Introduction

Nucleic acids are the biomolecules that are essential for every form of life present on the earth. They are present in all organisms from small viruses and bacteria to large and complex animals like humans and whales. 

The word nucleic acid is used collectively for DNA and RNA. Nucleic acids range in size from small biomolecules to large biopolymers. They are made up of repetitive subunits called nucleotides. 

The two nucleic acids differ in their structure, function, properties, and location within the cell.  We will discuss these nucleic acids in detail in this section.  

DNA

DNA also known as deoxyribonucleic acid is the most important biological molecule present in living cells. All the genetic information is stored in the cell in the form of DNA. 

Occurrence

DNA is present in all the living cells in one or another form. 

The eukaryotic cells have a nucleus that is made of entirely DNA and the proteins associated with it. DNA is also present in other organelles of eukaryotic cells such as mitochondria and chloroplast.

In bacteria, DNA is present in the form of a single circular molecule called chromosome. Some bacteria also have extra-chromosomal DNA in the form of plasmids.  

A large category of viruses called DNA viruses contain genetic information in the DNA. 

Structure

DNA is a polymer of deoxyribonucleotides having a double helix structure. The deoxyribonucleotides in DNA are linked together via phosphodiester bonds to form two polynucleotide chains. These polynucleotide chains are held together via hydrogen bonds between the nitrogenous bases. The two polynucleotide chains are coiled around each other in an anti-parallel fashion forming long DNA fibers.  

The DNA fibers are then wound around basic proteins called histone proteins forming nucleosomes. Nucleosomes then form chromatin fibers that are condensed together to form chromosomes. 

(This is only a summary of the structure of DNA. For more details about DNA structure, read notes on Structure of DNA)

Read more about DNA and RNA

Replication

DNA replication is the process by which a new identical copy of DNA is created. In the process of DNA replication, the two DNA strands are separated exposing the individual nucleotides. A new daughter DNA strand with a complementary structure is created in front of each parent strand.  As a result, a new DNA molecule is formed. 

This process is called semi-conservative replication because the new DNA molecule has one parent strand and one new strand or daughter strand. Thus, half of the parent DNA molecule is conserved in each new molecule. 

Breakdown 

The DNA breakdown involves two steps; 

• Separation of DNA strands
• Release of nucleotides

The process of DNA strands separation is called denaturation of DNA. It involves breaking the hydrogen bonds between the nitrogenous bases. DNA denaturation is favored by;

• Acidic or alkaline pH
• High temperature
• Enzymes like DNA helicase

After strand separation, the individual nucleotides are released by breaking the phosphodiester bonds. Two types of enzymes are involved in this process; endonucleases and exonucleases. 

• Endonucleases act on the middle of the DNA strand and break the phosphodiester bonds between the individual nucleotides. 
• Exonuclease cleave the phosphodiester bonds starting from one end of the DNA molecule. 

DNA breakdown occurs in various processes like apoptosis, autophagy, necrosis, phagocytosis, etc. An example of DNA breakdown is the digestion of DNA taken in their diet by humans. 

DNA enter human diet in the form of beans, liver, spleen, and meat of other highly cellular organs. Breakdown of DNA starts from the stomach where the DNA strands are separated by the acidic environment. The nucleotides of the individual strands are then separated by endonucleases and exonucleases present in the intestine. 

Functions

The functions of DNA can be divided into biological functions and non-biological functions.

Biological functions

These are the functions performed by DNA within the living organisms. Biological functions of DNA are as follows;

• The genetic information is packed in cells in the form of DNA. 
• All the structural and functional information of an organism is present in the form of DNA. 
• DNA encodes the synthesis of all types of proteins.
• The genetic information is transferred to the next generation of cells in the form of DNA.

Non-biological functions

These include the uses of DNA outside living organisms for the welfare of mankind. Non-biological functions of DNA are listed below;

• DNA is used by genetic engineers to make vaccines, hormones, and other important medical products. 
• DNA profiling is a technique used by forensic experts to find criminals. It is a process of identifying criminals by matching the DNA obtained from the crime scene with the DNA of the accused or the suspected person.
• DNA is used to make hybrid plants in agriculture. 
• DNA sequencing is done to rule out genetic disorders. 
• It is used in gene therapy. 
• DNA is used for the diagnosis of various viral diseases. 

RNA

Ribonucleic acid (commonly known as RNA) is the second most important nucleic acid present in the living bodies. It is a polymer of ribonucleotides having ribose as pentose sugar. In most of the living cells, DNA and RNA work collectively to perform their functions. 

Occurrence

RNA is also present in almost all living cells. 

In eukaryotic cells, it is present in the nucleus as well as in cytoplasm. It is an important component of ribosomes present in the cytoplasm of eukaryotic cells in free form as well as attached to the rough endoplasmic reticulum. 

In bacteria, it is present in the cytoplasm of the cell as well as in the bacterial ribosomes.

Like DNA viruses, RNA viruses are also present that have their entire genetic information in the form of RNA.

Structure

RNA is a polymer of ribonucleotides. These nucleotides are also attached via phosphodiester bonds to form a long chain of polynucleotide.

RNA is a single-stranded molecule. The single chain of ribonucleotides may fold onto itself resulting in the formation of a coil or a helix. When the polynucleotide chain folds, hydrogen bonds are formed between the nitrogenous bases. 

Synthesis 

RNA is synthesized from DNA in a process known as transcription. 

In the process of transcription, the two DNA strands are first separated by an enzyme called helicase.  One of the DNA strand acts as a template. The nucleotide sequence of this strand is copied by RNA polymerase enzyme. This enzyme joins the RNA nucleotides complementary to the sequence of the template strand. in this way, an RNA copy of the nucleotide sequence is created that is carried to the cytoplasm later on. 

Types

There are three types of RNA.

• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Ribosomal RNA (rRNA)

The details of these types are discussed below. 

Messenger RNA (mRNA)

As the name indicates, messenger RNA as a messenger between the DNA and the cytoplasm. It carries genetic information for protein synthesis present in the DNA of the nucleus to the ribosomes in the cell cytoplasm. 

The information for protein synthesis is present in the nucleus in the form of genes. A gene is a specific nucleotide sequence of DNA that codes for the synthesis of a particular protein. This nucleotides sequence is copied into the mRNA by RNA polymerase.  

The information for protein synthesis is present in mRNA in the form of genetic code. A genetic code is a specific sequence of three nucleotides that codes for one particular amino acid.

The size of mRNA depends on the size of the protein it encodes. For example, if a protein is made up of 2000 amino acids, the mRNA encoding this protein will have at least 6000 nucleotides, because 3 nucleotides code for one amino acid. 

Messenger RNA is present in the cytoplasm of both eukaryotic and prokaryotic cells. It makes only 3 to 4% of the total RNA present in a cell. 

Transfer RNA (tRNA)

This type of RNA transfers amino acids to the ribosomes so that they can be assembled into proteins. The information in mRNA is translated into amino acid sequence by transfer RNA. This type of RNA reads the code on mRNA and carries that specific amino acid to the ribosome so that it can be incorporated into the proteins. 

The transfer RNA is also made of a single chain of ribonucleotides. However, this chain is folded onto itself to form three hairpin loops. 

One of these three loops is called the anticodon loop. This loop carries a specific sequence of three nucleotides known as anticodon. This sequence of anticodon is complementary to the codon or genetic codon on mRNA for a specific amino acid. 

As specific codon are present for each amino acid, anticodon for each amino acid are also specific. One tRNA also carries only one anticodon. This means that only one specific amino acid can be transferred by one specific tRNA. Different transfer RNAs are present in the cytoplasm for different types of amino acids. 

The 3’ end of the tRNA is called the acceptor stem. The amino acid attaches to the tRNA at this end by reacting with the 3’ hydroxyl group. 

The transfer RNA is the shortest form of nucleic acids. It is made up of only 80 to 90 nucleotides.  

Ribosomal RNA (rRNA)

It is the most abundant form of RNA present in cells. It is an essential component of ribosomes present in bacterial and eukaryotic cells. 

Within the ribosomes, the ribosomal RNA arranges itself into two subunits, a larger ribosomal subunit and a smaller ribosomal subunit. The structure of these two subunits differs in prokaryotes and eukaryotes. 

In case of prokaryotes, 

• The larger ribosomal subunit contains one large molecule of rRNA and one small molecule of rRNA. The large molecule contains around 3000 nucleotides while the small molecule contains around 1500 nucleotides. 
• The smaller ribosomal subunit contains only one small molecule of rRNA containing around 1500 nucleotides. 

While in the case of eukaryotes, 

• The larger ribosomal subunit contains one large molecule of rRNA (having around 5000 nucleotides) and two small molecules of rRNA (each having around 1800 nucleotides). 
• The smaller ribosomal subunit contains only one small molecule of rRNA with around 1800 nucleotides.

In both eukaryotes and prokaryotes, the rRNA molecules in ribosomal subunits are combined with specific proteins called ribosomal proteins to form ribosomes. 

Ribosomes are the assembly houses where mRNA and tRNA interact to form proteins. 

Small RNAs

In addition to the types of RNA discussed above, some other RNA are also present in the human body. 

One of the most important small RNAs is small nuclear RNA (snRNA). This RNA is present within the nucleus. It is involved in the post-transcriptional modification of mRNA. It removes the non-coding part of mRNA also known as exons. The snRNA is assisted by some special ribonucleoproteins in this process known as snRNPs (small nuclear ribonucleoproteins).

Summary

Nucleic acids are the polymers of nucleotides. 

They are present in all the living cells. 

They are the most important biopolymers present in living cells as they control all the processes taking place in them. 

They broadly include DNA and RNA.

DNA is made up of nucleotides having deoxyribose as sugar. 

• It is present in the nucleus, mitochondria, and chloroplast of cells.
• It is also present in bacteria and fungi. 
• New copies of DNA are created by the process of DNA replication. 
• The breakdown of DNA takes place in two steps;

1. Denaturation of Strands
2. Cleavage of phosphodiester bonds

• It transfers genetic information to the next generation and also controls all the processes in the cell.
• It is also used in medical labs, forensic labs, and the agriculture industry. 

RNA is made of nucleotides having ribose as pentose sugar. 

• It is present abundantly in cytoplasm in free form and as a part of ribosomes. 
• It is synthesized in the nucleus from DNA, by the process of transcription. 
• Three types of RNA completely differ from one another in structure and function. These types include;

1. messenger RNA
2. transfer RNA
3. ribosomal RNA

• in addition to these three types, some important small RNAs are also present like snRNA that is involved in splicing. 

Frequently Asked Questions

What are the two important nucleic acids present in the human body?

The two important nucleic acids which are found in the human body are DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid)

What is the function of DNA in living organisms?

DNA consists of all the information required for the functioning of the human body. It also consists of genetic information that is passed from one generation to other.

How many types of RNA are found in humans?

There are three major types of RNA That are found in human beings. These are mRNA, rRNA, and tRNA.

What is the function of RNA in living organisms?

The main function carried out by RNA in living organisms is protein synthesis. RNA synthesizes proteins through a process called translation.

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