AP Biology 1.6 Nucleic Acids Study Notes - New Syllabus Effective 2025
AP Biology 1.6 Nucleic Acids Study Notes- New syllabus
AP Biology 1.6 Nucleic Acids Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the structure and function of DNA and RNA.
Key Concepts:
- Nucleic Acids
1.6.A.1
The genetic information of a cell is stored in molecules of deoxyribonucleic acid (DNA). The DNA, in turn, passes its genetic instructions to ribonucleic acid (RNA) for directing various metabolic activities of the cell. DNA is a polymer of nucleotides. A DNA nucleotide consists of three parts—a nitrogen base, a five-carbon sugar called deoxyribose, and a phosphate group. There are four DNA nucleotides, each with one of the four nitrogen bases, as follows.
- Adenine—a double-ring base (purine).
- Thymine—a single-ring base (pyrimidine).
- Cytosine—a single-ring base (pyrimidine).
- Guanine—a double-ring base (purine).
Pyrimidines are single-ring nitrogen bases, and purines are double-ring bases. You can remember which of these bases are purines because only the two purines (adenine and guanine) end with nine (and the two rings of a purine have nine unshared carbon bonds and one shared bond). The first letter of each of these four bases is often used to symbolize the respective nucleotide (A for the adenine nucleotide, for example).
Deoxyribonucleic acid (DNA)
● sugar=deoxyribose
● Contains genetic/hereditary information
● Provides directions for its own replication
● Directs synthesis of messenger RNA (mRNA), and through mRNA, controls protein synthesis
Ribonucleic acid (RNA)
● sugar=ribose
● Essential for protein synthesis
- 2 families of nitrogenous bases
■ Pyrimidines
● Single 6-membered ring
● Ex. - Cytosine
- Thymine (only DNA
- Uracil (only RNA)
■ Purines
● 6-membered ring fused to a 5-membered ring - Nucleotide Polymers
■ Nucleotide polymers linked together to build a polynucleotide
■ Adjacent nucleotides are joined by covalent bonds that form between the$ -OH$ group on the 3’ carbon of one nucleotide and the phosphate on the 5’ carbon on the next
● Links create a backbone of sugar-phosphate units with nitrogenous bases as appendages
■ RNA molecules usually exist as single polypeptide chains
■ DNA molecules have 2 polynucleotides spiraling around an imaginary axis, forming a double helix
● Two backbones run in opposite 5’→3’ directions from each other (antiparallel)
● One DNA molecule contains many genes
● Nitrogenous bases pair up and form hydrogen bonds - Adenine-Thymine
- Guanine-Cytosine
- Complementary base pairing
- In RNA, thymine is replaced by uracil, so A and U pair
1.6.A.2
How two strands of nucleotides, paired by weak hydrogen bonds between the bases, form a double stranded DNA. When bonded in this way, DNA forms a two-stranded spiral, or double helix. Note that adenine always bonds with thymine and guanine always bonds with cytosine (always a purine with a pyrimidine). The two strands of a DNA helix are antiparallel, that is, oriented in opposite directions. One strand is arranged in the 5′ → 3′ direction; that is, it begins with a phosphate group attached to the fifth carbon of the deoxyribose (5′ end) and ends where the phosphate of the next nucleotide would attach, at the third deoxyribose carbon (3′). The adjacent strand is oriented in the opposite, or 3′ → 5′ direction. RNA differs from DNA in the following ways.
- The sugar in the nucleotides that make an RNA molecule is ribose, not deoxyribose as it is in DNA.
- The thymine nucleotide does not occur in RNA. It is replaced by uracil. When pairing of bases occurs in RNA, uracil (instead of thymine) pairs with adenine.
- RNA is usually single-stranded and does not form a double helix as it does in DNA.
- Linear Polymers:
Nucleic acids, like DNA and RNA, are long chains (polymers) built from repeating units called nucleotides.
- Nucleotide Structure:
Each nucleotide consists of a sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, thymine in DNA, or uracil in RNA).
- Sugar-Phosphate Backbone:
The sugar and phosphate groups form the backbone of the nucleic acid chain, with the nitrogenous bases attached to the sugar.
- Phosphodiester Linkages:
Nucleotides are linked together by phosphodiester bonds, which form between the 3′ hydroxyl group of one nucleotide and the 5′ phosphate group of the next.
- Directionality:
The 3′ and 5′ ends of the nucleic acid chain give it directionality or polarity, with the 5′ end having a phosphate group and the 3′ end having a hydroxyl group.
- 3′ End Extension:
During nucleic acid synthesis (replication or transcription), new nucleotides are added to the 3′ end of the growing strand.
- Covalent Bond Formation:
The incoming nucleotide’s 5′ phosphate group forms a phosphodiester bond with the 3′ hydroxyl group of the last nucleotide in the chain, extending the strand.
- Enzymes:
DNA and RNA polymerases are enzymes that catalyze the synthesis of nucleic acids, ensuring that the correct nucleotides are added in the right order.
- Template-Dependent:
Nucleic acid synthesis is template-dependent, meaning that the sequence of the new strand is determined by the sequence of the template strand.
- 5′ and 3′ Ends:
The 5′ end of a nucleic acid strand has a phosphate group attached to the 5′ carbon of the sugar, while the 3′ end has a hydroxyl group attached to the 3′ carbon.
- Antiparallel Strands:
In double-stranded nucleic acids (like DNA), the two strands run in opposite directions, with one strand having a 5′ to 3′ orientation and the other having a 3′ to 5′ orientation.
- Complementary Base Pairing:
In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).
- RNA differences:RNA uses ribose sugar and uracil instead of thymine.
1.6.A.3
- Double Helix:DNA’s structure is a double helix, resembling a twisted ladder.
- Backbone:Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups.
- Bases:Attached to each sugar are one of four bases: adenine (A), cytosine (C), guanine (G), or thymine (T).
- Base Pairing:The two strands are connected by chemical bonds between the bases: adenine (A) bonds with thymine (T), and cytosine (C) bonds with guanine (G).
- Antiparallel:The two strands of the helix are antiparallel, meaning they run in opposite directions.
1.6.A.4
- DNA:Contains deoxyribose, a sugar molecule where the second carbon lacks an oxygen atom compared to ribose.
- RNA:Contains ribose, a sugar molecule with an oxygen atom on the second carbon.
- DNA:Contains the bases adenine (A), guanine (G), cytosine (C), and thymine (T).
- RNA:Contains the bases adenine (A), guanine (G), cytosine (C), and uracil (U).
- Uracil replaces thymine in RNA.
- Uracil replaces thymine in RNA.
- DNA: Typically a double-stranded molecule, forming a double helix structure.
- RNA: Usually single-stranded, although it can fold into complex 3D structures.
- DNA:More stable due to the deoxyribose sugar and the double-stranded structure, which provides protection from enzymes and hydrolysis.
- RNA:Less stable than DNA, as the ribose sugar is more susceptible to hydrolysis and it is usually single-stranded.
- DNA:Long, double-stranded molecules that contain a vast amount of genetic information.
- RNA:Shorter, single-stranded molecules that are used for various functions, including protein synthesis and gene regulation.