1. Genomes can be found in viruses, prokaryotes, and eukaryotes. Viruses have the smallest genomes consisting of either DNA or RNA. Prokaryotes like bacteria generally have a single circular chromosome while eukaryotes have multiple linear chromosomes within the nucleus. 2. Prokaryotes and eukaryotes differ in their genome organization. Prokaryotes lack membrane-bound nuclei and histones, have circular DNA, and generally do not contain introns. Eukaryotes have nuclei, linear chromosomes, and genes that can contain introns and exons. 3. In addition to chromosomes, bacteria can contain extra-chromosomal DNA called plasmids which

1. BTC 520
BACTERIAL, VIRAL
GENOME
ORGANISATION

2. GENEORGANISATION
• The word “genome,” coined by German botanist
Hans Winkler in 1920, was derived simply by
combining gene and the final syllable of
chromosome.
• An organism’s genome is defined as the complete
haploid genetic complement of a typical cell.
• In diploid organisms, sequence variations exist
between the two copies of each chromosome
present in a cell.
• The genome is the ultimate source of information
about an organism.

3. PROKARYOTES AND EUKARYOTES
GENOME
Prokaryotes Eukaryotes
Single cell Single or multi cell
No nucleus Nucleus
One piece of circular DNA Chromosomes
No mRNA posttranscriptional
modification
Exons/Introns splicing

4. CHROMOSOMEDIFFERENCE
[PROKARYOTICANDEUKARYOTICCELL]
 PROKARYOTICCELL
 Lacksamembrane bound nucleus.
 Circular DNAand supercoileddomain.
 Histones not present.
 Prokaryotic genomesgeneraly contains one largecircular
piece of DNArefered to asaCHROMOSOME.
 Somebacteria havelinear chromosome.
 Many bacteria havesmal circular DNAstr.CaledPLASMIDS
which can be swapped between neighbours and across
bacterial species.

5. Procaryotic genomes
Generally 1 circular chromosome (dsDNA)
Usually without introns
Relatively high gene density (~2500 genes per mm
of E. coli DNA)
length of E.coli genome: 1.7 mm
Often indigenous(natural) plasmids are present

6. Plasmids
Extra chromosomal circular DNAs
Found in bacteria, yeast and other fungi
Size varies form ~ 3,000 bp to 100,000 bp.
Replicate autonomously (origin of replication)
May contain resistance genes
May be transferred from one bacterium to another
May be transferred across kingdoms
Multicopy plasmids (~ up to 400 plasmids/per cell)
Low copy plasmids (1 –2 copies per cell)
Plasmids may be incompatible with each other
Are used as vectors that could carry a foreign gene of
interest (e.g. insulin)
-lactamase
ori
foreign gene

7. PLASMID
o The term plasmid was first
introduced by the American
molecular biologist Joshua
Lederberg in 1952.
o A plasmid is separate form,
and can replicate
independently of, the
chromosomal DNA.
o Plasmid size varies from 1 to
over 1,000 (kbp).

8. Classification andtypesof
PLASMID
•Plasmids can be broadly classified into
conjugative plasmids andnon-conjugative
plasmids.
•In the complexprocessof conjugation,plasmid
may be transferred from one bacterium to
another via sexpili encodedby someof
the tragenes(transfer operon).
•Non-conjugative plasmids are incapable of
initiating conjugation, hence they can be
transferred only with the assistanceofconjugative
plasmids.

9. Another way to classify plasmidsisby
function. Thereare five mainclasses:
• Fertility F-plasmids,which contain tragenes.Theyarecapable
of conjugation andresult in the expressionofsexpili.
• Resistance(R)plasmids,which contain genesthat provide
resistanceagainstantibiotics or poisons.
• Colplasmids,which contain genesthat code
for bacteriocins(p), proteins that cankill other bacteria.
• Degradative plasmids, which enable the digestion of unusual
substances, e.g. toluene(aromatic hydrocarbon) and salicylic
acid(aromatic acid)
• Virulence plasmids, which turn the bacteriumintoapathogen.

10. Bacterial genes
Most do not have introns
Many are organized in operons: contiguous
genes, transcribed as a single polycistronic
mRNA(bac and chloroplast mrna), that encode
proteins with related functions
Polycistronic mRNA encodes several proteins

11. Viral genomes
Viral genomes: ssRNA, dsRNA, ssDNA, dsDNA, linear or ciruclar
Viruses with RNA genomes:
•Almost all plant viruses and some bacterial and animal viruses
•Genomes are rather small (a few thousand nucleotides)
Viruses with DNA genomes (e.g. lambda = 48,502 bp):
•Often a circular genome.
Replicative form of viral genomes
•all ssRNA viruses produce dsRNA molecules
•many linear DNA molecules become circular
Molecular weight and contour length:
• duplex length per nucleotide = 3.4 Å
• Mol. Weight per base pair = ~ 660

12. VIRALGENOME
A virus has either a DNA or an
RNAgenomeandiscalled aDNAvirusor an
vast majority
genomes. Plant
RNA virus, respectively.
The of viruses RNA
viruseshavetendtohave single-stranded
RNA genomes and bacteriophages tend to
have double-strandedDNAgenomes.

13. Mitochondrial genome (mtDNA)
Multiple identical circular chromosomes
Size ~15 Kb in animals
Size ~ 200 kb to 2,500 kb in plants
Over 95% of mitochondrial proteins are encoded in
the nuclear genome.
Often A+T rich genomes.
Mt DNA is replicated before or during mitosis

15. Mitochondrial DNA contains 37 genes, all of which are
essential for normal mitochondrial function. Thirteen of
these genes provide instructions for making enzymes
involved in oxidative phosphorylation. Oxidative
phosphorylation is a process that uses oxygen and simple
sugars to create adenosine triphosphate (ATP), the cell's
main energy source.
The remaining genes provide instructions for making
molecules called transfer RNA (tRNA) and ribosomal
RNA (rRNA), which are chemical cousins of DNA. These
types of RNA help assemble protein building blocks
(amino acids) into functioning proteins.

16. Chloroplast genome (cpDNA)
Multiple circular molecules
Size ranges from 120 kb to 160 kb
Similar to mtDNA
Many chloroplast proteins are encoded in the
nucleus (separate signal sequence)

17. “Cellular” Genomes
VirusesProkaryotes Eukaryotes
Viral genome Bacterial
chromosome
Plasmids
Chromosomes
(Nuclear genome)
Mitochondrial
genome
Chloroplast
genome
Genome: all of an organism’s genes plus intergenic DNA
Intergenic DNA = DNA between genes
Capsid
Nucleus

18. Size of genomes
Epstein-Barr virus 0.172 x 106
E. coli 4.6 x 106
S. cerevisiae 12.1 x 106
C. elegans 95.5 x 106
A. thaliana 117 x 106
D. melanogaster 180 x 106
H. sapiens 3200 x 106

19. KARYOTYPE
o The study of chromosomes, their structure and
their inheritance is known asCytogenetics.
o Each species has a characteristic number of
chromosomes and this is known askaryotype.

21. GENES–GENETICINFORMATION
• One gene : one enzyme
hypothesis: summarizes
that a gene is a stretch of
DNA coding for one or
more isoforms of a single
enzyme.(Beadle 1941)
• One gene : one
polypeptide hypothesis: a
gene is responsible for the
production of a single
polypeptide.(Beadle 1945)