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What is upstream and downstream of the structural genes?

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In molecular biology, the terms  upstream  and  downstream  refer to the positions of sequences relative to structural genes in the DNA, which encode proteins or functional RNAs. These terms are based on the  direction of transcription.  Transcription occurs from the  5′ to 3′ direction  on the coding strand and these orientations help determine the functional significance of various DNA regions. What is Upstream of the Structural Genes? Upstream refers to the DNA sequences that are located before the structural genes, in the  5′ direction.  These regions do not directly code for  proteins  but play critical roles in regulating gene expression by influencing the process of transcription. Key components upstream of structural genes: 1. Promoter The promoter is the region where the  RNA polymerase  binds to  initiate transcription. In prokaryotes,  the promoter region often contains consensus sequences like th...
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Describe Elemental Composition of Earth's Crust

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The Earth's crust is the thin, outermost solid layer of our planet which forms both the continents and the ocean floors. It lies above the mantle and makes up less than 1% of Earth's total volume but it is the most accessible and studied layer because all landforms, soils and rocks we see are part of the crust. The crust is composed mainly of solid rocks and minerals, which in turn are made from various chemical elements. These elements do not exist in pure form but are combined in complex compounds like silicates, oxides, carbonates etc. Although more than 90 naturally occurring elements are found in the crust, only a small group of them dominate its total mass. The composition of the crust reflects its geochemical origin, its formation history and the processes like weathering, erosion, and plate tectonics that continuously shape it. Each of these elements has specific roles in rock formation and Earth’s surface structure. Their proportions are usually expressed by mass pe...
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What is the difference between excitatory and inhibitory postsynaptic potentials

When a nerve impulse reaches the axon terminal of a presynaptic neuron, neurotransmitters are released into the  synaptic cleft.  These neurotransmitters bind to specific receptors present on the membrane of the postsynaptic neuron. This binding leads to the opening of ion channels and causes changes in the membrane potential of the postsynaptic neuron. These changes can either increase or decrease the likelihood of generating an action potential. If the change leads to depolarization, it is called  Excitatory Postsynaptic Potential (EPSP)  and if it leads to hyperpolarization, it is known as  Inhibitory Postsynaptic Potential (IPSP).  Both are types of graded potentials and are crucial for the integration of synaptic inputs in the central nervous system. Here is the detailed comparison of EPSP and IPSP based on different criteria: 1. Based on Definition and Nature of Response EPSP (Excitatory Postsynaptic Potential) is a type of postsynaptic potential that...
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What is depurination and deamination? Describe the repair systems that operate after depurination and deamination

Depurination  and  deamination  are two types of  spontaneous mutations  that occur naturally within the DNA of cells. These are not caused by external agents but happen due to chemical instability in DNA under normal cellular conditions. Both processes can lead to mutations if not repaired in time, but cells have efficient repair mechanisms, especially the  Base Excision Repair (BER)  pathway, that work to correct these damages and maintain the genetic integrity of the organism. Depurination Depurination refers to the loss of a  purine base,  which is either adenine or guanine, from the DNA. This occurs when the N-glycosidic bond between the purine base and the deoxyribose sugar breaks due to hydrolysis. As a result, the base is removed but the sugar-phosphate backbone of the DNA remains intact. The site from where the purine base is lost is called an  apurinic site (AP site). If this AP site is not repaired before DNA replication, the ...
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Define Recon, Muton and Cistron

The terms Recon, Muton and Cistron were introduced by  Seymour Benzer  during the 1950s to study the detailed structure and function of genes at the molecular level. He worked on the rII region of T4 bacteriophage and used bacteriophage genetics to analyze how small changes in DNA affect phenotypes. At that time, the gene was considered as a single indivisible unit. But  Benzer  showed that a  gene has a finer internal  structure and can be divided into smaller functional units. Based on this, he proposed three molecular units:  Recon, Muton and Cistron,  each having a specific role related to recombination, mutation and expression. Recon Recon is defined as the  smallest unit of recombination.  It refers to the smallest segment of DNA within which crossing over cannot occur, but recombination can occur between two such units. According to modern molecular understanding, recombination between two genes or within a gene occurs at the leve...
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What are epigenetic modifications? Give examples

Epigenetic modifications are heritable and reversible changes in gene expression that occur  without altering the DNA sequence  itself. These changes play a major role in how genes are  turned on or off  in different cells and at different times. Epigenetics helps explain how the same DNA sequence can produce different types of cells like skin cells, nerve cells and liver cells in the same organism. These modifications are very important during development, cellular differentiation, X-chromosome inactivation in females, genomic imprinting, aging and in many diseases such as cancer. There are the following three main types of epigenetic modifications: 1. DNA Methylation This is the most studied and well-understood form of epigenetic modification. In this process, a methyl group (–CH₃) is added to the cytosine base in DNA, mainly at CpG dinucleotides. These CpG regions are often found in clusters called CpG islands, which are located near  gene promoters.  DN...
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Define lethal allele. Explain with a suitable example the molecular basis of lethality

A lethal allele is a type of  gene mutation  that causes death to an organism when present in a certain genotype. Usually, lethal alleles are  recessive,  meaning that they cause death only when an individual inherits two copies of the lethal allele (homozygous condition). However, some lethal alleles can also act in a  dominant form  and cause death even when only one copy is present. Lethal alleles generally affect essential biological processes like metabolism, development and organ formation. These alleles usually arise due to mutations that severely disrupt the function of an essential gene. Molecular Basis of Lethality One of the best-known classical examples of a lethal allele is found in mice involving the  yellow coat color gene. This trait is controlled by a single gene with two alleles: A⁺ (normal wild-type allele) Aʸ (mutant yellow allele) The  Aʸ allele  causes  yellow coat color  and is dominant for this trait. But if ...
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What are non-coding genes? Give examples

Non-coding genes are segments of DNA that do not code for any protein but still play very important roles in the cell. These genes are transcribed into functional RNA molecules instead of mRNA. These RNAs do not get translated into proteins but perform regulatory, structural, or catalytic roles directly as RNA. Non-coding genes are a major part of the eukaryotic genome and help in many cellular processes such as gene regulation, RNA processing and maintaining genome stability. Many people think that only protein-coding genes matter, but non-coding genes also play a crucial role in regulating cellular activities. They help control when and how genes are turned on or off, assist in processing other RNAs and participate in building cell machinery like ribosomes and spliceosomes. There are several important types of non-coding genes, which can be grouped based on the RNA they produce and their functions. The main categories are: 1. rRNA Genes (Ribosomal RNA Genes) These genes produce ribos...
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What are the differences between gene enhancers and gene silencers? How do enhancers and silencers regulate eukaryotic gene expression?

Enhancers and silencers are two important types of regulatory DNA sequences that play opposite roles in controlling gene expression in eukaryotic cells. They are non-coding DNA elements which do not produce proteins but control when, where and how much a gene is expressed. They work by interacting with transcription factors and RNA polymerase to either increase or decrease the level of transcription. Differences Between Gene Enhancers and Gene Silencers Gene enhancers and gene silencers are both regulatory DNA elements found in eukaryotic genomes. Their main role is to control the level of gene expression, but they work in opposite directions. The differences between them can be explained based on the following criteria: 1. Based on Function Enhancers   increase  the transcription of a gene. They make the gene more active and allow it to produce more RNA. On the other hand,  silencers   reduce  or  completely block  transcription. They stop or decrease...
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Write the significance of Km and Vmax in enzyme activity

Enzymes are specialized biological molecules that play a critical role in accelerating biochemical reactions. They function by lowering the activation energy required for substrate conversion into products. The study of enzyme kinetics provides a quantitative understanding of how efficiently enzymes work under various conditions. Two of the most important parameters that describe enzyme activity are the  Michaelis constant (Km)  and the  maximum reaction velocity (Vmax),  both derived from the  Michaelis-Menten equation.  These parameters help in understanding enzyme efficiency, substrate binding strength, catalytic turnover, metabolic regulation and response to inhibitors. 1. Significance of Km (Michaelis Constant) The Michaelis constant (Km) is a fundamental concept in enzyme kinetics that represents the substrate concentration at which an enzyme-catalyzed reaction proceeds at  half of its maximum velocity (Vmax/2).  It is a crucial indicator of...
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