Define the following terms: I band, A band and H-zone

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 end of one thick filament in a sarcomere to the beginning of a thick filament in the adjacent sarcomere. This band is bisected by the Z line (or Z disc), which serves as the point of anchorage for the thin filaments from two neighboring sarcomeres. During muscle contraction, the I band shortens significantly as the thin filaments slide over the thick filaments, bringing the Z lines closer together.

2. A Band (Anisotropic Band):

The A band is the dark band seen in a sarcomere and represents the entire length of the thick filaments (myosin). This region also includes areas where thick and thin filaments overlap, which contributes to its darker appearance. Unlike the I band, the A band remains unchanged in length during contraction because the thick filaments do not change their length. The central part of the A band contains only thick filaments (forming the H-zone), while the lateral parts include overlapping thin filaments.

3. H-Zone (Hensen's Zone):

The H-zone is the lighter central part of the A band where there is no overlap between thick and thin filaments. It contains only thick filaments (myosin) and is visible only when the muscle is in a relaxed state. During muscle contraction, as the actin filaments slide inwards, the H-zone becomes narrower or even disappears because thin filaments move into this region and begin to overlap with the thick filaments.






Comments

Post a Comment

Popular posts from this blog

Elucidate the basic structure of nucleosome

Image
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...
Read more

What are the characteristics of sclerenchyma cells?

Image
Sclerenchyma is a type of simple permanent tissue found in higher plants, mainly responsible for providing mechanical strength and support. These cells are specially adapted to offer rigidity to plant parts that are no longer elongating or growing. The word "sclerenchyma" is derived from the Greek word scleros, meaning  "hard",  which reflects the tough nature of these cells. The following are the key characteristics of sclerenchyma cells: 1. Dead at Maturity: Sclerenchyma cells are non-living when they reach maturity. They lose their protoplasm, making them metabolically inactive. This feature is important because their primary role is to provide support, not participate in physiological processes. 2. Highly Thickened Cell Walls: One of the most important features of sclerenchyma is the presence of very thick secondary cell walls. These thick walls are uniformly thickened and are rich in  lignin,  a complex and hard organic substance that adds rigidity and impermea...
Read more

PYQ – MZO-002: Genetics and Animal Biotechnology (Solved Q&A) | MZO-002 | MSCZOO | M.Sc.Zoology | IGNOU | December 2024

Image
M.Sc. (Zoology) (MSCZOO) Term-End Examination December, 2024 MZO-002 : GENETICS AND ANIMAL BIOTECHNOLOGY Time : 2 Hours| Maximum Marks : 50 Note:  Attempt any five questions.  All questions carry equal marks. 1. (a) Discuss the different categories of transcription factors. (5 Marks) Transcription factors are proteins that help in controlling gene expression by regulating transcription. They do not make RNA themselves but guide RNA polymerase to start or stop the process. They are important in development, cell cycle, stress response and hormone signaling. Transcription factors are mainly divided into two categories: 1. Based on Their Function There are two types: a) General Transcription Factors These transcription factors are needed for the transcription of almost all protein-coding genes. They help in forming the transcription initiation complex near the promoter region. They guide the RNA polymerase II to attach at the correct place and begin transcription. These factors a...
Read more

What is a Cell? What Are the Essential Characteristics of Cells?

Image
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. Discovery of the First Living Cell The first living cell was discovered by A...
Read more

What is the cortical cytoskeleton?

The cortical cytoskeleton is a specialized and dynamic part of the cell's cytoskeleton located just beneath the plasma membrane. It is primarily composed of  actin filaments  (microfilaments) along with associated proteins like spectrin, ankyrin, filamin and ERM (ezrin, radixin, moesin) proteins. This region forms a dense network of filamentous proteins that help maintain the cell's shape, provide mechanical support and regulate interactions with the external environment. The cortical cytoskeleton plays a key role in cellular processes like endocytosis, cell motility, signal transduction and adhesion, especially in animal cells. Structure of the Cortical Cytoskeleton The main structural element of the cortical cytoskeleton is  F-actin,  which forms a thin, mesh-like layer closely attached to the inner surface of the plasma membrane. This layer is cross-linked by actin-binding proteins such as filamin and is anchored to membrane proteins via adaptor proteins like spec...
Read more

Which tissue occurs in the outermost cell layer of plant organs?

Image
In all primary plant organs such as roots, stems, leaves, flowers and fruits, the outermost protective covering is formed by the  epidermal tissue system.  This system is a collective term used to describe the outer cell layers that protect the internal tissues of the plant. The  epidermis  is the main and most prominent component of this system. In simpler terms, we can say that epidermal tissue is the entire tissue system, while the epidermis is the actual single outer layer of cells that we directly observe in plant organs. The epidermal tissue system not only includes the epidermis, but also contains various specialized structures like stomata (with guard cells), trichomes (hair-like projections) and root hairs, each of which plays a functional role in plant physiology. It forms the first line of defence of the plant body. Structure of Epidermis The epidermis generally consists of a  single  layer of  parenchymatous cells,  which are flat, com...
Read more

Write a brief account on the origin of mitochondria and chloroplast

Image
Mitochondria and chloroplasts are essential organelles in eukaryotic cells. Mitochondria are involved in aerobic respiration and ATP production, while chloroplasts are the site of photosynthesis in plant and algal cells. The widely accepted explanation for their origin is the  Endosymbiotic Theory. The earliest idea related to the symbiotic origin of plastids was proposed by  Konstantin Mereschkowski  in 1905, who suggested that chloroplasts evolved from  autotrophic cyanobacteria  like ancestors through a process of symbiogenesis. Later in the 1920s,  Ivan Wallin,  an American biologist, extended this idea to mitochondria, proposing that they originated from  aerobic bacteria.  However, these early ideas received little acceptance at that time. A major revival and refinement of the endosymbiotic theory came with  Lynn Margulis  in 1967, who in her seminal paper  "On the Origin of Mitosing Cells"  provided detailed evidenc...
Read more

PYQ – MZO-004: Systematics, Biodiversity and Evolution (Solved Q&A) | MZO-004 | MSCZOO | M.Sc.Zoology | IGNOU | December 2024

Image
M.Sc. (Zoology) (MSCZOO) Term-End Examination December, 2024 MZO-004 : SYSTEMATICS, BIODIVERSITY AND EVOLUTION  Time : 2 Hours| Maximum Marks : 50 Note: (i)  Attempt any five questions.            (ii) All questions carry equal marks. 1. Explain the fundamental principles of evolution that are the foundations of modern evolutionary biology. (10 Marks) Modern evolutionary biology is based on certain basic principles that explain how populations change over time and how new species originate. These principles help us understand the mechanisms behind evolutionary processes. Though evolution can involve many complex interactions, there are five main principles that form the foundation of this field: 1. Genetic Variation Genetic variation is the raw material for evolution. It means the differences in DNA among individuals of a population. These differences arise due to mutation, recombination during meiosis and sometimes gene flow from other population...
Read more

Define the term: "rigor mortis"

Rigor mortis is a  postmortem physiological  phenomenon in which the muscles of a dead body become stiff and rigid due to a biochemical condition that prevents the relaxation of muscle fibers. It is directly related to the structure and function of sarcomeres, especially the role of actin and myosin filaments in muscle contraction. Under normal living conditions, muscle contraction is an active process that requires ATP for both the contraction and relaxation phases. After death, the production of ATP ceases completely, leading to an irreversible binding between the actin and myosin filaments in the sarcomere, causing the muscles to remain in a contracted state. This permanent cross-bridge formation without ATP results in the stiffness that characterizes rigor mortis. The process of rigor mortis begins approximately 2 to 6 hours after death, depending on environmental conditions like temperature, cause of death and metabolic activity at the time of death. The stiffness first a...
Read more

Solenoid Model

Image
The solenoid model is a well-known hypothesis that explains the  secondary level of DNA packaging  in eukaryotic cells. After the  primary level of compaction,  where DNA is wrapped around histone proteins to form nucleosomes, further folding is required to fit the large eukaryotic genome inside the nucleus. In 1976, scientists  Finch and Klug  proposed the solenoid model to describe how nucleosomes organize into a higher-order helical structure known as the  30 nm chromatin fiber.  This model plays a crucial role in making chromatin more compact, yet still accessible for essential processes like replication and transcription. Structure In the solenoid model, nucleosomes are arranged in a spiral or helical fashion forming a hollow tube-like fiber. About  six  nucleosomes are present per turn of the solenoid. The linker DNA, which connects each nucleosome, bends in such a way that it helps the nucleosomes to pack closely. Histone  H1...
Read more