Bacterial Genetics
In this lecture, we will talk about:
 Bacterial chromosome:
 Structure
 Replication
 Expression into proteins
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Plasmids
Transposons
Bacteriophages
Bacterial variation
 Phenotypic
 Genotypic (mutation & gene transfer)
 Genetic recombination
Before we start, we should know some important definitions:
o Genetics:
 The science that studies the transmission of genetic
information from the parents to the offsprings.
o Gene:
 Segment of DNA that carries the genetic information for
a specific structure or character.
o Genotype:
 The set of genes within the cell.
o Phenotype:
 The observable structural and physiological properties
of the bacterial cell.
Bacterial
chromosome
Structure
 Single circular double stranded DNA molecule.
 The 2 strands wind around each other to form a double
helix.
 Each strand consists of deoxyribose sugar.
 The deoxyribose sugar is a pentose which has 5 carbon
atoms.
 The pentose sugars are joined together by phosphate
groups between the 3rd and 5th carbon atoms of the
adjacent sugars.
 Thus, each strand has a free 5` end with terminal
phosphate group and a free 3` with terminal hydroxyl
group (OH).
 To the 1st carbon atom of each sugar attached a
nitrogenous base which is projecting inward from the
chain.
 The nitrogenous bases are:
 Purine: adenine (A) and guanine (G)
 Pyrimidine: thymine (T) and cytosine (C)
 The two DNA strands are held together by hydrogen
bonds between bases projecting at the same level from
each strand:
 Adenine is joined to thymine by two hydrogen bonds.
 Guanine is joined to cytosine by three hydrogen bonds.
 Thus, the two strands are complementary to each other
(When A exists in one strand, its complementary in the
other strand must be T)
 Each two complementary bases are known as a base
pair.
 The length of a DNA molecule is expressed in kilo base
pair (kbp).
 For example E. coli chromosome is 4000 kbp.
Sugar + nitrogenous base= nucleoside
BUT
Sugar + nitrogenous base + phosphate= nucleotide
Nucleotide
DNA structure
DNA versus RNA
Item
DNA
RNA
Sugar
Purines
Deoxyribose
Ribose
Adenine (A) & guanine (G)
Adenine (A) & guanine (G)
Pyrimidines
Strandedness
Cell distribution
Cytosine (C) & thymine (T)
Cytosine (C) & Uracil (U)
Double stranded
Single
• Chromosome: nucleoid
• Plasmid: cytoplasm
• Transposons: jumping within
the chromosome and in
between the chromosome and
plasmids.
Mainly in the cytoplasm:
• Messenger RNA: m-RNA
• Transfer RNA: t-RNA
• Ribosomal RNA: r-RNA
Replication
 Chromosome replication begins when helicase enzyme
breaks the hydrogen bonds between the 2 DNA strands
making the replication fork.
 DNA polymerase enzyme can add nucleotides only in 5`
to 3` direction of the new strand.
 Synthesis of one strand called the leading strand
proceeds continuously in 5` to 3` direction.
 But, synthesis of the other strand which is called the
lagging strand is more complex because DNA polymerase
enzyme can add nucleotides to only in 5` to 3` direction.
 So, RNA primase enzyme attaches to the DNA and
synthesizes a short RNA primer.
 DNA polymerase then add nucleotides to the 3` end of
RNA primer.
 Another DNA polymerase enzyme then removes RNA
primers and replaces with DNA.
 Finally, DNA ligase enzyme will seal the gaps in the
lagging strand.
 During DNA replication, the leading strand is synthesized
continuously while the lagging strand is synthesized
discontinuously.
Notice that DNA replicates in a semi-conservative
manner.
This means that each old strand (template) forms a
double helix with one newly synthesized strand.
Expression
 DNA expression means the process by which the
nucleotide sequence in a gene determines the sequence
of amino acids in a protein.
1- Transcription
2 stages
2- Translation
1- Transcription
• The 2 strands of the chromosome are separated.
• One strand acts as template for synthesis of messenger
RNA (mRNA) by the RNA polymerase enzyme.
• Each triplet of bases on mRNA is called codon.
2- Translation
• The mRNA attaches to the ribosomes.
• Transfer RNA (tRNA) is found in the cytoplasm. It
carries at one end a triplet of bases called anticodon
and at the other end a specific amino acid.
t-RNA structure
• mRNA and tRNA come together on the surface of
ribosome.
• Each tRNA finds its complementary codon on mRNA and
attaches to it by its anticodon.
• The amino acids carried by tRNAs on the ribosome are
then linked together to form polypeptide chain.
• As the ribosome moves along the mRNA, the
polypeptide chain grows sequentially until the entire
mRNA is translated.
• The synthesized protein is then released.
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