Bacterial Recombination and Bacterial Genetics

DNA

• the blueprint for a cell’s proteins, including enzymes

• obtained either from another cell in the same generation or from a parent cell during cell division

• can be expressed within a cell or transferred to another cell through recombination and replication

expression

genetic information is used within a cell to produce the proteins needed for the cell to function

recombination

genetic information can be transferred horizontially between cells of the same generation

replication

genetic information can be transferred vertically to the next generation of cells

MRSA

Methicillin-Resistant Staphylococcus aureus

genetics

the study of what genes are and how they carry information, how their information is expressed and how they are replicated and passed to subsequent generations or other organisms

genome

the genetic information in the cell

chromosomes

structures containing DNA that physically carry hereditary information; contains the genes

Genes

segments of DNA (except in some viruses which are made of RNA) the code for functional products that are usually proteins (rRNA, tRNA, microRNA).

Nucleotides

repeating units consists of a nucleobase (adenine, thymine, cytosine or guanine), deoxyribose, phosphate group

Base pairs

adenine-thymine, cytosine-guanine

Genetic code

the set of rules that determines how a nucleotide sequence is converted into the amino acid sequence of a protein.

Central Dogma

Theory by Francis Crick in 1956

Francis Crick

he first proposed that the sequence of nucleotides in DNA determines the

sequence of amino acids in a protein

DNA gyrase

relaxes supercoiling ahead of the replication fork

DNA ligase

makes covalent bonds to join DNA strands; okazaki fragments, and new segments in excision repair

DNA polymerase

synthesize DNA; proofread and facilitate repair of DNA

endonucleases

cut DNA backbone in a strand of DNA; facilitate repair

Helicase

unwinds double-stranded DNA

methylase

adds methyl group to selected bases in newly made DNA

photolyase

uses visible light energy to separate UV-induced pyrimidine dimers

primase

an RNA polymerase that makes RNA primers from a DNA template

ribozyme

RNA enzyme that removes introns and splices exons together

RNA polymerase

• copies RNA from a DNA template

• required in the process of transcription

snRNP

RNA-protein complex that removes introns and splices exons together

topoisomerase or gyrase

relaxes supercoiling ahead of the replication fork; separates DNA circles at the end of DNA replication

transposase

cuts DNA backbone, leaving single-stranded '“sticky ends”

Transcription in Prokaryotes

the synthesis of a complementary strand of RNA from a DNA template

rRNA

integral part of ribosomes; the cellular machinery for protein synthesis

mRNA

• carries the coded information for making specific proteins from DNA to ribosomes; where proteins are synthesized

• during transcription, there is the synthesis of strand of mRNA

translation

• process in which the mRNA serves as the source of information for the synthesis of proteins

• the language of mRNA is in the form of codons

codons

group of 3 nucleotides

AUG

• specifies the amino acid methionine

• the start of protein synthesis

• start codon

stop codon

• UAA

• UAG

• UGA

DNA → mRNA → protein → function

typical chain of events described by central dogma

mutations

• a permanent change in the base sequences of DNA

• such change can cause a change in the product encoded by the gene

Base substitution/point mutation

• Most common type of mutation involving single base pairs

• A single base at one point in the DNA sequence is replaced with a different base

missense mutation

When the change of a single base pair causes the substitution of a different amino acid in the resulting protein

nonsense mutation

Base substitutions that create a stop (nonsense) codon the prevents the synthesis of a complete functional protein

frameshift mutation

• One or a few nucleotide pairs are deleted or inserted in the DNA

• This can shift the “translation reading frame”

Silent Mutation

• Occur when one nucleotide is substituted for another in the DNA which results to new codon that might still code for the same amino acid

• If the amino acid is changed, the function of the protein may not change if the amino acid is in a nonvital portion of the protein

Spontaneous mutations

mutations that arise in the absence of known mutagens

Induced mutations

mutations that occur following treatment with a mutagen

Mutagenesis

the process by which a mutation is produced

Mutagens

agents that increase the frequency of mutation

nitrous acid

• convert the base adenine to a form that pairs with cytosine instead of the usual thymine

• alters DNA at different location

• result: base substitution

nucleoside analog

• randomly incorporated in DNA

• results:

  • causes mistakes in base pairing during DNA replication

  • base-pair substitutions in the progeny cells

• e.g. 

  • 2-aminopurine

  • 5-bromouracil

intercalating agents

• inserts between base pairs

• result: addition of base pairs

• e.g. ethidium bromide; acridine orange

x rays and gamma rays

• forms radiation that are potent mutagens

• ionize atoms and molecules 

• penetrating rays cause electrons to pop out of their usual shells which will cause more damage and some ions oxidizes bases in DNA

• results:

  • errors in DNA replication → mutation

  • breakage of covalent bond

  • causes physical breaks in chromosome

ultraviolet (UV)

• formation of harmful covalent bonds between pyrimidine bases

• thymine dimer foundation

• results: thymine dimers causes problem in transcription or replication of the DNA

genetic recombination

Refers to exchange of genes between two DNA molecules to form new combinations of genes on a chromosome

Vertical Gene Transfer

Occurs when genes are passed from an organism to its offspring

Horizontal Gene Transfer

• Pass their genes laterally to other microbes

• Ex. Between normal microbiota and pathogens in spread of antibiotic resistance

• Donor cell

• Recipient cell

Donor cell

gives a portion of its total DNA

Recombinant

The recipient cell that incorporated donor DNA into its own DNA

Transformation

Process where in genes are transferred from one bacterium to another as “naked” DNA in solution

Conjugation

• Requires direct cell-to-cell contact

• Conjugating cells must generally be of opposite mating type

• Gram-negative – sex pili

• Gram-positive- sticky surface molecules

Transduction

Bacterial DNA is transferred from a donor cell to recipient inside a virus that infects bacteria called bacteriophage or phage.