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Life Cycle of some

genetically

important

organisms

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 Lambda phage

Order Caudovirales

Family Siphoviridae

Genus

 λ-like viruses 

Species Enterobacteria phage 

λ  

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 Morphology 

Head: Icosahedral symmetry

Tail: Helical Symmetry

& one tail fiber 

Capsid not enveloped

Linear dsDNA ( phage) 

Infect E. coli cell

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Genome

genome contains about 50,000 nucleotide pairs

encodes 50-60 different proteins

Genome is 54% of virion by weight

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The ends of the genome have sticky ends 12 bp long => cos sites

(cohesive ends) 

The ends are joined by bacterial enzyme, Ligase

Closed circle dsDNA ( E.coli) 

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Linear genome becomes circularized

once inside E. coli  

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Mode of infection in E.coli cell

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Life cycle of lambda phage

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Life Cycle of Escherichia coli

E. coli includes both asexual and sexualreproduction in its life cycle.

 Asexual reproduction is achieved by simple binaryfission.

Sexual reproduction is achieved by conjugation of 

some genetically different stains of E. coli.

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Binary Fission

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During cell division, two new poles are formed, one in

each of the progeny cells (new poles, shown in blue).

The other ends of those cells were formed during a

previous division (old poles, shown in red). 

The number of divisions since each pole was formed is

indicated by the number inside the pole. Using thenumber of divisions since the older pole of each cell

was formed, it is possible to assign an age in divisions

to that cell, as indicated. Similarly, cells that

consecutively divided as a new pole are assigned a

new pole age, based on the current, consecutive

divisions as a new pole cell. 

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In 1965 Sydney Brenner settled on the small nematode worm

Caenorhabditis elegans to study the important questions of 

development and the molecular basis of behavior, because it

contained a variety of suitable characteristics.

 And due to its simplicity and experimental accessibility, it is now one of the most completely understood metazoan.

 THE NEMATODE WORM,

Caenorhabditis elegans  

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C. elegans has a very rapid life

cycle  At 25℃ fertilized embryos of C. elegans complete development in 12 hours

and hatch into free-living animals capable of complex behaviors.

 The first stage juvenile(L1) passes through four juvenile stages(L1-L4) over

the course of 40 hours to become a sexually mature adult.

Under stressful conditions, the L1 stage animal can enter an alternativedevelopmental stage in which it forms what is called a dauer.

Dauers are resistant to environmental stresses and can live many months while waiting for environmental conditions to imptove.

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The life cycle of the worm, C. elegans

Numbers indicate

time in HOURS

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  THE FRUIT FLY, Drosophila 

melanogaster 

The salient features of the Drosophila life

cycle are a very rapid period of embryogenesis, followed by three period of 

larval growth prior to metamorphosis

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The Drosophila life cycle

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One of the key processes that occurs during larval development is the growth of the imaginal

disks, which arise from invaginations of the

epidermis in mid-stage embryos.

Imaginal disks differentiate into their

appropriate adult structures during 

metamorphosis (or putation).

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Imaginal disks in

Drosophila

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Life cycle of a frog

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Life cycle in a plant