Molecular Information Flow and Protein Processing in Microbiology

Molecular Information Flow and Protein Processing

Introduction

This section introduces the fundamental processes by which genetic information is stored, transmitted, and expressed in microbial cells. Understanding these processes is essential for comprehending how microorganisms function, adapt, and survive.

DNA and Genetic Information Flow

Genetic Elements and Genes

• Gene: The functional unit of genetic information, typically encoding a protein or functional RNA.

• Genetic elements: Large DNA molecules and/or chromosomes that contain genes.

• Nucleic acids: Macromolecules (DNA and RNA) that store and transmit genetic information via sequences of nucleotides.

• Polynucleotides: DNA (the genetic blueprint) and RNA (the transcription product) are polymers of nucleotides.

• Informational macromolecules: Include nucleic acids and proteins, which are essential for cellular structure and function.

Central Dogma of Molecular Biology

• Genetic information flows from DNA to RNA to protein.

• Replication: DNA is duplicated by DNA polymerase.

• Transcription: Information from DNA is transferred to RNA by RNA polymerase.

• Translation: Information in RNA is used to build polypeptides (proteins).

Diagram: The central dogma is often represented as:

$\text{DNA} \xrightarrow{\text{Replication}} \text{DNA} \xrightarrow{\text{Transcription}} \text{RNA} \xrightarrow{\text{Translation}} \text{Protein}$

Nucleic Acids: Structure and Components

Nucleotides and Nucleosides

• Nucleotide: Consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• Nucleoside: A nitrogenous base attached to its sugar, but lacking a phosphate group.

• Four nucleotides in DNA:

  • Adenine (A): Purine

  • Guanine (G): Purine

  • Cytosine (C): Pyrimidine

  • Thymine (T): Pyrimidine

  • Uracil (U): Pyrimidine found in RNA instead of thymine

• Nitrogen bases are attached to the pentose sugar by a glycosidic linkage between carbon atom 1 of the sugar and a nitrogen atom in the base (N1 in pyrimidines, N9 in purines).

DNA and RNA Structure

• Backbone: Alternating phosphates and the pentose sugar (deoxyribose in DNA, ribose in RNA).

• Phosphodiester bonds: Connect the 3'-carbon of one sugar to the 5'-carbon of the adjacent sugar.

• Primary structure: The sequence of nucleotides that encodes genetic information.

• Double helix: DNA is double-stranded and held together by hydrogen bonding between complementary bases (A-T and G-C).

• Antiparallel strands: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

• Major and minor grooves: Protein-DNA interactions usually occur in the major groove.

Base Pairing and Complementarity

• Adenine (A) pairs with Thymine (T) in DNA (or Uracil (U) in RNA) via two hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

Size and Supercoiling of DNA

• DNA size is expressed in base pairs (bp), kilobase pairs (kbp), or megabase pairs (Mbp).

• Example: Escherichia coli genome is 4.64 Mbp.

• Linear DNA length is much longer than the cell, so DNA is compacted by supercoiling.

• Topoisomerases: Enzymes that insert or remove supercoils.

• Negative supercoiling: Twisted in the opposite sense to the right-handed double helix; found in most cells.

• DNA gyrase: Introduces negative supercoils into DNA via double-strand breaks.

• Positive supercoiling: Helps prevent DNA melting at high temperatures (common in Archaea).

Examples and Applications

• Jumbo Proteins: While the average protein size in all three domains of life is less than 500 amino acids, some organisms like Haloquadratum walsbyi encode much larger proteins, such as halomucin (9,159 amino acids).

• Biological Information Flow: The genetic blueprint is responsible for the distinctive attributes and survival mechanisms of microorganisms, as it directs the synthesis of proteins from DNA.

Table: Comparison of DNA and RNA Components

Component	DNA	RNA
Sugar	Deoxyribose	Ribose
Pyrimidine Bases	Cytosine, Thymine	Cytosine, Uracil
Purine Bases	Adenine, Guanine	Adenine, Guanine
Strandedness	Double-stranded	Single-stranded (usually)
Function	Genetic blueprint	Transcription product, protein synthesis

Additional info:

• Supercoiling is essential for DNA compaction and accessibility in prokaryotes, which lack a nucleus.

• Proteins involved in replication and transcription must access tightly packed DNA, which is facilitated by DNA supercoiling and the action of topoisomerases.

• Prokaryotic DNA replication is highly efficient, occurring at rates up to 1,000 nucleotides per second, while transcription occurs at about 40 nucleotides per second.