Ch 4 review

Nucleic Acids: Structure and Function

1. Nitrogenous Bases in DNA and RNA

Nucleic acids, such as DNA and RNA, are composed of sequences of nitrogenous bases that encode genetic information.

• DNA contains four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

• RNA contains Adenine (A), Uracil (U) (instead of Thymine), Cytosine (C), and Guanine (G).

• Example: In DNA, a sequence might be ATCG; in RNA, the equivalent would be AUCG.

2. Structure of a Nucleotide

A nucleotide is the basic building block (monomer) of nucleic acids.

• Three main parts of a nucleotide:

  1. Phosphate group

  2. Pentose sugar (deoxyribose in DNA, ribose in RNA)

  3. Nitrogenous base (A, T/U, C, or G)

• Example: The nucleotide adenosine monophosphate (AMP) contains a phosphate, ribose sugar, and adenine base.

3. The Central Dogma of Molecular Biology

The central dogma explains the flow of genetic information within a biological system.

• Key steps:

  1. DNA is transcribed into RNA

  2. RNA is translated into protein

• Summary: Information flows from DNA → RNA → Protein.

• Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

4. Formation of Polynucleotide Chains

During the synthesis of nucleic acids, nucleotides are joined to form long chains (polynucleotides).

• Incoming nucleotide: The 5' phosphate group of the new nucleotide binds to the 3' hydroxyl group of the existing chain.

• Directionality: Chains are synthesized in the 5' to 3' direction.

• Example: In DNA replication, DNA polymerase adds nucleotides to the 3' end of the growing strand.

5. Purines and Pyrimidines

Nitrogenous bases are classified as either purines or pyrimidines based on their structure.

• Purines: Double-ring structures; Adenine (A) and Guanine (G)

• Pyrimidines: Single-ring structures; Cytosine (C), Thymine (T) (DNA), and Uracil (U) (RNA)

• Structural difference: Purines have a fused double-ring, while pyrimidines have a single ring.

• Table: Classification of Nitrogenous Bases

Type	Bases	Structure
Purines	Adenine (A), Guanine (G)	Double-ring
Pyrimidines	Cytosine (C), Thymine (T), Uracil (U)	Single-ring

6. Sugar Components of Nucleotides

The sugar component of a nucleotide differs between DNA and RNA.

• DNA: Contains deoxyribose (lacks an oxygen atom at the 2' carbon)

• RNA: Contains ribose (has an -OH group at the 2' carbon)

• Structural difference: Ribose has a hydroxyl group (-OH) at the 2' position; deoxyribose has only a hydrogen (-H) at this position.

• Example: The presence of the 2' -OH in RNA makes it more reactive and less stable than DNA.

7. DNA Double Helix Structure

The DNA double helix consists of two strands forming a twisted ladder-like structure.

• Backbone: Formed by alternating phosphate and sugar groups (the 'sides' of the ladder)

• Rungs: Formed by paired nitrogenous bases (the 'steps' of the ladder), which face inward and connect the two strands via hydrogen bonds

• Orientation: The two strands run in opposite directions (antiparallel)

8. Covalent Linkage Between Nucleotides

Nucleotides within a single strand are joined by strong covalent bonds.

• Phosphodiester bond: The covalent linkage between the 3' carbon of one sugar and the 5' phosphate of the next nucleotide

• Equation: $\text{Sugar}_{3'} - \text{Phosphate} - \text{Sugar}_{5'}$

9. Non-Covalent Linkages Between DNA Strands

The two complementary strands of DNA are held together by non-covalent interactions.

• Hydrogen bonds: These bonds form between complementary nitrogenous bases on opposite strands

• Example: Adenine pairs with Thymine via two hydrogen bonds; Guanine pairs with Cytosine via three hydrogen bonds

10. Complementary Base Pairing

Specific pairing between nitrogenous bases ensures accurate replication and transcription.

• DNA: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C)

• RNA: Adenine (A) pairs with Uracil (U); Guanine (G) pairs with Cytosine (C)

• Table: Complementary Base Pairs

Nucleic Acid	Base 1	Base 2
DNA	Adenine (A)	Thymine (T)
DNA	Guanine (G)	Cytosine (C)
RNA	Adenine (A)	Uracil (U)
RNA	Guanine (G)	Cytosine (C)