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SAQ 1  i. What is a Cell? What Are the Essential Characteristics of Cells? A cell is the basic structural, functional, and biological unit of all living organisms. It is the smallest unit of life and is often referred to as the  "building block of life."  Cells carry out essential functions such as metabolism, energy production, and reproduction. Organisms can be unicellular (made of a single cell) or multicellular (composed of many cells). Discovery of the Cell Robert Hooke discovered the cell structure in 1665 by observing a thin slice of cork, specifically from the cork cambium of the cork oak tree (Quercus suber). The cork cambium is a layer of tissue found in the bark of plants that produces cork. Under his microscope, Hooke saw small, hexagon box-like compartments, which he named "cells" because they resembled the small rooms. These compartments were the cell walls of dead plant cells, a key observation that laid the foundation for the study of cell biology. D...

The cell is often called the "building block of life," and its discovery has shaped our understanding of biology and life itself. This journey of discovery began centuries ago, driven by curiosity, simple experiments and eventually the invention of more advanced tools. The discovery of cells has transformed how we understand life. It has led to breakthroughs in treating diseases, developing vaccines, and understanding how organisms grow and reproduce. Every living thing, from the smallest bacterium to the largest whale, is built from cells. Early Ideas About Life In ancient times, people had no idea about the microscopic structures that make up living things. Philosophers like Aristotle believed that life consisted of basic elements like earth, water, air and fire. For centuries, the theory of  spontaneous generation,  which suggested that life could arise from nonliving matter, was widely accepted. This theory suggested that life could appear suddenly from non-living matter...

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...

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...

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 ...

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...

In eukaryotic cells, the DNA is extremely long and must be efficiently packed to fit within the confines of the nucleus. This packaging is achieved without tangling or damaging the genetic material with the help of the nucleosome, which is the basic unit responsible for this organization. It is not only responsible for compacting the DNA but also plays an essential role in regulating access to genetic information during processes such as transcription, replication and DNA repair. The nucleosome acts as the  first level  of chromatin organization and serves as a structural and functional framework for further compaction. Core Structure of the Nucleosome The nucleosome consists of two main components:  the core particle and the linker DNA.  The core particle is built around a  histone octamer,  which is made up of two molecules each of histones  H2A, H2B, H3 and H4.  These histones are small, positively charged proteins that interact with the negati...

In genetics, traits can be influenced or expressed differently depending on the sex of the individual. Some traits are linked to sex chromosomes, while others are affected by hormonal or physiological differences between males and females. To describe these traits more precisely, geneticists use three main terms:  sex-linked, sex-limited and sex-influenced traits.  Although these terms may sound similar, they refer to different types of genetic expression related to sex. Understanding the distinction among these three is important for grasping how certain traits are inherited and expressed differently in males and females. 1. Sex-linked characters: These are traits controlled by genes that are located on the  sex chromosomes,  usually on the X chromosome in humans. Because males have only one X chromosome (XY) and females have two (XX), the pattern of inheritance and expression is different in both sexes. Most sex-linked traits are X-linked, and very few are Y-linked...

Lysosomes are small membrane-bound organelles found in most animal cells. They are absent in most plant cells, although plant cells do have similar structures like vacuoles that perform some of the same functions. Lysosomes are an important part of the eukaryotic endomembrane system. These organelles were first discovered by the Belgian scientist  Christian de Duve  in 1955. Lysosomes are known as the  digestive system of the cell,  because they contain hydrolytic enzymes that help in the breakdown of biological substances like proteins, lipids, carbohydrates and nucleic acids. Lysosomes are also called the  "suicidal bags"  of the cell. This is because under certain conditions like cellular damage, stress, or ageing, lysosomes may rupture and release their enzymes into the cytoplasm. These enzymes digest the cell's own components, leading to  autolysis or self-destruction . Types of Lysosomes Functionally, lysosomes can be divided into three main type...

In striated muscle fibers such as skeletal and cardiac muscles, the basic contractile unit is the  sarcomere,  which is the region between two Z-lines. Each sarcomere contains two main types of filaments:  thin filaments  (actin) and  thick filaments  (myosin). These filaments are arranged in a specific, overlapping pattern that produces the characteristic alternating light and dark bands seen under the microscope. Based on the arrangement and degree of filament overlap, the sarcomere displays three main horizontal zones: I band, A band, and H-zone. Understanding these zones is essential for grasping how muscles contract and how structural changes are translated into mechanical force. 1. I Band (Isotropic Band): The I band is the  lighter region  of the sarcomere that contains only  thin filaments  (actin). It does not have any thick filaments, and for this reason, it appears less dense under the microscope. The I band stretches from the...