BIO Exam 1: Ch 5

Genetics

Introduction to Genetics

Genetics is the study of genes, heredity, and genetic variation in living organisms. In microbiology, understanding genetics is essential for exploring how microorganisms function, adapt, and evolve.

• Genome: The complete set of genetic information in an organism; serves as the organism's genetic blueprint.

• Chromosome: DNA molecules wrapped around proteins (histones in eukaryotes), carrying hundreds of genes. Prokaryotes lack histones.

• Gene: A specific DNA sequence that codes for a particular trait or function (e.g., eye color).

• DNA: The molecule that stores genetic instructions; found in the nucleus of eukaryotes and in the nucleoid region of prokaryotes.

• RNA: Functions in protein synthesis, primarily in the cytoplasm.

• Central Dogma: The flow of genetic information: DNA → RNA → Protein.

Example: The gene for eye color is a specific DNA sequence that, when expressed, results in the production of pigment proteins in the eye.

Categories of Genes

• Structural genes: Code for proteins that perform cellular functions.

• Genes for RNA machinery: Code for RNA molecules involved in protein synthesis (e.g., rRNA, tRNA).

• Regulatory genes: Control gene expression, ensuring proteins are produced only when needed. Lack of regulation can lead to diseases such as cancer.

• Genotype: The sum of all gene types; the organism's unique genetic makeup.

• Phenotype: The observable traits or functions resulting from gene expression.

Historical Context: Gregor Mendel

• Known as the "Father of Modern Genetics" for his work with pea plants.

• Demonstrated that traits are heritable and introduced the concepts of genotype and phenotype.

Genomes in Cells and Viruses

• Cells: Always have DNA genomes.

• Viruses: May have either DNA or RNA genomes.

Prokaryotic vs. Eukaryotic Genomes

• Complexity: Not determined by the number of genes, but by gene regulation and diversity.

• Eukaryotes:

  • Multiple linear chromosomes in the nucleus.

  • DNA organized with histones.

  • Multiple replication origins.

• Prokaryotes:

  • 1-3 chromosomes, usually circular, in the nucleoid region.

  • Single replication origin.

• Plasmids: Small, circular DNA molecules that can carry additional genes, such as antibiotic resistance.

Structure of Nucleic Acids

DNA Structure

• Double-stranded helix with anti-parallel strands (3'-5' and 5'-3').

• Sugar-phosphate backbone forms the sides; nitrogenous bases form the rungs.

• Base pairing: Thymine (T) pairs with Adenine (A), Cytosine (C) pairs with Guanine (G).

• Strands held together by hydrogen bonds.

RNA Structure

• Single-stranded helix; can fold into complex structures.

• Contains Uracil (U) instead of Thymine; U pairs with A, C pairs with G.

• Ribose sugar instead of deoxyribose.

• Types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).

DNA Replication

Overview

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

• Facilitated by various enzymes (e.g., DNA polymerase, helicase, ligase).

• Ensures genetic information is accurately passed to daughter cells.

Example: Human cells have about 210 different cell types, each receiving instructions from DNA during replication.

Gene Expression and Protein Synthesis

Central Dogma: Transcription and Translation

• Transcription: DNA is copied into RNA by RNA polymerase.

• Translation: Ribosomes decode mRNA to build proteins.

• Many antibiotics target steps in protein synthesis.

Transcription

• Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).

• Steps: Initiation, Elongation, Termination.

• Main enzyme: RNA polymerase.

• Regulation can occur before (pre-transcriptional) or after (post-transcriptional) transcription.

mRNA Splicing (Eukaryotes)

• Removes non-coding regions (introns) and joins coding regions (exons).

• Ensures only necessary information is translated into protein.

Translation

• Occurs in ribosomes.

• Uses codons (three-nucleotide sequences) to specify amino acids.

• Start and stop codons define the beginning and end of protein synthesis.

• There are 64 codons for 20 amino acids.

Mutations

Types of Mutations

• Silent: No effect on protein sequence.

• Missense: Changes one amino acid in the protein.

• Nonsense: Introduces a premature stop codon.

• Frameshift: Insertion or deletion of bases shifts the reading frame.

Causes of Mutations

• Induced: Caused by chemicals, physical agents (e.g., radiation), or biological factors (e.g., viruses).

• Spontaneous: Occur naturally during DNA replication; rare events.

Repair Mechanisms

• DNA proofreading during replication.

• Specialized repair systems (e.g., UV damage repair, excision repair).

• Mutations are permanent and heritable if not repaired.

Mutation and Adaptation

• Some mutations provide advantages in changing environments.

• Ames Test: Used to assess the mutagenic potential of chemical compounds.

Comparison of Prokaryotic and Eukaryotic Transcription and Translation

Feature	Prokaryotes	Eukaryotes
Transcription Location	Cytoplasm	Nucleus
Translation Location	Cytoplasm	Cytoplasm
Simultaneity	Transcription and translation occur simultaneously	Transcription and translation are separated
mRNA Processing	None	Splicing, capping, polyadenylation

Gene Regulation and Expression

• Not all genes are expressed at all times.

• Housekeeping genes: Always expressed; essential for basic cell function.

• Facultative genes: Expressed only when needed.

Gene Transfer Mechanisms

Vertical Gene Transfer

• Transfer of genetic material from parent to offspring during reproduction.

Horizontal Gene Transfer (HGT)

• Transfer of genetic material between organisms other than by descent.

• Common in prokaryotes; contributes to genetic diversity.

Mechanism	Description	Key Features
Conjugation	Direct transfer of DNA via a pilus	Requires F plasmid; donor and recipient cells
Transformation	Uptake of free DNA from the environment	No pilus; requires competent cells
Transduction	Transfer of DNA by bacteriophages (viruses)	Virus-mediated
Transposons	"Jumping genes" that move within the genome	Can cause mutations and gene rearrangements

• Plasmids: Often carry genes for antibiotic resistance (R plasmids) and can be used in biotechnology.

Additional info: Many antibiotics target bacterial protein synthesis by interfering with ribosomal function, making understanding these processes crucial for medical microbiology.