Only a fraction of the genes in a cell are expressed.
Gene expression is the process by which the genetic code is used by cells to synthesise polypeptide chains of amino acids which form proteins.
Gene expression involves the transcription and translation of DNA sequences.
Transcription and translation involves three types of RNA (mRNA, tRNA and rRNA).
The second type of nucleic acid is RNA. RNA is single-stranded and is composed of nucleotides containing ribose sugar, phosphate and one of four bases: cytosine, guanine, adenine and uracil.
In RNA uracil — U — replaces thymine and is complementary to adenine.
| RNA | DNA | |
|---|---|---|
| number of nucleotide strands present in one molecule | one | two |
| complementary base partner of adenine | uracil | thymine |
| sugar present in a nucleotide | ribose | deoxyribose |
The three types of RNA are as follows.
Messenger RNA (mRNA): —
→ Each triplet of bases on the mRNA molecule is called a codon and codes for a specific amino acid.
→ mRNA is transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm.
→ 3. Messenger RNA (mRNA) carries a copy of the DNA code from the nucleus to the ribosome.
Transfer RNA (tRNA): —
→ Transfer RNA (tRNA) folds due to complementary base pairing.
→ A tRNA molecule has an anticodon (an exposed triplet of bases) at one end and an attachment site for a specific amino acid at the other end.
→ 4.a. Each tRNA molecule carries its specific amino acid to the ribosome.
→ tRNA is involved in the translation of mRNA into a polypeptide at a ribosome.
Ribosomal RNA (rRNA): —
→ Ribosomal RNA (rRNA) and proteins form the ribosome.
An anticodon is an exposed triplet of bases on tRNA that is complementary to a specific codon in mRNA.
A codon is a triplet of bases on mRNA that codes for a specific amino acid and is complementary to a specific anticodon found on a tRNA molecule.
The codon on the mRNA is the triplet of bases that is used to identify what amino acid is added to the polypeptide when using a codon table.
The process of transcription of DNA into a primary mRNA transcript is as follows:
