Is dna sequencing done before switching to fish ifnot then how the repeated sequences are known to block them

Krishan k jakhad

no DNA sequencing required there are various methods to find repeated sequences e.g. Study of the maize (Zea mays) somatic (2n = 20) n=10 has been difficult because of a lack of distinguishing characteristics. To identify all maize chromosomes, a multicolor fluorescence in situ hybridization procedure was developed. The procedure uses tandemly repeated DNA sequences to generate a distinctive banding pattern for each of the 10 chromosomes. Fluorescence in situ hybridization screening trials of nonsubtracted or subtracted PCR libraries resulted in the isolation of microsatellite 1-26-2, subtelomeric 4-12-1, and 5S rRNA 2-3-3 clones. These three probes, plus centromeric satellite 4 (Cent4), centromeric satellite C (CentC), knob, nucleolus-organizing region (NOR), pMTY9ER telomere-associated sequence, and tandemly repeated DNA sequence 1 (TR-1) were used as a mixture for hybridization to root-tip chromosomes. All 10 chromosomes were identified by the banding and color patterns in the 14 examined lines. There was significant quantitative variation among lines. The same probe mixture identifies meiotic pachytene, late prophase I, and metaphase I chromosomes. other method is Chromosomal detection of simple sequence repeats (SSRs) using nondenaturing FISH (ND-FISH) SSRs are known to be scattered and present in high number in eukaryotic genomes. We demonstrate that dye-labeled oligodeoxyribonucleotides with repeated mono, di, trinucleotide motifs (15-20 nucleotides in length) have an unexpected ability to recognize SSR target sequences in non-denatured chromosomes. This novel and remarkable property of binding SSR oligonucleotides to duplex DNA targets permitted the development of a non-denaturing fluorescence in situ hybridization method that quickly and efficiently detects SSR-enriched chromosome regions. These results have implications for genome analysis and for investigating the roles of SSRs in chromosome structure and function.