases. Purines have a double ring structure and include adenine and guanine. Pyrimidines have a single ring structure and include cytosine, thymine, and uracil. A purine always pairs with a pyrimidine in double-stranded nucleic acids.

Question 5

What are the base pairing rules in DNA?

Accepted Answer

In DNA, adenine always pairs with thymine through two hydrogen bonds, and guanine always pairs with cytosine through three hydrogen bonds. This complementary base pairing is the foundation of DNA replication and transcription, ensuring accurate copying and reading of genetic information.

Question 6

What is a phosphodiester bond?

Accepted Answer

A phosphodiester bond is the covalent bond that links nucleotides together in a nucleic acid chain. It connects the phosphate group of one nucleotide to the sugar of the next, forming the sugar-phosphate backbone. These bonds are formed through dehydration synthesis.

Question 7

What does it mean that DNA is antiparallel?

Accepted Answer

Antiparallel means the two strands of a DNA double helix run in opposite directions. One strand runs from the five-prime end to the three-prime end while the other runs from three-prime to five-prime. This orientation is essential for proper base pairing and for the enzymes that copy and read DNA.

Question 8

What is the five-prime to three-prime directionality of DNA?

Accepted Answer

The numbers refer to specific carbon atoms in the deoxyribose sugar. The five-prime end has a free phosphate group while the three-prime end has a free hydroxyl group. New nucleotides can only be added to the three-prime end during replication, which has major implications for how DNA is copied.

Question 9

Who discovered the structure of DNA?

Accepted Answer

The double helix structure of DNA was published in 1953 by James Watson and Francis Crick, building on the X-ray crystallography work of Rosalind Franklin and Maurice Wilkins. The discovery is one of the most famous in the history of biology and earned Watson, Crick, and Wilkins the Nobel Prize in 1962.

Question 10

What are the main types of RNA and what do they do?

Accepted Answer

The three main types of RNA are messenger RNA, which carries the genetic message from DNA to ribosomes; transfer RNA, which brings amino acids to the ribosome during translation; and ribosomal RNA, which is a structural and catalytic component of the ribosome. All three are essential for protein synthesis.

Question 11

How does the structure of DNA enable accurate replication?

Accepted Answer

Because DNA is double-stranded with complementary base pairing, each strand contains all the information needed to rebuild the other. During replication, the two strands separate and each serves as a template for synthesizing a new complementary strand, producing two identical daughter molecules.

Question 12

Why is the sequence of bases in DNA important?

Accepted Answer

The sequence of bases in DNA encodes genetic information. Specific sequences of three bases, called codons, specify which amino acid will be added during protein synthesis. The order of bases ultimately determines the order of amino acids in proteins, and therefore the structure and function of every protein an organism makes.

Question 13

How is the directionality of nucleic acids related to their function?

Accepted Answer

The five-prime to three-prime directionality determines how DNA polymerase and RNA polymerase read templates and synthesize new strands, always adding nucleotides to the three-prime end. This constraint shapes how replication and transcription occur, including the need for leading and lagging strand synthesis during DNA replication.

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