Makindo Medical Notes

Related Subjects: |DNA and RNA short notes |DNA replication |DNA structure in Nucleus |Mitosis and Meiosis |Cell Cycle

Nucleus Structure

The cell nucleus is a membrane-bound organelle found in eukaryotic cells. It houses the cell's genetic material, DNA, and coordinates activities such as growth, metabolism, protein synthesis, and cell division. The nucleus is often referred to as the "control center" of the cell.

Structure of the Nucleus

• Nuclear Envelope: A double membrane that surrounds the nucleus, separating it from the cytoplasm.
  • Each layer of the nuclear envelope is a lipid bilayer.
  • The outer membrane is continuous with the endoplasmic reticulum (ER), often studded with ribosomes.
  • The envelope contains nuclear pores that regulate the passage of molecules in and out of the nucleus.
• Nuclear Pores: Protein channels embedded in the nuclear envelope that allow the transport of proteins, RNA, and other molecules between the nucleus and the cytoplasm.
  • Each nuclear pore complex is highly selective, controlling which molecules can enter or leave the nucleus.
  • Small molecules pass freely, but larger molecules require active transport.
• Nucleoplasm: The semi-fluid matrix inside the nucleus, where the nucleolus and chromatin are suspended.
  • Contains dissolved ions, nucleotides, and other small molecules necessary for nuclear function.
• Chromatin: A complex of DNA and proteins (mainly histones) that packages and organizes DNA within the nucleus.
  • Exists in two forms: euchromatin (less condensed, actively transcribed) and heterochromatin (more condensed, transcriptionally inactive).
  • During cell division, chromatin condenses further to form visible chromosomes.
• Nucleolus: A dense, round structure within the nucleus, not membrane-bound.
  • Primary site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
  • Contains DNA, RNA, and proteins specifically related to ribosome production.

Function of the Nucleus

• DNA Storage and Protection: The nucleus protects DNA from damage and maintains its structure for replication and repair.
• Gene Expression Regulation: Controls when and how genes are transcribed into RNA, influencing cell function.
• RNA Synthesis: Transcribes DNA into messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
• Ribosome Production: The nucleolus is the site of ribosome synthesis, which are then transported to the cytoplasm.

Clinical Relevance

• Nuclear Envelope Disorders: Defects in nuclear envelope proteins can lead to diseases such as Emery-Dreifuss muscular dystrophy.
• Genetic Disorders: Abnormalities in chromatin organization or nuclear function can result in genetic diseases and cancers.
• Nucleolus Dysfunction: Disruption in nucleolus function affects ribosome production and is linked to diseases like nucleolar myopathies.

Overview of DNA Structure in the Cell Nucleus

• DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. In eukaryotic cells, DNA is housed within the nucleus. The structure of DNA in the cell nucleus is highly organized, allowing it to fit within the confined space while remaining accessible for replication, transcription, and repair.

Double Helix Structure of DNA

DNA is composed of two long strands that coil around each other to form a double helix. Each strand consists of a backbone made of alternating sugar (deoxyribose) and phosphate groups, with nitrogenous bases attached to the sugar molecules.

• Nitrogenous Bases:
  • Adenine (A)
  • Thymine (T)
  • Cytosine (C)
  • Guanine (G)
• Base Pairing:
  • Adenine pairs with thymine (A-T) via two hydrogen bonds.
  • Cytosine pairs with guanine (C-G) via three hydrogen bonds.

Chromatin Structure

In the cell nucleus, DNA is packaged into chromatin to fit within the limited space and to regulate gene expression. Chromatin is composed of DNA, histone proteins, and non-histone proteins.

• Nucleosome:
  • The basic unit of chromatin.
  • Consists of a segment of DNA wrapped around a core of eight histone proteins (histone octamer).
  • Approximately 147 base pairs of DNA are wrapped around each nucleosome.
• Histones:
  • Proteins that play a crucial role in DNA packaging.
  • Core histones: H2A, H2B, H3, and H4 form the histone octamer.
  • Linker histone: H1 binds to the DNA between nucleosomes, helping to compact the chromatin further.
• Chromatin Fiber:
  • Nucleosomes are coiled into a 30 nm fiber, further compacting the DNA.
  • This fiber can be further looped and folded to form higher-order structures.

Chromosomes

During cell division, chromatin condenses to form chromosomes, ensuring the accurate segregation of DNA to daughter cells. Each chromosome contains a single, continuous DNA molecule.

• Chromatid:
  • One half of a duplicated chromosome, joined to its sister chromatid at the centromere.
• Centromere:
  • A region of the chromosome where the sister chromatids are joined.
  • Plays a crucial role in the movement of chromosomes during cell division.
• Telomeres:
  • Repetitive DNA sequences at the ends of chromosomes.
  • Protect the ends of chromosomes from deterioration and prevent them from fusing with other chromosomes.

Organization of DNA in the Nucleus

The organization of DNA within the nucleus is dynamic and allows for efficient regulation of gene expression and DNA replication.

• Nuclear Envelope:
  • Double membrane structure that encloses the nucleus.
  • Contains nuclear pores that regulate the transport of molecules between the nucleus and the cytoplasm.
• Nucleolus:
  • Dense region within the nucleus where ribosomal RNA (rRNA) synthesis and ribosome assembly occur.
• Nuclear Matrix:
  • Network of fibers within the nucleus that provides structural support and organizes the chromatin.

Functions of DNA in the Nucleus

• Genetic Information Storage:
  • DNA contains the genetic instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses.
• Gene Expression:
  • DNA is transcribed into RNA, which is then translated into proteins. These proteins carry out various functions in the cell.
• DNA Replication:
  • DNA replicates during the S phase of the cell cycle, ensuring that each daughter cell receives an identical copy of the genetic material.
• DNA Repair:
  • Mechanisms exist to detect and repair damage to DNA, maintaining the integrity of the genetic information.

Summary

DNA in the cell nucleus is organized into a highly structured and compact form to fit within the limited space while remaining accessible for essential cellular processes. The double helix structure of DNA, wrapped around histone proteins to form nucleosomes, further condenses into chromatin fibers and chromosomes. This organization ensures efficient regulation of gene expression, DNA replication, and repair, maintaining the integrity of the genetic information crucial for the cell's function and survival.