Bacterial Genetics

Genetics is the study of inheritance of the different characters from parents to offspring's who usually have the same characters as the parents. The same with bacteria where the daughter cells inherit the same characters as the parent forms.

Genes Genes are the units of inheredity. They are segments on the chromosome or DNA that carry the information for a specific character or a specific structure.

Bacteria have closed, circular DNA Genome: genetic material in an organism E. coli 4 million base pairs 1 mm long (over 1000 times larger that actual bacterial cell) DNA takes up around 10% of cell volume Bacterial Chromosome

DNA is a long polymer of simple units called nucleotides, held together by a sugar phosphate backbone.polymer nucleotidessugarphosphate Attached to each sugar molecule is a molecule of one of four bases; adenine (A), thymine (T), guanine (G) or cytosine (C), and the order of these bases on the DNA strand encodes information.basesadeninethymineguaninecytosine In most organisms, DNA is a double-helix (or duplex molecule) consisting of two DNA strands coiled around each other, and held together by hydrogen bonds between bases. Because of the chemical nature of these bases, adenine always pairs with thymine and guanine always pairs with cytosine.double-helixhydrogen bonds

Genes essential for bacterial growth are carried on the chromosome. Few genes associated with specialized functions are carried on plasmids.

Chromosomal Replication

Protein Synthesis DNA  mRNA  protein transcription translation Central Dogma of Molecular Genetics

Transcription One strand of DNA used as a template to make a complimentary strand of mRNA Promoter/RNA polymerase/termination site/5’ to 3’ Ways in which RNA & DNA differ: –RNA is ss –RNA sugar is ribose –Base pairing-A-U

Types of RNA Three types: –mRNA: messenger RNA Contains 3 bases ( codon) –rRNA: ribosomal RNA Comprises the 70 S ribosome –tRNA: transfer RNA Transfers amino acids to ribosomes for protein synthesis Contains the anticodon (3 base sequence that is complimentary to codon on mRNA)

Genetic Code DNA: triplet code mRNA: codon (complimentary to triplet code of DNA) tRNA: anticodon (complimentary to codon)

Genetic Code Codons: code for the production of a specific amino acid 20 amino acids 3 base code Degenerative: more than 1 codon codes for an amino acid Universal: in all living organisms

Genetic Code

Translation Three parts: –Initiation-start codon (AUG) –Elongation-ribosome moves along mRNA –Termination: stop codon reached/polypeptide released and new protein forms rRNA=subunits that form the 70 S ribosomes (protein synthesis occurs here) tRNA=transfers amino acids to ribosomes for protein synthesis)

Mutations Changes in base sequence of DNA/lethal and inheritable Can be: –Harmful –Lethal –Helpful –Silent

Genetic Transfer in Bacteria Genetic transfer-results in genetic variation Genetic variation-needed for evolution Three ways: –Transformation: genes transferred from one bacterium to another as “naked” DNA –Conjugation: plasmids transferred 1 bacteria to another via a pilus –Transduction: DNA transferred from 1 bacteria to another by a virus

Research on E. coli revealed that these bacteria have a sexual mechanism that can bring about the combining of genes from two different cells This discovery led to the development of recombinant DNA technology –a set of techniques for combining genes from different sources From E.Coli to a Map of Our Genes

DNA technology has many useful applications –The Human Genome Project –The production of vaccines, cancer drugs, and pesticides –Engineered bacteria that can clean up toxic wastes

Transformation, the taking up of DNA from the fluid surrounding the cell 12.1 In nature, bacteria can transfer DNA in three ways BACTERIA AS TOOLS FOR MANIPULATING DNA Figure 12.1A DNA enters cell Fragment of DNA from another bacterial cell Bacterial chromosome (DNA)

Transduction, the transfer of bacterial genes by a phage Conjugation, the union of cells and the DNA transfer between them Fragment of DNA from another bacterial cell (former phage host) Phage Sex pili Mating bridge Donor cell (“male”) Recipient cell (“female”)

The transferred DNA is then integrated into the recipient cell’s chromosome Donated DNA Recipient cell’s chromosome Crossovers Degraded DNA Recombinant chromosome

An F factor is a DNA segment in bacteria that enables conjugation and contains an origin of replication 12.2 Bacterial plasmids can serve as carriers for gene transfer F factor (integrated) Male (donor) cell Origin of F replication Bacterial chromosome F factor starts replication and transfer of chromosome Only part of the chromosome transfers Recipient cell Recombination can occur

An F factor can exist as a plasmid, a small circular DNA molecule separate from the bacterial chromosome Figure 12.2B, C F factor (plasmid) Male (donor) cell Bacterial chromosome F factor starts replication and transfer Plasmid completes transfer and circularizes Cell now male Plasmids

Some genetic properties in the bacterial cell are carried on plasmids. However, these properties are not essential for growth. Plasmids are extrachromosomal double straned, cercular DNA molecules smaller than the chromosome. They replicate autonomously

Plasmids Multiple copies of the same plasmid may be present in each bacterial cell. Different plasmids may co-exist within the same bacterium. They are inherited by daughter cells. Some plasmids can transfer to other bacteria of the same or different species.

Plasmids Transfer takes place normally by conjugation. Since plasmids can transfer from cell to cell, they are widely used as vectors for cloning DNA in yeast and bacteria.

Bacterial properties carried on plasmids Include: Drug resistance Virulence Production of antimicrobial agents Sex factor plasmids

Transposons “Jumping genes” These are genitic elements that contain several Kbp of DNA. Can move extremly readly from plasmid to plasmid or plasmid to chromosome (and vice versa), hence the “jumping genes”.

Bacterial variation Genotype is the set of genitic determinants within the cell. The changes is heritable. Genitic variation occurs through: 1-Mutation 2-Gene transfer

Bacterial variation Bacterial variation may be phenotypic or genotypic variation. Phenotype is the observable structural and physiologic properties of the cell. The changes is not heritable

Plasmids are key tools for DNA technology –Researchers use plasmids to insert genes into bacteria 12.3 Plasmids are used to customize bacteria: An overview

Figure 12.3 Plasmid isolated 1 Bacterium Bacterial chromosome Plasmid 2 DNA isolated Cell containing gene of interest DNA Gene of interest 3 Gene inserted into plasmid Recombinant DNA (plasmid) 4 Plasmid put into bacterial cell Recombinant bacterium 5 Copies of geneCopies of protein Clones of cellGene for pest resistance inserted into plants Gene used to alter bacteria for cleaning up toxic waste Protein used to dissolve blood clots in heart attack therapy Protein used to make snow form at higher temperature Cell multiplies with gene of interest

Restriction enzymes cut DNA at specific points DNA ligase “pastes” the DNA fragments together The result is recombinant DNA 12.4 Enzymes are used to “cut and paste” DNA Figure 12.4 DNA 1 Restriction enzyme recognition sequence Restriction enzyme cuts the DNA into fragments Sticky end Restriction enzyme cuts the DNA into fragments Addition of a DNA fragment from another source Two (or more) fragments stick together by base-pairing DNA ligase pastes the strand Recombinant DNA molecule

Bacteria take the recombinant plasmids and reproduce This clones the plasmids and the genes they carry –Products of the gene can then be harvested 12.5 Genes can be cloned in recombinant plasmids: A closer look

Figure 12.5 Isolate DNA from two sources 1 E. coli Cut both DNAs with the same restriction enzyme 2 Plasmid Human cell DNA Gene V Sticky ends Mix the DNAs; they join by base-pairing 3 Add DNA ligase to bond the DNA covalently 4 Recombinant DNA plasmid Gene V Put plasmid into bacterium by transformation 5 Clone the bacterium 6 Bacterial clone carrying many copies of the human gene

Recombinant DNA technology allows the construction of genomic libraries –Genomic libraries are sets of DNA fragments containing all of an organism’s genes Copies of DNA fragments can be stored in a cloned bacterial plasmid or phage 12.6 Cloned genes can be stored in genomic libraries Figure 12.6 Genome cut up with restriction enzyme Recombinant plasmid OR Recombinant phage DNA Bacterial clone Phage clone Phage library Plasmid library

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