Gene Definition: Structure, Expression, Facts, Characteristics, Functions

What Is a Gene?

A gene is an elementary unit of heredity across all living organisms. It is identified as a definite arrangement of nucleotides in the sequence of DNA (or RNA) that acts as a coded blueprint for the manifestation or transmission of a hereditary trait from one generation of organisms to the next. Genes are found on chromosomes, with very different sizes, and huge differences in functions, they control everything about biological processes and features.

Concept and Significance of Genes

Genes are at the top of the hierarchy of biological functioning of living organisms. They direct the synthesis of proteins, enzymes, structural parts, and signal molecules, among others, that organize complex biochemical reactions holding life together.

These proteins are encoded in genes, meaning that genes determine the phenotype of an organism, such as the colour of eyes, height, or disease sensitivity.

Historical Background of Gene Concept

The historical background of gene concept includes:

Gregor Mendel

• The study of genes was initiated in the middle of the 19th century with the pioneering work of Gregor Mendel, who determined principles of heredity using pea plants.

• Mendel's laws of inheritance were laid down for the real science of genetics.

Molecular Discovery

• The finding of the structure of the DNA double helix by James Watson and Francis Crick in 1953 truly was the breakthrough.

• Later progress in genetic research, including the Human Genome Project, increased manyfold the understanding of gene sequences and operations in health, diseases, and evolution.

Molecular Structure of a Gene

The molecular structure of a gene is discussed below:

DNA and RNA as Genetic Material

• The two forms of nucleic acids that encode genetic information are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA is a double helix and consists of two long chains of polymers, running anti-parallel to each other.

• RNA is a single stranded chain.

• Both DNA and RNA are polynucleotides

Nucleotide Composition

• A nucleotide is formed from a compound of a sugar molecule, a phosphate group, and a nitrogenous base.

Nitrogenous Bases

• In DNA, the nitrogenous bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

• RNA contains uracil (U) instead of thymine.

• Adenine pairs with thymine (or uracil in RNA), and cytosine pairs with guanine.

• These pairing rules are consistent with the way DNA is replicated and how genetic information is transcribed into RNA in the cell.

Genes and Chromosomes

Genes and chromosomes are discussed below:

Chromosomes

• Chromosomes are long, thread-like structures made of DNA and associated proteins located in the nucleus of eukaryotic cells.

• Each chromosome contains many genes, regulatory sequences, and other nucleotide sequences.

Human Chromosome Number

• Humans have 46 chromosomes, consisting of 22 pairs of autosomes and one pair of sex chromosomes, one member inherited from each parent.

• The genes themselves are located along specific segments of the chromosomes and code for proteins or RNA molecules, which play a fundamental role in determining an organism's growth, appearance, and physiology.

Gene NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Molecular structure of gene

• Genes and Chromosomes

Practice Questions for NEET

Q1. Alleles are:

1. different molecular forms of a gene

2. heterozygotes

3. different phenotype

4. true breeding homozygotes

Correct Answer: 1) different molecular form of gene

Explanation:

Alleles are different gene variants or molecular forms that occur at the same location (locus) on a chromosome and are caused by mutations. Different characteristics or variances in an organism can be caused by alleles.

People who have two distinct alleles at a specific gene locus (such as Aa) are known as heterozygotes.
Various phenotypes Although alleles are not distinct phenotypes, phenotypes are the observable qualities or characteristics of an organism that arise from the expression of alleles.
Although this is not the definition of alleles, true-breeding homozygotes are organisms that have two identical alleles for a particular attribute (AA or aa, for example).

Hence, the correct answer is option 1)different molecular forms of a gene.

Q2. What is a phenotype?

1. The genetic makeup of an individual

2. The physical characteristics of an individual

3. The reproductive cells of an individual

4. The expression of a single gene

Correct Answer: 1) different molecular form of gene

Explanation:

A phenotype is the observable physical and biochemical traits of an individual, such as height, hair color, and blood type. It is the result of the interaction between an individual's genetic makeup (genotype) and the environment. The phenotype can also include behavioral characteristics and susceptibility to certain diseases. The genotype refers to the genetic makeup of an individual, including all of the individual's genes and genetic variants. The reproductive cells of an individual are called gametes, and the expression of a single gene is just one part of the complex process that leads to the development of a phenotype.

Hence the correct answer is option 2, The physical characteristics of an individual.

Q3 What is a gene?

1. A unit of heredity that is passed from parent to offspring

2. A type of protein that regulates gene expression

3. A section of DNA that codes for a specific protein

4. A type of RNA that carries genetic information

Correct answer: 3) A section of DNA that codes for a specific protein

Explanation:

A gene is a specific sequence of DNA that encodes the information required to make a functional protein or RNA molecule. This sequence of DNA contains the instructions for the sequence of amino acids that make up the protein, as well as the regulatory sequences that control when and where the gene is expressed. These proteins perform various functions in the body, such as enzymes that catalyze chemical reactions, structural proteins that form the framework of cells, and signalling proteins that regulate cellular communication. Mutations in genes can lead to changes in protein function, which can result in genetic disorders or diseases. In summary, a gene is a fundamental unit of heredity that carries the genetic information necessary to produce a functional protein or RNA molecule.

Hence, the correct answer is option 3) A section of DNA that codes for a specific protein.

Also Read:

• MCQs on Mendelian Genetics

• Mendel’s Laws of Inheritance

• Complementary Genes

Video recommended on Genetic Mutation