Genomes and Genetic Variation

Definition and Comparison of Genomes

The genome is the complete set of genetic material present in a cell or organism. In prokaryotes and eukaryotes, genomes differ in size, composition, chromosome structure, and chromosome location.

• Prokaryotic genomes are typically smaller, often consisting of a single circular chromosome located in the nucleoid region.

• Eukaryotic genomes are larger, organized into multiple linear chromosomes found within a membrane-bound nucleus.

• Chromosome structure: Prokaryotes lack histones (except some Archaea), while eukaryotic DNA is wrapped around histone proteins.

Genotype vs. Phenotype

Genotype refers to the genetic makeup of an organism, while phenotype is the observable physical or biochemical characteristics determined by both genotype and environment.

• Example: The gene for eye color (genotype) determines the actual eye color seen (phenotype).

Genetic Variation

Genetic variation arises from several mechanisms, including mutation, recombination, and horizontal gene transfer.

• Spontaneous mutations occur naturally during DNA replication.

• Induced mutations result from exposure to mutagens such as chemicals or radiation.

• Horizontal gene transfer allows genetic material to move between organisms, increasing diversity.

Structure and Function of DNA and RNA

Basic Functions and Structural Characteristics

• DNA (Deoxyribonucleic acid) stores genetic information and is double-stranded with complementary base pairing (A-T, G-C).

• RNA (Ribonucleic acid) is usually single-stranded and functions in protein synthesis (mRNA, tRNA, rRNA).

• DNA and RNA differ in sugar (deoxyribose vs. ribose) and bases (thymine in DNA, uracil in RNA).

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• Replication: DNA is copied to produce identical DNA molecules.

• Transcription: DNA is transcribed into RNA.

• Translation: RNA is translated into proteins.

Gene Expression and Regulation

Stages of Protein Synthesis

• Transcription: Synthesis of RNA from a DNA template.

• RNA processing: (mainly in eukaryotes) includes splicing, capping, and polyadenylation.

• Translation: Synthesis of proteins from mRNA by ribosomes.

Types of RNA

• mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Genetic Code and Redundancy

• The genetic code is redundant (degenerate), meaning multiple codons can code for the same amino acid.

• This redundancy helps protect against some mutations.

Regulation of Protein Synthesis

• Protein synthesis is regulated at transcriptional, post-transcriptional, translational, and post-translational levels.

• Examples: Operons in prokaryotes (e.g., lac operon), alternative splicing in eukaryotes.

Post-Translational Modifications

• Modifications after translation, such as phosphorylation or glycosylation, alter protein function and activity.

• Example: Addition of phosphate groups can activate or deactivate enzymes.

Mutations and DNA Repair

Types of Mutations

• Silent mutations: Do not change the amino acid sequence.

• Missense mutations: Change one amino acid in the protein.

• Nonsense mutations: Introduce a premature stop codon.

• Frameshift mutations: Insertions or deletions that shift the reading frame.

DNA Repair Mechanisms

• Direct repair: Reverses DNA damage directly.

• Excision repair: Removes and replaces damaged DNA segments.

• Mismatch repair: Corrects errors introduced during DNA replication.

Ames Test

The Ames test is used to assess the mutagenic potential of chemical compounds using bacteria.

• Bacteria are exposed to the test substance; increased mutation rates indicate mutagenicity.

Genetic Exchange in Microorganisms

Horizontal Gene Transfer

• Transformation: Uptake of naked DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA between bacteria via cell-to-cell contact.

Generalized vs. Specialized Transduction

• Generalized transduction: Any bacterial gene can be transferred by a lytic phage.

• Specialized transduction: Only specific genes near the prophage insertion site are transferred by a lysogenic phage.

Transposons

Transposons are mobile genetic elements that can move within and between genomes, contributing to genetic diversity and evolution.

• They can disrupt genes or regulatory regions, leading to mutations.

• Transposons can carry antibiotic resistance genes between plasmids and chromosomes.

Summary Table: Types of Mutations

Additional info:

• Some details, such as specific examples of operons or post-translational modifications, were added for academic completeness.

• Table content inferred from standard mutation types in genetics.