Chapter 6

Ch. 6 Molecular Biology & Genetics

Introduction

This chapter covers the molecular foundations of genetics, focusing on the structure and function of nucleic acids, the processes of DNA replication, transcription, and translation, and the molecular mechanisms underlying gene expression in microorganisms. Understanding these concepts is essential for studying microbial genetics and biotechnology.

Nucleic Acids: Structure and Components

Nucleotides: The Building Blocks

Nucleotides are the monomeric units that make up nucleic acids such as DNA and RNA. Each nucleotide consists of three main components:

• Phosphate Group: A negatively charged group containing phosphorus and oxygen. It links nucleotides together via phosphodiester bonds.

• Pentose Sugar: A five-carbon sugar. In DNA, this is deoxyribose; in RNA, it is ribose.

• Nitrogenous Base: A ring structure containing nitrogen. There are two types:

  • Pyrimidines: Cytosine (C), Thymine (T, in DNA), and Uracil (U, in RNA)

  • Purines: Adenine (A) and Guanine (G)

Example: The nucleotide adenosine monophosphate (AMP) contains a phosphate group, a ribose sugar, and the base adenine.

Comparison of Ribose and Deoxyribose

• Ribose: Found in RNA; has a hydroxyl group (-OH) on the 2' carbon.

• Deoxyribose: Found in DNA; lacks the 2' hydroxyl group (has only a hydrogen atom at this position).

Additional info: The absence of the 2' hydroxyl group in DNA increases its stability compared to RNA.

DNA Structure and Function

Double Helix and Base Pairing

DNA is composed of two antiparallel strands forming a double helix. The strands are held together by hydrogen bonds between complementary nitrogenous bases:

• Adenine (A) pairs with Thymine (T) via two hydrogen bonds.

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

The backbone of each strand is formed by alternating phosphate and sugar groups, with the bases projecting inward.

Antiparallel Orientation: One strand runs 5' to 3', while the other runs 3' to 5'.

DNA Compaction in Prokaryotes

• Bacterial and archaeal DNA is highly compacted to fit within the cell.

• Supercoiling, initiated by the enzyme gyrase, helps condense the DNA.

Additional info: Eukaryotic DNA is compacted using histone proteins, while most bacteria use nucleoid-associated proteins.

Genetic Information Flow: The Central Dogma

Overview

The central dogma of molecular biology describes the flow of genetic information:

• DNA → RNA → Protein

This involves two main processes:

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of proteins using the information in mRNA.

Reverse transcription (RNA → DNA) occurs in some viruses.

Types of Nucleic Acids in Microorganisms

Element	Type of Nucleic Acid	Structure
Virus genome	Single or double-stranded DNA or RNA	Short, circular or linear
Bacterial chromosome	Double-stranded DNA	Long, usually circular
Eukaryotic chromosome	Double-stranded DNA	Long, linear
Plasmid	Double-stranded DNA	Short, circular or linear
Transposable element	Double-stranded DNA	Found inserted in another DNA molecule

DNA Replication

Mechanism

DNA replication is the process by which a cell copies its DNA before cell division. It is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Initiation: Begins at the origin of replication.

• Elongation: DNA polymerase synthesizes new DNA in the 5' to 3' direction.

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

• Enzymes involved: Helicase (unwinds DNA), DNA polymerase (synthesizes new DNA), primase (synthesizes RNA primers), ligase (joins Okazaki fragments).

Equation:

$\text{(dNTP)}_n + dNTP \rightarrow \text{(dNTP)}_{n+1} + PP_i$

Additional info: DNA polymerase requires a primer to initiate synthesis and can only add nucleotides to the 3' end.

Transcription: DNA to RNA

Process and Enzymes

Transcription is the synthesis of RNA from a DNA template. It is catalyzed by RNA polymerase, which binds to promoter regions on DNA.

• Initiation: RNA polymerase binds to the promoter.

• Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction.

• Termination: RNA synthesis ends at a terminator sequence.

Differences between Prokaryotes and Eukaryotes:

• Prokaryotes: Transcription and translation are coupled; mRNA is often polycistronic (encodes multiple proteins).

• Eukaryotes: Transcription occurs in the nucleus; mRNA is monocistronic and undergoes processing (capping, splicing, polyadenylation).

Main Classes of RNA

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes.

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

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

Translation: RNA to Protein

Genetic Code and tRNA

The genetic code is read in triplets (codons) on mRNA, each specifying an amino acid. tRNA molecules have anticodons that pair with codons and carry the corresponding amino acid.

• Start Codon: AUG (codes for methionine in eukaryotes, formylmethionine in prokaryotes).

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

Wobble Position: The third base in a codon can often vary without changing the amino acid specified, allowing for redundancy in the genetic code.

Ribosomes and Protein Synthesis

• Ribosomes are composed of rRNA and proteins; they facilitate the assembly of amino acids into polypeptides.

• Translation occurs in three stages: initiation, elongation, and termination.

• In prokaryotes, multiple ribosomes can translate a single mRNA simultaneously (polysomes).

Protein Structure

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Local folding (α-helices, β-sheets) stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape of a single polypeptide.

• Quaternary Structure: Association of multiple polypeptide chains.

Gene Organization in Prokaryotes

Operons and Regulons

• Operon: A cluster of genes transcribed as a single mRNA, allowing coordinated expression (e.g., lac operon in E. coli).

• Regulon: A group of operons or genes regulated by the same regulatory protein (e.g., heat shock regulon).

Example: The lac operon enables bacteria to metabolize lactose only when it is present.

Summary Table: Key Differences in Genetic Processes

Feature	Prokaryotes	Eukaryotes
DNA Structure	Circular, supercoiled	Linear, chromatin-associated
Transcription Location	Cytoplasm	Nucleus
mRNA Processing	Rare (no capping, splicing)	Common (capping, splicing, poly-A tail)
Translation	Coupled with transcription	Separate from transcription
Operons	Common	Rare

Conclusion

Understanding the molecular mechanisms of genetics is fundamental to microbiology. The structure of nucleic acids, the processes of replication, transcription, and translation, and the organization of genes in prokaryotes and eukaryotes form the basis for genetic regulation, inheritance, and biotechnology applications.