Question

Describe the classes of repetitive elements that are present in the human genome?

Answer 1

Ans:

There are two classes of repetitive DNA, The tandem repeats and The interspread repeats.Below are their various forms and their respective structure's in human DNA :

1. Telomere repeats [(TTAGGG)n]- They are located at the extreme end of linear DNA molecules in both humans as well as vertebrate chromosomes.These are tandemly repeating units.Structure wise,these are different from DNA and extends from 5K to 12K base pairs. Shortening of chromosomes during replication is prevented by enzyme telomerase, by replicating the chromosomal ends in an unsual manner.
2. Subtelomeric repeats- These are spread along the last 500K bases of non-repetitive DNA adjacently located to telomere.While some of the sequences are specific to choromoses,others are found near the chromosomal ends in the humans.
3. Microsatellite repeats -These are tandem repeats found in the euchromatic arms of chromosomes.The most common form of this repeat is the dinucleotide repeat,GTn. Their repeat range size is nearly 20 to 60 base pairs.
4. Minisatellite repeats - These are also class of tandem repeats with repeat size of 30-25 base pairs with sequences varying from base to base but have a core sequence of 10-15 base pair length.
5. Alu repeats -This is the most commonly found interspread repeat in the human DNA.They are 300 base pair in length.They are found abundantly in light bands when compared to dark bands.
6. L1 repeats - These are long repeats with 1K to 7K long base pairs. Apart from humans these are found in other mammals too.These can code reverse transcriptase ,thus helping the L1 copy themselves and re-inserted nto the genes. On an average, for about one lac copy , these repeats occur on every 28K base pairs in humans.