Genetic Material, Gene Transfer, and Gene Regulation in Microbiology

Genetic Material and DNA Structure

A. DNA as Genetic Material

DNA is the primary genetic material in most organisms, with the exception of some viruses that use RNA. The genetic material carries the information necessary for the structure and function of living cells.

• DNA: Deoxyribonucleic acid, the molecule that stores genetic information.

• RNA: Ribonucleic acid, genetic material in some viruses.

B. Basic Building Blocks of DNA

• Nucleotides: The monomers of DNA, each consisting of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

C. Structure of DNA

DNA is a double helix composed of two strands of nucleotides.

• Two strands of nucleotides form a double helix.

• Sugar-phosphate backbone provides structural support.

• Antiparallel orientation: One strand runs 5' to 3', the other 3' to 5'.

• Complementary base pairing: Adenine pairs with Thymine (A-T), Guanine pairs with Cytosine (G-C), connected by hydrogen bonds.

D. DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division.

• Bacteria have closed, circular DNA.

• Genome: The complete genetic material in an organism.

• Replication is semiconservative: Each new DNA molecule contains one original strand and one newly synthesized strand.

E. Major Requirements for Replication

• Template: Single-stranded DNA (parental DNA).

• RNA primer: Short segment of RNA that initiates DNA synthesis.

• Raw materials: Deoxynucleoside triphosphates (dNTPs) are assembled into the new strand.

• Enzymes: DNA polymerase is the main enzyme that synthesizes new DNA.

F. Direction of Replication

• New nucleotides are added to the 3' end of the growing strand.

• DNA polymerase can only add nucleotides in the 5' to 3' direction.

• Leading strand is synthesized continuously; lagging strand is synthesized discontinuously (Okazaki fragments).

Example: Synthesis on one template proceeds from left to right, then synthesis on the other strand must proceed from right to left.

Genetic Variation and Gene Transfer

A. Genetic Variation

Genetic variation arises from changes in the genetic material and is essential for evolution and adaptation.

B. Vertical vs. Horizontal Gene Transfer

• Vertical gene transfer: Transmission of genetic material from parent to offspring.

• Horizontal gene transfer: Transfer of genetic material between organisms that are not parent and offspring.

C. Mechanisms of Genetic Transfer in Bacteria

• Transformation: Uptake of free DNA from the environment.

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

• Transduction: Transfer of DNA from one bacterium to another via bacteriophages (viruses that infect bacteria).

Flow of Genetic Information: Protein Synthesis

A. Central Dogma of Molecular Genetics

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

• DNA → mRNA → protein

• Transcription: DNA is used as a template to make RNA.

• Translation: RNA is used to synthesize proteins.

B. Transcription

• Process by which DNA is used as a template to make a complementary strand of RNA.

• RNA differs from DNA: RNA contains ribose sugar and uracil instead of thymine.

• Three types of RNA:

  • mRNA (messenger RNA): Temporary copy of a gene, used as a template for protein synthesis. Contains codons.

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

  • tRNA (transfer RNA): Transfers amino acids to ribosomes for protein synthesis. Contains anticodon (complementary to codon on mRNA).

C. Genetic Code

• Set of rules that determine how a nucleotide sequence is converted into an amino acid sequence.

• Codons: Triplets of nucleotides on mRNA that code for specific amino acids.

• There are 64 codons, 20 amino acids, and start/stop codons.

Codon	Amino Acid	Function
AUG	Methionine	Start codon
UAA, UAG, UGA	None	Stop codons
UUU	Phenylalanine	Standard codon
Additional info: ...	Other amino acids	Standard codons

D. Translation

Translation is the process by which the information in mRNA is decoded to build a functional protein.

• rRNA and tRNA are important for translation.

• Translation occurs in three main stages:

  1. Initiation: Ribosome assembles on mRNA, tRNA brings the first amino acid (usually methionine).

  2. Elongation: tRNAs bring amino acids to the ribosome, peptide bonds form, and the polypeptide chain grows.

  3. Termination: Stop codon is reached, and the completed polypeptide is released.

Example: Peptidyl transferase catalyzes peptide bond formation during elongation.

Gene Regulation in Prokaryotes

A. Operons

Operons are clusters of genes under the control of a single promoter and operator, allowing coordinated regulation.

• Genes can be turned on/off as needed.

• Operons control multiple genes involved in a common function.

B. Model of Prokaryotic Gene Regulation: The Lac Operon

• Components of an operon:

  • Structural genes: Code for proteins (e.g., enzymes for lactose metabolism).

  • Control region: Promoter (where RNA polymerase binds) and operator (where repressor binds).

  • Regulatory gene: Produces repressor protein.

• Inducible operon: Transcription is "off" unless an inducer (e.g., lactose) is present.

• Lac operon is inducible; when lactose is present, the repressor is inactivated, and transcription occurs.

• Transcription of the lac operon requires absence of glucose and presence of lactose.

Errors and Mutations in Genes

A. Mutations

Mutations are changes in the genetic material of a cell and can be neutral, beneficial, or harmful.

B. Mutagens

• Agents that cause mutations (e.g., chemicals, radiation).

C. Spontaneous Mutations

• Occur in the absence of a mutagen.

D. Types of Mutations

• Point mutations:

  • Chemical change in one or a few base pairs of a single gene.

  • Substitutions: Replacement of one nucleotide with another.

  • Missense mutations: Change in amino acid sequence.

  • Nonsense mutations: Change resulting in a stop codon.

• Insertions and deletions:

  • Insertion: Addition of one or more nucleotides.

  • Deletion: Loss of one or more nucleotides.

  • Frameshift mutations: Change the reading frame, usually causing nonfunctional proteins.

  • Exception: Insertions or deletions of a group of three nucleotides may not cause frameshift.

Example: Sickle cell anemia is caused by a point mutation in the hemoglobin gene.

Additional info: The notes have been expanded with academic context and examples for clarity and completeness.