Genetic research - model organisms

Genetic research


GENETIC RESEARCH

Reasons for genetic research..

To understand better the cell functions of living organisms including humans, there is need to understand the genetic makeup of the organism

Knowledge of the genetic makeup (genome) of any organism allows for possible manipulation for the improvement of their functionality
With respect to humans, the results of genetic research findings have led to improvement in the aspect of health, food, environment, industry.

Successive improvements in molecular biology techniques and recombinant DNA ( rDNA ) technology have led to an improved and more confirmatory genetic research

These techniques include genetic engineering, cloning, gene expression and analysis, sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS PAGE), Polymerase chain reaction (PCR), hybridization, DNA sequencing, bioinformatics analysis.

MODEL ORGANISMS


For the molecular technique and rDNA technology application to be effective, the cloned organism or the expression system (organism used to express the target gene) should possess some characteristic features.

Organisms with these desired features are referred to as model organisms

Genetic findings on these organisms give insight on other categories of organisms they represent because they possess highly conserved regions in their genome maintained through the path of evolution

Characteristics of a model organism


Easily manipulated in the laboratory
Have shorter generation time
High level of proliferation
Usually possess lower percentage of non-coding regions on their DNA
Can easily be maintained in the laboratory
Study on their complete genome have been completed
High compatibility with some proteins in many organisms
Have small genome

Major Examples of Model Organisms

Prokaryotes: Escherichia coli, Bacillus subtilis
Eukaryotes
Fungi: Saccharomyces cerevisiae, Aspergilus sp
Plant: Arabidopsis thaliana, maize (Zea mays)
Animal
Invertebrates: Caenorhabditis elegans, Drosophila melanogaster
Vertebrates: Rat (Mus musculus)

Saccharomyces cerevisiae

Saccharomyces cerevisiae

A unicellular fungi that reproduce sexually by binary fusion (haploid form) during stress condition and asexually by budding (diploid form) during high nutrient availability.

In unfavourable condition, the diploid organism undergo meiosis to produce 4 haploid ascus (ascospores).

When the environment is favourable, the haploid spores germinate and can reproduce by:
budding through mitosis to produce haploid offspring (no of chromosomes=16)
fusion of male and female to produce diploid offspring (no of chromosomes=32).

Number of genes: 5800 equivalent of 13.5 million bases of DNA
Cells in the diploid state can easily resist harsh environmental condition
Length: 5 – 10 ┬Ám in diameter
Life cycle: approximately 90 mins

Utilizes glucose for metabolism either by aerobic respiration or anaerobic fermentation
Commonly referred to as baker’s or brewer’s yeast.

Many of their proteins are similar in sequence and function to those found in other organisms, studies performed in yeast can help us to determine how a particular gene or protein functions in higher eukaryotes (including humans).

Study (using yeast-two hybrid method for instance) on some human enzymes were done using this specie of organism because these enzymes from humans have homology with that from the organism
Have been employed extensively in the study of
DNA repair mechanism (eg discovery of cyclin-dependent kinases as key regulators of cell division)
Eukaryotic cell regulatory systems (eg discovery of prevention of DNA degeneration by telomeres)
Reproduction by mating implies possible gene recombination, hence,
Allows for study of gene manipulation using plasmids or gene combination

Drosophila melanogaster

Drosophila melanogaster

Commonly known as fruit fly
Length: 3 mm in diameter
Life cycle: 2 weeks
Belong to the order: Diptera, family: Drosophilidae
Used specially for study of genetics, evolution and microbial pathology
Most early studies on heredity (especially those discoveries that led to modifications of Mendelian theory) like epistasis, multiple alleles, sex-linked inheritance, were performed with Drosophila
Undergo morphogenesis.

The development of their eggs is dependent on temperature; higher temperature increases the development time of egg to adult
At 25 0C, they develop within 2 weeks
At18 0C, they develop as long as 4 weeks
Within 12 hours of becoming adult, are fertile
Their genome is made of three autosomal chromosomes and one sex chromosome
Though they have XY system of sex determination, the sex of offsprings are determined by X:A ratio and not the presence of the Y-chromosome as in humans
Have 1 sex chromosome and 3 autosomes
No of genes: 14,000 equivalent of 165 million bases of DNA
Additional features other than the general features on why they are considered as model organism is that:
Males do not show meiotic recombination
Mature larva have giant chromosomes that can easily show gene activity like transcription process
There are distinct features that differentiate males from females; also female virgins from female non-virgins (this attribute facilitates genetic crossing)
They possess unique genetic marker (polythene chromosome) as well as good genetic markers like body pigmentation, wing colour and shape, eye colour etc.
50% of their protein sequence have human homologue

Caenorhabditis elegans

Caenorhabditis elegans

A multicellular unsegmented free-living nematode that inhabits temperate soil environment

Length: 1 – 5 mm
Life cycle: 3 days
Lifespan: 2 - 3 weeks
Phylum: nematode
Number of genes: 17,800 equivalent of100 million bases of DNA
They have false body cavity (pseudo coelomate) lacking circulatory and respiratory systems but possess mouth, pharynx, intestine, gonad, collagenous cuticle and four muscles (81 cells) that run down the entire body cavity

They are propelled (move) by
the four longitudinal bands of muscles paired sub-dorsally and sub-ventrally (front and back respectively)
Flexing and relaxation of dorsal-ventral waves along the body

Possess 300 neurons
All its sense organs are in the head
Their gut has granules (similar to lysozymes) with acidic interior and high endocytosis capacity and can emit bluish fluorescence under ultraviolet light

Females are mainly hermaphrodites and generally possess two ovaries and a uterus. Males have tail with spicules and a single-lobed gonad (vas deferens)

The female lays internally fertilised eggs that pass through four stages of molting.

At unfavourable condition, their egg goes into Dauer (stress resistant larvae)
Earlier use as model organism was applied in the study of neural development because it is one of the simplest organisms with a nervous system.

Their transparent nature improves the study on cellular differentiation and development.
Their use in genetic research brought about better understanding of:
Asymmetric cell division.

Cell apoptosis (elucidation of apoptotic gene)
Allows for study of functional genomics that employs the use of interference RNA ( RNAi ).

Arabidopsis thaliana


Commonly referred to as thale cress or mouse-ear cress
An angiosperm, dicot from mustard family
They are annual herb with basal leaves and short leaf stalk ( petioles )

Have flowers on hairless sepals and spatulate petals

Have six stamen (male organ) and one gynoecium (female organ)

Length: 6 – 12 inches

Life cycle: 5 – 6 weeks

Can produce 10,000 seeds per plant

Number of genes: 27,407 genes equivalent of 157 million bases of 

DNA
Herbaceous flowering dicot plant with lots of genetic and epigenetic diversities (allowing for existence of many variants)
Can cross-pollinate but usually self-pollinate • Their SRK and SCR genes that are responsible for preventing self pollination are usually inactive

Applied in researches for better understanding of plant biology including flower development, light sensing, plant-pathogen interaction and non-Mendelian inheritance studies

Grows by having cluster of leaves at the base of the stem and few leaves on the flowering stem
The evolution of Arabidopsis involved a whole-genome duplication, followed by subsequent gene loss and extensive local gene duplications, giving rise to a dynamic genome enriched by lateral gene transfer from a cyanobacteria-like ancestor of the plastid.

Transformation of the plant during genetic research employs specifically Agrobacterium tumefaciens
  

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