Structural, Functional and Evolutionary Roles of Major Motifs in DNA Sequences
With the first sequences of complete eukaryotic chromosomes published in the closing years of the 20th century, it is an appropriate time to consider the relationship of the linear sequence to chromosomal organization and function. The majority of the DNA in most genomes is made up of recognizable repetitive DNA sequence motifs, ranging in length from a single base to thousands of bases, repeated many hundreds or thousands of times in the genome. Individual sequence motifs have particular characteristics and organization along the linear chromosomal DNA molecule. Some repetitive motifs are highly conserved across eukaryotes and even prokaryotes, while others are so variable that they provide valuable markers for biodiversity. Some are present as tandem repeats in long and relatively homogeneous stretches of repetitive DNA motifs, while others are widely dispersed in the genome. Repetitive sequences may have clearly defined functions (e.g. telomeres), functions whose role in the genome are unclear (retroelements), or no known function. Within the nucleus, DNA is modified by addition of methyl groups, and most of the DNA double-helix is wrapped around the histone proteins, forming nucleosomes, the first and fundamental structural subunit of chromosomes. The remodelling of chromatin and modification including methylation and to its packaging are important features with wide involvement in developmental control. The organization of chromosomes in the three dimensions of the interphase nucleus - nuclear architecture - and the relationship of genome organization to gene expression and development is an exciting area of current research. The multiple levels of organization of the chromatin, including the repetitive DNA sequences, provide functional regulators of DNA behaviour: packing and unpacking, replication, repair, mutation and transcription. Individual cells behave differently, and the architecture is highly dynamic. The analysis and significance of these will be discussed.
Further details and citations to the work described are available at http://www.jic.bbsrc.ac.uk/staff/pat-heslop-harrison/index.htm.
J. S. Heslop-Harrison
Department of Cell Biology John Innes Centre Colney,