Nucleic Acids – DNA and RNA

We know that DNA is the genetic material present in living cells and is responsible for the traits of an organism. The sequence of this nucleic acid present determines everything from the hair color, height, metabolism rate, and even the tendency to develop cancer.

Let us learn about genetic material in more detail.

Introduction to Nucleic Acids

The crucial role of DNA in life was not known to scientists until the mid- the 1900s. Genes were first analyzed by Gregor Mendel in 1865. He also reported their role in inheritance and transmission of information from one generation to another.

However, the nature and molecular structures of these factors were not known. The location of these factors was also unknown.

Later, Friedrich Griffith discovered the presence of a “transformation factor” in 1928 with his famous experiment on Streptococcus pneumonia. He stated that this factor transferred from dead bacteria thereby transforming the traits of the live bacteria.

The transformation factors were thought to be the genetic material but the biochemical nature of this factor was then unknown, and highly thought of to be a protein.

Later, detailed experiments by Oswald Avery and colleagues in 1944 determined the biochemical nature of the transformation principle to be a DNA. This is when nucleic acids came into the limelight. Extensive research on the molecule provided the molecular basis for inheritance.

Cell and Nucleus

Cells are the structural and functional unit of an organism. They consist of a cytoplasm surrounded by a cell membrane and are capable of surviving and performing metabolic functions independently.

They may have further membrane-bound organelles such as mitochondria or a chloroplast to perform various functions and a nucleus. There are two types of cells based on the presence and absence of a nucleus-

Prokaryotic – The cells lack a nucleus and other membrane-bound organelles. The DNA is present in the cytoplasm organized as a nucleoid. Eg- Bacteria

Eukaryotic – The cells are well differentiated and have a nucleus and other membrane-bound organelles. The DNA in such organisms is present with a nucleus organized into a chromosome. Eg- Plant and animal cell

Nucleic Acids

Nucleic Acids are large biopolymers formed by multiple repetitions of their monomeric unit- nucleotides.

On the basis of monomeric unit composition, they can be either DNA or RNA. The nucleotides consist of three components – a 5 carbon sugar (Ribose in RNA and Deoxyribose in DNA), a phosphate group, and a nitrogenous base (Purines –A and G, Pyrimidines – C, T, and U).

The nucleotides for DNA go by the name of Deoxyribo-nucleotides. And the nucleotides for RNA go by the name of Ribo-nucleotides.

In DNA, the nitrogenous bases are Adenine (A), Cytosine (C), Guanine (G), and Thymine (T)

In RNA, Uracil (U) replaces the Thymine.

The bases attach to the sugar by a 1’ N-glycosidic linkage to form a nucleoside. The phosphate group is then attached to form the monomeric nucleotide.

Two monomers attach to each other by a 3’-5’ phosphodiester linkage. The phosphate group of one nucleotide (at 5’ position) bonds to the 3’-OH of another nucleotide.

The main function of nucleic acids is the storage and stable transmission of genetic information from one generation to the other. This mediates through the following functions of nucleic acids

• DNA is the genetic material for most of the life forms except some viruses and is responsible for transferring genes from parents to offsprings.
• It is responsible for the synthesis of proteins in our cells.
• Loss of DNA content or mutations finds connection with many diseases such as cancer or haemophilia.
• DNA fingerprinting is a method used to determine paternity and identify criminals using evidence at crime scenes such as hair.
• RNA mainly functions as a template of protein synthesis (mRNA), a catalyst for protein synthesis (rRNA) or as an adaptor molecule for amino acids (tRNA).
• Recently discovered RNA molecules can also act as a regulator in the process of protein synthesis.

DNA

DNA stands for deoxyribonucleic acid as the sugar ring of the nucleotides comprising the DNA lack an OH at its 2’ position. This absence of the oxy- group makes it more stable and has allowed the information to pass on over hundreds of generations.

The length of nucleotide repeats describes DNA which is mentioned in bp (base pairs) – the number of monomeric units repeated. It is a characteristic of a species. The bacteria, E.coli have about 4.6 million bp while humans have 3.3 billion bp.

The sequence of arrangement of these monomers is a characteristic of the individual. And this characteristic forms the blueprint of the individual.

James Watson and Francis Crick elucidated the DNA structure using the Chargaff rules and X-Ray studies by Rosalind Franklin in 1928.

DNA is a double helical molecule with two polynucleotide strands running antiparallel to each other. One chain runs in the 3’-5’ direction while the other runs in the 5’-3’ direction.

The sugar and phosphates form the backbone of the helix while the nitrogenous bases protrude inside. These bases between strands are attached to each other through a hydrogen bond that stabilizes the entire molecule.

Purines and Pyrimidines are always present in an equal amount as the A is always found attached to a G and the C found attached to a T in the other strand.

Each turn of the double helix is 3.4nm apart and has roughly 10 bases per turn

The process of protein synthesis, the relative speed, the protein structure and proper processing of the protein formed heavily depend upon the sequence of DNA present in an individual. Alteration to this sequence results in MUTATION.

Mutation can occur due to various physical or chemical agents known as mutagens. Such mutations are the known cause of various genetic disorders such as Haemophilia or Sickle cell anemia and cancer.

Packaging of DNA

Since the length of the complete human DNA would roughly be 2.2 meters in length, it is impossible to pack the entire DNA within each and every cell without proper organization.

In prokaryotes (Organisms lacking a nucleus –Eg. bacteria), DNA is present in the cytoplasm in a supercoiled state known as the nucleoid.

In Eukaryotes, the process of packing is multi-fold and complex. The protein known as histones stabilizes the negative charges of the phosphate group.

This allows tight packing of the DNA which is found in the cells condensed as chromatin (When not replicating) and as Chromosomes (While replicating).

What are Chromosomes?

Chromosomes are thread-like structures present in the nucleus. In humans there are 23 pairs of such chromosomes containing the genes encoding for various protein molecules.

RNA

RNA stands for Ribonucleic acid and is of three major types within a cell –mRNA, tRNA, or rRNA.

mRNA or messenger RNA plays the role of an intermediate in the process of genetic expression. The central DOGMA of molecular biology states that the information stored in DNA converts into a single-stranded mRNA molecule before forming a protein.

Also, the mRNA is complementary to the DNA strand.

What are Genes?

A long DNA molecule contained in a cell has many segments of regions known as a gene that can form a protein. Every gene codes for an intermediate mRNA molecule that is later translated into its corresponding protein.

Eg- A gene coding for insulin will produce an insulin mRNA that would finally result in the production of the hormone insulin. This process is highly regulated and many processes control the final rate of production of the protein.

Every gene has two or more alternate forms which code for the same characteristic. These forms go by the name of alleles. And the final expression of the character depends upon both the alleles present in a cell.

• The mRNA is later translated into proteins by ribosomes in the cytoplasm of the cell
• tRNA or transfer RNA plays the role of an adapter molecule and brings the amino acid corresponding to the sequence in mRNA during protein synthesis
• rRNA is present within the ribosomes and often plays a structural or catalytic role in the process.

Protein Synthesis

The mRNA formed from a DNA molecule is further translated to form a protein. This takes place in the cytoplasm of a cell within organelles known as ribosomes. Proteins are polymers made up of amino acids. There are 20 commonly known AA within every cell.

The mRNA formed from a gene in the DNA also contains polymers of nucleotide sequences. The mechanism of formation of a sequence of AA from a sequence of nucleotides can be well explained by the concept of codons.

Codons and Amino Acids

Codons are base triplets. Three adjacent nucleotides combine to form a codon. Each codon corresponds to a specific amino acid. Since there are 64 possible ways to arrange the four nucleotide bases and only 20 AA, there will be more than one codon coding for an amino acid.

We obtain this information from a codon table. Within the ribosome, the mRNA bases read in groups of three (codon) and the corresponding amino acids join to form a chain. The function and structure of the protein formed determine from the sequence of the amino acids joined.

Since the change of a single base in the gene can lead to a different amino acid in a protein, it may affect its function. Diseases such as Sickle cell anemia are the result of such single base mutations.

Recombinant DNA

Recombinant DNA technology is the process of genetic engineering where DNA molecules from two or more sources join and insert into a host organism. The aim is to obtain favorable genetic traits that may benefit society.

Since the DNA molecules from various species share a common chemical structure, they can be joined together by various laboratory techniques. One such technique is molecular recombination to form a recombinant DNA (rDNA).

The most commonly used host for rDNA techniques is the bacteria Escherichia coli due to its being a short replication cycle and ease to grow and maintain in a laboratory.

To carry out the process, experts use certain enzymes such as –
1. Restriction Endonucleases (Cuts a DNA strand at conserved sequences known as restriction sites eg. EcoRI Restriction endonuclease).
2. Ligases (Joins two ends of DNA fragments).

Example – In earlier days, insulin obtained from pigs was supplied to diabetic patients. However, this led to allergic responses and ethical concerns due to the number of pigs that needed to be killed.

To overcome this obstacle, the insulin protein gene from human DNA was isolated and engineered into E.coli bacteria. The insulin protein thus produced shows no allergic response, as it is obtained from the same species and does not require the culling of animals.

Procedure

• Bacteria contain closed circular pieces of DNA in their cytoplasm known as plasmids. rDNA technology is often performed on such plasmids as it is easy to insert into the host.
• A plasmid was isolated from the E.coli cells and cut open by restriction endonucleases.
• The human insulin gene was isolated and PCR (Polymerase chain reaction) used to make multiple copies of it.
• Ligases used to recombine the two fragments of DNA together.
• This engineered plasmid was then transformed/cloned into host cells using the heat-shock method. (Other processes such as gene gun or microinjection are in use).
• The transformed cells were then grown in large containers containing the favorable medium, temperature, and pH to produce the human insulin protein in large volumes.
• This can further processed and used in medical procedures.

Various rDNA products develop by such rDNA techniques using a biological sustainable method. They can be used to correct genetic diseases by replacing a faulty gene with a normal one. It is also used in genetically modifying animals to help them overcome environmental stresses and diseases.

The regulated use of this technique can help solve various problems faced by human society in a sustainable manner and at a lower cost.

Conclusion

In this article, we learned about the genetic material in significant detail. Firstly we learned about cells and nucleus followed by nucleic acids. We then learned about DNA and its packaging followed by RNA and protein synthesis.

We then ended up discussing Recombinant DNA technology and its procedure. Try to recall each one of them. Happy learning!