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Noncoding DNA in Eukaryotes : Introduction
Each cell in our bodies has about 6 feet of DNA stuffed into it.
-However, less than one inch is devoted to genes! Non-coding DNA describes components of an organism’s DNA
sequences that do not encode for protein sequences. In many eukaryotes, a large % of an organism’s total genome size
is non-coding DNA. Amount of non-coding DNA & the proportion of coding versus non-
coding DNA varies greatly between species. Much of this DNA has no known biological function & was referred
to as “Junk DNA”.
Types of non- coding DNA sequences
Non- coding functional RNA
Cis- and Trans- regulatory elements
Introns
Pseudogenes
Repeat sequences, transposons and viral elements
Telomeres
1.Non- coding Functional RNA
The RNA molecules which are not translated into proteins.
For eg:- Ribosomal RNA, Transfer RNA &
Micro RNA
2.Cis- and Trans- Regulatory Elements
Those are sequences that control the transcription of a nearby gene.
Located within 5’ or 3’ untranslated regions or within introns.
trans-regulatory element control the
transcription of a distant gene.
3. Introns
They are non-coding sections of a gene.
Transcribed in the precursor m-RNA sequence
but is ultimately removed by RNA splicing.
Most of the introns appear to be mobile
genetic elements.
4. Pseudogenes
They are related to known genes, that have lost their
protein-coding ability or are otherwise no longer
expressed in the cell.
Arise from retrotransposition or genomic duplication of
functional genes.
Therefore become “Genomic Fossils” : non-functional.
5.Repeat Sequences, Transposons & Viral Elements
Transposons & Retrotransposons are mobile genetic element.
Retrotransposons : LINEs, SINEs – account for large proportion of the
genomic sequences in many species.
Over 8% of the human genome is made up of endogenous retrovirus
sequences as a part of over 42% fraction that is recognizably derived
of retrotransposons.
Remaining 3% can be identified to be the remains of DNA
transposons.
6. Telomeres
Telomeres are regions of repetitive DNA.
Located at the end of a chromosome.
They provide protection from chromosomal
deterioration during DNA replication.
FUNCTIONS OF NON-CODING DNA
They have strong biological functions : some regions that are highly conserved are under evolutionary pressure & positive selection. Some specific sequences of non-coding DNA are essential for chromosome
structure, centromere function & homolog recognition in meiosis. From study over 300 prokaryotic & 30 eukaryotic genomes, eukaryotes
appear to require less amount of non-coding DNA. Apart from this : 1. protection of the genome 6. Enhancers
2. Genetic switches 7. Silencers
3. Regulation of gene expression 8. Promoters
4. Transcription factor sites 9. Insulators
5. Operators
REFERENCES
www.Wikipedia.org www.google.com www.Princeton.edu T. A. Brown