Genetic Materials

Introduction to Genetics

Genetics is the science of heredity that deals with the mechanism of transmission of characters from parents to offspring.

• The term Genetics was introduced by W. Bateson in 1906.
• Genetics is a science that deals with heredity and variation.
• The father of Genetics is Gregor Johann Mendel.

Genetic Material

The substance which stores biological information in a coded form and transfers it to the next generation, and causes its expression in the offspring, is called genetic material.

Essential requirements of genetic material

The genetic material should have the following characteristics:

1. It should be present in every cell.
2. It should be able to duplicate itself, forming its carbon copy.
3. It should be able to faithfully pass its copies into the progeny.
4. It should be able to occasionally develop inheritable change (mutation) to allow adaptation and evolution to occur in organisms.
5. It should be able to express its information in the progeny.

Heredity, Inheritance and Variation

Heredity is the tendency of an individual to resemble its parent. Heredity may be defined as the transmission of characters from parents to offspring, i.e. through egg and sperm.

Inheritance is a process by which characters are passed on from parent to progeny. It is the basis of heredity.

Variation: children only resemble their parents, they are not identical. These differences shown by individuals of a species are termed variation. Variation means the differences that exist among the individuals of a species, i.e. the degree by which progeny differs from parents.

Significance of variation

1. It develops variations in a species.
2. It brings changes in the individual and helps them adapt to the new environment.
3. It may cause evolution.

Chromosome and Gene

Chromosome: a thread-like structure of nucleic acid and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.

Gene: a gene is a sequence of nucleotides (DNA) which codes for a particular character.

Properties of a gene

• Each gene occupies a fixed position in a specific chromosome. The position of a gene is called the locus of the gene.
• A gene which moves from one locus to another locus is called a transposable/mobile gene, or a jumping gene.
• Genes are arranged in a linear position.
• Each gene has at least two forms known as alleles. Alleles must have similar loci in homologous chromosomes. Genes present on different chromosomes are called non-alleles.
• A few genes have more than two alleles, called multiple alleles. Example: blood group genes.
• Genes are capable of being copied, which is also called replication.
• A gene can undergo modification to form a new gene with a different character; this process is called mutation.

Function of a gene

Genes code for special proteins which determine the various characters of a cell, so the character of an organism is determined by genes. They are responsible for the passage of characters from parent to offspring.

Some Important Terms

1. Cistron: the functional unit of DNA. It is a gene consisting of a number of nucleotides which is capable of synthesizing a polypeptide chain. The term was coined by S. Benzer.
2. Muton: the smallest unit of DNA which can undergo mutation. It represents a change in a pair of nucleotides.
3. Recon: the smallest unit of DNA that can undergo recombination and crossing over. It can be as small as one nucleotide pair of DNA.

Central Dogma of Molecular Genetics

The Central Dogma is the "unidirectional flow of genetic information from DNA to protein via RNA."

DNA --(Replication)--> DNA --(Transcription)--> RNA --(Translation)--> Protein

Reverse Central Dogma: here genetic information gets transferred from RNA to DNA and then to protein through mRNA formation, by retroviruses (RNA --(Reverse transcription)--> DNA --(Transcription)--> mRNA --(Translation)--> Protein).

DNA is the Genetic Material: Griffith's Experiment

Griffith's experiment on bacterial transformation, performed by Frederick Griffith in 1928, provided the evidence that DNA is the hereditary material.

Griffith worked with Streptococcus pneumoniae, a bacterium that causes pneumonia in mammals including humans, commonly called pneumococcus. Griffith found two strains of pneumococcus:

1. Non-capsulated or non-virulent strain (R-type): non-pathogenic bacteria without a capsule.
2. Capsulated or virulent strain (S-type): pathogenic bacteria that causes infection when injected into mice. These bacteria produce smooth colonies (polysaccharide) in culture media, so are called S-type ('S' stands for smooth).

Griffith's observations:

• He injected live rough (R-type) bacteria into mice; the mice did not die.
• He injected mice with live smooth (S-type) bacteria; the mice died.
• He injected mice with heat-killed smooth bacteria; the mice survived.
• Finally, he injected mice with a mixture of live rough bacteria and heat-killed smooth bacteria. As these bacteria alone are harmless, but their mixture caused pneumonia and killed the mice. He found live smooth bacteria in the dead mice.

He concluded that some "transforming principle" (a chemical substance) from the dead smooth bacteria had entered the living rough bacteria and transformed it later into smooth bacteria. Thus transformation was a permanent genetic change, i.e. the evidence that DNA is the (only) genetic material in the organism.

Types of Nucleic Acid

There are two types of nucleic acid:

1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)

Deoxyribonucleic acid (DNA)

DNA is a polymer of deoxyribonucleotides that contains many deoxyribonucleotides covalently linked by phosphodiester bonds. DNA is generally double stranded, except in certain viruses where single stranded DNA is present, e.g. Parvovirus.

DNA is the genetic material and forms the molecular basis of heredity in all organisms. In certain viruses, such as Tobacco Mosaic Virus (TMV), RNA is the genetic material. In prokaryotes DNA occurs in the cytoplasm, whereas in eukaryotic cells DNA is largely confined to the nucleus and is the main component of the chromosome, hence called nuclear DNA.

DNA molecules show primary, secondary and tertiary structure, like a protein molecule, being coiled in three orders for accommodation in a small space.

DNA shows polarity (direction): one end of each DNA strand is called the 5' end and the other end is the 3' end.

DNA molecules are of two types: linear and circular.

• Linear DNA is found in the nuclei of eukaryotic cells; it is associated with protein in about a 1:1 ratio.
• Circular DNA is found in many viruses, prokaryotes, and in mitochondria and chloroplasts of eukaryotic cells, and has little protein.