repetitive and non repetitive dna.pptx

occurences of DNA sequence is major three forms.

1. Repetitive and Non-repetitive DNA FBT-601 Advances in Molecular and Cell Biology

2. Our genome is made of a chemical called DNA and its contains 4 basic building blocks (A,T,G,C). The order or sequence of these bases form the instruction in the genome. Eukaryotic DNA has numerous DNA segments which have the identical sequences of nitrogenous bases. These sequences are repeated several times and clustered together( Repetitive DNA). DNA sequences occurs in a three forms: 1. Highly Repetitive DNA 2. moderately repetitive DNA 3.Non-repetivive DNA

3. Repetitive DNA The DNA is made up of sequences that are present in more than in a million copies/ genome. It ranges approximately 50% in human genome. Highly repetitive sequences: - These sequences present in >10 5 to several million copies/genome. Short sequences Different density from the rest of chromosomal DNA called satellite DNA Located in heterochromatic region of the chromosome – Centromeric region They are not transcribed many satellite DNA consists of tandem repeats of short sequences of 2-8 bp. These repeats are identical. Variation of this type arise due to different in the number of units which are highly variable. These DNA are called minisatellites or VNTRs/ DNA fingerprinting.

4. Genome that contain these different classes of sequences reanneal in a different manner than genomes with only single copy sequences. Instead of having a single smooth “Cot” curve, three distinct curves can be seen, each representing a different repetition class. The first sequences to reanneal are the highly repetitive sequences because so many copies of them exist in the genome, and because they have a low sequence complexity. The second portion of the genome to reanneal is the middle repetitive DNA and the final portion to reanneal the single copy DNA or Unique DNA sequence.

5. Tandem repeats It occur in DNA when a pattern of two or more nucleotides is repeated and the repetitions are adjacent to each other. Form different density band on density gradient centrifugation (from bulk DNA)- satellite Tandem repeats: Satellite DNA : > 100 bp bp depending on species, not code for proteins and convey functional genetic information, Generally heterochromatic, Centromeric DNA, Telomeric DNA, There are at least 10 distinct human types of satellite DNA15 (alphoid DNA, beta, satellite 1,2 and 3) Microsatellite: Short tandem repeats, Simple sequence length polymorphism, simple sequence repeat (SSRs, STRs) 1-6 bp 10-100 times Generally Euchromatin Most useful marker for population level studies Minisatellites: 10-100 bp 20-50 times Euchromatin DNA fingerprints, Tandemly repeated but often in dispersed clusters. Also called VNTR’s (variable number tandem repeats)

6. Interspersed repetitive DNA Interspersed repetitive DNA accounts for 25-40% of mammalian DNA They are scattered randomly throughout the genome Units are 100-1,000 base pairs long Copies are similar but not identical to each other Interspersed repetitive genes are not stably integrated in the genome; they move from place to place They can sometimes mess up good genes Retrotransposons (class I transposable elements) (copy and paste), copy themselves to RNA and then back to DNA (using reverse transcriptase) to integrate into the genome. Long terminal repeat: any transposon flanked by long terminal repeat also called retrovirus like elements. None are active in humans, some are mobile in mice Long interspersed nuclear elements (LINEs) encodes RT, two open reading frames (ORF1 and 2), 5,00,000 bp Short interspersed nuclear elements (SINEs) uses RT from LINEs. Ex . Alu ( A rthrobacter luteus RE) made up of 350 bp long, recognized by the RE AluI (non- autonomus ), <500 bp Role of LINEs and SINEs It mainly knowns as Junk DNA in some case both were incorporated into novel genes so as to evolve new functionality. The distribution of these elements has been implicated in some genetic diseases and cancers. Transposons (class II TEs) (cut and paste) uses transposases to makes a staggered sticky cut.

7. 2. Moderately repetitive sequences : - It present in 2 to < 10^5 % genome It makes 20-80 % of chromosomal DNA in eukaryotes In this family the repeated sequences are not identical but are related to each other. The degree of relationship varies among the members of the family (in southern blots). The two major ribosomal RNA molecules come from tandem pair of genes that is repeated several hundred times in most eukaryotic genomes. The genomes of all eukaryotes also contain multiple copies of the histone genes. Non repetitive DNA: These sequences are also called single copy DNA sequences or Unique sequences. It present in only one or in a small number of copies in a genome. It has the reassociation kinetics expected from unique sequences and is characterized by a high t value. It generally consists of protein coding genes or a plasmid in only one copy per genome. 3. Unique sequences M ost gene sequences and the adjacent nucleotide sequences required for their expression are contained in the unique sequences component. With minor exceptions (ex- the repetition of one or a few genes), the genomes of viruses and prokaryotes are composed entirely of single –copy sequences; in contrast, such sequences constitute only 38% of the total genome in some sea urchin species, more than 50% of human genome, and 70% of D. melanogaster genome.

8. Abundant mRNA-1000-10,000 copies per cell Scarce mRNA- <10 copies/ cell Therefore, abundant mRNA’s represent genes whose proteins are major product of the concerned cell type. House keeping genes (constitutive genes) Luxury genes

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