A gene is defined as the basic unit of heredity, meaning it’s what parents pass down to their offspring. Genes are sections of DNA that contain instructions for making proteins, which help control traits and functions in the body. Genes determine features like blood type, eye colour, and even the risk of certain diseases. This topic is covered in the Class 12 chapter Molecular Basis of Inheritance in Biology.
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What is a Gene?
Gene Structure and Composition
Chromosomes and Genes
Gene Definition
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 sometimes 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.
Genes are at the top of the hierarchy of biological functioning of living organisms. They command the making of proteins, which have very few exceptions in cells for playing essential roles. Proteins are 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. Knowledge of genes and individual gene functions is the foundation of many scientific disciplines, including genetics, molecular biology, and medicine.
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. The finding of the structure of the DNA double helix by James Watson and Francis Crick in 1953 truly was the breakthrough in giving a molecular basis for the storage and transmission of genetic information. Later progress in genetic research, including the Human Genome Project, increased manyfold the understanding of gene sequences and operations in health, diseases, and evolution.
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A gene is a segment of DNA that contains the instructions for making a specific protein or set of proteins. It is the basic unit of heredity, passing traits from parents to offspring. Genes are composed of nucleotides and are located on chromosomes within the cell nucleus.
Q: What is a cistron, and how does it relate to the concept of a gene?
A:
A cistron is a genetic unit that codes for a single polypeptide chain. It is essentially equivalent to a gene in prokaryotes. In eukaryotes, a single gene can contain multiple cistrons due to alternative splicing. The concept of a cistron helps clarify the relationship between genes and their protein products, especially in more complex genetic systems.
Q: What is a gene family, and why is it important in evolution?
A:
A gene family is a group of genes that share a common ancestral gene and have similar sequences and functions. Gene families are important in evolution because they can expand through gene duplication events, allowing for the development of new functions while maintaining the original function. This process contributes to genetic diversity and the evolution of new traits.
Q: How do genes interact with the environment to influence traits?
A:
Genes provide the blueprint for traits, but environmental factors can influence how these traits are expressed. This interaction, known as gene-environment interaction, can affect the penetrance (likelihood of a gene causing a trait) and expressivity (degree to which a trait is expressed) of genetic traits. For example, diet and exercise can influence how genes related to metabolism and body composition are expressed.
Q: What is a polygenic trait, and how does it differ from a single-gene trait?
A:
A polygenic trait is influenced by multiple genes, each contributing a small effect to the overall phenotype. This results in a continuous range of phenotypes, such as height or skin color. In contrast, single-gene traits are determined by one gene and often show discrete phenotypes, like blood type. Polygenic traits are more complex and often show a normal distribution in populations.
Gene Structure and Composition
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 of each other. The two strands are held together by the formation of hydrogen bonds in pairs between the bases: adenine (A), thymine (T), guanine (G), and cytosine (C). RNA acts mostly as a single polynucle. Both DNA and RNA are polynucleotides. A nucleotide is formed from a compound of a sugar molecule, a phosphate group, and a nitrogenous base.
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.
Commonly Asked Questions
Q: How does gene structure relate to its function?
A:
Gene structure typically includes regulatory regions (promoters and enhancers), coding regions (exons), and non-coding regions (introns). The regulatory regions control when and where the gene is expressed, while the coding regions contain the actual instructions for protein synthesis. This structure allows for precise control of gene function and expression.
Q: What are introns and exons?
A:
Introns are non-coding sequences within a gene that are removed during RNA processing, while exons are the coding sequences that remain in the mature mRNA and are translated into protein. The presence of introns allows for alternative splicing, increasing the diversity of proteins that can be produced from a single gene.
Q: How does alternative splicing contribute to genetic diversity?
A:
Alternative splicing is a process where different combinations of exons from the same gene can be included in the final mRNA, resulting in different protein isoforms. This mechanism greatly increases the number of proteins that can be produced from a limited number of genes, contributing to genetic diversity and complexity in organisms.
Q: What is a promoter region, and why is it important?
A:
A promoter is a regulatory DNA sequence located upstream of a gene that serves as a binding site for RNA polymerase and transcription factors. It is crucial for initiating gene transcription and controlling the rate of gene expression, allowing cells to respond to various environmental and developmental signals.
Q: How do enhancers differ from promoters in gene regulation?
A:
While promoters are located near the transcription start site and are essential for initiating gene expression, enhancers can be located far from the gene they regulate and can increase the rate of transcription. Enhancers can act on multiple genes and work in conjunction with promoters to fine-tune gene expression.
Chromosomes and Genes
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. 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 in turn play a fundamental role in determining an organism's growth, appearance, and physiology.
Genes are the entire DNA sequence that codes for a particular trait, while alleles are different versions of the same gene. For example, the gene for eye color has multiple alleles, such as those for blue, brown, or green eyes. Alleles can be dominant, recessive, or codominant in their expression.
Q: How do genes contribute to genetic disorders?
A:
Genetic disorders can result from mutations in one or more genes, chromosomal abnormalities, or complex interactions between multiple genes and environmental factors. These genetic changes can lead to the production of abnormal proteins, insufficient protein levels, or altered gene regulation, resulting in various inherited diseases or predispositions to certain conditions.
Q: What is the central dogma of molecular biology, and how does it relate to gene expression?
A:
The central dogma of molecular biology describes the flow of genetic information in cells: DNA is transcribed into RNA, which is then translated into proteins. This process is fundamental to gene expression, where the information in genes is used to produce functional gene products, typically proteins.
Q: How do genes determine phenotype?
A:
Genes determine phenotype by coding for proteins that carry out specific functions in the body. These proteins can be structural components, enzymes, or signaling molecules that influence various traits. The interaction between multiple genes and environmental factors ultimately shapes an organism's observable characteristics or phenotype.
Q: What is a gene mutation, and how can it affect protein function?
A:
A gene mutation is a permanent change in the DNA sequence of a gene. Mutations can be point mutations (changing a single nucleotide), insertions, deletions, or larger chromosomal changes. These alterations can affect protein function by changing the amino acid sequence, creating a premature stop codon, or altering gene expression, potentially leading to changes in phenotype or genetic disorders.
Frequently Asked Questions (FAQs)
Q: How do genes contribute to the development and function of the immune system?
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Genes are crucial for immune system development and function:
Q: How do genes contribute to the circadian rhythm?
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Genes play a central role in regulating circadian rhythms through a complex feedback loop:
Q: How do genes contribute to the development of the nervous system?
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Genes play crucial roles in nervous system development through:
Q: How do genes contribute to antibiotic resistance in bacteria?
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Genes contribute to antibiotic resistance in bacteria through several mechanisms:
Q: How do genes contribute to aging?
A:
Genes contribute to aging through various mechanisms:
Q: How do genes influence behavior?
A:
Genes influence behavior through several mechanisms:
Q: How do genes contribute to the immune system's ability to recognize and fight pathogens?
A:
Genes play a crucial role in the immune system by:
Q: What is a gene regulatory network, and why is it important?
A:
A gene regulatory network is a collection of molecular regulators that interact with each other and with other substances in the cell to govern the gene expression levels of mRNA and proteins. These networks are important because they:
Q: How do genes contribute to the development of cancer?
A:
Genes contribute to cancer development through several mechanisms:
Q: What is genetic imprinting, and how does it affect gene expression?
A:
Genetic imprinting is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. This means that either the maternal or paternal copy of the gene is silenced, while the other is expressed. Imprinting is achieved through DNA methylation and histone modifications and plays crucial roles in development and certain genetic disorders.
