Microbial Genetics and Genetic Engineering: Study Notes

Microbial Genetics and Genetic Engineering

Introduction to Genetics and Genes

Genetics is the study of inheritance, or heredity, in living organisms. It explores how traits are passed from one generation to the next through genetic material. The fundamental unit of heredity is the gene, which is located on a chromosome and provides information for specific cellular functions. The genome is the sum total of genetic material in an organism, which may include chromosomes, plasmids, and, in some cases, organelles such as chloroplasts and mitochondria.

• Gene: Segment of DNA responsible for a specific trait or function.

• Genome: Complete set of genetic material in a cell or organism.

• Eukaryotic chromosomes: Linear in structure.

• Prokaryotic chromosomes: Typically a single, circular DNA molecule.

Locations and Forms of the Genome in Cells and Viruses

The genome can be found in various cellular and viral structures. In eukaryotes, genetic material is located in the nucleus (chromosomes), mitochondria, and chloroplasts. In prokaryotes, it is found in the cytoplasm as a single circular chromosome and sometimes as plasmids. Viruses may contain either DNA or RNA as their genetic material.

• Plasmids: Small, circular DNA molecules found in some bacteria and eukaryotic microbes, often carrying genes for advantageous traits such as antibiotic resistance.

• Chromosomes: Main genetic structures in both prokaryotes and eukaryotes.

Genome Size Examples

• Escherichia coli genome: Single chromosome with 4,288 genes; if stretched linearly, it would be about 1 mm long.

• Human genome: 46 chromosomes with approximately 25,000 genes; total length of DNA is about 6 feet per cell if unraveled.

DNA Structure

Nucleotide Structure

The basic unit of DNA is the nucleotide, which consists of a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, thymine, cytosine, or guanine). DNA is a double helix formed by two complementary strands held together by hydrogen bonds between paired bases.

• Base pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

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

DNA Replication

Overview of Replication

DNA replication is the process by which a cell copies its DNA before cell division. It involves the coordinated action of many enzymes to separate the DNA strands and synthesize new complementary strands. The process is semi-conservative, meaning each new DNA molecule contains one original (parental) strand and one newly synthesized strand.

• Key enzymes: DNA polymerase, helicase, primase, ligase.

• Replication fork: The Y-shaped region where the DNA is split into two strands for copying.

Gene Expression: From DNA to Protein

Central Dogma of Molecular Biology

The central theme of biology is the flow of genetic information from DNA to RNA to protein. This process involves two main stages: transcription and translation.

• Transcription: The synthesis of RNA from a DNA template, catalyzed by RNA polymerase. The resulting RNA is called messenger RNA (mRNA).

• Translation: The process by which the mRNA sequence is used to assemble amino acids into a protein at the ribosome.

The Genetic Code

The genetic code consists of codons, which are groups of three nucleotides on mRNA that specify particular amino acids. There are 64 possible codons but only 20 amino acids, so some amino acids are specified by more than one codon (redundancy). Start and stop codons signal the beginning and end of translation.

• Start codon: AUG (codes for methionine)

• Stop codons: UAA, UAG, UGA (do not code for amino acids)

Genetic Variation and Recombination

Recombination in Bacteria

Recombination is the process by which genetic material is exchanged between different DNA molecules, resulting in new genetic combinations. In bacteria, this often involves plasmids or chromosomal fragments and leads to the creation of recombinant strains with new traits.

• Plasmids: Can carry genes for antibiotic resistance and other advantageous traits.

• Recombinant: A cell or organism that has acquired new genetic material from another source.

Horizontal Gene Transfer Mechanisms

Bacteria can exchange genetic material through three main mechanisms: conjugation, transformation, and transduction.

• Conjugation: Direct transfer of DNA from one bacterium to another via a pilus or cell-to-cell contact. Both Gram-positive and Gram-negative bacteria can perform conjugation.

• Transformation: Uptake of free DNA fragments from the environment by a bacterial cell. This process does not require direct contact between cells.

• Transduction: Transfer of bacterial DNA from one cell to another via a bacteriophage (virus that infects bacteria).

Conjugation: Bacterial "Sex"

Conjugation involves the transfer of plasmids or other genetic material through a direct connection between donor and recipient cells. In Gram-negative bacteria, a pilus forms to connect the cells, while in Gram-positive bacteria, an opening forms between adjacent cells.

Transformation: Capturing DNA from the Environment

Transformation is the process by which bacteria take up free DNA fragments from their surroundings. This DNA can integrate into the bacterial chromosome, leading to genetic changes. Transformation is a key tool in recombinant DNA technology.

Transduction: Viral-Mediated DNA Transfer

Transduction occurs when a bacteriophage transfers DNA from one bacterium to another. During the infection process, fragments of bacterial DNA can be packaged into new phage particles and delivered to other cells, facilitating genetic exchange.

Summary Table: Mechanisms of Horizontal Gene Transfer

Mechanism	Description	Requirement for Cell Contact	Genetic Material Transferred
Conjugation	Direct transfer of DNA via pilus or cell contact	Yes	Plasmids, sometimes chromosomal DNA
Transformation	Uptake of free DNA from environment	No	DNA fragments
Transduction	DNA transfer via bacteriophage	No	Any gene (generalized), specific genes (specialized)

Additional info: These mechanisms contribute to genetic diversity in microbial populations and play a significant role in the spread of antibiotic resistance and other traits.