Genome Structure and Chromatin in Eukaryotes and Prokaryotes

Genome Types: Prokaryotic vs. Eukaryotic

• Terminology Clarification:

  • The term "prokaryotic" is considered dated. While historically meaning "before nucleus" and referring to organisms lacking a nucleus, the more accurate term for the group often discussed is bacteria.
  • Archaea is the other prokaryotic group but will be largely ignored due to their unique, transitional, and complex nature.
  • The primary focus will be on bacteria and eukaryotes.

• Model Organisms:

  • Much of our molecular genetic knowledge of bacteria comes from studying E. coli.
  • Much of our molecular genetic knowledge of eukaryotes comes from studying yeast, which are single-celled eukaryotes and serve as an excellent model.
  • While many principles discussed are fundamental and similar across groups (e.g., replication, transcription, translation), it's important to remember that not every process has been studied in every organism, so some differences will exist.

• Key Differences in Genomes:

  • Size: Bacterial genomes are generally small compared to eukaryotic genomes.
  • Number of DNA Pieces:
    • Bacteria typically have a single, small piece of DNA (one chromosome).
    • Eukaryotic genomes, being much larger, are broken into multiple pieces of DNA (chromosomes).
  • Shape:
    • Bacterial DNA is circular, meaning it has no ends. The two antiparallel strands are wrapped around and attached to each other.
    • Eukaryotic DNA pieces are linear, meaning they have distinct ends.
  • Aside from these major differences, the fundamental DNA structure (double-stranded, antiparallel, directionality) is largely the same.

Eukaryotic Chromosomes and Chromatin

• Definition of Chromatin:

  • Chromosomes are made of chromatin, which is simply DNA plus associated proteins.
  • This discussion focuses on eukaryotic cells (plants, fungi, animals).

• DNA Packaging Challenge:

  • The amount of DNA in eukaryotic cells is immense; for example, the DNA from a single human cell, stretched end-to-end, measures $2$ meters.
  • The total DNA from all human cells could reach to the moon and back.
  • To fit this vast amount of DNA into the tiny nucleus, it must be tightly packaged with proteins.

• Functions of Chromatin:

  • Packaging: Allows $2$ meters of DNA to fit into the nucleus.
  • Stabilization: Proteins help stabilize the DNA structure.
  • Protection: Guards the DNA from degradation.
  • Organization: Helps organize the DNA within the nucleus.

• Chromatin Proteins:

  • All proteins associated with DNA in chromatin are DNA-binding proteins because their function involves binding to DNA.
  • The black looped structure in diagrams represents DNA, while the core structures it wraps around are histones (a specific type of DNA-binding protein).

Chromatin Compaction and the Cell Cycle

• Variable Compaction:

  • The level of chromatin compaction (how tightly packed the DNA is) is not constant; it changes throughout the eukaryotic cell cycle.
  • Compaction levels reflect where the cell is in its cycle and can also reflect gene expression (different genome parts can be packaged differently at different times).

• Compaction for Cell Division:

  • DNA becomes most compacted just before a cell enters mitosis (cell division).
  • Analogy: Imagine trying to separate long, sticky strands of spaghetti; they get tangled and might break. Similarly, long, uncompacted DNA strands would be difficult to separate accurately.
  • High compaction ensures that chromosomes, once replicated, can be separated into daughter cells as individual, tightly packed units, preventing errors in chromosome distribution.
  • This is why chromosomes are often depicted as compact 'X' shapes in illustrations, especially visible under a basic microscope when stained during the mitotic phase.

Regulation by Chromatin

• The degree of chromatin compaction and the specific proteins associated with DNA directly influence transcription (gene expression) and replication.
• Cells precisely control these processes, as they don't occur randomly.

Proteins: A Molecular Overview

• Proteins: Often called the "workhorse of the cells," proteins are a major class of biological macromolecules.
• Polymers and Monomers:
  • Proteins are polymers (also called polypeptides).
  • Their monomers are amino acids.
• Amino Acid Structure:
  • Each amino acid has a central carbon atom bonded to:
    • An amino group ($NH_2$)
    • A carboxyl group ($COOH$)
    • A hydrogen atom
    • A unique R group (side chain), which determines the specific characteristics of each of the $20$ different amino acids. This R group is analogous to the bases in DNA.
• Polypeptide Formation:
  • Amino acids are linked together by peptide bonds (covalent bonds) through a dehydration synthesis reaction (releasing water).
  • A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next.
• Directionality: Similar to nucleotides, amino acids have distinct ends, giving polypeptides directionality with an amino end and a carboxyl end.

Histones and Nucleosome Structure

• Histones - Eukaryotic Feature:

  • Histones are proteins found exclusively in eukaryotic cells; bacterial cells lack histones.
  • They form a core structure around which DNA is wound for packaging.

• Nucleosome Core Particle (Histone Octamer):

  • The functional core unit around which DNA wraps is called a nucleosome core particle.
  • It is an octamer, meaning it consists of eight individual histone polypeptides.
  • These eight polypeptides are: two Histone H3s, two Histone H4s, two Histone H2As, and two Histone H2Bs.
  • Assembly: Histone H3 and H4 first associate to form a tetramer (containing two H3s and two H4s). H2A and H2B also associate, and then all these components assemble to form the complete octamer.

• DNA Winding:

  • Approximately $147$ base pairs of DNA are wrapped around each histone octamer (nucleosome core particle).

• Linker DNA:

  • The stretches of DNA found between individual nucleosomes are known as linker DNA.

• Beads-on-a-String Model (10 nm Fiber):

  • The arrangement where nucleosomes (the "beads") are connected by linker DNA (the "string") is known as the beads-on-a-string model of chromatin.
  • This structure is also referred to as the $10$ nanometer fiber because the diameter of the nucleosomes and the overall fiber in this state is approximately $10$ nanometers.