Comparison of model genetic organisms for analysis of gene function by means of mutants

  Summary Page  

1.  INTRODUCTION  3

2.  DISCUSSION  3

2.1.  Caenorhabditis elegans  3

2.1.1.  General Description  3

2.1.2.  The Genome  4

2.1.3.  Transposable Elements  5

2.1.4.  Integrative Transformation  5

2.1.5.  Cell lineage  5

2.1.6.  Hermaphrodite-specific genes Male-specific genes  6

2.2.  Mus musculus  6

2.2.1.  General Description  6

2.2.2.  The Genome  6

2.2.3  Transgene Animals  7

2.2.4  Knock-out Mice  8

2.3  Arabidopsis thaliana  8

2.3.1.  General Description  8

2.3.2.  The Genome  9

2.3.3.  Arabidopsis Mutants  10

3.  CONCLUSION  11

4.  REFERENCES  11

5.  COMPARISON OF MODEL ORGANISMS (TABLE)  14

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Comparison of model genetic organisms for analysis of gene function by means of mutants

Benjamin Thimm

University of Glasgow

1. Introduction

Biomedical research with experiments on plants and animals is used if we wish to understand human disease, growth and development. Appropriate models must be used to improve animal or plant production, control diseases or pests, etc. Therefore model organisms took precedence because large numbers of researchers chose to build on existing knowledge and recognised the unique attributes of the particular system.

Following will illustrate the connections between a group of model organisms in the form of a table as well as insight is given in more detail in three specific ones (Nematode, Vertebrate, and Plant).

2. Discussion

2.1 Caenorhabditis elegans (Nematode)

hermaphrodites and males, are about 1mm in length but differ in appearance as adults. Hermaphrodites produce both oocytes and sperm and can reproduce by self-fertilisation. Males, which arise at low frequency, can fertilize hermaphrodites; latter cannot fertilize each other. After hatching C.elegans runs through four larval stages and four molt stages to end in the adult state. [13]

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A hermaphrodite lays about 300 eggs and has exactly 959 somatic nuclei, the adult male exactly 1031, respectively. The haploid genome size is 8x10 7 bp. The wild-type and mutant strains can be stored indefinitely in the frozen state. The animals are transparent throughout lifecycle, so that development can be followed at the cellular level. Mutations are readily obtained following chemical mutagenesis or exposure to ionising radiation. Because of its favourable experimental attributions (simplicity, transparency, ease of cultivation, short life cycle, suitability of genetic analysis, and small genome) C. elegans is a useful experimental metazoan organism for investigation of a variety of problems. [1] [2]

2.1.2 The Genome

80% of the C.elegans genome is composed of single-copy sequences , and 20% are primarily moderately repetitive sequences , including a transposable element Tc1.

Genes of C.elegans can be mapped into six haploid chromosomes. Each chromosome is holocentric. The haploid set, seen with dyes like Hoechst 33258, includes five autosomes and one sex chromosome. Sex is determined chromosomally, depending on the X/A ratio. Hermaphrodites are diploid for all six chromosomes (XX), whereas males are diploid for the autosomes but only have one X chromosome. Males arise spontaneously in hermaphrodite populations by X- chromosome non-disjunction at meiosis, with a frequency of about 1/500 animals. [3]

Chemical mutagens such as EMS induce mutations at high frequency. All mutants, however, will come out as heterozygotes because mutation in the same gene is rare. Some transposable elements are mobile in the germ line and allow mutagenesis by transposon insertions. A mutation present in the heterozygous state in a hermaphrodite will be homozygous in one quarter of that animal’s self-progeny, so that recovery of recessive alleles is convenient. Males are useful for double mutant analysis. [3] Sophisticated genetic tools have become available, including balancer chromosomes for maintaining lethal mutations, unstable duplications for mosaic analysis, temperature-sensitive mutants, and nonsense suppressors. The genetic approach has let to explicit models of many developmental processes in the

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nematode, such as vulva formation, sperm maturation, sex determination, muscle assembly, neural differentiation, and dauer larva formation. Identification of genes is possible through transposon tagging (Tc1 and Tc3), mutation mapping and complementation. [1] [2]

2.1.3 Transposable Elements

C.elegans contains several families of transposable elements, called Tc elements. Tc1 and Tc3 transpose into the germ line. The structure of Tc elements resembles that of P elements in Drosophila and Ac/Ds elements in maize. The most straightforward method for isolating a gene of interest is to take advantage of a transposon-induced allele. [3]

2.1.4 Integrative Transformation

Cloned genes can be reintroduced into C.elegans by microinjection of DNA into the syncytal ovary or into the oocyte nucleus. The DNA either integrates into a chromosome or forms unstable, extrachromosomal tandem arrays. Fluorescent marking and body shape alteration reveal integrated genes. Also E.coli plasmids can be used for gene induction. However, genes can’t be knocked out in C.elegans but can be silenced by double-stranded RNA inserts. [3], [14]

2.1.5 Cell lineage

Because of its relative invariance, cell lineage occupies a special place. The fate of each cell is determined in advance. Programmed cell death, mostly in neuroblasts and mesodermal cells accounts for one in eight somatic cells produced. It does not occur at random rather follows a well-defined course. Apoptosis mutants can be spotted by cells non-capable of apoptosis. [4]

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