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Agriculture LibreTexts

Section A


Structure of DNA :-A gene is a region of DNA that controls a hereditary characteristic.DNA, or deoxyribonucleic acid, is the hereditary material present in almost all organisms. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T) . DNA, which consists of a polysugar-phosphate backbone possessing projections of purines (adenine and guanine) and pyrimidines (thymine and cytosine), forms a double helix that is held together by hydrogen bonds between these purines and pyrimidines (adenine to thymine and guanine to cytosine). Nitrogen bases lie at right angles to the frame and attached to sugar residues.The two chains run anti parallel and possess complementary nitrogen bases with A opposite T and C opposite G, and vice versa T opposite A and G opposite C to form units called base pairs, and each DNA is made up of 50 to 250 million bases located in a chromosome.

The bases A and G are identical in structure and belong to the group of the purines and the other two bases C and T are also identical structure belongs to pyrimidines. A combination of three nucleotides forms a triplet or a codon.. Each three-letter DNA sequence, or codon, encodes a specific amino acid.he code has several key features:-

  • All genes begin with the "start" codon, ATG.
  • There are three "stop" codons that signify the end of the gene.
  • Different codons can encode the same amino acid.

For example, the triplet codons for the amino acid isoleucine are AUU, AUC, AUA, and AUG.

An important feature of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases.

Mutation :-Mutation is an error in the genotype that create new alleles during DNA replication that results in a change in the sequence of deoxyribonucleotide bases in the DNA.And can result in a variety of genetic disorders.In the living cell, DNA undergoes frequent chemical change, especially when it is being replicated (in S phase of the eukaryotic cell cycle),a DNA gene is damaged or permanent changed in such a way as to alter the genetic message carried by that gene causes mutation.Mutations range in size from a single DNA building block (DNA base) to a large segment of a chromosome

Since genetic information is encoded by the order of the nucleotide bases of DNA, adenine (A), thymine (T), guanine (G), and cytosine (C), a mutation represents some sort of change in that order.

Example:- G.M. Beadle and E.L. Tatum discovered that if spores of the red bread mold, Neurospora were treated with X- rays or ultra-violet radiation (induced mutation) then some of them were unable to grow on the minimal culture medium But when certain amino acids were added, they would grow. From this experiment they derived that due to the affect of the radiations, they lost the ability to synthesize certain amino acids. The next generation grown from these spores were also unable to synthesize these amino acids. This shows that their inability to synthesize those amino acids was inherited by successive generations , just like any other genetic characters., such sudden changed characters are called mutation.

In eukaryotic organisms there are two types of primary cell :- germ(sex cell) and somatic (vegetative). Mutations may occur in both somatic and sex cells.

  • Germinal mutation:- If a gene is altered in a germ cell, the mutation is termed a germinal mutation.Only mutations that occur in sex cells can be passed from parent to offspring.
  • Somatic mutations:-Mutations in somatic cells are called somatic mutations. Because somatic mutation do not occur in cells that give rise to gametes, the mutation is not passed along to the next generation by sexual means.