Genetics and Evolution: Structure of DNA and RNA and Genome Evolution

Genetics is a branch of biology that deals with heredity and genetic variation. The two word heredity and variation are main. Heredity is passing of traits or characters from parents to their offsprings. Genetic variation means differences that make one organism different from its parents. Evolution is the process of a species feature changing over numerous generations through natural selections. Humans are made up of cells. Fibers are made up of DNA. Every time a cell divides, its DNA is replicated and apportioned equally to two daughter cells. 

Key Terms: Genetics, Evolution, DNA, Genome, RNA, Inheritance, Variation, Heredity, Genetic variation, Genome evolution

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Evolutionary Genetics

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Evolutionary genetics deals with the studies of how genetics leads to evolutionary change. Genome evolution is the process by which the size and structure of the genome change with time.

The majority of studies have concentrated on two of nature's most notable patterns:

(1) adaptation, or the 'fit' between organism and environment; and

(2) polymorphism, or the persistence of two or more phenotypic or genetic forms in a single population. In support of Darwin, the first studies attempted to document the action of natural selection in wild populations. 

While natural selection is the sole evolutionary force that can account for adaptation, a polymorphism can be maintained, at least temporarily, by various evolutionary processes operating alone or in combination. As a result, establishing causal agency for polymorphism explanations is far more challenging than it is for adaptation explanations.

The material of the initial analyses of natural populations was conspicuous phenotypic variations, such as the spotting patterns on butterfly wings or the banding patterns on snail shells. Natural selection was chosen as the most likely explanation for the persistence of polymorphisms among the four evolutionary processes that modify gene frequencies in these research.

Molecular ecological genetics attempts to investigate a less selection bias of genetic polymorphisms, such as allozymes and single nucleotide polymorphisms. But it retains the early emphasis on natural selection as the most important evolutionary force shaping the hereditary material.

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Principles of Inheritance and Variation

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Gregor Mendel studied the inheritance of different traits in peas, which he grew himself. His experiment involved interbreeding plants that showed different traits. For E.g: Short plants were bred with tall plants to see how the traits were inherited by offsprings. 

After careful analysis of his experiment, he postulated that existence of hereditary factors were responsible for the trait, which is known as genes. Each of the genes is associated with a different trait. For example Plant height, colour, seeds, etc. Genes exist in different forms, known as alleles. Mendel highlighted that hereditary factors, or genes, are distinct entities.

Fig. Pea Experiment by Mendel

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Molecular Basis of Inheritance

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• Structure of DNA

The DNA molecule is made up of two strands that form a double helix structure when they coil around each other. Each strand's backbone consists of sugar (deoxyribose) and phosphate groups that alternate. Each sugar is linked to one of four bases: adenine (A), cytosine (C), guanine (G), and thymine (T).

Bonds between the bases hold the two strands together; adenine bonds with thymine, while cytosine bonds with guanine. The nucleotide sequence along the backbones serves as a set of instructions for putting protein and RNA molecules together.

The central information storage system of most animals and plants, as well as certain viruses, is DNA, or deoxyribonucleic acid. The term originates from its structure, which consists of a sugar and phosphate backbone with bases—so-called bases—protruding from it. As a result, "deoxyribo" refers to sugar, while "nucleic acid" refers to phosphate and bases. 

Adenine, cytosine, thymine, and guanine, sometimes known as A, C, T, and G, are the four bases. The structure of DNA is extremely basic. It's a polymer of four bases—A, C, T, and G—but the pattern of those bases, one after the other, enables for enormous complexity to be encoded.

Genes are composed of nucleic acid. Nucleic acids are made up of elementary building blocks called nucleotides. Each nucleotides contains three components:

1. A sugar molecule
2. A phosphate molecule
3. Nitrogen containing molecule

In cellular organisms the genetic material is DNA.This material must be able to replicate so that copies can be transmitted from cell to cell and from parents to offspring.

Fig. Detailed Image of Structure of DNA

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• Structure of RNA

RNA is a single-stranded molecule made up of ribonucleotides linked together by phosphodiester bonds.Ribose (the pentose sugar), one of the four nitrogenous bases (A, U, G, and C), and a phosphate group make form a ribonucleotide in the RNA chain. The slight structural difference between the sugars offers DNA more stability, making it better suited to storing genetic information, whilst RNA's relative instability makes it better suited to its more transient tasks. 

Instead of the DNA base thymine, the RNA-specific pyrimidine uracil forms a complementary base pair with adenine.Despite the fact that RNA is a single-stranded molecule, the majority of RNA molecules have significant intramolecular base pairing between complementary sequences inside the RNA strand, resulting in a predictable three-dimensional structure that is essential for their function.

Fig. Structure of RNA

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Mechanism of Genome Evolution

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• Gene Duplication

Duplication is a form of mutation in which one or more copies of a gene or chromosome region are produced. Duplications of genes and chromosomes occur in all species, but they are more common in plants. Gene duplication is an important evolutionary mechanism.

Fig. Gene duplication

• Transposable elements

Transposable elements (TEs) are DNA sequences that travel (or jump) from one site in the genome to another. They're also known as transposons or "jumping genes."

Fig. Image of Transposable Elements

• Mutation

When the sequence of DNA changes, it is called a mutation. Mutations can happen as a result of DNA copying errors during cell division, ionising radiation exposure, mutagen exposure, or virus infection. Germline mutations occur in eggs and sperm and can be passed onto offspring, but somatic mutations occur in body cells and are not passed onto offspring.

Fig. Causes of Mutation

• Exon shuffling

Exon shuffling is a molecular technique for creating new genes in which two or more exons from different genes are recombined between introns, resulting in rearranged genes with varied functions. Exon shuffling can occur through a variety of pathways, including transposon-mediated exon shuffling, crossover during parental genome sexual recombination, and illicit recombination.

Fig. Exon shuffling

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Things to Remember

• Gregor Mendel proposed the existence of particulate factors, which we now refer to as genes.
• DNA is the hereditary material in all life forms except some types of viruses, in which RNA is the hereditary material.
• Mutation can alter the DNA sequence of genes.
• Mutation creates genetic variety, which is the foundation for biological evolution.
• When DNA replicates, each strand of the duplex molecule serves as the template for the synthesis of a complementary strand.

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