Biology Reference
In-Depth Information
biochemical reactions taking place within the cell. Transport proteins carry impor-
tant molecules throughout the body. Regulatory proteins control and coordinate
biochemical reactions in the cell and the organism as a whole. Protective proteins
(antibodies) protect against infectious agents and injury. Storage proteins store
products for future use. However, the dogma that “one gene equals one protein”
needs modification because not all genes code for proteins.
Much of the genome (up to 80%) contains DNA that does not code for pro-
teins, and much of this DNA has been considered to be junk or parasitic DNA.
Debate has raged as to why the genome would retain this expensive DNA if it
had no function. However, geneticists have determined that much of this so-
called junk DNA is transcribed into ncRNAs and that these ncRNAs are involved
in a variety of essential functions, including gene regulation and development
( Storz 2002 ). The discovery that ncRNAs are important in a variety of cellular
functions largely resolves why eukaryotes have so much DNA ( Box 2.1 ).
Box 2.1 RNAs Have Multiple Functions. Additional RNAs and Functions May Yet be
Discovered
Name
Function
Abbreviation
Size
Protein synthesis
Pre-messenger RNA
Transcript from the DNA
pre-mRNA
Variable
Messenger RNA
Codes for proteins
mRNA
Variable
Ribosomal RNA
Part of ribosome structure, involved
in translation
rRNA
Several
Transfer RNA
Involved in translation, transport of
amino acids
tRNA
75-80 nt
Posttranslational modification
or DNA replication
Small nuclear
Splicing of RNA in nucleus
snRNA
Small nucleolar
Modification of rRNAs
snoRNAs
60-300 nt
Telomerase RNA
Telomere synthesis
Regulatory RNAs
Long noncoding
Various
long ncRNA
Various
Micro
Gene regulation, inhibiting
translation by binding to mRNAs
miRNA
21-26 nt
Piwi-interacting
Defense against transposons and ?
piRNA
29-30 nt
Small interfering
Gene regulation; one strand of ds
siRNA incorporates into RISC,
inhibiting transcription of mRNAs
with complementary sequence
siRNA
20-25 nt
 
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