BACTERIAL GENETICS

1. BACTERIAL GENETICS DR. A.S.AL-KHATTAF

2. 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.

3. 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 complementarybase pairs, the hydrogen bonds from which connect strands of DNA: adenine with thymine, and cytosine with guanine. • Information on DNA can be transcribed into RNA (transcription) and in turn, translated into protein (translation).

4. 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.

5. 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.

6. 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.

7. DNA Replication • Short strands of DNA are then joined into continuous DNA by DNA ligaseenzymes. • 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.

8. 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. • 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.

9. 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.

10. 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.

11. 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.

12. 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).

13. Mutagens • Many chemicals and radiation bring about mutations; these are called mutagens. • Nitrous acid (HNO2) 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.

14. 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 Aspergillusflavus is a frame mutagen. • Acridine dyes used against herpesvirus infections.

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.