chap 5A

Genetics and Heredity Basics

Introduction to Genetics

Genetics is the study of genes, their function, and how variations arise in genomes. It is fundamental to understanding how traits are inherited and expressed in microorganisms.

• Genotype: The genetic makeup of an organism; the set of genes it carries.

• Phenotype: The observable physical and physiological traits of an organism, determined by its genotype.

• Relationship: The genotype influences the phenotype, meaning the genetic instructions determine the traits expressed.

• Example: Mendel’s pea plant experiments demonstrated how traits are passed from one generation to the next.

Genomes: Definition and Organization

What is a Genome?

The genome is the complete set of genetic material in a cell or virus. It contains all the instructions necessary for the structure, function, and regulation of the organism.

• Cells: Typically have deoxyribonucleic acid (DNA) genomes.

• Viruses: May have either DNA or ribonucleic acid (RNA) genomes.

Size and Organization of Genomes

Genomes are organized into chromosomes, which are long strands of DNA associated with organizational proteins. The complexity and organization differ between prokaryotic and eukaryotic cells.

• Prokaryotic cells: Usually have a single, circular chromosome located in the nucleoid region.

• Eukaryotic cells: Have multiple, linear chromosomes housed in the nucleus. Organizational proteins called histones help package DNA.

• Plasmids: Small, circular pieces of DNA that exist outside the chromosomal DNA, often conferring advantages such as antibiotic resistance.

Genome Complexity and Gene Number

• More complex organisms tend to have more genes.

• Example: Escherichia coli (E. coli) has about 4,400 genes; humans have about 24,000 genes.

• Note: The number of chromosomes does not necessarily indicate organism sophistication.

Table: Genome Characteristics

Factor	Prokaryotic Genomes	Eukaryotic Genomes
Complexity	Simple	More complex
Genome can include	Chromosomal DNA and plasmids	Chromosomal DNA, plasmids, and DNA in mitochondria and chloroplasts
Chromosomes	Few (generally only one), usually circular	Many nuclear chromosomes are linear
Location of chromosomes	Nucleoid region	Nucleus
DNA organized by	Histone-like proteins	Histones

DNA and RNA: Structure and Function

Nucleotides: The Building Blocks

Nucleic acids (DNA and RNA) are polymers made up of nucleotides. Each nucleotide consists of three components:

• Phosphate group

• Sugar: Deoxyribose in DNA, ribose in RNA

• Nitrogen base: Adenine (A), Guanine (G), Cytosine (C), Thymine (T) in DNA; Uracil (U) replaces Thymine in RNA

Classification of Nitrogen Bases

Nitrogen bases are classified as either purines or pyrimidines based on their chemical structure.

Nitrogen Base	Family	Pairs with	Found in
Adenine (A)	Purine	Thymine (T)	DNA and RNA
Guanine (G)	Purine	Cytosine (C)	DNA and RNA
Cytosine (C)	Pyrimidine	Guanine (G)	DNA and RNA
Thymine (T)	Pyrimidine	Adenine (A)	Only DNA
Uracil (U)	Pyrimidine	Adenine (A)	Only RNA

• Base pairing rules: A pairs with T (or U in RNA), G pairs with C.

DNA Structure

DNA is a double-stranded molecule with an antiparallel arrangement, forming a twisted ladder (double helix).

• Sugar-phosphate backbone: Forms the "rails" of the ladder.

• Nitrogen bases: Form the "rungs" of the ladder, held together by hydrogen bonds.

• Complementary base pairing: Ensures accurate replication and transcription.

Phosphodiester Bonds

Phosphodiester bonds link the sugar and phosphate groups of adjacent nucleotides, forming the backbone of DNA and RNA.

• Directionality: DNA and RNA strands have a 5' to 3' direction, which is important for replication and transcription.

• Formation: The bond forms between the 3' hydroxyl group of one nucleotide and the 5' phosphate group of the next.

Equation for phosphodiester bond formation:

$\text{Nucleotide}_1\text{-OH} + \text{Nucleotide}_2\text{-phosphate} \rightarrow \text{Nucleotide}_1\text{-O-PO}_4\text{-Nucleotide}_2 + \text{H}_2\text{O}$

Central Dogma of Molecular Biology

Flow of Genetic Information

The central dogma describes the flow of genetic information from DNA to RNA to protein.

• DNA is transcribed into RNA.

• RNA is translated into protein.

• This process is essential for gene expression and cellular function.

Equation for central dogma:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Additional info: These notes cover foundational concepts in microbial genetics, including genome organization, nucleotide structure, and the central dogma, which are essential for understanding microbial physiology and molecular biology.