Genetic information is stored in DNA within the nucleus, in the format of chromosome. This information needs to be replicated precisely and transmitted into next generation (heredity). Genetic transmission from cell to cell is through mitosis, whereby DNA is replicated and chromosome duplicates and then segregates evenly into two daughter cells. Genetic transmission from organism to next generation is through meiosis and gamete formation. This transmission follows Mendel’s Law of segregation and Law of Independent Assortment.
DNA is present in everyday life. DNA is made of a sugar-phosphate backbone (deoxyribose-phosphate), connected with the four bases of life: adenine, guanine, cytosine, and thymine. DNA is a double-stranded double helix running anti-parallel, with A bonding with G and C bonding with T which holds the double helix together.
DNA replicates itself during the S phase of the cell cycle, prior to mitosis. The double-helix comes apart at the replication fork. DNA copies from the 3’ to 5’ end with the help of the protein DNA polymerase. One of the ends is the leading strand, and the other is the lagging strand, which has to replicate backwards in strips known as Okazaki fragments.
The Central Dogma and Genetic Code
DNA can replicate itself according to base-paring mechanism. RNA is made from DNA template, also via base-paring mechanism. mRNA is read in triplet code called genetic code. Three adjacent nucleotides on mRNA determine an amino acid on protein. The code starts from a fixed point and continues undisrupted. Genetic codes are degenerative because there are only 20 amino acids and 64 combinations of 4 nucleotides.
Mutation, Genetic disease and Gene Therapy
Many factors may cause mutations in DNA. The consequence is that gene function is compromised or stopped. Mutations occurring in germ line cells may cause genetic diseases. For these mutated DNA, gene therapy can provide an effective cure.
Transcription is converting DNA to mRNA using RNA polymerase, followed by editing to cut out the introns and splice together the exons. The mRNA then leaves the nucleus. The point of transcription is to read the master code and bring it to a new location for production.
Translation is the process of changing mRNA into the actual protein. The ribosome reads the mRNA, 1 codon at a time (3 RNA bases at a time). Each codon will code for an amino acids. When a codon is read, the tRNA brings the appropriate amino acid to the ribosome to add to the growing chain. A protein is made when many amino acids are assembled together.
There are three main causes of mutations: natural errors during DNA replication, DNA damage caused by radiation or chemicals, and chromosomal errors during meiosis. Mutations lead to malfunctioned gene products, which can lead to abnormal phenotype, genetic diseases, or death.