Genome Structure, Mutation, and Cell Division: Key Concepts and Mechanisms

Genome Structure, Mutation, and Cell Division

Bases

The genetic material of all living organisms is composed of nucleic acids, which are polymers of nucleotides. Each nucleotide contains a nitrogenous base, which is essential for encoding genetic information.

• Bases: The nitrogenous bases found in DNA and RNA are classified into two groups: purines and pyrimidines.

• Purines: Adenine (A) and Guanine (G) are double-ringed structures.

• Pyrimidines: Cytosine (C), Thymine (T) (in DNA), and Uracil (U) (in RNA) are single-ringed structures.

Nucleosides and Nucleotides

Nucleosides consist of a nitrogenous base attached to a sugar molecule, while nucleotides are nucleosides with one or more phosphate groups.

• Nucleoside: Base + Sugar

• Nucleotide: Nucleoside + Phosphate group(s)

• Example: Adenosine (nucleoside) vs. Adenosine triphosphate (ATP, nucleotide)

DNA and RNA Polymers

DNA and RNA are long chains of nucleotides linked by phosphodiester bonds. The sequence of bases encodes genetic information.

• DNA Polynucleotide Strand: Composed of deoxyribonucleotides (A, T, G, C)

• RNA Polynucleotide Strand: Composed of ribonucleotides (A, U, G, C)

Prokaryotic vs. Eukaryotic Genomes

Genomes differ in organization and complexity between prokaryotes and eukaryotes.

• Prokaryotic: Typically have a single, circular chromosome located in the nucleoid region.

• Eukaryotic: Possess multiple, linear chromosomes contained within a membrane-bound nucleus.

Base Pairing and Mutations

Base pairing is fundamental to the structure of DNA and RNA. Mutations can arise from errors in base pairing or external factors such as UV light.

• Base Pairing: In DNA, A pairs with T, and G pairs with C. In RNA, A pairs with U.

• 5-bromouracil: An analog of thymine that can pair with guanine, leading to mutations.

• Example: 5-bromouracil is used experimentally to induce mutations by mispairing during DNA replication.

DNA Repair Mechanisms

Cells possess several mechanisms to repair DNA damage and maintain genome integrity.

• Photoreactivation: The enzyme photolyase repairs thymine dimers caused by UV light.

• Nucleotide Excision Repair: Removes bulky DNA lesions, such as thymine dimers, and replaces the damaged section with newly synthesized DNA.

Summary Table: DNA Repair Mechanisms

Repair Mechanism	Main Function	Example of Damage Repaired
Photoreactivation	Directly reverses thymine dimers	UV-induced thymine dimers
Nucleotide Excision Repair	Removes and replaces damaged DNA segments	Bulky lesions, thymine dimers

Additional info: DNA repair is essential for preventing mutations that can lead to diseases such as cancer. The efficiency and accuracy of these repair mechanisms are critical for genetic stability.