Genetics: Structure and Function of Genetic Material

Key Concepts in Genetics

Genetics is the study of genes, their functions, and how variations arise in genomes. The genome is the entire collection of genetic material in a cell or virus, while a gene is a heritable unit of genetic material that determines a particular trait. The genotype refers to the genetic makeup of an organism, and the phenotype is the observable collection of traits.

• Genotype: The inherited set of genetic instructions contained in the genome.

• Phenotype: Observable physical and physiological characteristics.

• Genome: All DNA in a cell, or DNA/RNA in a virus.

• Gene: Sequence encoding a functional product (protein or RNA).

Organization of Genetic Material in Cells

Genetic material is organized differently in eukaryotic and prokaryotic cells.

• Eukaryotic cells:

  • Numerous linear chromosomes housed in the nucleus

  • Organizational proteins called histones

  • Chromosomal DNA, plasmids, and DNA in mitochondria and chloroplasts

• Prokaryotic cells:

  • 1-3 chromosomes, usually circular

  • Located in the nucleoid region

  • Histone-like proteins organize DNA

  • Chromosomal DNA and plasmids

Plasmids

Plasmids are pieces of DNA that exist outside of chromosomal DNA in bacteria and some eukaryotic cells. They often carry genes conferring antibiotic resistance.

Structure of DNA and RNA

Nucleotide Structure

Nucleotides are the building blocks of DNA and RNA, each consisting of three parts:

• Phosphate group

• Sugar (deoxyribose in DNA, ribose in RNA)

• Nitrogen base (Adenine, Guanine, Cytosine, Thymine, or Uracil)

Nitrogenous Bases: Families and Pairing

The nitrogenous bases are classified as purines or pyrimidines and pair specifically in DNA and RNA.

DNA Structure

DNA is a double-stranded molecule forming a double helix, resembling a twisted ladder. The "rungs" are complementary nitrogen bases (A-T, G-C), and the "side rails" are alternating sugar and phosphate groups linked by phosphodiester bonds.

• DNA has 5’ to 3’ directionality

• Double helix is antiparallel: one strand runs 5’ to 3’, the other 3’ to 5’

• Hydrogen bonds hold complementary strands together

RNA Structure

RNA is usually single-stranded, contains ribose sugar, and uses uracil (U) instead of thymine (T). RNA can fold into helical and loop structures.

• RNA nucleotides: phosphate group, ribose sugar, nitrogen base (A, G, C, U)

• RNA has 5’ to 3’ directionality

• RNA is often single-stranded

Central Dogma of Molecular Biology

Flow of Genetic Information

The central dogma describes the flow of genetic information:

• Transcription: DNA → RNA

• Translation: RNA → Protein

DNA directs production of RNA, and RNA directs assembly of proteins, which are essential for cellular functions.

DNA Replication

Overview and Enzymes

DNA replication is the process by which a cell copies its genome before division. It is semiconservative: each new DNA molecule contains one original strand and one newly made strand.

• Replication is fast and accurate

• Occurs in the 5’ → 3’ direction

Main Enzymes and Their Functions

Leading vs. Lagging Strand

• Leading strand: Synthesized continuously toward the replication fork

• Lagging strand: Synthesized discontinuously away from the fork in Okazaki fragments

Transcription and Translation

Transcription

Transcription is the first stage of protein synthesis, where information in DNA is copied into RNA by RNA polymerase.

• Initiation: RNA polymerase binds to promoter and unwinds DNA

• Elongation: RNA polymerase lays down complementary ribonucleotides

• Termination: RNA polymerase reaches termination sequence and releases RNA

RNA Types and Functions

Translation

Translation is the process in which ribosomes decode mRNA to build proteins. Ribosomes have two subunits and three sites: exit (E), peptidyl (P), and acceptor (A).

• Translation begins at a start codon (AUG)

• Ribosome reads mRNA in codons (three nucleotides)

• tRNA brings correct amino acid to ribosome

• Polypeptide chain is formed

Mutations and Their Effects

Types of Mutations

• Silent: Codon changes but specifies same amino acid

• Missense: Codon specifies a different amino acid

• Nonsense: Codon becomes a stop codon

• Frameshift: Insertion or deletion shifts reading frame

Causes and Repair of Mutations

• Spontaneous mutations: Occur naturally during DNA replication

• Induced mutations: Result from mutagens (chemical, physical, biological agents)

• Excision repair: Damaged DNA is cut out, replaced, and sealed

Gene Transfer in Bacteria

Vertical and Horizontal Gene Transfer

• Vertical gene transfer: Genetic information passed to next generation

• Horizontal gene transfer: Includes conjugation, transformation, and transduction

Mechanisms of Horizontal Gene Transfer

• Conjugation: DNA transfer via pilus

• Transformation: Uptake of free DNA from environment

• Transduction: DNA transfer via bacteriophage

Regulation of Gene Expression

Pre-Transcriptional Regulation

• Operons

• Quorum sensing

• Epigenetic control (DNA methylation)

• Recruitment of transcription factors

Post-Transcriptional Regulation

• Recruiting ribosomes to mRNA

• Controlling mRNA stability

• Small noncoding RNAs

• Riboswitches

• RNA processing and nuclear export (eukaryotes)

Summary Table: DNA and RNA Comparison

Additional info: The notes above expand on brief points with academic context, definitions, and examples to provide a comprehensive study guide for genetics and molecular biology relevant to microbiology students.