Q.1.If majority of our DNA is junk DNA then is it
completely useless or it has some function?
Can’t the junk DNA be cut off and still organisms
can survive and evolve?
–Bushra, Mumbai
Ans. Junk DNA also called selfish DNA includes those
regions of DNA, that apparently have no function, and
exist between the regions of DNA, that represent the
genes. They are non-coding DNA sequences i.e., do not
encode protein sequences. Though some non-coding
DNA is transcribed into functional non-coding RNA
molecules (transfer RNA, ribosomal RNA and regulatory
RNAs). The amount of non-coding DNA varies greatly
among species, for example 98% of human genome
is non-coding DNA, while only about 2% of a typical
bacterial genome is non-coding DNA.
Junk DNA sequences include non-coding functional
RNA, cis-regulatory elements, introns, telomeres,
repeat sequences, transposons, centromeres and viral
elements.
The term ‘Junk DNA’ was first used in 1960’s but
was formalised by Susumu Ohno in 1972. Earlier, junk
DNA, as the name suggests, was considered to be of
no advantage, but advent of new molecular biology
techniques has changed the perception.
Many non-coding DNA sequences have important
biological functions, as indicated by comparative
genomic studies that report some regions of non-
coding DNA are highly conserved, sometimes on time
scales, representing hundreds of millions of years,
implying that these non-coding regions are under
strong evolutionary pressure and positive selection.
Linkage mapping often identifies chromosomal
regions associated with a disease, with no evidence of
functional coding variants of genes within the region,
suggesting that disease causing variants lie in the
non-coding DNA.
The significance of non-coding DNA mutations was
explored in cancer. In a study of colorectal cancer, it
has been found that mutations in junk DNA are equally
important as mutations in coding DNA for development
of cancer. Some specific sequences of non-coding DNA
maybe essential for chromosome structure, centromere
function and homologue recognition in meiosis.
Life scientists continue to identify new functions of the
so-called junk DNA. Recently, a research by US Center
for Disease Control and Prevention emphasised that
junk DNA:
(i) Has a nucleo-skeletal role that helps to establish
the volume of the cell’s nucleus. This nucleo-
skeletal hypothesis states that the amount of DNA
in a cell dictates the nuclear volume. The cell will
die, if the ratio of the nuclear volume, relative to
overall cell volume deviates too much.
(ii) Acts as a mutational buffer, that protects the
genome from mutations, resulting from transposons
and retroviral DNA insertion activity.
These non-informational functions help to account for
the existence of abundant junk DNA in the genomes
of humans and other organisms.
Hence, it is clear that junk DNA cannot be cut off, as
it serves an important purpose in organisms, although
more researches and discoveries are yet to be made,
to understand the functions of junk DNA.
Q. 2.Why do our eyes blink?- Akash Jack
Ans. Blinking is semi-autonomous rapid opening and
closing of the eye. It is an essential function of eye that
helps to remove irritants from the surface of eye.
Blinking keeps to keep the cornea moist, as every time
we blink, our eyelids spread oils and mucous secretions
across the surface of eye, secreted by about 20-30
sebaceous glands located between eye lashes.
Normal length of a blink is 300-400 milliseconds.
Blinking rate is controlled by the ‘blinking center’
but it can also be affected by external stimuli.
Blinking centre is located in globus pallidus of
lenticular nucleus within basal ganglia. Blinking of
eye can be voluntary or involuntary.
There are 2 types of involuntary blinking-spontaneous
and reflex blinking.
Spontaneous blinking is a type of involuntary blinking
without any external stimulus or internal factor. This
type of blinking is controlled by the premotor brain
stem and requires no conscious effort, like breathing
and digestion.
Reflex blinking is also a type of involuntary blinking.
The blink is the response of external stimuli or contact
of a foreign body with cornea. It is a type of ocular
motor response due to irritation of cornea, or for
changes in the visual stimulus. It is initiated by sensory
stimuli that activate afferent neurons (somato-sensory
stimuli for eye blink reflex).