Nucleic Acids: Class 12 Chemistry Notes, Definition, Types, Functions & Examples

In the NCERT textbook, nucleic acid is defined as

The particles in the nucleus of the cell, responsible for heredity, are called chromosomes, which are made up of proteins and another type of biomolecule called nucleic acids.

Nucleic acids are biomolecules that store and share genetic information. They act like a blueprint and messenger of life, instructing cells what to do and how to do it. Nucleic acids are polynucleotides because they are long-chain polymers made up of repeating monomer units called nucleotides. Classified into two types:

• DNA (Deoxyribonucleic Acid)
• RNA (Ribonucleic Acid)

 

Nucleic acids are classified into two main types:

• DNA (Deoxyribonucleic Acid)
• RNA (Ribonucleic Acid)

Deoxyribonucleic Acid (DNA)

DNA is found in the cell's nucleus and guides the cell's growth, function, and reproduction. It stores and transmits genetic information. The appearance of DNA is like a twisted ladder called a double helix. DNA contains sugar β-D-2-deoxyribose alongwith cyclic bases: adenine (A), guanine (G), cytosine (C), thymine (T). 

Ribonucleic Acid (RNA)

The structure of RNA is single-stranded. It consists of sugar called β-D-ribose. The heterocyclic bases adenine (A), uracil (U), cytosine (C), and guanine (G) are available in RNA. There are three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).

DNA and RNA work as a team to store, pass on, and use the genetic information so living organisms can survive and grow.

Some Important Links:

NCERT Class 11 notes	NCERT Class 11 Physics notes
NCERT Class 11 Chemistry notes	NCERT Class 11 Maths notes

We know that DNA and RNA are made up of a long chain of nucleotides. It contains three parts: sugar, phosphate group, and nitrogenous base. The backbone of nucleic acid is formed by sugar and phosphate, while the nitrogenous base sticks like letters. These letters store the genetic information.

DNA structure is double-stranded, called a double helix. Two strands twist around each other and are held together by hydrogen bonds between base pairs. Adenine (A) forms a hydrogen bond with thymine, and cytosine forms a hydrogen bond with guanine.

RNA is a single-stranded structure. It can be folded into three-dimensional shapes to perform various functions in the cells. The sequence of bases along DNA or RNA acts like a code. This code determines the traits of an organism and guides the production of proteins.

The basic building blocks of nucleic acids (DNA & RNA) are called nucleotides. The main parts of nucleotides are the phosphate group, the sugar, and the nitrogenous base. The sugar present in DNA is deoxyribose, and in RNA, ribose is present.

Image credit: The source of the image is the NCERT textbook.

There are four types of nitrogenous bases present in DNA, including adenine (A), thymine (T), guanine (G), and cytosine (C). While in RNA, thymine is replaced by uracil. A nucleotide in the absence of a phosphate group is called a nucleoside. It consists only of sugar and a nitrogenous base. Nucleotides connect through phosphate and sugar to form long chains, forming the backbones of DNA or RNA. The bases stick out and form the letters that carry genetic information.

Difference between Nucleotide and Nucleoside

Features	Nucleotide	Nucleoside
Function	Forms DNA & RNA	Build blocks for nucleotide
Components	Phosphate group, sugar, and nitrogenous base	Nitrogenous base and sugar
Contain Phosphate	One or more phosphate groups	No phosphate group
Examples	ATP (Adenosine Triphosphate, GTP, dATP	Adenosine, Guanosine

Chemical Composition of Nucleic Acids

When we hydrolyse DNA (or RNA), it forms pentose sugar, phosphoric acid, and nitrogen. It contains heterocyclic compounds called bases. The sugar moiety in DNA is β-D-2-deoxyribose, and in RNA sugar moiety is β-D-ribose.

The bases like adenine (A), guanine (G), cytosine (C), and thymine (T) are present in DNA. While the bases in RNA are similar to DNA, except thymine, it is replaced by uracil (U).

DNA's full form is "Deoxyribonucleic Acid". It is like a recipe book that tells our body how to grow, look, and work. DNA decides our hair types, eye colour, and how the body fights germs.

 

Sort History of DNA

1. In 1869, Friedrich Miescher discovered a strange substance in the white blood cells.
2. In a further experiment, it was found that this substance carried genetic information.
3. It was 1953, when the group of scientists, Watson and Crick, with the help of Rosalind Franklin and Maurice Wilkins, introduced the famous double helix shape - the DNA twisted ladder model.

Also Check: 

NCERT Class 12 notes	NCERT Class 12 Chemistry
NCERT Class 12 Maths notes	NCER Class 12 Physics notes

The Building Blocks of DNA

We have already discussed that DNA is made of small pieces called nucleotides. Every nucleotide has three parts:

1. A sugar (deoxyribose)
2. Nitrogen base (A, T, G or C)
3. Phosphate

Types of DNA Bases

There are four types of bases

1. A (Adenine)
2. T (Thymine)
3. G (Guanine)
4. C (Cytosine)

DNA Structure

The structure of DNA is like a twisted ladder. On the side of the ladder, phosphate and sugar are present, and the steps are base pairs.

1. A pairs with T (A-T)
2. G pairs with C (G-C)

DNA Replication - Copying the Code

When cells break, DNA make a copy so that each daughter cell gets a complete set.

• Semiconservative Replication
  • In the Meselson-Stahl experiment, it was confirmed that each new DNA double helix carries the old strand and the newly synthesised strand. This explains DNA's high fidelity.

DNA Replication

1. Helicase - It unwinds the double helix like a zipper by splitting double-stranded nucleic acid into single strands
2. Single-strand binding proteins (SSBs) - It is an enzyme that prevents strands from binding together.
3. Primase - A crucial enzyme that synthesises short RNA primers on a DNA template.
4. DNA polymerases - It is an enzyme that synthesises the new DNA strand. Polymerase adds nucleotides in the 5′→3′ direction, which is guided by the base pair. 
5. Ligase - A Class of enzymes that seal nicks and stitch fragments into a continuous strand.

DNA and Inheritance

DNA is carried from parents to children, which is known as genes.

Dominant, Recessive, Sex-Linked, and Mitochondrial Inheritance

1. Autosomal dominant
2. Autosomal recessive
3. Sex-linked (X- or Y-linked)
4. Mitochondrial inheritance.

DNA in Health and Medicine

1. Changes in DNA can result in genetic disease and cancer.
2. The study of DNA helps to create personalised medicine.
3. Pharmacogenomics helps in prescribing the right drug to the right patient.

The full form of RNA is "Ribonucleic Acid". The role of RNA is to copy the genetic information from DNA. In the absence of RNA, DNA would be ideal in the nucleus, unread.

Structure of RNA

The structure of RNA is single-stranded. In RNA, ribose sugar is present, and all the bases are similar to DNA except thymine (T). It is replaced by uracil (U).

Types of RNA and Their Functions

RNA is of various types, including messenger RNA, transfer RNA, Ribosomal RNA, etc. Let's discuss different types of RNA below.

1. Messenger RNA (mRNA)

• mRNA role is to carry instructions from DNA to the ribosome. It acts like a photocopy. mRNA instructs the cells what protein to make.

2. Transfer RNA (tRNA)

• It carries amino acids to the ribosome. tRNA acts like a delivery truck; it picks up and drops the right proteins to the right spot.

3. Ribosomal RNA (rRNA)

• It builds ribosomes, where proteins are made. rRNA holds everything together during protein synthesis.

Other Types of RNAs

1. snRNA (small nuclear RNA)
2. lncRNA (long non-coding RNA)
3. miRNA & siRNA (micro/small interfering RNA)

How RNA is Made - Transcription

RNA is made directly from DNA. Helicase unwind the DNA. RNA polymerase class of enzymes, copies one DNA strand into RNA. Then the RNA strand is released and processed (in eukaryotes) before being used.

The entire process is known as transcription. In this process, nucleotides stay the same - just copied from DNA to RNA.

Why is RNA important?

RNA helps to make proteins that do most of the work in cells. It acts as a regulator that turns genes on or off. Some viruses, like influenza and HIV, store their genetic material as RNA.

Nucleic acids are present in every human mankind and hold a different level of importance. Some of the major functions of nucleic acids are as follows:

1. The synthesis of protein in our body is regulated due to the presence of nucleic acids in our body.
2. The transfer of intrinsic personality from a parent to a child takes place due to the presence of nucleic acids. Physical personalities of children depend upon the parents, which is possible through the nucleic acids
3. The digital fingerprinting of DNA can identify the parent of the child.

Also Read: NCERT Notes for Class 11 & 12