Lec. 8: The Nucleus, Chromatin and Chromosomes PDF

College of Medicine, University of Mosul/ Department of Anatomy Subject: Medical Biology Stage: 1 st Lec. 8 Title: The Nucleus, Chromatin and chromosomes The nucleus is a vital organelle in animal cells and serves as the control center...

College of Medicine, University of Mosul/ Department of Anatomy Subject: Medical Biology Stage: 1 st Lec. 8 Title: The Nucleus, Chromatin and chromosomes The nucleus is a vital organelle in animal cells and serves as the control center of the cell. It is surrounded by a double membrane called the nuclear envelope, which contains pores to regulate the passage of molecules in and out of the nucleus. The primary function of the nucleus is to store and protect the cell's genetic material, which is in the form of DNA. The nucleus plays a crucial role in gene expression, cell division, and overall cellular function. According to the number of nuclei present, cells are classified as mononucleate (e.g.; smooth muscle cell). Binucleate (e.g.; liver cells, cartilage cells) and polynucleated (syncytial cells) (e.g.; osteoclast). The shape of the nucleus normally related to the shape of the cell, but certain nuclei are almost regular in shape. In a tumor, the presence of nuclei with irregular features (e.g., variable size, atypical chromatin patterns). The capacity to invade neighboring tissues are the main morphologic characteristic used by pathologists to estimate the malignancy. The nucleus is the largest cellular organelle in animal cells. In mammalian cells, the average diameter of the nucleus is approximately 6 (µm), which occupies about 10% of the total cell volume. The viscous liquid within it is called nucleoplasm or karyolymph, and is similar in composition to the cytosol found outside the nucleus. The composition by dry weight of the nucleus is approximately: DNA 9%, RNA 1%, Histone Protein 11%, Residual Protein 14%, and Acidic Proteins 65%. cell-nucleus-anatomy-diagram-illustration-cell-nucleus-anatomy-diagram-121064342 Structure of the nucleus The nucleus of an animal cell is a complex organelle with several key components. Its main compositions include: 1 1. Nuclear Envelope: The nucleus is enclosed by a double-membrane structure known as the nuclear envelope. This envelope consists of two lipid bilayers with nuclear pores that regulate the exchange of molecules between the nucleus and the cytoplasm. 2. Nuclear Pores: These are protein-lined channels in the nuclear envelope that control the transport of molecules, such as RNA and proteins, between the nucleus and the cytoplasm. 3. Chromatin: Chromatin is a complex of DNA and proteins, primarily histones. It's the material that makes up the chromosomes. Chromatin can exist in two forms: euchromatin, which is less condensed and allows for gene expression, and heterochromatin, which is highly condensed and typically repressive. 4. DNA: The nucleus contains the cell's genetic material, which is in the form of DNA. DNA carries the instructions for the cell's structure and function, including the synthesis of proteins and other molecules. 5. Nuclear Lamina: This is a fibrous network of proteins that lines the inner surface of the nuclear envelope. It provides structural support to the nucleus and helps maintain its shape. 6. Nuclear Matrix: The nuclear matrix is a network of protein filaments that helps organize the contents of the nucleus, including chromatin and nucleoli. These components work together to ensure the proper functioning of the nucleus, including the regulation of gene expression, DNA replication, and the protection of the genetic material. The nucleus is often referred to as the "control center" of the cell because of its central role in cellular activities and the storage and management of genetic information. 7- The nucleolus is a distinct substructure found within the nucleus of eukaryotic cells, and it primarily consists of three main components: 1. Fibrillar Center (FC): The fibrillar center is the innermost region of the nucleolus. It contains DNA that codes for the production of ribosomal RNA (rRNA). The FC is the site where initial transcription of rRNA genes occurs. 2. Dense Fibrillar Component (DFC): Surrounding the FC is the dense fibrillar component. It contains factors and enzymes responsible for modifying and cleaving the initial rRNA transcripts to form mature ribosomal RNA. 3. Granular Component (GC): The outermost part of the nucleolus is the granular component. This region contains ribosomal proteins and the partially 2  assembled ribosomal subunits. These subunits are later exported to the cytoplasm, where they combine to form mature ribosomes involved in protein synthesis. Nucleus Nucleosome: A nucleosome is the basic structural unit of chromatin, which is the material that makes up eukaryotic chromosomes. Nucleosomes play a fundamental role in packaging and organizing DNA within the cell's nucleus. They consist of DNA wrapped around a core of histone proteins. Nucleosomes are responsible for regulating access to the genetic information stored in DNA. Composition: The core composition of a nucleosome includes: 1. Histone Proteins: There are four main types of histone proteins that make up the core of the nucleosome: H2A, H2B, H3, and H4. These histones are highly positively charged due to an abundance of positively charged amino acids, such as lysine and arginine. This positive charge allows them to interact with the negatively charged DNA. 2. DNA: The DNA strand wraps around the histone core. The DNA segment is associated with a single nucleosome is approximately 147 base pairs in length. The wrapping of DNA around the histones not only condenses and compacts the genetic material but also protects it from damage. In addition to the core histones and DNA, nucleosomes can also interact with another histone called H1, or linker histone. H1 binds to the DNA between nucleosomes and helps stabilize the overall structure of chromatin. It plays a role in the further compaction of chromatin into higher-order structures. Nucleosomes are stacked along the length of DNA, and they facilitate the efficient packaging of DNA into the confined space of the cell nucleus. Importantly, nucleosomes are not fixed structures; They can be modified and repositioned which is crucial for gene regulation and various cellular processes such as transcription, replication and repair. 3 Chromosomes are single-stranded groupings of condensed chromatin. During the cell division processes of mitosis and meiosis, chromosomes replicate to ensure that each new daughter cell receives the correct number of chromosomes. A duplicated chromosome is double-stranded and has the familiar X shape. The two strands are identical and connected at a central region called the centromere. Human cells contains 46 chromosomes arranged in 23 pairs, each pair are identical with each other. Genetic defects in nuclear envelope Genetic defects in the nuclear envelope can lead to a group of rare and often severe disorders known as nuclear envelopathies or nuclear envelope disorders. These genetic defects typically result from mutations in genes that encode various proteins associated with the nuclear envelope. Some of the most well-known nuclear envelope proteins associated with these disorders include lamins, emerin, and nesprins. Here are a few examples of nuclear envelopathies and their associated genetic defects: 4  1. Emery-Dreifuss Muscular Dystrophy (EDMD):  Genetic Defect: Mutations in the EMD gene, which encodes the protein emerin.  Symptoms: EDMD primarily affects muscle function and is characterized by muscle weakness, joint contractures, and cardiac issues.  2. Laminopathies:  Genetic Defect: Mutations in the LMNA gene, which encodes lamins A and C.  Symptoms: Laminopathies can manifest in various ways, including muscle weakness.. 3. Nesprin-Related Disorders:  Genetic Defect: Mutations in genes such as SYNE1 and SYNE2, which encode nesprin proteins.  Symptoms: These disorders can affect both muscles and the nervous system and are characterized by muscle weakness, cardiac issues, and other neuromuscular symptoms. Summary: The chromatin is the main part of the cell for transmitting the hereditary information. 5