AP Biology Unit 1: Nucleic acids

Nucleic acids store and transmit the hereditary information of every living cell, making their structure the foundation for both Unit 1 biochemistry and Unit 6 gene expression. On the AP Biology exam, you will routinely interpret unlabeled strand diagrams to determine $5'$ to $3'$ orientation, compare the stability of DNA strands based on base composition, and predict how structural modifications affect polymer synthesis.

What the exam asks

Section I (Multiple Choice): Expect diagram-based questions showing a segment of DNA or RNA where you must label the

5'

and

3'

ends, identify where the next incoming nucleotide will attach, or calculate the percentage of a given base using Chargaff’s rules.

Section II (Free Response): Especially in Question 5 (Analyze Model or Visual Representation), you may be asked to draw or label a model showing antiparallel strands, explain why a chemical change to the

3'

hydroxyl group halts strand elongation, or relate GC content to the thermal stability of a DNA molecule.

Core ideas

Nucleotide monomers and information storage

Nucleic acids (DNA and RNA) are polymers made of nucleotide monomers. Every nucleotide consists of three covalently bonded components:

A five-carbon (pentose) sugar (deoxyribose in DNA; ribose in RNA).

A phosphate group attached to the

5'

carbon of the sugar.

A nitrogenous base attached to the

1'

carbon of the sugar.

When nucleotides link together by dehydration synthesis, they form a sugar-phosphate backbone held together by covalent phosphodiester bonds. This backbone is identical in every molecule of DNA or RNA; biological information is encoded strictly in the linear sequence of nitrogenous bases projecting from the backbone.

Directionality (

5'

3'

) and strand growth

The carbons of the pentose sugar are numbered

1'

through

5'

. This numbering gives every nucleic acid strand a distinct chemical polarity (directionality):

$5'$ end: Has a free phosphate group attached to the

5'

carbon.

$3'$ end: Has a free attached to the carbon.

Text

5'-[Phosphate]—(5' Sugar 3')—[Phosphate]—(5' Sugar 3')—OH-3'
                    |                         |
                 [Base]                    [Base]

During nucleic acid synthesis, new nucleotides are always added to the free $3'$ $-\text{OH}$ end of a growing strand. The

5'

phosphate of the incoming nucleotide forms a covalent bond with the

3'

hydroxyl group of the existing chain, meaning strands always grow in the .

Nucleic acids: nucleotides, strand direction and base pairing

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What the exam asks

Core ideas

Nucleotide monomers and information storage

Directionality ( $5'$ to $3'$ ) and strand growth

By the end you’ll be able to

Upgrade to Pro to unlock the rest

Still to come

What the exam asks

Core ideas

Nucleotide monomers and information storage

Directionality (5′5'5′ to 3′3'3′) and strand growth

Directionality ( $5'$ to $3'$ ) and strand growth