Chromatin, Chromosomes, and Cell Membranes: Structure and Function

Posted on Nov 29, 2024 in Biology

Chromatin and Chromosomes

Chromosomes are composed of DNA and proteins, forming the complex called chromatin.

Genetic Components

  • Genes: Determine individual characteristics.
  • Genome: A complete set of DNA containing information to synthesize proteins (approximately 30,000).
  • Karyotype: An ordered arrangement of chromosomes.
  • Homologous Chromosomes: One pair of parental and one paternal chromosome.

Chromosome Structure

A chromosome consists primarily of centromeres. Types include:

  • Metacentric: Centromere in the middle, resulting in equal arm lengths.
  • Submetacentric: One arm is longer than the other.
  • Acrocentric: One short arm (P) and one very long arm (Q).
  • Telocentric: Only one visible arm, centromere at the end.

Interspersed DNA, often called junk DNA, includes repetitive sequences. Chromosome 22, one of the smallest, represents 1.5% of the human genome and contains about 40 genes.

Nucleoplasm

Nucleoplasm is a gel-like substance within the nucleus, containing water, proteins, RNA, and ions. It lacks microtubules and microfilaments. Processes like RNA synthesis and DNA replication occur here.

Chromatin Components

  • Histones: Primary, low molecular weight proteins in equal proportion to DNA.
  • Non-histones: A diverse group of proteins, mostly enzymes involved in transcription and DNA replication.

Chromatin Types

  • Euchromatin: Less condensed chromatin, where genetic information is transcribed.
  • Heterochromatin: Inactive chromatin, located at the nucleus periphery.
    • Constitutive: Lacks genetic information, representing 10-20%.
    • Optional: Contains genes that are not expressed (80-90%).

Nucleosome

A nucleosome consists of eight histone proteins. It is released from chromatin by nuclease digestion. A 146-nucleotide-pair length of DNA wraps around the histone core 1.65 times.

Biological Membranes

Biological membranes are primarily composed of lipids, proteins, and fewer carbohydrates, varying by cell type and function. They are crucial for energy conversion processes like photophosphorylation and oxidative phosphorylation.

Membrane Features

  • Selectively exchange materials.
  • Participate in receiving external stimuli and generating signals.
  • The extracellular matrix provides protection and intercellular cooperation.

In 1972, Singer and Nicolson proposed the fluid mosaic model:

  • A) Membranes are fluid and asymmetric.
  • B) Components form a mosaic.
  • C) Composed of a lipid bilayer, proteins, and carbohydrates.

Membrane Proteins

  • Integral Proteins: 70%, partially or completely cross the membrane.
  • Peripheral Proteins: Located on the inner surface, including enzymes bound to integral proteins or lipids.

Carbohydrates on the outer membrane form the glycocalyx, covalently bonded to proteins and lipids.

Lipid Bilayer

The lipid bilayer is a selective barrier, 4-5 nm thick, composed of phospholipids, sphingolipids, and cholesterol. These are amphipathic molecules.

  • Cholesterol: Acts as a “fluid damper,” preventing crystallization at low temperatures and reducing fluidity at high temperatures.

Self-assembly: In aqueous environments, lipids form bilayers with hydrophobic tails hidden and hydrophilic heads exposed.

Self-sealing: Membranes spontaneously close, forming airtight compartments.

Fluidity: Diffusion, flexion, and rotation are caused by hydrophobic interactions and temperature changes. Factors increasing fluidity include unsaturated fatty acids, low cholesterol, high temperature, and short hydrocarbon tails. Factors increasing viscosity include high saturation, long hydrocarbon tails, and low temperature.

Membrane Dynamics

  • Dissemination and protein interaction.
  • Lipid distribution after synthesis.
  • Membrane fusion.
  • Equitable distribution of molecules during cell division.

Protein Function

  • Transport of nutrients, ions, and metabolites.
  • Connect molecules across the membrane.
  • Receive and transmit chemical signals.
  • Catalyze reactions as enzymes (proteins comprise 50% of the membrane, with 50 lipids per protein molecule).

Membrane Carbohydrates

Located on the outer surface of eukaryotic cells, comprising 2-10% of dry weight. Function:

  • Protect the cell surface.
  • Mediate cell recognition and adhesion.
  • Provide negative charges to attract cations and water.

Immunological Properties: Blood group differences are determined by oligosaccharides on red blood cell membranes, differing only in terminal monomers.