How does DNA code for proteins?
The triplet code
The code carried by the DNA is a triplet code. Each adjacent group of 3 bases codes for an amino acid. Several triplets code for the same amino acid; others are start signals or stop signals (chain terminators).
From DNA to proteins
The DNA in our chromosomes carries the genetic information from one generation to he next. It carries the codes that help determine the structure and function of cells by telling the cells which proteins to make. But DNA is in the nucleus, and proteins are made in the cytoplasm. DNA cannot pass through the membranes surrounding the nucleus into the cytoplasm. A 'copy of the DNA is made. This 'copy' is made from another type of nucleic acid called, ribonucleic acid, RNA. This RNA can leave the nucleus, carrying the information to the cytoplasm where it is used in the manufacture of proteins.
What is the difference between DNA and RNA?
An RNA molecule has a single strand made of a string of RNA molecules. These are very similar in structure to DNA nucleotides except they contain ribose sugar and not deoxyribose. Another difference is that in RNA nucleotides, the base Uracil (U), replaces thymine, so RNA never contains thymine.
There are 3 types of RNA involved in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
Transcription takes place in the nucleus.The DNA double helix unwinds and hydrogen bonds between the bases break, allowing the strands to partly separate. The sequence of one of the strands, the template strand, is used in the production of mRNA molecule. This mRNA is built from free RNA nucleotides which line up against the DNA template strand. Every triplet code on DNA gives to a complementary codon on mRNA.
The template strand is also known as the antisense strand because once transcribed it makes an mRNA molecule with the same base sequence as the DNA coding strand.
The completed mRNA molecule leaves the nucleus through a pore in the nuclear envelope and enters the cytoplasm.
Translation takes place on ribosomes and happens in the cytoplasm. Ribosomes are small organelles made of rRNA and protein. Ribosomes are found free in the cytoplasm or attached to endoplasmic reticulum, a system of flattened, membrane-bound sacs. A tRNA molecule carrying an amino acid molecule has 3 bases called an anti-codon and these pair with bases on the mRNA codon. Then the amino acids that the tRNA carry join by means of peptide bonds.