BIOL211 – Fundamentals of Microbiology Notes

BIOL211 – Fundamentals of Microbiology

Instructor Information

  • Instructor: Dr. Professor Mercado

  • Semester: Fall 2025

  • Poll Everywhere Username: sdsumicrobes

  • Sign in with SDSU email

Chapter 7: Microbial Genetics

  • Date: Oct 1st, 2025

Learning Objectives

  • Understand DNA replication in eukaryotic and prokaryotic cells

  • Identify the “players” in DNA replication

  • Understand transcription and translation in eukaryotic and prokaryotic cells

  • Identify the “players” in transcription and translation

  • Define operons and understand their role for gene expression in bacteria

DNA Replication

Key Concepts

  • Parent vs Daughter Strand

    • The parental DNA strands serve as templates for the synthesis of new, complementary daughter DNA strands.

    • The term complementary refers to the mirror image of base pairing rules:

    • Adenine (A) pairs with Thymine (T)

    • Cytosine (C) pairs with Guanine (G)

    • DNA strands “unzip” to begin the replication process, facilitated by the enzyme helicase.

Origin of Replication

  • Definition: Site where DNA replication begins.

  • Characteristics:

    • Recognized by initiator proteins

    • Contains A-T rich sequences (approximately 250 base pairs in length)

Role of DNA Polymerase

  • Overview:

    • DNA polymerase is a crucial enzyme in DNA replication that moves along the DNA strand, reading the sequence of the parental strand to add nucleotides to the growing daughter strand.

    • Structure: The replication fork forms a Y-shaped structure.

    • Error Rate: DNA polymerase can make errors during the process, leading to potential mutations.

Summary of DNA Replication

  • DNA replication:

    • Begins at the origin of replication

    • Initiator proteins recognize the start point

    • DNA polymerase synthesizes the complementary daughter strand following base pairing rules

    • DNA consists of purines (A, G) and pyrimidines (T, C)

    • Eukaryotic DNA: Has multiple origins of replication

    • Prokaryotic DNA: Has a single origin of replication

Gene Expression

Key Concepts

  • Definition: Gene expression is the biological conversion of a nucleotide sequence (DNA) into a protein.

  • Process Overview:

    • Transcription converts DNA to RNA

    • Translation converts mRNA to Protein

Transcription: DNA to RNA

  • The process involves using DNA to create a complementary RNA strand.

  • Types of RNA:

    • Messenger RNA (mRNA): A temporary single-stranded RNA molecule

  • Composition: RNA is made of nucleotides: Adenine (A), Cytosine (C), Guanine (G), and Uracil (U).

  • Sequence Length: Transcription produces short sequences of RNA.

Role of RNA Polymerase

  • Function:

    • Recognizes the promoter region, which marks the beginning of a gene.

    • In prokaryotic cells, RNA polymerase directly binds to the promoter.

    • In eukaryotic cells, RNA polymerase binds to transcription factors that interact with the promoter sequence.

    • Note: Transcription only occurs on the coding strand of DNA.

Eukaryotic Genes

  • Eukaryotic genes contain both coding regions (exons) and noncoding sequences (introns).

    • Introns: Noncoding sequences that are removed during RNA processing.

    • Exons: Coding sequences that are transcribed into mRNA.

  • Example:

    • Initial Transcription (Pre-mRNA):

    • Contains introns and exons.

    • Processed mRNA:

    • Final mRNA contains only exons (e.g., Exon 1, Exon 2, Exon 3, etc.) moving forward into translation.

Summary of Transcription

  • Process: RNA polymerase reads the DNA sequence and converts it into mRNA through base pairing rules.

  • Composition: mRNA is composed of nucleotides (A, G, C, U).

  • Promoter Recognition: RNA polymerase identifies the promoter region of the coding strand.

  • In eukaryotic DNA, only exons are needed to construct the final mRNA molecule.

Translation: mRNA to Protein

  • The process converts mRNA into an amino acid sequence, resulting in protein synthesis.

  • Amino Acids: Building blocks of proteins, connected in a specific sequence.

  • Role of Ribosomes:

    • Ribosomes bind to the mRNA molecule and recognize RNA sequences as codons (three bases at a time).

  • The translation process continues until a stop codon, which does not encode for an amino acid, is reached.

Ribosomes and tRNA

  • Function of tRNA:

    • Transfer RNA (tRNA) is responsible for bringing the anticodon that is complementary to a specific codon on the mRNA strand, facilitating the correct positioning of amino acids in the growing polypeptide chain.

Genetic Code

  • The first codon is always AUG, which codes for Methionine (Met).

  • Stop Codons:

    • Three stop codons: UAA, UAG, UGA.

  • The genetic code specifies which amino acids correspond to each codon.

Summary of Translation

  • Ribosomes read the mRNA sequence codon by codon to produce the corresponding amino acid sequence.

  • Start codons indicate where the translation should begin, while stop codons signify termination.

  • tRNA brings the appropriate anticodon linked to the amino acid that matches the codon on the mRNA.

Differences in Eukaryotic and Prokaryotic Transcription and Translation

Eukaryotic Cells

  • Transcription Location: Nucleus

  • mRNA produced must exit the nucleus before reaching ribosomes in the cytoplasm.

  • Transcription and translation are separate processes, enhancing regulatory mechanisms.

  • Ribosomes are larger and consist of more proteins and rRNA than prokaryotic ribosomes.

Prokaryotic Cells

  • Transcription Location: Cytoplasm, direct binding to the DNA.

  • Transcription and translation occur simultaneously, promoting rapid protein production.

  • Ribosomes are smaller and structurally different from eukaryotic ribosomes, allowing a different approach to protein synthesis.