Genetics Syllabus: DNA Structure, Mutation, and Repair Mechanisms

UNIT I: DNA and RNA Structure, Composition, and Replication

DNA and RNA: Structure and Types

This unit introduces the fundamental structures and functions of nucleic acids, focusing on DNA and RNA. It covers the various forms of DNA and the different types of RNA, as well as the mechanisms of DNA replication.

• Types of DNA: A-DNA, B-DNA, and Z-DNA are structural variants of the DNA double helix, differing in helical twist and handedness.

• Types of RNA: mRNA (messenger RNA), rRNA (ribosomal RNA), and tRNA (transfer RNA) each play distinct roles in gene expression and protein synthesis.

DNA Replication Mechanisms

• Replication Types: Includes unidirectional and bidirectional replication, as well as semi-conservative replication (each daughter DNA contains one parental and one new strand).

• Models of Replication: Meselson and Stahl experiment demonstrated semi-conservative replication. Other models include continuous and discontinuous synthesis, and the rolling circle model.

• Key Enzymes: DNA polymerases (synthesize new DNA), DNA ligase (joins DNA fragments), primase (synthesizes RNA primers), helicase (unwinds DNA), and topoisomerase (relieves supercoiling).

Example: The Meselson-Stahl experiment used isotopic labeling to show that DNA replication is semi-conservative.

UNIT II: Mutation

Origin, Classification, and Mechanisms of Mutation

This unit explores the sources, types, and molecular mechanisms of genetic mutations, as well as methods for their detection and analysis.

• Mutation: A heritable change in the DNA sequence. Can be spontaneous (arising naturally) or induced (caused by mutagens).

• Mutagens: Classified as physical (e.g., radiation) or chemical (e.g., base analogs, alkylating agents).

• Molecular Mechanism: Includes base substitutions, insertions, deletions, and DNA damage (e.g., thymine dimers from UV light).

• Detection Methods: Ames test (detects mutagenicity), cytogenetic analysis (chromosome-level changes), and in vitro/in vivo host-mediated assays.

Example: The Ames test uses bacteria to assess the mutagenic potential of chemical compounds.

UNIT III: DNA Repair and Recombination Mechanisms

DNA Repair, Transposons, and Transcription

This unit covers the cellular mechanisms for repairing DNA, the role of transposable elements, and the processes of transcription and post-transcriptional modification.

• DNA Repair Mechanisms: Include direct repair, excision repair (base and nucleotide), mismatch repair, and recombinational repair.

• Transposons: DNA sequences that can move within the genome, affecting gene function and genome stability.

• Transcription: The process by which RNA is synthesized from a DNA template. Includes differences between prokaryotic and eukaryotic transcription.

• Post-Transcriptional Modifications: mRNA capping, splicing (removal of introns), and polyadenylation (addition of poly-A tail).

Example: In eukaryotes, pre-mRNA undergoes capping, splicing, and polyadenylation before becoming mature mRNA.

Comparison Table: Prokaryotic vs. Eukaryotic Transcription

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Nucleus
RNA Polymerases	One type	Three types (I, II, III)
mRNA Processing	None	Capping, splicing, polyadenylation
Transcription-Translation Coupling	Yes	No