Microbiology Study Guide: Heredity, DNA Replication, Gene Expression, Mutations, and Genetic Variation

Section 5.1: Heredity Basics

What is a gene?

A gene is a segment of DNA that contains the instructions for synthesizing a specific protein or functional RNA molecule. Genes are the fundamental units of heredity and are passed from parents to offspring.

• Genotype: The genetic makeup of an organism; the set of genes it carries.

• Phenotype: The observable characteristics or traits of an organism, resulting from the interaction of its genotype with the environment.

Prokaryotic vs. Eukaryotic Chromosomes

Chromosomes are structures that organize and package DNA. Prokaryotes typically have a single, circular chromosome, while eukaryotes have multiple, linear chromosomes.

• Prokaryotic chromosomes: Usually circular, found in the cytoplasm.

• Eukaryotic chromosomes: Linear, located in the nucleus, associated with histone proteins.

Nucleic Acids

Nucleic acids are polymers made up of nucleotides. The two main types are DNA and RNA.

• DNA (Deoxyribonucleic acid): Contains the genetic blueprint for life.

• RNA (Ribonucleic acid): Involved in protein synthesis and gene regulation.

The four nucleotides that comprise DNA are:

• Adenine (A)

• Thymine (T)

• Cytosine (C)

• Guanine (G)

RNA contains Uracil (U) instead of Thymine.

DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double-stranded	Single-stranded
Nitrogen Bases	A, T, C, G	A, U, C, G
Function	Genetic information storage	Protein synthesis, gene regulation

Types of RNA

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

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

• rRNA (ribosomal RNA): Forms the core of ribosome structure and catalyzes protein synthesis.

Transcription is the process by which RNA is synthesized from a DNA template. Translation is the process by which proteins are synthesized from mRNA.

• Transcription: DNA → RNA

• Translation: RNA → Protein

Some RNAs are functional and not translated into proteins (e.g., rRNA, tRNA).

Section 5.2: DNA Replication

What is DNA replication?

DNA replication is the process by which a cell copies its DNA before cell division. It ensures that each daughter cell receives an identical set of genetic information.

• Replication is semiconservative: each new DNA molecule consists of one old strand and one new strand.

• Replication errors can lead to mutations.

Enzymes in DNA Replication

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

• Helicase: Unwinds the DNA double helix.

• Primase: Synthesizes RNA primers needed to start replication.

• Ligase: Joins Okazaki fragments on the lagging strand.

Leading vs. Lagging Strand

• Leading strand: Synthesized continuously in the direction of the replication fork.

• Lagging strand: Synthesized discontinuously, forming short segments called Okazaki fragments.

Okazaki fragments are joined together by DNA ligase.

Section 5.3: Protein Synthesis (Gene Expression)

What is gene expression?

Gene expression is the process by which the information encoded in a gene is used to direct the synthesis of a protein or functional RNA. The end product of gene expression is typically a protein.

Transcription

Transcription is the synthesis of RNA from a DNA template. The enzyme responsible for transcription is RNA polymerase.

Section 5.4: Regulating Protein Synthesis

What is an operon?

An operon is a cluster of genes under the control of a single promoter and regulatory elements, allowing coordinated expression in prokaryotes.

• Example: The lac operon in Escherichia coli regulates lactose metabolism.

Gene Regulation

• Gene expression can be regulated at the transcriptional, translational, and post-translational levels.

• Environmental signals, such as nutrient availability, can affect gene expression.

Section 5.5: Mutations

What is a mutation?

A mutation is a change in the nucleotide sequence of DNA. Mutations can be spontaneous or induced by external factors.

• Point mutation: Change in a single nucleotide.

• Insertion: Addition of one or more nucleotides.

• Deletion: Removal of one or more nucleotides.

• Frameshift mutation: Insertion or deletion that alters the reading frame of a gene.

Mutations can be beneficial, neutral, or harmful, depending on their effect on protein function.

Section 5.6: Genetic Variation Without Sexual Reproduction

Horizontal Gene Transfer

Horizontal gene transfer is the movement of genetic material between organisms other than by descent from parent to offspring. It is a major source of genetic diversity in prokaryotes.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

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

Transposons

Transposons are DNA sequences that can change their position within the genome, sometimes creating or reversing mutations and altering the cell's genetic identity.

• Cut-and-paste transposons: Move by excising themselves from one location and inserting into another.

• Copy-and-paste transposons: Replicate themselves and insert the copy elsewhere in the genome.

Transposons can carry antibiotic resistance genes and contribute to genetic variation.

Additional info: The study guide covers foundational topics in microbial genetics, including gene structure, DNA replication, gene expression, mutation types, and mechanisms of genetic variation in prokaryotes. These concepts are essential for understanding microbial evolution, adaptation, and biotechnology applications.