Concept 5.5: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

Introduction to Nucleic Acids

Nucleic acids are essential biomolecules responsible for the storage, transmission, and expression of genetic information in all living organisms. The sequence of amino acids in a polypeptide is determined by genes, which are composed of DNA, a type of nucleic acid.

• Gene: A discrete unit of inheritance that encodes the amino acid sequence of a polypeptide.

• Nucleic acid: A polymer made up of monomers called nucleotides.

The Roles of Nucleic Acids

Types and Functions

There are two main types of nucleic acids, each with distinct roles in genetic information management and protein synthesis.

• Deoxyribonucleic acid (DNA): Stores genetic information and provides instructions for its own replication.

• Ribonucleic acid (RNA): Functions in the expression of genetic information by directing protein synthesis.

• Gene expression: The process by which DNA directs the synthesis of messenger RNA (mRNA), which in turn guides protein synthesis.

• Central dogma: The flow of genetic information follows the sequence: DNA → mRNA → Protein.

Flow of Genetic Information

Inheritance and Replication

Genetic information is inherited from parent organisms and is faithfully replicated before cell division to ensure continuity.

• Inheritance: Organisms inherit DNA from their parents, with each DNA molecule containing hundreds or more genes.

• Replication: Before cell division, DNA is copied so that each daughter cell receives an exact copy, ensuring genetic continuity.

Transcription and Translation

Gene expression involves two key processes: transcription and translation.

• Transcription: Each gene on a DNA molecule directs the synthesis of a specific messenger RNA (mRNA).

• Translation: The mRNA interacts with the cell's protein-synthesizing machinery (ribosomes) to determine the order of amino acids in a polypeptide.

Central Dogma Equation:

Ribosomes and Protein Synthesis

Protein synthesis occurs on ribosomes, which are cellular structures found in both eukaryotes and prokaryotes.

• Eukaryotes: DNA is located in the nucleus; most ribosomes are in the cytoplasm. mRNA acts as an intermediary, carrying instructions from the nucleus to the cytoplasm.

• Prokaryotes: Lack a nucleus but use mRNA to carry messages from DNA to ribosomes in the cytoplasm.

The Components of Nucleic Acids

Nucleic Acid Structure

Nucleic acids are polymers called polynucleotides, composed of repeating nucleotide monomers.

• Polynucleotide: A chain of nucleotides.

• Nucleotide: Consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

Nucleoside vs. Nucleotide

• Nucleoside: Nitrogenous base + Sugar (no phosphate group).

• Nucleotide: Nucleoside + Phosphate group.

Nitrogenous Bases

Nitrogenous bases are classified into two groups based on their structure:

• Pyrimidines (single 6-membered ring):

  • Cytosine (C)

  • Thymine (T) – found only in DNA

  • Uracil (U) – found only in RNA

• Purines (6-membered ring fused to a 5-membered ring):

  • Adenine (A)

  • Guanine (G)

Pentose Sugars

The pentose sugar component differs between DNA and RNA:

• Ribose: Found in RNA.

• Deoxyribose: Found in DNA; lacks an oxygen atom at the 2' carbon compared to ribose.

Atoms in the sugar are labeled with a prime (') to distinguish them from atoms in the base. Key positions:

• 2' carbon: Lacks oxygen in deoxyribose.

• 5' carbon: Extends from the ring and usually binds to the phosphate group.

Summary Table: Nitrogenous Bases in DNA and RNA

Additional info:

• The central dogma is a foundational concept in molecular biology, describing the directional flow of genetic information from DNA to RNA to protein.

• Polynucleotide chains are formed by phosphodiester linkages, which are covalent bonds between the phosphate group of one nucleotide and the sugar of another.

• DNA is double-stranded and forms a double helix, while RNA is typically single-stranded and can fold into complex shapes due to internal base pairing.