Category: Biology Second Paper

All codons that initiate protein synthesis are called start codons or initial codons. The start codon is AUG. Amino acids that initiate protein synthesis are methionine (true cells) and formyl methionine (primitive cells).

All the codons that provide signals for the creation of amino acids are called sense codons. There are 61 sense codons for 20 amino acids.

There are 3 types of genetic codons.
1. Sense codon: All the codons that provide signals for the creation of amino acids are called sense codons. There are 61 sense codons for 20 amino acids.
2. Start codon: All codons that initiate protein synthesis are called start codons or initial codons. The start codon is AUG. Amino acids that initiate protein synthesis are methionine (true cells) and formyl methionine (primitive cells).
3. Stop codon: Those codons that terminate protein synthesis are called termination or stop codons. The stop codons are UAA, UAG and UGA. They do not provide any signal for the formation of amino acids. Hence they are called non sense codons.

The code that carries the hereditary characteristics of an organism is called genetic code. The code formed by the combination of three adjacent nitrogen bases in the DNA molecule is called the genetic code. It is also called mRNA code. The genetic code is the biochemical basis of heredity. Scientist Francis Crick proved that the genetic code is a triplet or 3-letter code. Nirenberg was able to synthesize artificial mRNA using poly U (polyuridylic acid) in a cell-free or in vitro system. Indian scientist Hargobind Khorana synthetically synthesized the 5GUGUGUGUGU3 nucleotide chain and used it to create the valine-cysteine valine-cysteine polypeptide chain. Nirenberg and Hargobind Khorana won the Nobel Prize in 1969 for this discovery.

The unit of gene expression in a primitive cell is called an operon. Jacob and Mond (1961) proposed the operon model. According to them, an operon is formed by the combination of several genes and their interaction in primitive cells results in the expression of functional genes. They discovered the lactose operon. The operon consists of four segments.
1. Structural genes: It synthesizes enzymes.
2. Driver Gene: It controls the production of protein.
3. Regulatory Gene: A gene that controls the functions of an operon is called a regulatory gene. An increase in the transcription of a gene in a cell is called induction and a decrease is called repression.
4. Promoter or stimulator gene: RNA polymerase enzyme is attached to it.

In 1950, scientist Erwin Chargaff mentioned the principles about the structure of DNA through research called Chargaff’s rule. Scientist Chargaff found that DNA molecules have equal amounts of adenine and thymine. Again, the amounts of Guanine and Cytosine are also equal. This is called Chargaff principle. Half of the nitrogenous bases in the DNA molecule will be purines (A+G) and half will be pyrimidines (T+C). Hence A pairs with T and G with C.

Several nucleotides are linked together by phospho-diester bonds to form a long chain. It is called polynucleotide. In polynucleotides, the phosphate molecule is attached to the 5th carbon of the pentose sugar on one side and to the 3rd carbon of the pentose sugar on the other side. Nucleotides in polynucleotides are arranged in a 5→3 carbon orientation. Each single helix of a DNA molecule is a polynucleotide chain.

Two molecules of nucleotide together are called dinucleotide. The 5th carbon of the pentose sugar of one nucleotide and the 3rd carbon of the pentose sugar of the other nucleotide are joined by a phospho-diester bond to form a dinucleotide.

If the nitrogenous base forming the nucleotide is pyrimidine, it is called pyrimidine nucleotide. It is of three types. Thymidine nucleotide, cytidine nucleotide and uridine nucleotide. Pentose sugars and thymine combine to form thymidine nucleotides, pentose sugars and cytosine combine to form cytidine nucleotides, and pentose sugars and uracil combine to form uridine nucleotides.

If the nitrogenous base forming the nucleotide is purine, it is called purine nucleotide. It is of two types. Adenosine nucleotide and guanosine nucleotide. Pentose sugars and adenine combine to form adenosine nucleotides and pentose sugars and guanine combine to form guanosine nucleotides.