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NAJRAN UNIVERSITY College of Medicine

NAJRAN UNIVERSITY College of Medicine. Microbiology &Immunology Course Lecture No. 5. By. Dr. Ahmed Morad Asaad Associate Professor of Microbiology. Bacterial genetics. HEREDITY :

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NAJRAN UNIVERSITY College of Medicine

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  1. NAJRAN UNIVERSITY College of Medicine Microbiology &Immunology Course Lecture No. 5 By Dr. Ahmed MoradAsaad Associate Professor of Microbiology

  2. Bacterial genetics • HEREDITY : • -It is the transmission of information (genetic determinants) from parent to offspring (or from generation to generation). • -Members of the new generation formed by division of an organism are identical to the parent cell with rare occurrence of variations (mutation).

  3. Genes: • -These are the units of heredity. • -Genes are located on the chromosome. • -A gene is a segment of DNA that carries in its nucleotide sequence information for a specific biochemical or physiologic property. • Genotype: • -It is the set of genetic (heritable) determinants within the cell. • Phenotype: • -It is the manner in which the inherited genetic determinants express themselves under the influence of the environment (i.e. the observable characters).

  4. -The genetic determinants are made of nucleic acid • -The nucleic acid is either DNA or RNA • -The nucleic acid is composed of : • Phosphoric acid • Sugar (pentose) residue: ribose (in RNA) or deoxyribose (in DNA). • Nitrogenous bases: • Purine bases: {adenine (A) and Guanine (G)} • Pyridmine bases: {thymine (T), cytosine (C) and uracil (U)}.

  5. Bacterial chromosome It is a single circular molecule of double ‑stranded DNA (about 1 mm long). The 2 strands form a double helix. Each strand consists of: -a Backbone of phosphate and deoxyribose groups -To each sugar group a nitrogenous base is attached and is projecting inwards from the chain. (A-T & G-C) -The 2 chains are held together by hydrogen bonds between pairs of bases -The 2 chains are complementary (the sequence in one chain determines the sequence in the other) -Every 3 successive bases on the gene form a base triplet (a codon), which codes for one amino acid during the process of protein synthesis

  6. Gene expression The mechanism by which the sequence of nucleotides in a gene determines the sequence of amino acids in a protein occurs in 2 steps as follows : Transcription 5’ G A T C C T T T C G A 3’ 3’ C T A G G A A A G C T 5’ 5’ G A T C C T T T C G A 3’ 3’ C T A G G A A A G C T 5’ 5’ G A T C C T T T C G A 3’ 3’ C T A G G A A A G C T 5’ Polymerase transcriptase messenger RNA (mRNA)

  7. Translation • The mRNA passes out of the nuclear body and is associated with the ribosome to form a ribosomal template on which amino acids will be arranged. • Amino acids are carried by specific RNA molecule called transfer RNA (tRNA) • Each tRNA carries an amino acid by one end and recognizes a specific codon on the mRNA by a complementary triplet (anticodon) present on its other end. • After the process of lining up of amino acids is completed, the amino acids are joined by peptide bonds to form a polypeptide chain. Assembly of polypeptide chains to form proteins and macromolecules then occurs.

  8. BACTERIAL VARIATION I- Phenotypic Variations : These are reversible modifications in the appearance of the organism in response to environmental conditions. Phenotypic variations are not heritable, i.e. they will not be transmitted among generations. Examples of Phenotypic Variations 1. Morphological variations: a. Exposure to phenol interferes with the development of flagella. b. Exposure to penicillin interferes with cell wall synthesis. 3. Metabolic variation, i.e. change in the enzymatic and biochemical activities.

  9. II- GenotypicVariations : These are permanent (irreversible) variations, which are heritable, i.e. they will be transmitted among generations. They may be due to: 1- mutation 2- gene transfer

  10. 1- MUTATION Biochemically, a mutation is an alteration in the nucleotide sequence at some point in the organism’s DNA. It results from 4 types of molecular changes: 1‑Base substitution: One base pair is replaced by another. Replacement could be: a‑ Transition: An original purine is substituted by a purine, or an original pyrimidine is replaced by a pyrimidine. b‑ Transversion: An original purine is replaced by a pyrimidine or an original pyrimidine is replaced by a purine. 2‑ Insertion: Transposonsare integrated into the DNA. 3‑ Deletion: A base pair is erased. 4‑Frame shift mutation: This is a result of either deletion or insertion mutation.

  11. Genetic transfer Bacterial DNA may be transferred within or between bacterial cells. A. Transfer of DNA within bacterial cells: Transposition: Transposons or jumping genes can move from one site in a DNA molecule to other target chromosomal or plasmid sites in, the same or a different DNA molecule. This process is termed transposition and it results in insertion or deletion mutations.

  12. B. Transfer of DNA between bacterial cells: Bacterial DNA may be transferred between bacterial cells by three mechanisms: transformation, transduction, and conjugation.

  13. Transformation: • The transfer of DNA from one cell to another by either of 2 mechanisms: • 1- In nature, dying bacteria may release their DNA which may be taken by another cells • 2- In the lab, DNA may be extracted from one bacterial cell into another one

  14. Transduction: • Transfer of bacterial DNA by means of a bacterial virus (bacteriophage). • 2 types of transduction: generalized and specialized • Generalized transduction: • This occurs when the bacterial virus carries a segment from any part of the bacterial chromosome. This occurs because cell DNA is fragmented after phage infection and a piece of DNA is incorporated into the virus.

  15. Specializedtransduction: • This occurs when the bacterial virus that has integrated into the cell DNA is excised and carries with it an adjacent part of the cell DNA

  16. Conjugation: • 1- It is the mating of 2 bacterial cells during which DNA is transferred from the donor to recipient cell. • 2- The mating process is controlled by F (fertility) plasmid (F factor) which carries genes for synthesis of pilin and form sex pilus. • 3- Mating begins when the sex pilus of donor male bacteria carrying F factor (F+) attach to to recipient female bacteria (F-). • 4- Cleavage of F factor DNA: one strand to recipient cell. • The process is completed by synthesis of a complementary strand in each cell

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