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DNA and RNA
Structure

Explore the molecular architecture of nucleic acids: how nucleotides assemble, how the double helix forms, and what makes RNA structurally distinct from DNA.

AP Biology

What Are Nucleic Acids?

Nucleic acids are macromolecules that store and transmit hereditary information. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both are polymers built from repeating monomer units called nucleotides.

The Nucleotide Monomer

Each nucleotide consists of three components joined together. These monomers link end-to-end to form the long chains of DNA and RNA.

Phosphate Group
A phosphorus atom bonded to four oxygen atoms. Gives nucleic acids their negative charge and forms the backbone linkage.
Pentose Sugar
A five-carbon sugar: deoxyribose in DNA or ribose in RNA. The sugar connects the phosphate to the base.
Nitrogenous Base
A ring-shaped molecule containing nitrogen. It carries the genetic code and determines how nucleotides pair.

Anatomy of a Nucleotide

Phosphate Group Pentose Sugar Nitrogenous Base 5' bond 1' bond 5' 1' 3' Nucleotide = Phosphate + Sugar + Base Linked by phosphodiester bonds to form polynucleotide chains

The Five Nitrogenous Bases

Purines
Adenine (A) and Guanine (G): double-ring structures with 9 atoms in the ring system. Larger molecules.
Pyrimidines
Cytosine (C), Thymine (T) in DNA, Uracil (U) in RNA: single-ring structures with 6 atoms. Smaller molecules.

Base Pairing Rules

A T H H 2 hydrogen bonds G C H H H 3 hydrogen bonds A U In RNA: U replaces T DNA: A=T, G≡C | RNA: A=U, G≡C

The Double Helix Structure

A T G C T A C G 5'→3' 3'→5' Base pairs Sugar-phosphate backbone Antiparallel strands

The Sugar-Phosphate Backbone

The backbone of each strand is formed by alternating sugar and phosphate groups connected by phosphodiester bonds. These are covalent bonds linking the 3' carbon of one sugar to the 5' carbon of the next sugar through a phosphate group.

1
Phosphodiester bonds are strong covalent bonds that give each strand structural integrity.
2
Hydrogen bonds between complementary bases hold the two strands together but are individually weak, allowing strand separation during replication.
3
The negative charges on phosphate groups make DNA hydrophilic on the outside, while the hydrophobic bases are tucked inside the helix.

DNA vs. RNA: Key Differences

DNA RNA
Sugar Deoxyribose (no OH at 2') Ribose (OH at 2')
Strands Double-stranded Single-stranded
Bases A, T, G, C A, U, G, C
Location Nucleus (mainly) Nucleus + Cytoplasm
Function Long-term genetic storage Gene expression, regulation
Stability Very stable Less stable, short-lived

Types of RNA

mRNA
Messenger RNA: Carries the genetic code from DNA to ribosomes. Acts as the template for protein synthesis.
tRNA
Transfer RNA: Delivers amino acids to ribosomes during translation. Has an anticodon that matches the mRNA codon.
rRNA
Ribosomal RNA: Structural and catalytic component of ribosomes. Makes up about 80% of total RNA in a cell.

Deoxyribose vs. Ribose

O 1' 2' 3' 4' 5' H Deoxyribose Found in DNA Only H at 2' carbon vs. O 1' 2' 3' 4' 5' OH Ribose Found in RNA OH group at 2' carbon The 2' carbon difference determines stability and function

How to Identify DNA vs. RNA

1
Check the sugar: Does it have a hydroxyl (-OH) at the 2' carbon? If yes, it is ribose (RNA). If only H, it is deoxyribose (DNA).
2
Look at the bases: Is thymine (T) present? Then it is DNA. Is uracil (U) present? Then it is RNA.
3
Count the strands: Two complementary strands forming a helix indicates DNA. A single strand (possibly folded) indicates RNA.
4
Consider location: Found only in the nucleus? Likely DNA. Found in both nucleus and cytoplasm? Likely RNA.

Example: Complementary Strand

Given a DNA template strand reading 5'-ATCGGA-3', determine the complementary strand with correct directionality.

5' A T C G G A 3' Template Strand 3' T A G C C T 5' Complementary Strand Answer: 3'-TAGCCT-5' (or written 5'-TCCGAT-3')

Example: Transcribing DNA to mRNA

Given a DNA template strand reading 3'-TACGGC-5', write the corresponding mRNA sequence.

3' T A C G G C 5' DNA Template Strand Transcription → 5' A U G C C G 3' mRNA Strand mRNA: 5'-AUGCCG-3'

Example: Chargaff's Ratios

A sample of DNA is analyzed and found to contain 22% adenine. What are the percentages of the other three bases?

1
\(A = T\) (Chargaff's rule): If \(A = 22\%\), then \(T = 22\%\).
2
Total \(A + T = 44\%\). Since all four bases total 100%, \(G + C = 100\% - 44\% = \mathbf{56\%}\).
3
\(G = C\) (Chargaff's rule): \(G = 56\% / 2 = \mathbf{28\%}\) and \(C = \mathbf{28\%}\).

Common Mistakes

✕ Pairing T with RNA
Thymine (T) is found only in DNA. In RNA, adenine pairs with uracil (U), not thymine.
✕ Ignoring Directionality
DNA strands are antiparallel. Writing both strands in the same 5' to 3' direction gives an incorrect complementary sequence.
✕ Chargaff's on RNA
Chargaff's base-pairing ratios (\(A=T, G=C\)) apply only to double-stranded DNA. Single-stranded RNA does not follow these ratios.

Summary: Nucleic Acid Structure

Feature DNA RNA
Full Name Deoxyribonucleic acid Ribonucleic acid
Sugar Deoxyribose Ribose
Unique Base Thymine (T) Uracil (U)
Structure Double helix Single strand
Bonds (inter-strand) Hydrogen bonds Intramolecular folding
Backbone Bond Phosphodiester Phosphodiester
Primary Role Information storage Gene expression

Question 1 of 5

Which component of a nucleotide is responsible for carrying the genetic code?

Question 2 of 5

A DNA sample contains 30% cytosine. What percentage of the sample is adenine?

Question 3 of 5

What structural feature makes RNA less stable than DNA?

Question 4 of 5

If a DNA template strand reads 3'-GCATTAGC-5', what is the resulting mRNA sequence?

Question 5 of 5

Why do G-C base pairs contribute more to the thermal stability of DNA than A-T base pairs?