Genetics: Structure, Function, and Variation of Genetic Material

Introduction to 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. A gene is a heritable unit of genetic material that determines a particular trait. The genotype refers to the genetic makeup of an organism, while the phenotype is the observable collection of traits.

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

• Phenotype: The organism’s observable traits (physical and physiological characteristics).

Organization of Genetic Material

Prokaryotic vs. Eukaryotic Cells

Genetic material is organized differently in prokaryotic and eukaryotic cells:

• Eukaryotic cells:

  • Numerous linear chromosomes housed in the nucleus

  • Organizational proteins called histones prevent DNA tangling

  • Genetic material found in chromosomal DNA, plasmids, mitochondria, and chloroplasts

• Prokaryotic cells:

  • 1-3 chromosomes (usually 1 circular chromosome) in the nucleoid region

  • Histone-like proteins organize DNA

  • Genetic material found in chromosomal DNA and plasmids

Plasmids

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

Structure of DNA and RNA

Nucleotide Structure

Both DNA and RNA are polymers of nucleotides. Each nucleotide consists of:

• Phosphate group

• Sugar (deoxyribose in DNA, ribose in RNA)

• Nitrogen base (Adenine, Guanine, Cytosine, Thymine in DNA; Uracil replaces Thymine in RNA)

Nitrogenous Bases: Classification and Pairing

Nitrogenous bases are classified as purines (double-ring) or pyrimidines (single-ring):

DNA Structure

• Double-stranded molecule forming a double helix (twisted ladder)

• "Rungs" are complementary nitrogen bases (A-T, G-C)

• "Side rails" are alternating sugar and phosphate groups, linked by phosphodiester bonds

• Antiparallel arrangement: one strand runs 5’ to 3’, the other 3’ to 5’

RNA Structure

• Usually single-stranded

• Contains ribose sugar

• Uracil (U) replaces thymine (T)

• Can fold into helical and loop structures

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 the production of RNA, which then directs the assembly of proteins.

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 and one new strand.

• Helicase: Unwinds the DNA helix

• Gyrase/Topoisomerases: Relieve supercoiling ahead of the replication fork

• Primase: Lays down RNA primers

• DNA Polymerase III: Main enzyme that adds nucleotides (5’→3’ direction)

• DNA Polymerase I: Removes RNA primers and replaces them with DNA

• Ligase: Seals nicks in the sugar-phosphate backbone, joining Okazaki fragments

• Single-strand DNA-binding proteins: Stabilize separated DNA strands

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 (DNA → RNA)

• Initiation: RNA polymerase binds to the promoter and unwinds DNA

• Elongation: RNA polymerase adds complementary ribonucleotides

• Termination: RNA polymerase reaches a termination sequence and releases the RNA transcript

Types of RNA produced:

• mRNA (messenger RNA): Carries genetic instructions to ribosomes

• tRNA (transfer RNA): Brings amino acids to ribosomes; contains an anticodon loop

• rRNA (ribosomal RNA): Forms part of the ribosome and catalyzes peptide bond formation

RNA Processing (Eukaryotes)

• Splicing: Removal of introns (non-coding regions); exons (coding regions) are joined together

Translation (RNA → Protein)

• Ribosomes decode mRNA in sets of three nucleotides (codons)

• Each codon specifies an amino acid or a stop signal

• tRNA molecules bring the correct amino acids to the ribosome

• Ribosomes have large and small subunits, with E (exit), P (peptidyl), and A (acceptor) sites

• Multiple ribosomes can translate a single mRNA simultaneously (polysomes)

Genetic Code

• There are 64 codons (triplets of nucleotides)

• Genetic code is redundant: multiple codons can specify the same amino acid

• Start codon: AUG (methionine)

• Stop codons: UAA, UAG, UGA

Gene Regulation

Pre-Transcriptional Regulation

• Operons (in prokaryotes): clusters of genes regulated together

• Quorum sensing, epigenetic control (e.g., DNA methylation), recruitment of transcription factors

Post-Transcriptional Regulation

• Control of mRNA stability and translation

• Small noncoding RNAs, riboswitches

• RNA processing and nuclear export (eukaryotes)

Mutations and Genetic Variation

Types of Mutations

• Insertion: Addition of one or more nucleotides

• Deletion: Loss of one or more nucleotides

• Substitution: Replacement of one nucleotide with another

• Silent: No change in amino acid sequence

• Missense: Changes one amino acid

• Nonsense: Changes a codon to a stop codon

• Frameshift: Insertion or deletion not in multiples of three, shifting the reading frame

Causes of Mutations

• Spontaneous mutations: Natural errors during DNA replication

• Induced mutations: Caused by mutagens (chemical, physical, or biological agents)

• Carcinogens: Mutagens that promote cancer development

DNA Repair Mechanisms

• Proofreading: DNA polymerases correct errors during replication

• Excision repair: Damaged or mismatched nucleotides are removed and replaced

Horizontal Gene Transfer in Bacteria

Mechanisms

• Conjugation: Direct transfer of DNA via a pilus

• Transformation: Uptake of free DNA from the environment

• Transduction: Transfer of DNA by bacteriophages (viruses)

Transposons

• "Jumping genes" that can move within the genome

• Retrotransposons use reverse transcriptase to copy RNA into DNA

• DNA transposons can move by "copy-and-paste" or "cut-and-paste" mechanisms

Summary Table: Nitrogenous Bases

Key Equations and Concepts

• Phosphodiester bond formation: Covalent bonds linking nucleotides in DNA and RNA

• Directionality: DNA and RNA are synthesized in the 5’ → 3’ direction

• Central Dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Genetic Code: $4 \times 4 \times 4 = 64$ codons

Additional info: These notes integrate foundational concepts from Chapter 5 (Genetics) and related molecular biology topics, providing a comprehensive overview suitable for exam preparation in a college-level microbiology course.