BOTANY Higher Secondary Second Year - Textbooks Online

Role of RNA in protein synthesis
There are three types of RNA in the organism namely mRNA, tRNA
and rRNA. mRNA brings the message in the form of codons from DNA to
the site of protein synthesis ie. ribosome. The tRNA molecule performs a
number of functions. The most important one is to carry specific aminoacids
to the site of protein synthesis. The rRNA is found in ribosomes. During
protein synthesis, rRNA provides the site where aminoacids are assembled.
3.9. Genetic code
Aminoacids are the basic units of proteins. Different types of proteins
are synthesized using 20 different aminoacids. But number of aminoacids
and their arrangements vary in different types of proteins. These features
ultimately determined by the nucleotide sequence in the DNA. At the time
of protein synthesis, the sequence of information in the form of three
successive nucleotide bases in the mRNA, which codes for specific
aminoacid in a polypeptide is called genetic code. It is transferred from
DNA to mRNA.
The unit of genetic code is codon, which has three nucleotide bases.
Hence, it is also known as triplet. The four-nucleotide bases adenine,
guanine, cytosine and uracil combine with each other in different
combination to form 64 codons. The mRNA carries number of codons
depending upon the type of protein to be synthesized. For example, if a
protein consists of 100 aminoacids, there will be 300 nucleotide bases in
the genetic code (100 codons).
Characteristics of genetic code
Out of 64 codons, three codons are called stop codons UAA, UAG
and UGA. Remaining 61 codons code 20 different aminoacids. For
example, lysine has two codons AAA and AAG. Proline has four
codons CCA, CCG, CCC and CCU.
In the codons of a particular aminoacid, the first and the second
aminoacids remain the same and the third nucleotide only changes.
The initial or starting codon is AUG, which also code for methionine.
The genetic code is non-overlapping i.e. adjacent codons do not
overlap. The non-overlapping code means that the same letter is not
used to code two different aminoacids. For example, UUU codes for
phenylalanine and CCC for proline.
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