Microbial Genetics ref. Talaro 5e Chapter 9
Genes and the=
Genetic
Material
Genetics is the study of heredity which can be studied= at the level of the genome, the chromosome, the gene and deoxyribonucleic acid (DNA).
Genes contain the instructions to build peptides / pro= teins which carry out many structural and metabolic functions.
A eukaryotic chromosome is composed of a linear, double helix stand of DNA tightly coiled, and supercoiled around special nucleoproteins (histones)
A prokar= yotic “chromosome” (chromatin body) consists of a single molecule of double helix DNA.
Two theories for the replication of DNA have been offe= red (1) the “conservative theory” and the “semiconservative theory”). Research to d= ate strongly supports the “semiconservative theory”
Gene Structur=
e and
Replication
A DNA molecule is a polymer of nucleotides. The DNA molecule is described as b= eing both directional and antiparallel. <= /span>
Each of the strands consists of a = sugar( deoxyribose)–phosphate “backbone” with nucleotide bases attached to the sugar residue at the number 1’ Carbon ( “one pr= ime carbon”), and extending into the double helix similar to the rungs on= a ladder.
The strands are said to antiparallel in that the sugar-phosphate backbone of one strand is in the 5’ → 3’ direction whil= e the complementary strand is in the 3’ → 5’ direction.
Chargaff’s rule, a.k.a. “base-pairing rule”, states that Adeni= ne pairs with Thymine, and Cytosine pairs with Guanine (abbreviated: A-T with two hydrogen bonds;
C-G with three hydrogen bonds).
Gene Function=
Structurally, a gene consists of a specific sequence of nucleotides on a DNA molecule that code for a peptide or RNA. The nucleic acid sequence transcri= bes
instructions to messenger R= NA (rRNA), which is subsequently translated into a specific sequence of amino acids comprising a peptide, and ultimately a protein.
Ribonucleic acid (RNA) is a single-stranded polynucleo= tide with a
ribose (sugar) – phos= phate backbone, and containing Uraci= l instead of Thymine.
Three types of RNA in cells include messenger RNA (mRN= A), ribosomal RNA (rRNA) and transfer RNA (tRNA).
Genetic instructions in DNA are transcribed as Codons (three-base triplets) in mRNA , with each codon = coding for a specific amino acid, or functioning as a “stop” or “start” code. Complimentary “anticodons” on each tRNA brings a specific amino acid to the rRNA (a.k.a. “ribozyme”) to be assembled by t= he formation of peptide bonds. <= /p>
“Gene expression” involves copying genetic instructions from DNA, via transcription, to mRNA, followed by translation = of the coded message into a specific sequence of amino acids resulting in the production of a polypeptide, and in turn, functional proteins.
During transcription the mRNA is synthesized in the 5&= #8217;→ 3’ direction, by = adding complimentary nucleotides to the 3’ end of the mRNA strand.
Eukaryotic DNA and mRNA include exons (expressed seque= nces) and introns (intervening sequences). = Eukaryotic gene expression requires that the “primary mRNAR= 21; have the introns excises and the exons spliced together to form the “mature mRNA” molecule before it leaves the nucleus and transla= tion can begin.
Virus Genetics – Animal Model
Viral genomes are represented by a number of forms = 211; some not seen in animal viruses, including dsDNA. ssDNA= , dsRNA and ssRNA.
As a rule of thumb DNA viruses tend to replicate in the nucleus.
RNA viruses tend to replicate in the cytoplasm.
Retroviruses (RNA viruses having reverse transcriptase) synthesize dsDNA from ssRNA, a feature that goes against the central dogma = of gene expression in cells.
Some viruses are able to insert their DNA into the gen= ome of the host cell. Insertion of by oncogenic viruses results in transformation of the host cell into a cancero= us cell.
Regulation of= Gene Function
Protein synthesis (the culmination of gene expression)= is regulated at the genetic level by gene induction (inducible genes) or gene repression (repressible genes)
.
The informational / regulatory unit of genetics is the= operon, which regulates metaboli= sm by controlling synthesis of mRNA, and thereby controls gene expression.
An operon consists of:
(1) the regulator, genes = that codes for a repressor protein.
=(2) the promoter, a gene recog= nized by RNA polymerase to signal initiation of =
&nb= sp; transcription.
(3) the structural locus, a group of genes that code for different enzymes.
Inducible Gen= es are normally turned off, but can be turned on by the presence
of a suitable substrate.=
&nbs=
p; Example:
lac (lactose) operon of E. coli [see fig. 9.22, p. 274]=
Repressible G= enes are normally turned on, but can be turned off when the
product of th= eir expression is in excess or is no longer needed.
&nbs= p; Example: arginine operon [see fig 9.23, p. 275]
Interfering with transcription or translation is one o= f the mechanisms of certain antibiotics [see Fig. 12.1, p. 353]
Gene Mutation=
A permanent ch= ange in an organism’s genome is called a mutation.
Some mutations are spontaneous, result from errors in DNA replication due to unknown causes.
An induced mu= tations result from chemical or physical agents (mutagens) that disrupt normal DNA in some way.
Cells pos= sess remarkable abilities to repair mut= ations (e.g. mismatched bases, pyrimidine dimers see fig. 11.9, p. 332) using norm= al intracellular enzymes.
The Ames Test is a means of evaluating the potential for chemicals to cause mutations in bacteria.
According to the premise (hypothesis) the Ames Test assumes that if a chemi= cal agent causes mutations in the bacteria it will also cause mutation in mamma= ls, including humans. It should be noted that the strains of bacteria used demonstrate a certain level of spontaneous back-mutation of the t= arget gene.
Recombinant D= NA
Organisms that reproduce asexually produce progeny tha= t are genetically identical to the parent cell, resulting in essentially no genet= ic recombination.
Bacteria have devised other means to provide opportuni= ties to increase their genetic variation, and thereby enhance survivability as w= ell as contribute to natural selection (“evolution”).
Three principle mechanisms by which bacteria experience genetic recombination into new genetic variables include;
(1) Bacterial conjugation, via a pilu= s used to transfer a plasmid
(2) Bacterial transformation, via nonspecific acceptance of small DNA
fragments = from the surrounding environment
· some bacterial exhibit apoptosis, programming some of the population to die and release their DNA into the environment,
where it can be assimilated by other cells
(3) Transduction, via a Bacteriophage “vector” carrying DNA from one host cell
to another
(4) A fourth possibility is that ment= ioned above, i.e. mutation
· most mutations are fatal
· some don’t make any difference
· occasionally a mutation provides a survival factor