Prokaryote vs. Eukaryote Genetics

Prokaryote vs. Eukaryote Genetics

• Prokaryotes are haploid, containing a single circular chromosome.
• As a result of their single circular chromosome, prokaryotes have one copy of each gene
• Prokaryotes contain small circular DNA molecules called plasmids that confer useful properties like drug resistance.
• Only circular DNA molecules can replicate in prokaryotes.
• Eukaryotes are diploid, having a pair of each chromosome and thus two copies of each gene.
• Eukaryotes have linear chromosomes, usually more than one.
• In eukaryotes, transcription of genes into RNA occurs in the nucleus.
• Translation of RNA into protein occurs in the cytoplasm in eukaryotes.
• In eukaryotes, transcription and translation are separated processes.
• In prokaryotes, translation is coupled to transcription, starting translation of the new RNA molecule before transcription finishes.
• In diploid cells, one allele of a gene may be dominant and expressed as a protein, while another allele may be recessive and not expressed.
• In haploid cells, any mutated gene that is not expressed results in a cell that has lost that trait.

Bacterial Genetics

• Other genetic elements in bacteria include plasmids and transposons.
• Reproduction in bacteria is asexual.
• Bacteria need other mechanisms for diversity due to asexual reproduction.
• Bacteria display a poor ability to control their environment.

Genetic Elements: Plasmids

• Plasmids are small, circular pieces of double-stranded DNA.
• Plasmids replicate independently of the bacterial chromosome.
• Conjugative plasmids contain F factor (fertility factor) from E. coli.
• F factor carries genes encoding the sex pilus for physical transfer of genetic material
• R factors, also known as MDR plasmid, confer multiple drug resistance.
• Hfr is a conjugative plasmid integrated into the chromosome that has a high frequency of recombination, and passes this trait to receptive bacteria during conjugation.

Genetic Elements: Transposons

• Transposons are DNA segments.
• Simple transposons are insertional sequence elements (IS elements) like Tn3, flanked by reverted repeat sequences that facilitate insertion into the bacterial genome
• Composite transposons contain drug resistance gene core area, flanked by IS elements, like Tn9, conferring multiple drug resistance by genetic "cassette" transfer.

Genes and Proteins

• Transcription and translation are key processes for protein synthesis.
• DNA is transcribed to mRNA via RNA polymerase.
• mRNA is translated to protein via a ribosome.
• Genes code for proteins.
• Gene expression and the effects of expressed genes need regulation.
• Genes are not always expressed, or the effect is not always apparent.

Genotype and Phenotype

• Genotype comprises all the genetic elements in a cell.
• Phenotype is the physical or observable effect of the genotype.
• Phenotype depends on the context of observation.
• Phenotype depends on the level of expression of certain genes.
• Two visibly different colonies on a plate have different phenotypes and probably different genotypes.
• Two visibly identical colonies on a plate have same phenotype, but the genotype could be the same.

DNA Replication

• Bacterial chromosome replicates before binary fission.
• Replication starts at a fixed point, the origin of replication.
• Replication is usually bidirectional, with two V-shaped replication forks forming and moving in opposite directions from the origin.
• DNA polymerase is an enzyme that synthesizes a new DNA strand from a DNA template.
• DNA ligase is an enzyme that binds together short DNA fragments.

Protein Synthesis

• DNA undergoes transcription to create RNA, which is then translated into protein.

Transcription

• RNA polymerase synthesizes an RNA polynucleotide from the DNA template.
• Types of RNA include mRNA (encodes different genes/message and contains codons), rRNA (framework of ribosome), and tRNA (contains anticodons).

Translation

• Translation occurs as the ribosome moves along the mRNA.
• Codons in mRNA are exposed to sites within the ribosome.

Stages of Translation

• Chain initiation occurs, the tRNA anticodon recognizes the start codon (AUG).
• Chain elongation occurs, enzyme attaches increase of chain, enzyme attaches AA by a peptide bond which ATP and Guanosine triphosphate GTP supply the energy to the reaction.
• Chain termination continues until the ribosome reaches a stop codon (UGA, UAA, UAG).

Mutations

• A mutation changes the base sequence of DNA, often leading to the insertion of a different amino acid into a protein and an altered phenotype.
• Mutations result from base substitution, frameshift mutation or transposons/insertion sequences.

Base Substitution

• Base substitution occurs when one base is inserted in place of another.
• Missense mutation results the base substitution leads to a codon causing insertion of a different amino acid
• Nonsense mutation results when the base substitution generates a termination codon and stops protein synthesis prematurely, almost always destroying protein function.

Frameshift Mutation

• Frameshift mutation occurs when one or more base pairs are added or deleted.
• The shift happens on the reading frame in the ribosome.
• Frameshift mutations result in incorporation of the wrong amino acids downstream from the mutation and results in the production of inactive protein

Transposons or Insertion Sequences

• Transposons/insertion sequences occur when transposes (jumping genes) or insertion sequence are integrated into the DNA.
• Newly inserted pieces of DNA by either cause large variations within gene itself or adjacent genes

Bacterial Mutation

• Mutation can be either spontaneous or induced.

Spontaneous Mutation

• Spontaneous happen due to naturally heritable base sequence changes in DNA.
• Causes for it happening include radiation penetrating the atmosphere
• Radiation penetrating the atmosphere or even errors taking place during DNA replication are types to note.
• Spontaneous Mutation can occur for anything.
• The mutant will undergo survive, multiply and emerge as a dominant.
• A example of a type of Spontaneous Mutation is Neisseria gonorrhoeae

Induced Mutations

• Induced Mutations planned experiments.
• The plan is to subject bacteria chemical or physical agents.

Causes for Induced Mutations

• Ultraviolet light (UV): induces adjacent thymine(cytosine)
• Nitrous acid: chemical mutagen converts DNA's adenine molecule hypoxanthine(replication)
• Base analog: 5-bromouracil (Acyclovir: antiviral)

Point Mutations

• Base-Pair happens if there an incorrect in the mRNA sequence of codons
• Silent mutation is where no change happens
• Missence Mutation leads to the wrong AA inserion
• Nonsence Mutation generates a stop codon because bacterial will be stopped

More Point Mutations

• Base-Pair deletion or insertion is where bases can be both lose or add (frameshift mutation)
• Transposons or Insertion sequences can be integrated into the DNA.
• The DNA changes affects the genes into which they insert and in adjacent genes

Recombination

• Recombination is the process by which an organism picks up “new” DNA.
• Recombination involves entirely new genes or collections of genes Transfer is horizontal.
• Horizontal Transfer causes medical implications like ability spread more or have increased resistant to antibiotics.

Bacteria Exchange Genetic Material

• Transformation is when genetic material is taken up from their environment.

Transformation - Bacteria take up DNA from their environment.

• Bacteria incorporate the genetic material into their genome (Griffith experiment).
• Conjugation involves direct transfer of DNA by bacteria through plasmid.
• Transduction transports DNA between bacteria through viruses.

Transformation

• Transformation is when small linear strands of DNA are taken into target cells.
• The transfer depends to a small range of organisms and a set level of competency
• Process is in stages of "Uptake, Transport, Integration"

Transduction 2

• Transformation is an environment cell ability to take up DNA from their environment.
• Natural is a place where chromosomal genes active are under certain environment conditions
• Can not enter the competent which cause it to be permeable: in artificial transformation
• Examples include Gram positive pneumococcus and Gram negative rod H. influenza

Conjugation

• Conjugation is plasmid-encoded and needs cell-contact.
• Cell contact requires cell-to-cell physical contact for the transmission
• Conjugation needs both cell-to-cell transfer and cell-to-cell receiver .
• "Larger pieces of DNA like ""plasmid"" conjugative can take place if tra gets mobilized by plasmid in an enviroment"
• Conjugation is extremely regulated with gram negative sides
• Positive sides do not have sex plus and cause clumping
• Examples of plasmids that rely on well-regulated is Mainly Gram negatives
• An environment that does not used sex plus is Gram positive

Mechanism of Conjugation

• F pilus and contact factors or clumping
• Movement is uni-directional
• Donor and recipient end up with DNA
• Mechanisms exist for plasmid DNA and chromosomal DNA to mix and be transferred during conjugation (high frequency recombination Hfr).

Transduction

• Transduction is a way that DNA can be transferred using a virus.
• How it works is during within the cell, and part pieces of bacterial DNA can integrate in
• Recipient cell can undergo either of these outcomes if have the phage DNA
• Virulent phage happens because because of the lytic cycle due to bacterial integration (lysogenic conversion

More Transduction points

• Generalized transduction: virus carries a segment from any part of the bacterial chromosome
• Specialized transduction: adjacent cellular genes that are transuded are usually specific to that virus