[Biology Class Notes] on Basics of Genetics Pdf

Definition of Genetics

Genetics is a branch of life science that deals with the study of genes, genetic variation and the transmission of the characters from parent to offspring. 

Genetics has opened up a new horizon in the field of life sciences research. For centuries, humans were eager to know how characters are transmitted from parents to the offsprings. Genetics has been able to give an answer to it and even in this age of advanced biosciences, where all the mysteries of life are being divulged one by one with the emergence of new branches of science such as bioinformatics, metagenomics, etc, only genetics has the ability to answer how organisms differ from each other and how characters are manifested in them. 

Heredity and a Brief History of Genetics

The study of genetics begins with understanding heredity. It is the mechanism by which physical and behavioural traits are transmitted from one generation to another. These traits are nothing but genetic factors. “How these traits are manifested in an organism”, “How are they transmitted through generations”- are some of the questions that genetics attempts to answer quite successfully? 

The origin of genetics can be traced back to the development of the theories of evolution by Darwin and Wallace which later on took a definitive turn when Gregor Johan Mendel, a scientist and Augustinian friar working in the 19th century started to study genetics scientifically. Mendel is called the ‘Father of Genetics.’ He selected pea plants, did experiments of monohybrid and dihybrid crosses with those plants, and successfully showed that the characters of pea plants get transmitted from one generation to another. His research was published in a German journal with the headline of “Experiments on Plant Hybrids.” 

Significance of Genetics: 

The significance of genetics in biology and medical science is huge. Following are some of the important points: 

1. Genetics helps us to know about genes and their (nucleotide) sequences. It helps us to study the diseases related to a particular gene.

2. It forms the molecular basis of life.

3. In medical science, Genetics is very much important to detect various syndromes and pave a way to the development of the diagnosis for those syndromes. 

Some Important Terminologies in Genetics:

The subject of genetics has a lexicon of its own. There are numerous technical terms which one must know to be able to understand the functioning of life. Some of the key terms in genetics are as follows:

Phenotype: The structural, functional and biochemical observable characteristics of an organism is called the phenotype. It is regulated by the interaction between genes and environment. 

Genotype: The structural organisation of genes of an organism is called the genotype. Phenotype is always regulated by Genotype. 

Allele: The region of the opposite characteristics of the same gene is called Allele. 

Gene: Gene is the unit of heredity. It controls all the mechanisms in our body. It transmits from parent to offspring. 

Chromosome: In the cell’s nucleus, thread-like structures which tightly hold the DNA in them and provide the structural support are called chromosomes. 

Genome: The total number of genes in a haploid set of chromosomes of an organism is called Genome.

Homozygosity: This is the condition of having similar alleles in a particular gene.

Monohybrid Cross: It is the cross between two individuals having homozygous genotype. 

Heterozygosity: This is the condition of having opposite alleles in a particular gene.

Dihybrid Cross: The cross between two individuals i.e. line of genes that represent heterozygosity in two observable traits. 

Linked Genes: The genes which are located in the same chromosome and tend to get transmitted together are called ‘linked genes.’

Dominant: The allele which is expressed in the heterozygous condition is called Dominant. 

Recessive: The allele which is not expressed in the heterozygous condition is called Recessive.

Euchromatin: The regions in the less tightly coiled chromosomes containing active DNA molecules are called Euchromatin.

Heterochromatin: The regions in that tightly coiled, inactive DNA is present is called Heterochromatin.

Sister Chromatids: Each of the two identical copies of a replicated chromosome which are linked in centromere are called sister chromatids.

Clone: In genetics, an exact replica or copy of a genetic molecule or organism is called a clone. The first cloned animal is Dolly, a sheep.

Genetic Recombination: It is the exchange of genetic material between different chromosomes of different organisms which results in the production of offsprings having combinations of traits that are different from those of their either parent.

Now that we are acquainted with the basic terminology of genetics let’s a have a look at the experiment that Mendel performed with the pea plants. 

Mendel’s Experiment 

Mendel took pea plants with seven pairs of opposite characters and crossed them in his garden. The experiments involved monohybrid and dihybrid crosses.

Establishment of Mendel’s Laws of Heredity Through the Experiment:

Mendel enunciated the following laws of heredity after concluding the experiments: 

1. Principle of Dominance: If there are two conditions of opposite characters present in a chromosome, the gene which is expressed is called dominant and the other is recessive. 

2. Law of Segregation: During gametogenesis, the two copies of each hereditary factor segregate so that offspring can get one factor from each parent and the probability of the chance of getting is equal.

3. Law of Independent Assortment: The law of independent assortment describes that the alleles of the different genes are segregated independently within the organisms during sexual reproduction. 

Nucleic Acids

Nucleic acids are central to all known forms of life. They are biopolymers composed of nucleotide monomers. Each nucleotide has three components: a nitrogenous base, a 5-carbon sugar, and a phosphate group. The polymer is RNA or ribonucleic acid if the sugar is a ribose compound; if the sugar is deoxyribose, then the polymer is DNA or deoxyribonucleic acid. 

The information encoded in the gene is basically contained in the nucleic acid sequence and conveyed via the same. It is interesting to note that the nucleic acid monomers are arranged in a ‘ladder-step’ pattern within each molecule of DNA or RNA forming a helical backbone. There is single helix in RNA and 2 helices in  DNA. The chains of nucleotides have bases (nuc
leobases) paired with each other. There are five such nucleobases: adenine, guanine, cytosine, and thymine (in DNA) or Uracil (in RNA). The specific sequences of these bases constitute the genes the instructions from which are transferred in coded format to the RNA. From RNA, the genetic instructions are further decoded to form proteins. Before we discuss this two-step method of genetic code transmission, here’s a brief outline of the characteristics of DNA:

1. DNA stands for Deoxyribonucleic Acid. DNA was first discovered by Scientist Watson and Crick.

2. DNA is the genetic material of cells.

3. DNA is self-replicating and the replication occurs in the time of Cell cycle.

4. DNA is a double helix structure with Nucleotides, hydrogen bonds and sugar-phosphate backbones.

5. From DNA, RNA is formed during transcription. 

6. The mutation is a phenomenon by which the sequence of DNA structure is changed by the influence of environmental and biochemical factors (mutagen).

Central Dogma

The Central Dogma is a mechanism by which from DNA to DNA, DNA to RNA and RNA to Protein are formed. The process in each step is known as replication, transcription and translation respectively. These three processes are together known as the Central Dogma of Molecular Biology.

DNA Replication:

This is a mechanism by which DNA is formed from a DNA during cell division. It occurs in the nucleus. The main enzyme of DNA replication is DNA polymerase. Besides this many enzymes and proteins are associated with this mechanism. 

Transcription: 

This is the mechanism by which RNA is formed from DNA, and it occurs in the cytoplasm of a cell.

Translation:

This is the mechanism by which protein is formed from RNA, and after this event, protein is modified and translocate in the different parts of the cell.

Human Genome: A Brief Idea

Human is a diploid organism with 23 pairs of chromosome. In these 23 pairs, 22 are autosomes and one pair constitutes the sex chromosomes. 

In the case of females, the genome is represented as 22 A +XX. 

In the case of males, the genome is represented as 22 A+XY.