A sequence of bases in DNA that .codes for the synthesis of One polypeptide is called gene. The genes must transmit their information from the nucleus to the cytoplasm. Therefore, protein synthesis occurs in cytoplasm. The synthesis of an RNA molecule from DNA is Called transcription and the formation )f a protein from RNA at the ribosome is called translation.
Three major kinds of RNA
There are three major kinds of RNA. Each RNA has a specific role in protein synthesis. All the RNA are produced in the nucleus from DNA.
a) Messenger RNA (mRNA) is a linear strand. It carries a set of genetic instructions for synthesizing proteins to the cytoplasm.
b) Transfer RNA (tRNA) picks up amino acids in the cytoplasm and carries them to ribosomes. It joins these amino acids for the synthesis of polypeptide. (c) Ribosomal RNA (RNA) with proteins makes up ribosomes.
The Genetic Code •
The genetic code is a sequence of three bases—a triplet code. DNA ‘must code for the 20 different amino acids. DNA has information carrying capabilities. These information are present in the sequence of nitrogenous bases of DNA. DNA transfers these sequences to mRNA. Each three-base combination is a codon. There are 64 possible codons. But there are only 20 amino acids. Hence more than one codon can specify same amino acid because. This characteristic of the code is called degeneracy. All codons does not code for amino acids. There are some initiation and termination Codes:
(a) Termination codons: The base sequences UAA, UAG, and UGA are stop codes. They indicate the termination point of the polypeptide synthesis.
(b) Initiation codon: The codon AUG codes for the amino acid methionine. It is a start codon.
he synthesis of messenger RNA from DNA is called transcription. The genetic information in DNA is not translated directly into proteins. These formation are first transcribed into mRNA. Many enzymes are involved in the process of transcription. These enzymes perform following functions:
They unwind a region of a DNA molecule.
They initiate and end mRNA synthesis.
• They also modify the mRNA after the completion of transcription.
Only one of a few genes are exposed during transcription. Only one of the two DNA strands is transcribed. There are following steps of transcription:
- Initiation: RNA polymerase is the important enzymes which take part in transcription. A section of DNA is unwound. RNA polymerase recognizes a specific sequence of DNA nucleotides. RNA polymerase attaches on these specific sites.
- Elongation: RNA polymerase starts joining complementary ribose nucleotides to the 3′ end of the DNA strand. The same complementary bases paired in DNA. But in RNA, the base uracil replaces the base thymine. Thus uracil complement to adenine.
- Termination: Transcription continues. Finally, RNA polymerase reaches the termination sequences. Thus transcription stops.
Processing of primary transcript: The newly transcribed mRNA is called the primary transcript. It is modified before leaving the nucleus. Some base sequences in newly transcribed mRNA do not code for proteins. RNA splicing cut out these noncoding regions. Thus the mRNA coding region can he read continuously at the ribosome.