The Genetic Code

The Genetic CodeGenes are made of DNA. A gene is a section of DNA required for the production of a particular protein molecule. Protein molecules are made up of combinations of hundreds or thousands of amino acids joined together in a specific sequence. Up to 20 different types of amino acids are used in proteins. This means that  a gene must carry a different code to control the assembly of each of the 20 different amino acids. DNA codes for each amino acid by using a sequence of three consecutive bases. Such sequence of three bases if called a triplet or a codon. This is similar to the way in Morse code uses a sequence of three dots or dashes to specify a letter. A long stretch of triplets that produce a protein form a gene. For example, the DNA triplet CAA is the code for an amino acid called Valine, while CGA is the triplet for an amino acid called, Alanine. If these triplets form part of a gene, they will cause a protein to form with the relevant amino acids in sequence. 

Non-Coding DNAUp to 97% of the DNA in human cells does not carry the code for the production of proteins. This so-called junk DNA is of two types - some of occurs between genes and the rest is found within genes. The exact functions of non-coding DNA is not known. However, the sequence of basis in non-coding DNA varies greatly from person to person. It is the DNA in these non-coding sections that is used to prepare DNA profiles. Some non-coding DNA is transcribed into functional non-coding RNAs (Such as transfer RNA, ribosomal RNA). 

Replication of DNAAt the end of mitosis, each new cell has single stranded chromosomes. Before these cells can divide again, the DNA in each chromosome must produce an exact copy of itself. The single stranded chromosomes must become double stranded chromosomes. This process is called DNA replication and it takes place in the nucleus during interphase. Please see notes on mitosis. Mechanism of DNA replication:1. The double helix unwinds (uncoils)2. An enzyme breaks the bonds between the base pairs. 3. DNA bases that are normally present in the cytoplasm enter the nucleus. The incoming bases attach to the exposed complementary bases. In this way, each side of the DNA molecule acts as a mould or template for the new DNA that is formed. 4. Each new strand is half new DNA and half old DNA; Identical to the original DNA strand and to the other new partner strand. 5. Each new piece of DNA rewinds to form a double helix. 

Significance of DNA replicationEach new DNA double helix (chromosome) will have exactly the same sequence of bases as the original. DNA is able to produce exact copies of itself. This means that the same DNA is passed on to each new generation of cells. For example, a human zygote is a single cell with 46 chromosomes. These chromosomes contain certain sequences of bases. The same sequence of bases is passed on, in the form of new chromosomes, to each body cell in a person due to DNA replication (and mitosis). 

DNA replication 

The Genetic Code