Q.1. After transcription, how the rRNA is synthesised
from hnRNA? – Bidhan Banerjee, West Bengal
Ans. Transcription is the process of copying genetic information
from template strand of DNa into rNa. rNa is of 3
types based on function-mrNa, rrNa and trNa. rrNa
is formed from nucleolar DNa and mrNa is formed from
nuclear DNa. This rNa brings instructions from DNa for
formation of particular type of polypeptide. Therefore, it is
called messenger rNa (mrNa) or informational rNa.
freshly released rNa formed after transcription, known
as primary transcript is also called heterogenous nuclear
rNa (hnrNa). It contains both exons (coding regions)
and introns (non-coding regions). This hnrNa undergoes
5 ′ capping, 3′ polyadenylation and splicing to form
mature mrNa. Capping of hnrNa involves addition of
7-methylguanosine to the 5′ end. The cleavage of 3′ end
is done and then 250 adenine residues are added to form
poly a tail. Then introns are removed and remaining exons
are connected to reform a single continuous molecule in
the process of splicing. Mature rNa thus obtained is
known as mrNa. On the other hand molecules of rrNa
are synthesised in a specialised region of cell nucleus
called nucleolus which contains the genes that encode
rrNa.
eukaryotic ribosomes contain four types of rrNa,
designated as 5S, 5.8S, 18S, and 28S rrNas. The 5.8S,
18S and 28S rrNa are transcribed as a single unit
within the nucleolus by rNa polymerase I, yielding a
45S ribosomal precursor rNa. The 45S pre-rrNa is
processed to the 18S rrNa of the 40S (small) ribosomal
subunit and to the 5.8S and 28S rrNas of the 60S (large)
ribsomal subunit. The rrNa genes are located in the
fibrillar centres, with transcription occurring primarily at
the boundary of the fibrillar centers and dense fibrillar
component. Processing of the pre-rrNa is inititated in the
dense fibrillar component and continues in the granular
component, where the rrNa is assembled with ribosomal
proteins to form nearly completed preribosomal subunits,
ready for export to the cytoplasm for further processing.Q.2. I am a CBSE Board class 12th student and will be
appearing for medical entrance exam in 2016 as
well. Can you help me in planning my studies so,
I can perform good in board as well as medical
entrance.- Radhika Sharma, Meerut
Ans. This is a crucial time for all the students appearing in
boards and medical entrance exams. a proper planning
will definitely help you fetch good marks in both.
boards and PMT exams are mainly based on NCer T
books. It is important to master NCerT. read and
understand every concept mentioned in the NCer T book
and then practice as many questions as possible and also
keep an eye on how you score. High scores will boost up
your confidence but do not let low scores demotivate
you, revise the chapter, practice more questions.
MTG NCer T @ your fingertips may come handy for
this. for boards, MTG excel in biology with chapterwise,
topicwise questions of last 15 + years’ board paper will
also help in getting ready for boards.
PMTs are different from boards in terms of the level and
pattern of questions. To get an idea of the questions
asked in a particular exam solve previous years’ papers of
that exam. every exam has a specific pattern of questions
asked solving previous years’ papers gives an insight into
this and helps you prepare better. adopt chapterwise
approach initially, and solve questions of a chapter just
after reading it. MTG Objective Zoology and Objective
botany (when you are preparing for multiple PMTs) or
MTG Chapterwise explorers for different exams (when
you are targeting any one or two of the PMTs) can
prove useful. When exams are close, then start solving
practice papers from entire syllabus in exam simulating
environment and within that time slot.
It is the time, when you should have covered your entire
syllabus and started revision. In case, any chapters are
left then finish them at earliest and then start revision
and practice. for distribution of time to different subjects,
you can follow the 3 + 2 + 2 rule where you give 3 days
a week to your weakest subject and 2 days each to the
other two subjects.
Hope this helps you. MTG team is here with all the
support possible and wishes you great success at both
the fronts.
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