Bacterial Genetics Flashcards
Genetic Material of Escherichia coli
The genetic material of Escherichia coli, a typical bacterium, is composed of a single circular DNA molecule. This DNA has a molecular weight of about 2 x 10^9 and consists of approximately 5 x 10^6 base pairs. This vast amount of genetic information is sufficient to code for around 2000 proteins, each with an average molecular weight of 50,000.
- To put this in perspective, imagine a long, continuous loop of DNA that contains all the instructions necessary for the bacterium to function and reproduce. The size and complexity of this DNA molecule are significant, given that it encodes thousands of proteins, each playing a critical role in the cell’s life processes.*
Haploidy in Bacteria
Bacteria are haploid organisms, meaning they have a single set of chromosomes and, consequently, a single copy of each gene. This is in contrast to diploid organisms (like humans), which have two copies of each gene. The haploid nature of bacteria implies that any mutation in a gene will directly affect the bacterium’s traits because there isn’t a second copy of the gene to compensate for the mutation. For example, if a gene responsible for antibiotic resistance mutates, the bacterium may lose its resistance to antibiotics, impacting its survival.
How do bacterials reproduce?
Binary Fission and Vertical Transmission
Bacteria reproduce through a process called binary fission, where a single bacterial cell divides into two identical daughter cells. This method of reproduction ensures that the genetic information is passed from the mother cell to the offspring, a process known as vertical transmission.
Think of binary fission as a form of cloning, where each new cell is an exact genetic copy of the original. This mode of reproduction is highly efficient and allows bacteria to rapidly colonize environments.
Pathogenicity Islands
Some bacterial species are particularly adept at causing diseases in higher organisms because they possess specific genes that encode pathogenic determinants—traits that enable them to infect and damage host tissues. These genes are often grouped together in the DNA, forming regions known as pathogenicity islands. These clusters of genes can be thought of as specialized tools that bacteria use to invade and survive within their hosts.
- For instance, a pathogenicity island might contain genes that allow the bacterium to produce toxins, evade the host’s immune system, or adhere to host cells. This clustering of pathogenic genes facilitates the coordinated expression of virulence factors, enhancing the bacterium’s ability to cause disease.
Housekeeping Genes and Plasmids
- Genes essential for basic bacterial growth and maintenance are typically located on the chromosome. These are often referred to as housekeeping genes because they are involved in fundamental processes like metabolism, DNA replication, and cell division.
- In addition to chromosomal DNA, many bacteria also possess plasmids—small, circular DNA molecules that exist independently of the chromosomal DNA. Plasmids often carry genes associated with specialized functions, such as antibiotic resistance or exotoxin production.
Imagine plasmids as bonus packs that bacteria can acquire, providing them with additional capabilities. For example, a plasmid might carry a gene that allows the bacterium to neutralize a specific antibiotic, giving it a survival advantage in environments where that antibiotic is present.
In summary, the genetic structure of bacteria like E. coli is a fascinating interplay of essential and specialized genes, allowing these microorganisms to thrive in diverse environments and sometimes cause disease in higher organisms.
What are bacteriophage?
Bacteriophages, often referred to simply as phages, are viruses that specifically infect prokaryotes (bacteria). Unlike viruses that infect eukaryotic cells (such as human cells), bacteriophages are uniquely adapted to hijack bacterial cellular machinery for their replication.
Describe the Structure of Bacteriophages
A typical bacteriophage consists of nucleic acid enclosed within a protein coat. This protein coat, also known as a capsid, provides protection to the nucleic acid and aids in the infection process by helping the phage attach to and penetrate bacterial cell walls
Variability in Nucleic Acid
One of the intriguing aspects of bacteriophages is the variability in their nucleic acid. This variability means that different phages can have different types of genetic material, which can be:?
Double-Stranded DNA (dsDNA): Many bacteriophages possess dsDNA as their genetic material. This form is similar to the DNA found in the chromosomes of higher organisms, consisting of two complementary strands that form a double helix.
Single-Stranded DNA (ssDNA): Some phages have ssDNA, which consists of a single strand of DNA. This type of genetic material is less common but still found in various phages.
Double-Stranded RNA (dsRNA): A few bacteriophages contain dsRNA. This type of genetic material consists of two complementary strands of RNA.
Single-Stranded RNA (ssRNA): Certain bacteriophages have ssRNA as their genetic material. Like ssDNA, this form consists of a single strand, but it’s made of RNA instead of DNA.
The diversity in the genetic material of bacteriophages influences their replication mechanisms and the ways they interact with their bacterial hosts. For instance:
- dsDNA Phages: These phages often follow a lytic or lysogenic cycle. In the lytic cycle, the phage injects its DNA into the bacterium, takes over the bacterial machinery to produce new phage particles, and eventually causes the bacterial cell to burst (lyse), releasing new phages. In the lysogenic cycle, the phage DNA integrates into the bacterial chromosome and replicates along with it until certain conditions trigger a switch to the lytic cycle.
- ssRNA and dsRNA Phages: These phages must bring their own enzymes or use the host’s enzymes to replicate their RNA genomes, as bacterial cells primarily work with DNA.
In summary, bacteriophages are viruses that infect bacteria, encapsulating their nucleic acid within a protective protein coat. The genetic material of phages shows significant variability, ranging from double-stranded DNA to single-stranded RNA, each form dictating the phage’s replication strategy and interaction with its bacterial host. This variability not only underscores the complexity of phage biology but also highlights their potential as tools in molecular biology and biotechnology, particularly in phage therapy and bacterial genetic research.
Bacteriophages, or phages, can be classified based on how they propagate within their bacterial hosts. Which are?
Understanding these modes of propagation is crucial for appreciating the different strategies phages use to replicate and survive.
Lytic Phages
Temperate Phages
Filamentous Phages
Explain the Lytic phages
Lytic phages are characterized by their aggressive replication strategy, producing numerous copies of themselves and ultimately causing the death of the host bacterial cell. This process is called the lytic cycle. The T-even phages, such as T2 and T4, which infect E. coli, are well-studied examples of lytic phages.
The life cycle of a lytic phage involves a highly coordinated sequence of events, starting with the infection of the bacterial cell. Upon infection, the phage injects its genetic material into the host, hijacking the host’s cellular machinery to produce viral components. These components are then assembled into new phage particles. Finally, the host cell is lysed (broken open), releasing the newly formed phages to infect other bacteria. The timing of viral gene expression is crucial in this process to ensure the proper assembly and release of new phage particles.
Explain the Temperate Phages strategy
Temperate phages have a more flexible replication strategy. They can enter a non-lytic state called the prophage state, where their genetic material integrates into the bacterial chromosome. In this state, the phage’s DNA is replicated alongside the host’s DNA during cell division. This means that the phage does not immediately kill the host cell; instead, it becomes a latent part of the host’s genome.
- The host bacteria carrying prophages are referred to as lysogenic. While in this lysogenic state, the phage does not produce new particles or harm the host. However, certain environmental or physiological signals can trigger the prophage to re-enter the lytic cycle, leading to the production of new phages and the eventual lysis of the host cell. The E. coli phage λ (lambda) is a well-characterized example of a temperate phage, illustrating how phages can balance between lysogenic and lytic cycles.
Explain the Filamentous Phages
Filamentous phages, such as the E. coli phage M13, exhibit unique characteristics that set them apart from lytic and temperate phages. These phages contain single-stranded DNA complexed with proteins, forming a filament-like structure. Unlike lytic phages, filamentous phages do not kill their host cells. Instead, they cause a chronic infection, where new phage particles are continuously produced and extruded from the host cell without causing cell lysis.
- During this process, the host cell is debilitated but remains viable. This means the host cell can continue to grow and divide, albeit at a slower rate due to the burden of producing new phages. Filamentous phages represent an interesting model of virus-host interaction, where the virus achieves a form of coexistence with the host.
The different modes of phage propagation have significant implications for both bacterial populations and potential therapeutic applications:
What are the use?
Lytic Phages: Because they kill their hosts, lytic phages can be used in phage therapy to target and destroy specific bacterial pathogens. This is particularly useful for treating antibiotic-resistant bacterial infections.
Temperate Phages: The ability of temperate phages to integrate into the bacterial genome can be exploited for genetic engineering purposes. For example, they can be used to insert specific genes into bacterial chromosomes, creating genetically modified bacteria for research or industrial applications.
Filamentous Phages: These phages offer insights into chronic viral infections and the balance between virus production and host cell viability. They are also useful in phage display technology, where foreign peptides or proteins are displayed on the surface of the phage for various biotechnological applications.
In summary, bacteriophages exhibit diverse strategies for replication and propagation, ranging from the destructive lytic cycle of T-even phages to the more symbiotic relationship seen with filamentous phages like M13. Understanding these differences is essential for harnessing the power of phages in medical and biotechnological fields.
Mutation refers to a change in the base sequence of DNA. This alteration can lead to the incorporation of a different amino acid into a protein, which may result in a change in the organism’s phenotype. The phenotype is the set of observable characteristics or traits of an organism, and mutations can lead to new traits that may be beneficial, neutral, or detrimental to the organism.
- Think of DNA as a set of instructions written in a specific language. A mutation is like a typo in these instructions, which can change the meaning of the message. Depending on where and how the typo occurs, the impact on the final product (the protein) can vary greatly.
What are the Types of bacterial Mutation
Base Substitution
Base substitution occurs when one base is replaced by another during DNA replication. This can happen due to errors made by DNA polymerase, the enzyme responsible for copying DNA. Alternatively, mutagens (substances that cause mutations) can alter the hydrogen bonding properties of the bases, leading to the incorporation of the wrong base into the new DNA strand.
- To illustrate, imagine a DNA sequence that reads A-T-G (where A pairs with T, and G pairs with C). During replication, if an A is mistakenly replaced with a G, the sequence might read G-T-G instead. This small change can have significant consequences, such as producing a protein with a different amino acid, potentially altering its function.
Frameshift Mutation
Frameshift mutation occurs when one or more base pairs are added or deleted from the DNA sequence. This type of mutation shifts the reading frame of the genetic code during translation on the ribosomes.
The genetic code is read in groups of three bases, known as codons, each of which specifies a particular amino acid. If the number of inserted or deleted bases is not a multiple of three, the reading frame shifts, changing every subsequent codon and leading to the production of a completely different protein.
- For example, consider a DNA sequence that reads ATG-CTG-ACA. If a base (say G) is added after the first codon, the sequence might read ATG-GCT-GAC-A. This insertion shifts the reading frame, changing the codons and potentially resulting in a nonfunctional protein.
What are the Impact of mutations on Proteins and Phenotypes
Mutations can have various effects on the resulting proteins and phenotypes:
Silent Mutations: Some mutations do not change the amino acid sequence of the protein due to the redundancy in the genetic code. These are called silent mutations and typically do not affect the organism’s phenotype.
Missense Mutations: When a mutation results in the substitution of one amino acid for another, it is called a missense mutation. This can alter the protein’s structure and function, potentially affecting the phenotype. Depending on the location and nature of the amino acid change, the impact can range from negligible to severe.
Nonsense Mutations: If a base substitution creates a stop codon, it results in a nonsense mutation. This type of mutation leads to premature termination of protein synthesis, usually producing a truncated, nonfunctional protein.
Frameshift Mutations: As previously explained, frameshift mutations can drastically alter the protein produced, often leading to a completely nonfunctional protein. The resulting phenotype can be significantly different from the original, and such mutations are usually detrimental.
Understanding mutations is critical for various fields, including genetics, medicine, and evolutionary biology. For instance?
Genetic Disorders: Many genetic disorders are caused by mutations. Sickle cell anemia, for example, is caused by a single base substitution in the hemoglobin gene, leading to misshaped red blood cells and various health issues.
Cancer: Mutations in certain genes can lead to uncontrolled cell growth and cancer. Understanding these mutations helps in developing targeted therapies to treat cancer.
Evolution: Mutations are a source of genetic variation, which is essential for evolution. Beneficial mutations can provide a selective advantage, leading to the evolution of new traits and species over time
In summary, mutations are changes in the DNA sequence that can have a wide range of effects on proteins and phenotypes. Base substitutions and frameshift mutations are two major types, each with distinct mechanisms and consequences. Studying mutations provides valuable insights into genetic diseases, cancer, and the process of evolution.
