Molecular basis of inheritance involves the study of genes, genetic variations and heredity. It explains how an offspring looks similar to the parents. DNA, RNA and genetic code form the basis of the molecular basis of inheritance.
Deoxyribonucleic Acid (DNA) is considered as the molecule of inheritance as it carries genetic information in all living organisms. It is a long polymer chain of deoxyribonucleotides. Its length depends on the number of nucleotide base pair present in it.
Watson and Crick were the first scientists who proposed a double-helical model for DNA, based on X-ray crystallography of the molecule. Each strand of DNA is a polymer of nucleotides, every nucleotide consists of a deoxyribose sugar, a nitrogen base and a phosphate.
A nucleotide consists of three elements – nitrogenous base, sugar and phosphate group. Nitrogenous bases are in the form of purines (Adenine, Guanine) and Pyrimidines (Cytosine and Thymine).
Cytosine is common for both DNA and RNA and thymine is present in DNA. Uracil is present in RNA at the place of thymine.
A nitrogenous base is linked to the pentose sugar through a N-glycosidic linkage to form a nucleoside, i.e. adenosine and guanosine, etc.
When a phosphate group is linked to 5′ —OH of a nucleoside through phosphodiester linkage, a corresponding nucleotide is formed.
Two nucleotides are linked through 3′ -> 5′ phosphodiester linkage to form a dinucleotide.
Several nucleotides can be joined to form a polynucleotide chain.
The backbone in a polynucleotide chain is formed due to sugar and phosphates.
The nitrogenous bases linked to sugar moiety project from the backbone.
The base pairs are complementary to each other.
Friedrich Meischer in 1869, first identified DNA as an acidic substance present in the nucleus and named it as ‘nuclein’.
James Watson and Francis Crick, proposed a very simple double helix model for the structure of DNA in 1953 based on X-ray diffraction data.
Erwin Chargaff proposed that for a double-stranded DNA, the ratios between adenine and thymine and guanine and cytosine are constant and equals to one.
DNA in prokaryotes is arranged as a large loop in the nucleoid region wherein the negatively charged DNA is firmly held by positively charged proteins.
DNA in prokaryotes is observed as a complex organisation of DNA in chromosomes. DNA is wound around the core of histone octamer (a unit with 8 histone molecules) to form a nucleosome.
Positively charged proteins in the form of histones are observed that are rich in basic amino acids, lysine and arginine. They are of 5 types – H1, H2A, H2B, H3 and H4. The histone octamer has 2 molecules of 4 histone proteins vital in gene regulation.
Nucleosome is a repeating unit in the chromatin preventing DNA from tangling containing nearly 200 bp of DNA.
The further packaging of chromatin is facilitated by NHC (Non-histone chromosomal proteins).
The replication of the genome is essential for the continuity of life. The molecular mechanism is very similar in all groups of organisms. Although the basics of replication are already well understood, researchers are still focusing on questions relating to DNA replication.
Watson and Crick proposed that the DNA replication is semiconservative.
Meselson and Stahl in 1958 proved experimentally that the DNA replicates semi conservatively.
Taylor et al in another experiment on faba beans (Vicia faba) using radioactive thymidine proved that the replication on DNA is semiconservative.
Enzyme DNA polymerase catalyses DNA replication. It can polymerise only in 5’→3’ direction.
Replication is continuous in a strand with 5’→3’ direction, called leading strand, where the template strand has 3’→5’ polarity, called leading strand template.
Replication is discontinuous in lagging strand template where the template strand has 5’→3’ polarity.
The genetic information present in DNA (one segment only) is copied into RNA. Adenine pairs with Uracil instead of Thymine in RNA. Transcription of DNA involves three regions – the structural gene, promoter, terminator. The RNA polymerase catalyses transcription and while the direction of transcription is the same as that of replication by DNA polymerase, i.e. 5’→3’ direction. Template strand has a 3’→5’ polarity acting as a template for RNA formation known as antisense strand. Coding strand has a 5’→3’ polarity and is also known as a sense strand. In addition is consists of the structural gene, promoter, terminator, exons and introns.
Genetic code is the term we use for the way that the four bases of DNA--the A, C, G, and Ts--are strung together in a way that the cellular machinery, the ribosome, can read them and turn them into a protein. In the genetic code, each three nucleotides in a row count as a triplet and code for a single amino acid.
Translation is the process of amino acid polymerisation. Amino acids are joined by peptide bonds. All three RNAs (mRNA, tRNA and rRNA) have a different role in the process of translation. First stage in this process of translation is aminoacylation of tRNA. Ribosomes are a protein manufacturing factory, acting as a catalyst in the formation of a peptide bond. Translation process is in the 5’→3’ direction always. There are two sites in the large subunit of a ribosome accommodating two tRNAs with amino acids close enough to form a peptide bond.
Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein or RNA).
Sophisticated programs of gene expression are widely observed in biology, for example to trigger developmental pathways, respond to environmental stimuli, or adapt to new food sources.
Expression of a gene to form polypeptide can be regulated at different levels in eukaryotes
Gene expression is regulated by environmental, physiological and metabolic conditions.
The development and differentiation of embryo is a result of coordinated regulation and expression of several sets of genes.
Control of gene expression in prokaryotes is mainly at the initiation of transcription.
The activity of RNA polymerase at the start site is regulated by regulatory proteins, which can be a repressor or activator.
The accessibility of the promoter region is regulated by an operator sequence adjacent to it, that binds with the specific protein, mostly a repressor. There is a specific operator and repressor protein in a specific operator.
The Human Genome Project (HGP) was an international scientific research project with the goal of determining the base pairs that make up human DNA, and of identifying and mapping all of the genes of the human genome from both a physical and a functional standpoint.
(i) Identify all the approximately 20,000-25,000 genes in human DNA;
(ii) Determine the sequences of the 3 billion chemical base pairs that make up human DNA;
(iiii) Store this information in databases;
(iv) Improve tools for data analysis;
(v) Transfer related technologies to other sectors, such as industries;
(vi) Address the ethical, legal, and social issues (ELSI) that may arise from the project.
DNA fingerprinting is a laboratory technique used to establish a link between biological evidence and a suspect in a criminal investigation. A DNA sample taken from a crime scene is compared with a DNA sample from a suspect. If the two DNA profiles are a match, then the evidence came from that suspect.
The technique of DNA Fingerprinting was initially developed by Alec Jeffreys. He used a satellite DNA as probe that shows very high degree of polymorphism. It was called as Variable Number of Tandem Repeats (VNTR). The technique, as used earlier, involved Southern blot hybridisation using radiolabelled VNTR as a probe. It included
(i) isolation of DNA,
(ii) digestion of DNA by restriction endonucleases,
(iii) separation of DNA fragments by electrophoresis,
(iv) transferring (blotting) of separated DNA fragments to synthetic membranes, such as nitrocellulose or nylon,
(v) hybridisation using labelled VNTR probe, and
(vi) detection of hybridised DNA fragments by autoradiography.