3.3 The Nucleus and DNA Replication

The Nucleus

Structure and Function of the Nuclear Membrane

The nuclear membrane (also called the nuclear envelope) is a double membrane that separates the cell's genetic material from the cytoplasm. Think of it as both a wall and a gate: it protects DNA while still allowing specific molecules to pass through.

• Outer membrane is continuous with the rough endoplasmic reticulum, which means the nucleus and the endomembrane system are physically connected. This helps with communication and transport between the nucleus and cytoplasm.
• Inner membrane contains its own unique proteins and lipids that help regulate nuclear processes and maintain structural integrity.
• Nuclear pores punctuate the envelope and allow selective, bidirectional transport of molecules like proteins and RNA. Each pore is regulated by a nuclear pore complex that recognizes specific cargo before letting it through.

The envelope also gives the nucleus its shape and helps it resist mechanical stress from the rest of the cell.

Key Components of the Nucleus

Nucleolus: This dense, spherical structure inside the nucleus is where ribosomal RNA (rRNA) is synthesized and ribosome subunits are assembled. It looks dark and prominent under a microscope because of its high concentration of rRNA and proteins. Since ribosomes are the machinery for protein synthesis, the nucleolus plays a direct role in the cell's ability to make proteins.

Chromatin: This is the combination of DNA and associated proteins (called histones) found in non-dividing cells. Chromatin exists in two forms:

• Euchromatin is loosely packed and transcriptionally active, meaning genes in these regions can be read and expressed.
• Heterochromatin is tightly condensed and transcriptionally inactive, so those genes are essentially "turned off."

The balance between these two forms is one way the cell regulates which genes are expressed at any given time.

Nuclear matrix: A protein scaffold that provides structural support for the nucleus. It also helps organize chromatin and localizes enzymes involved in DNA replication, transcription, and repair.

DNA Organization in the Nucleus

Your cells need to fit roughly 2 meters of DNA into a nucleus only about 6 micrometers across. That requires serious packaging, and it happens in stages:

1. DNA wraps around clusters of histone proteins to form nucleosomes, the basic unit of chromatin. Each nucleosome consists of eight histone proteins (two each of H2A, H2B, H3, and H4). Short stretches of linker DNA connect adjacent nucleosomes, and histone H1 associates with this linker region to help stabilize the structure.
2. Nucleosomes coil together into a thicker 30 nm chromatin fiber (sometimes called a solenoid structure). Interactions between nucleosomes and histone H1 drive this level of compaction.
3. These fibers fold into higher-order loops and domains that attach to the nuclear matrix. This organization both compacts the DNA efficiently and helps regulate which regions are accessible for transcription.

Telomeres cap the ends of each chromosome, protecting them from degradation and preventing chromosomes from fusing with each other. They shorten with each cell division, which is relevant to cell aging.

DNA Replication

DNA Replication Mechanism

Before diving into the steps, a few foundational principles are worth understanding:

• Replication is semiconservative: each new DNA molecule contains one original (parental) strand and one newly synthesized strand. This was demonstrated by the Meselson-Stahl experiment.
• Complementary base pairing (A pairs with T, G pairs with C) guides accurate synthesis. Each parental strand serves as a template for building its complement.
• Nucleotides in the new strand are joined by phosphodiester bonds, which form the sugar-phosphate backbone of DNA.

Steps of DNA Replication

1. Initiation

• Specific proteins recognize the origin of replication, a particular DNA sequence where replication begins.
• DNA helicase unwinds the double helix at this site, creating a Y-shaped structure called the replication fork.
• Single-stranded DNA binding proteins (SSBPs) coat the exposed single strands to keep them from snapping back together (reannealing).

2. Elongation

• DNA primase lays down short RNA primers on each template strand. These primers are necessary because DNA polymerase cannot start a new strand from scratch; it can only add nucleotides to an existing strand.
• DNA polymerase III extends the primers by adding complementary deoxyribonucleotides, always building in the $5' \to 3'$ direction.
  • The leading strand runs toward the replication fork, so it can be synthesized continuously as one long stretch.
  • The lagging strand runs away from the fork, so it must be synthesized in short segments called Okazaki fragments.
• DNA polymerase I then removes the RNA primers and replaces them with DNA nucleotides, filling in the gaps.

3. Termination

• DNA ligase seals the remaining nicks (breaks in the sugar-phosphate backbone) between Okazaki fragments on the lagging strand, creating a continuous double-stranded molecule.
• The result is two identical DNA molecules, each ready to be passed to a daughter cell.

Why DNA Replication Matters

• It ensures each daughter cell receives a complete, accurate copy of the genome during cell division.
• It maintains genome stability, which helps prevent mutations.
• It's essential for growth, tissue repair, and development, since every new cell needs its own copy of the genetic blueprint.