Microbial structure and function Flashcards
General structural features of bacterial cells
- Cell shape
- Cell Wall
- Cell membrane(s)
- Capsules
- Pili and/or Fimbriae
- Cytoplasmic inclusions
- Bacterial DNA and nucleic acids
- Ribosomes
- Flagella
- Spores
CHECK DIAGRAM FOR BACTERIA LABELLED
What is Cell membrane / plasma membrane / inner membrane
Thin structure lying inside the cell wall and enclosing the
cytoplasm of the cell.
Role:
1) Selective barrier through which materials exit and enter thecell = selective permeability
2) Large molecules e.g. proteins can not pass through
membrane
3) Allows entry of smaller molecules e.g. H20, CO2 and some simple sugars
What is the Outer Membrane
- Thin structure lying beyond the the cell wall. Only
Gram-negative bacteria.
Role:
1) Selective barrier
2) Contains proteins for transport
3) Lipopolysaccharide – bacterial defence
4) Allows selective uptake and efflux
What do archaea contain but also lack
Archaea contain polysaccharides and proteins but lack
peptidoglycan
- Gram stain used to classify bacteria by cell wall composition:
What is gram positive and negative bacteria
Gram-positive bacteria have simpler walls with a large amount of peptidoglycan
Gram-negative bacteria have less peptidoglycan and an outer
membrane that contains lipopolysaccharides
What is the capsule
Outer layer of polymer made of extracellular
polysaccharide and/or proteins is present in some
prokaryotes.
Role:
1) adherence to a substrate, cells, surface such as
teeth (e.g. Streptococcus mutans)
2) protect bacteria from the host immune system such
as phagocytosis and complement (e.g.
Streptococcus pneumoniae)
What are fimbriae
Hair-like thin appendages
Role: allow attachment to substrates/surfaces or
other cells
What are pili
longer than fimbriae
Role: allow prokaryotes to exchange DNA
(conjugation)
What are flagella
- Flagella are long filamentous appendages consisting of a
filament, hook and motor.
-Composed of 42 different types of proteins.
-Scattered across the surface or concentrated at one or
both ends of the cell.
Role: Movement, Attachment, Secretory apparatus
Flagella & Motility
- Rotates clockwise or counter-clockwise by rotation from
the motor unit. - Bacterial flagella rotate to push the cell.
- Motile bacteria exhibit taxis:
What is positive and negative taxis
positive taxis = movement towards an attractant
negative taxis= movement away from a repellent
Whats Mesosome
Bacterial cells may have one or more large, irregular
folds.
Role:
* Many functions have been proposed: respiration etc
* Still unknown if cell processing artefacts or true cell
structures.
Nucleoid / nuclear body / DNA / chromosome
Prokaryotic genome has less DNA than the eukaryotic genome.
Circular continuous chromosome of dsDNA, not contained in a
nucleus but in nucleoid region with no surrounding membrane.
Most also have smaller rings of independently replicating DNA
called plasmids.
Role:
* DNA replication, transcription.
-Differences between prokaryotes and eukaryotic DNA
replication, transcription, and translation allows use of
antibiotics to inhibit bacterial growth without harming host.
Ribosomes
Cytoplasm of a bacterial cell contains numerous 70S
ribosomes (consisting of 50S and 30S subunits). Consists
of rRNA and protein.
Role:
* Protein synthesis
Endospores
Dormant, tough, non-reproductive structure.
Role:
* Permits survival of nuclear material for later germination.
* To ensure the survival of a bacterium through
environmental stress.
* Resistant to ultraviolet and gamma radiation, desiccation,
lysozyme, temperature, starvation, and chemical
disinfectants.
* Commonly found in soil and water, where they may
survive for long periods of time
Cysts
Dormant cells with thickened cells walls
* Permits survival of nuclear
material for later germination.
* To ensure the survival of a
bacterium through stress.
* Resistant to desiccation and
some chemicals, but cannot
withstand high temperatures
like endospores.
Structure and function: Fungi
- Fungal filaments absorb nutrients from the
soil, transfer them to trees and receive sugars
in return. - Some even transfer sugars between trees of
different species.
What does the body of fungi form
- Body of fungi form networks of branched hyphae
adapted for absorption. - Hyphae have tubular cell walls strengthened with chitin.
What is a septa
- Most fungi have hyphae divided into cells by septa, with
pores allowing cell-to-cell movement of organelles.
- Fungal hyphae form an interwoven mass called
mycelium.
* The structure of a mycelium maximizes surface-to-volume
ratio, making feeding very efficient.
What is a virus
- A virus is an infectious particle consisting of
DNA/genes packaged in a protein coat. - Viruses are much simpler in structure than even
prokaryotic cells. - Viruses cannot reproduce or carry out metabolism
outside of a host cell. - Viruses exist in an area between life-forms and
chemicals, leading a “borrowed life”
Structure and function: viruses
- Viruses are not cells.
- Small infectious particles consisting of nucleic
acids enclosed in a protein coat (in some cases, a
membranous envelope). - Viruses are obligate intracellular parasites =
replicate only within a host cell. - Each virus has a host range, a limited number of
host cells that it can infect.
What is Viral genomes
Double- or single-stranded DNA or RNA.
Viruses are classified as DNA viruses or RNA viruses.
Viruses have between three and 2,000 genes in their
genome.
Capsids and Envelopes
*Capsid = protein shell (capsomeres) that encloses the
viral genome.
*Variety of structures = helical or icosahedral.
*Viral envelopes (derived from membranes of host cells)
surround the capsids of influenza viruses and many
others.
*Viral envelopes = combination of viral and host cell
molecules.
What are bacteriophages
- Bacteriophages, also called
phages, are viruses that infect
bacteria. - They have an elongated capsid
head that encloses their DNA. - A protein tail piece attaches the
phage to the host and injects the
phage DNA inside.
Image size/ real size = magnification
What are Mutations
Mutations are random, inheritable (usually small) alterations in the
sequences of a genome. Mutations can change the phenotype.
What are selectable mutations
Selectable mutations:
could be selected after placing of organisms under conditions where their
growth will be favoured.
What are Non-selectable mutations
Non-selectable mutations:
could be identified only after testing the whole population of an
organisms.
What are mutation rates
Different mutations occur at different frequencies.
Spontaneous mutation rates for a typical bacterium vary
from 10^–7
to 10^–11 per base pair
What are mutagens
Mutagens are physical or chemical factors that increase the rate of mutations (in this case they are called induced mutations).
Mutagens can modify DNA in different ways, but those
changes are considered as mutations when they are
inherited.
What is point mutation
Point mutation is a mutation that involves a single base pair change.
Point mutations could be of two general categories:
- base–pair substitutions (one nucleotide is substituted with another);
- base–pair insertions or deletions (the nucleotides are inserted [added]
or deleted)
Point mutations affect proteins differently. how?
Base–pair substitutions may lead to silent, missense, or
nonsense mutations.
Insertions and deletions are mutations that have much more dramatic,
disastrous effects on resulted proteins than substitutions do.
This is because mRNA is read as triplets of nucleotides mutation leads to a shift in genetic
message (a frameshift mutation).
What is a substitution mutation also called
A substitution is called a silent mutation if it
has no effect on the encoded protein
What is a missense mutation?
A substitution is a missense mutation if
the altered codon causes encoding for a
different amino acid
What is a nonsense mutation
A substitution is a nonsense mutation if a
codon for an amino acid is converted into
a stop codon
What is the ames test?
Ames test: detecting mutagenic compounds
What is recombination
Recombination:
the process by which parts or all of the DNA molecules from two
separate sources are exchanged or brought together into a single
DNA molecule.
Mechanisms of recombination in prokaryotes involve DNA transfer
during any kind of genetic exchange
(transformation, conjugation and transduction).
What is homologous recombination
Homologous recombination
allows to exchange DNA fragments
from two different DNA molecules that
have homology (identity) in some regions.
What does homologous recombination involve?
This process involves an
endonuclease, single-strand binding
(SSB) protein and RecA protein.
Discovery of conjugation
This question was answered largely by Joshua Lederberg with the
help of Edward Tatum in 1946.
What are bacterial plasmids
Biological roles of bacterial plasmids cover not only a genetic
control of conjugation and gene transfer that provide a wide range of
genetic recombination events for adaptation and evolution.
A range of adaptive mechanisms encoded by bacterial
plasmids includes:
- antibiotic resistance,
- toxin production,
- metal resistance,
- degradation of organic compounds,
- utilisation of metabolic pathways of other organisms.
Bacterial plasmids are main tools for what?
Bacterial plasmids are main tools for genetic engineering
(based on genetic transformation).
What is transduction
Transduction:
gene transfer in bacteria mediated by phages (bacterial
viruses).
What are the 2 different transductions?
Generalised transduction: potentially any donor bacterial gene could be transferred.
Specialised transduction: only certain donor genes could be
transferred
Generalized transduction
Phages occasionally carry random
fragments of the donor’s bacterial
chromosome to another bacterium
(the recipient). The transferred
DNA can be integrated into the
recipient genome by
recombination.
Transposition takes place:
- between plasmids and chromosomes;
- within and among chromosomes.
What are transposons
Transposons are a group of mobile genetic elements that are defined as a DNA sequence. Transposons can jump into different places of the genome
What do microbial species represent and what’s the main advantage for biomedical research
Microbial species represent prokaryotic and eukaryotic
genetic systems.
The main advantage for bio-medical research: they are easily and
rapidly cultivable in laboratory by using microbiological techniques
What do bacteria have?
Bacteria (e.g. E. coli, Pneumococcus) have
small haploid genomes
and a universal genetic code allowing principal
genetic phenomena discovered at molecular level to
be transferable to higher genetic systems like
humans.
The most important microbial models
for eukaryotic genetics:
Neurospora crassa (the orange bread mould);
Saccharomyces cerevisiae (budding yeast, or baker’s yeast);
Schizosaccharomyces pombe (fission yeast);
Chlamydomonas reinhardtii (green alga).
Three important aspects of cytoplasmic
inheritance:
- not predictable phenotypic expression
(due to the copy number of mutated DNA; - random cytoplasmic segregation;
- maternal inheritance in crosses.
Plasmid-borne resistance to antibiotics
is provided by different mechanisms:
*Inactivation of antibiotics by
- breakdown of the molecule
- modification of the molecule
*Efflux of antibiotics out of bacterial cells
(example: Tc-resistance of pBR322)
*Protection of ribosomes by
- blocking access of antibiotic to ribosome
- changing ribosomal confirmation
In practice, microbial biotechnology involves:
-using microbial cells in certain industrial technological
processes;
- cultivation of microbial cells in large amounts for
getting their commercial products;
- improvement of existing microbial species/strains
for increased production of their useful products
(mutagenesis, genetic modification, construction
genetically new strains/species).
What do Magnetotactic bacteria do
Magnetotactic bacteria convert iron to magnetite
These nanoparticles are
considered to be used as a contrast medium
to improve magnetic resonance tomography
and to detect cancers at early stages.
What do Rhisobia do
Rhisobia – for inoculation of legume seeds
to ensure nitrogen fixation by the plants.
What are Probiotic bacteria
Probiotic bacteria – for adding to dairy products
What are Diatoms
Diatoms create precise structures from
silica that can be chemically converted to
silicon and potentially used in
nanotechnologies
What are plasmids popular for
Plasmids of antibiotic resistance
are popular vectors for genetic
engineering
Plasmids can carry transposons from one cell to another.
