BACTERIAL GENETICS DR. A.S.AL-KHATTAF

Structure and Function of the Genetic Material Chromosomes are cellular structures made up of genes that carry hereditary information. Genetics is the study of genes carry information, how they are replicated and passed to other generations, and how they affect the characteristics of an organism. A gene is a segment of DNA that codes for a functional product. The genetic information in a cell is the genome.

Nucleotides DNA is composed of repeating nucleotides containing the bases adenine (A), thymine (T), cytosine (C), or guanine (G); a deoxyribose sugar; and a phosphate group. Bases occur is specific complementary base pairs, the hydrogen bonds from which connect strands of DNA : adenine with thymine, and cytosine with guanine. with guanine. Information on DNA can be transcribed into RNA (transcription) and in turn, translated into protein (translation).

Genotype and Phenotype The genotype is an organism’s genetic makeup, the information that codes for all the characteristics and potential properties of the organism. The genotype is its gene collection- its DNA. The phenotype refers to an organism’s actual expressed properties, such as its ability to perform a chemical reaction. The phenotype is the collection of enzymatic or structural proteins.

DNA and Chromosomes DNA in chromosomes is in the form of long double helix. In prokaryotes, DNA is not found within a nuclear membrane. The chromosome takes up only about 10% of the cell’s volume because the DNA is supercoiled by an enzyme called DNA gyrase.

DNA replication In DNA replication, the two helical strands unravel and separate from each other at a replication fork, where the synthesis of new strand begins. The complementary pairing of bases yields a complementary copy of the original DNA. Segments of new nucleotides are joined to form short strands of DNA by DNA polymerase enzymes.

DNA Replication Short strands of DNA are then joined into continuous DNA by DNA ligase enzymes. Because each new double-stranded DNA molecule has one original strand and one new strand, the process is called semi- conservative replication. In bacteria, replication begins at an origin of replication and in some cases two replication forks move in opposite directions. DNA replication makes few mistakes, largely due to the proofreading capability of DNA polymerase.

RNA and Protein Synthesis Transcription In transcription, a strand of messenger RNA (mRNA) is synthesized from the genetic information in DNA. Adenine in the DNA dictates the location of uracil, which replaces thymine in mRNA. Adenine in the DNA dictates the location of uracil, which replaces thymine in mRNA. If DNA has the bases sequence ATGCAT, the mRNA will have UACGUA. The region where RNA polymerase (needed for synthesis) binds to DNA and transcription begins is known as promoter site. The terminator site is where the RNA polymerase and newly formed mRNA are released from the DNA.-Endpoint.

Translation Protein synthesis is called translation. The language of mRNA is in codons, groups of three nucleotides such as AUG. Each codon codes for a particular amino acid. There are 64 possible codons, but only 20 amino acids. An amino acid has more than one codon (degeneracy) of the code. Sense codons code for amino acids; nonsense (stop) codons signal the end of synthesis of a protein.

Translation The site of translation are ribosomes that move along mRNA. The amino acids are transported to the ribosome by transfer RNA (tRNA). Each tRNA molecule is made specific for an amino acid by an anticodon that is complementary to a codon. The codon AUG would attach to anticodon UAC.

Repression and Induction An inducer is a substance (substrate) whose presence results in the formation, or increase in the amount of an enzyme. Such enzymes are called inducible enzymes; this genetically controlled response is termed enzyme induction. Lactase production in response to lactose – example. Genetic regulation that decreases enzymes synthesis is enzyme repression.

Mutation: Change in Genetic Material A mutation is a change in the bases sequence of DNA. The most common mutation is a base substitution- a single base in DNA is replaced with different one. This may create a stop codon, that stop protein synthesis before completion- a nonesense mutation. Deletion or addition of base pairs results in a frame-shift mutation. Leads to shift in translational reading frame (three-by-three).

Mutagens Many chemicals and radiation bring about mutations; these are called mutagens. Nitrous acid (HNO 2 ) is a base pair mutagen. It cases Adenine to pair with Cytosine not Thymine. Other mutagens are nucleoside analogs- structurally similar to bases and incorporated into DNA by error. Example- 2-Aminopurine analog to Adenine, 5- Bromouracil to Thymine.

Mutagenic drugs Some antiviral and anti-tumor drugs are nucleoside analogs. AZT (azidothymindine), one of the primary drugs used to treat HIV infection. Other chemical mutagens cause small deletions or insertions, which can result in frame-shifts. Example- benzpyrene, which present in smoke and soot. Aflatoxin – produced by Aspergillus flavus is a frame mutagen. Acridine dyes used against herpesvirus infections.

Radiation Ionizing radiation- such as X rays and gamma rays are mutagens and damage DNA. They cause electrons to pop out of their usual shells- producing ions and free radicals. Ultraviolet light (non- ionizing radiation) is also mutagen. Light-repair enzymes repair UV damage to DNA. Enzymes cut out distorted DNA and synthesize replacement.

Genetic Transfer and Recombination Genetic recombination is the rearrangement of genes to form new combinations. Crossing over happens when two chromosomes break and rejoined in such a way- genes are reshuffled between the two chromosomes. The donor cell gives a portion of its total DNA to a different recipient cell (recombinant). Vertical gene transfer occurs from an organism to its offspring. Horizontal gene transfer from bacteria to other microbe.

Transformation in Bacteria Transformation –naked DNA in solution is transferred from one bacteria to another. It occurs naturally among very few genera of bacteria, when donor and recipient are closely related- occurs in the log- phase of growth. Recipient cell must be competent- cell wall permeable to large DNA molecules.

Conjugation in Bacteria Conjugation requires contact between living cells of opposite mating types. The donor cell F + has plasmid called F (fertility factor). When F + cell mixed with F -, cells attach by sex pili. F factor is duplicated by donor and the new copy is transferred to F - cell.

Transduction in Bacteria In generalized transduction, the phage (bacteriophage) attaches to the bacterial cell wall and injects DNA into bacteria. Normally this synthesis new viruses. Occasionally part of bacterial chromosome is taken by the viral DNA. When new bacteria are infected by these viruses old bacterial DNA is incorporated in the new bacterial genome.