Blueprint of Life: DNA, RNA, Microbial Genetics, Fungi, and Viruses – Study Notes

Chapter 7: The Blueprint of Life, from DNA to Protein

DNA vs. RNA

DNA and RNA are nucleic acids essential for genetic information storage and transfer in all living organisms. They differ in structure, function, and location within the cell.

• DNA (Deoxyribonucleic Acid):

  • Double-stranded helix held by hydrogen bonds between complementary bases (A-T, G-C).

  • Contains deoxyribose sugar and phosphate backbone.

  • Found in the nucleus of eukaryotic cells.

• RNA (Ribonucleic Acid):

  • Single-stranded; bases are A, U, G, C (uracil replaces thymine).

  • Found both inside and outside the nucleus.

  • Types:

    • mRNA (messenger RNA): Carries genetic code from DNA to ribosome for protein synthesis.

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

    • tRNA (transfer RNA): Transfers amino acids to ribosome, matching codons in mRNA.

DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division, ensuring genetic continuity.

• Key Enzymes and Steps:

  • DNA Polymerase: Synthesizes new DNA strands by adding nucleotides to a template strand.

  • Helicase: Unwinds the double helix, separating the two DNA strands.

  • Ligase: Seals gaps between Okazaki fragments on the lagging strand.

  • Primase: Synthesizes short RNA primers needed to start DNA synthesis.

  • Gyrase: Relieves supercoiling ahead of the replication fork.

  • RNase: Degrades RNA primers after DNA synthesis.

  • Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Semiconservative Replication: Each new DNA molecule consists of one parental and one new strand.

DNA Polymerase

DNA polymerases are enzymes that synthesize DNA molecules from deoxyribonucleotides, the building blocks of DNA.

• They use one DNA strand as a template to synthesize a complementary strand.

• Nucleotides are added only to the 3' end of the growing DNA strand.

Transcription

Transcription is the process of synthesizing RNA from a DNA template, a key step in gene expression.

• RNA Polymerase: Synthesizes single-stranded RNA using DNA as a template.

• mRNA Transcript: The RNA molecule formed during transcription, complementary to the DNA template.

• rRNA: Forms part of the ribosome.

• tRNA: Carries amino acids to the ribosome during translation.

Translation and Post-Translational Modifications

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. Post-translational modifications further process the protein for functionality.

• Pre-mRNA Processing (Eukaryotes):

  • Exons (coding regions) and introns (non-coding regions) are present in the initial transcript.

  • Introns are removed, and exons are joined to form mature mRNA.

  • 5' cap and 3' poly-A tail are added for stability and ribosome binding.

• Translation:

  • Occurs in the cytoplasm at ribosomes.

  • mRNA codons are read in sets of three nucleotides, each coding for a specific amino acid.

  • tRNA brings amino acids to the ribosome, matching codons via its anticodon.

  • Polypeptide chains are formed and folded into functional proteins.

• Post-Translational Modifications: Proteins may be folded, cleaved, or chemically modified (e.g., phosphorylation, glycosylation) to become functional.

Genetic Code and Codons

The genetic code is the set of rules by which information encoded in mRNA is translated into proteins.

• Codon: A sequence of three nucleotides on mRNA that codes for a specific amino acid.

• Start Codon: AUG (codes for methionine).

• Stop Codons: UAA, UAG, UGA (signal termination of translation).

• Anticodon: Three-nucleotide sequence on tRNA complementary to mRNA codon.

• Polyribosome: Multiple ribosomes translating a single mRNA simultaneously.

Given a DNA Sequence: Determining the RNA Transcript

To determine the RNA transcript from a DNA sequence:

1. Identify the template (antisense) DNA strand.

2. Transcribe the sequence by replacing T with U and using complementary base pairing.

Eukaryotic vs. Prokaryotic Gene Expression

Gene expression differs between prokaryotes and eukaryotes, particularly in ribosome structure and the timing/location of transcription and translation.

Feature	Prokaryotes	Eukaryotes
Ribosome Size	70S (50S + 30S)	80S (60S + 40S)
Location	Cytoplasm	Cytoplasm (translation), Nucleus (transcription)
Antibiotic Sensitivity	Sensitive	Not sensitive
Transcription/Translation	Simultaneous	Separated

Key Takeaway: Eukaryotic ribosomes are larger and more specialized, allowing for more controlled protein synthesis.

Chapter 8: Microbial Genetics

Mutation

A mutation is a change in the nucleotide sequence of DNA, which can be inherited by daughter cells or transferred horizontally between organisms.

• Vertical Gene Transfer: DNA passed from parent to offspring.

• Horizontal Gene Transfer (HGT): DNA transferred between organisms by transformation, conjugation, or transduction.

Types of Mutations

• Silence Mutation: No change in the amino acid sequence.

• Missense Mutation: Base substitution resulting in a different amino acid.

• Nonsense Mutation: Change that creates a stop codon, truncating the protein.

• Frameshift Mutation: Insertion or deletion of nucleotides, altering the reading frame.

Intercalating Agents

Intercalating agents are chemicals that insert between DNA bases, increasing the likelihood of frameshift mutations by distorting the DNA helix.

Radiation: UV and X-ray

• UV Radiation: Causes thymine dimers, distorting DNA and blocking replication.

• X-rays: Cause single and double-strand breaks, leading to deletions.

The Ames Test

The Ames test screens chemicals for their potential to cause mutations (carcinogenicity). Developed by Bruce Ames.

Horizontal Gene Transfer Mechanisms

• Transformation: Uptake of naked DNA from the environment by a bacterial cell.

• Competent Cells: Cells capable of taking up DNA via transformation.

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

• Transduction: Transfer of DNA from one bacterium to another via bacteriophage (virus).

  • Generalized Transduction: Any gene can be transferred.

  • Specialized Transduction: Only specific genes near the phage integration site are transferred.

Plasmids and Transposons

• Plasmids: Small, circular DNA molecules carrying accessory genes, separate from the bacterial chromosome. F plasmid (fertility factor) enables conjugation.

• Transposons: DNA sequences that can move (transpose) within the genome, also called "jumping genes."

Fungi, Protozoa, and Viruses

Fungi

• Types: Yeasts (unicellular), molds (multicellular), mushrooms (reproductive structures).

• Cell Structure: Characterized by cell walls containing chitin; membranes contain ergosterol.

• Dimorphic Fungi: Can exist as either mold or yeast depending on environmental conditions.

• Growth Requirements: Moist environments, organic material, and specific temperature/pH ranges.

• Roles: Decomposers, pathogens, and symbionts.

Protozoa

• Unicellular eukaryotes, often motile, and found in aquatic environments.

• Some cause diseases such as malaria (Plasmodium), sleeping sickness (Trypanosoma), and amoebic dysentery (Entamoeba histolytica).

Slime Molds and Water Molds

Slime molds and water molds were once classified as fungi but are now considered protists due to differences in cell wall composition and life cycle.

Viruses

• Definition: Acellular infectious agents composed of nucleic acid (DNA or RNA) and a protein coat (capsid).

• Structure: May have an envelope, capsid, nucleocapsid, and spikes for attachment.

• Replication: Require host cells for replication; steps include attachment, penetration, transcription, replication, assembly, and release.

• Types of Infection: Lytic (productive), temperate (latent), lysogenic conversion, and filamentous phage infection.

• CRISPR System: Bacterial defense mechanism against phages, involving sequence-specific recognition and cleavage of foreign DNA.

• Prions: Infectious proteins causing neurodegenerative diseases.

Animal Viruses

• Replication involves antigenic shift and drift, leading to genetic variation.

• Can cause acute or persistent infections.

• Laboratory techniques include plaque assays and electron microscopy (EM).

Key Terms Table

Term	Definition
Capsid	Protein shell of a virus
Nucleocapsid	Capsid plus nucleic acid
Capsomere	Protein subunit of the capsid
Spikes	Attachment proteins on viral surface
Productive Infection	Virus actively replicates and produces new virions
Lytic Phage	Phage that lyses the host cell
Temperate Phage	Phage that can integrate into host genome (lysogeny)
Latent Infection	Viral genome persists without active replication
Lysogenic Conversion	Change in host phenotype due to prophage
Filamentous Phage	Phage that extrudes from host without lysis

Additional info:

• Some details, such as the full mechanism of CRISPR or the structure of prions, may require further reading for comprehensive understanding.