From NCERT NEET - 2006
(2) m-RNA
- TRANSLATION
form a polypeptide . The order and sequence of amino acids
are defined by the sequence of bases in the mRNA. The amino acids are
joined by a bond which is known as a peptide bond. Formation of a
peptide bond requires energy. Therefore, in the first phase itself amino
acids are activated in the presence of ATP and linked to their cognate
tRNA–a process commonly called as charging of tRNA or
aminoacylation of tRNA to be more specific.
- REGULATION OF GENE EXPRESSION
at various levels. Considering that gene expression results in the formation
of a polypeptide, it can be regulated at several levels. In eukaryotes, the
regulation could be exerted at
(i) transcriptional level (formation of primary transcript),
(ii) processing level (regulation of splicing),
(iii) transport of mRNA from nucleus to the cytoplasm,
(iv) translational level.
- Types of RNA and the process of Transcription
tRNA (transfer RNA), and rRNA (ribosomal RNA). All three RNAs are
needed to synthesise a protein in a cell. The mRNA provides the template,
tRNA brings aminoacids and reads the genetic code, and rRNAs play
structural and catalytic role during translation. There is single
DNA-dependent RNA polymerase that catalyses transcription of all types
of RNA in bacteria. RNA polymerase binds to promoter and initiates
transcription (Initiation).
- In bacteria, since the mRNA does not require any processing to become
compartment (there is no separation of cytosol and nucleus in bacteria),
many times the translation can begin much before the mRNA is fully
transcribed. Consequently, the transcription and translation can be coupled
in bacteria.
- The RNA polymerase I transcribes rRNAs
for transcription of tRNA, 5srRNA, and snRNAs (small nuclear
RNAs). The RNA polymerase II transcribes precursor of mRNA, the
heterogeneous nuclear RNA (hnRNA).
- The salient features of genetic code are as follows:
not code for any amino acids, hence they function as stop codons.
(ii) Some amino acids are coded by more than one codon, hence
the code is degenerate.
(iii) The codon is read in mRNA in a contiguous fashion. There are
no punctuations.
(iv) The code is nearly universal: for example, from bacteria to human
UUU would code for Phenylalanine (phe). Some exceptions to this
rule have been found in mitochondrial codons, and in some
protozoans.
(v) AUG has dual functions. It codes for Methionine (met) , and it
also act as initiator codon.
(vi) UAA, UAG, UGA are stop terminator codons.
- Pest Resistant Plants: Several nematodes parasitise a wide variety of
incognitia infects the roots of tobacco plants and causes a great reduction
in yield. A novel strategy was adopted to prevent this infestation which
was based on the process of RNA interference (RNAi). RNAi takes place
in all eukaryotic organisms as a method of cellular defense. This method
involves silencing of a specific mRNA due to a complementary dsRNA
molecule that binds to and prevents translation of the mRNA (silencing).
The source of this complementary RNA could be from an infection by
viruses having RNA genomes or mobile genetic elements (transposons)
that replicate via an RNA intermediate.
Using Agrobacterium vectors, nematode-specific genes were
introduced into the host plant . The introduction of DNA
was such that it produced both sense and anti-sense RNA in the host
cells. These two RNA’s being complementary to each other formed a double
stranded (dsRNA) that initiated RNAi and thus, silenced the specific mRNA
of the nematode. The consequence was that the parasite could not survive
in a transgenic host expressing specific interfering RNA. The transgenic
plant therefore got itself protected from the parasite.
- Ribosomes and Inclusion Bodies
the cell. They are about 15 nm by 20 nm in size and are made of two
subunits - 50S and 30S units which when present together form 70S
prokaryotic ribosomes. Ribosomes are the site of protein synthesis. Several
ribosomes may attach to a single mRNA and form a chain called
polyribosomes or polysome. The ribosomes of a polysome translate the
mRNA into proteins.
Inclusion bodies: Reserve material in prokaryotic cells are stored in
the cytoplasm in the form of inclusion bodies. These are not bound by
any membrane system and lie free in the cytoplasm, e.g., phosphate
granules, cyanophycean granules and glycogen granules. Gas vacuoles
are found in blue green and purple and green photosynthetic bacteria.
