Cell Nucleus and Cell Division

The Nucleus

• Contains DNA compacted into chromosomes
• DNA is transcribed into RNA in the nucleus
• In eukaryotes, transcription and translation are decoupled, with transcription occurring in the nucleus and translation occurring in the cytoplasm

Cell Division

• Mitosis is nuclear division, while cytokinesis is cellular division
• Mitosis can occur without cytokinesis, resulting in a cell with multiple nuclei (syncytium)
• In muscle cells, mitosis occurs without cytokinesis, producing multiple nuclei in one cell

Structure of the Nucleus

• The nucleus is a membrane-bound sac held together by an intermediate filament called the nuclear lamina
• The nucleus has a double membrane, with a perinuclear space between the membranes
• The nucleoplasm communicates with the cytoplasm through nuclear pores
• Dark patches in the nucleus are heterochromatin, while light patches are euchromatin

Chromatin

• Found during interphase, chromatin is the material that chromosomes are composed of (containing DNA, RNA, and protein)
• Can be highly compacted (heterochromatin) or loosely compacted with an open structure (euchromatin)
• Chromatin can have two states: when stored, it is compacted as heterochromatin, and when used, it is turned into euchromatin

The Nuclear Envelope

• The nucleus is 5-20µm in diameter and is demarcated by the nuclear envelope
• Inside the envelope is the nucleoplasm
• The perinuclear space is between the inner and outer membranes of the nuclear envelope and is continuous with the inside of the ER

Endoplasmic Reticulum

• Smooth ER is where membrane synthesis occurs
• Rough ER is one of the places where protein synthesis takes place

Nuclear Lamina

• The nuclear lamina is a mesh of fibers that supports the nuclear envelope
• Lamins are a type of intermediate filament composed of polymerized sub-units
• During mitosis, the nuclear lamina network must break down, but reforms at the end of mitosis
• Phosphorylation and dephosphorylation control the assembly and disassembly of lamin fibers

Nuclear Pores

• The interior of the nucleus (nucleoplasm) communicates with the cytosol via protein-bound nuclear pores
• Nuclear pores allow the free diffusion of small molecules but larger molecules must be targeted and actively transported
• Each pore has 8 or 9 separate channels that allow the diffusion of small molecules between the nucleoplasm and the cytoplasm

Transport through the Nuclear Pores

• RNA must be exported out of the nucleus, and proteins must be imported into the nucleus
• Proteins to be imported into the nucleus have Nuclear Localization Signals (NLS)
• NLS are the first 20 bases of a protein that provides a signal telling a protein to be taken into or out of the nucleus

The Nucleolus

• Site of transcription of rRNA
• A round structure present in the nucleus, with several nucleoli per nucleus
• The primary function of the nucleolus is the transcription of rRNA
• rRNA is ribosomal RNA and is an important part of the ribosome, which translates mRNA into protein

DNA in the Nucleus

• DNA is compacted with proteins, wrapped around histone proteins into chromosomes
• Inactive heterochromatin is tightly packed, while active euchromatin is loosely packed
• Histones help compact DNA, representing the lowest level of DNA compaction
• Histone modification has been implicated in the control of gene expression

Chromosomes

• Can become visible to light microscopy during the mitotic process
• Chromosomes can be counted during metaphase
• Humans have 22 pairs of chromosomes and 1 pair of non-homologous sex-determining chromosomes

Function

• Many organisms have more than one set of chromosomes
• Many single-celled organisms carry a single set of chromosomes (haploid), while most multi-cellular organisms carry two sets of chromosomes (diploid)
• The sets of chromosomes can have different alleles
• A gene can be the same on the different copies of the chromosome (homozygous), or different versions on different copies of a chromosome (heterozygous)

Transcription in the Nucleus

• The nucleus holds the genetic material (DNA)
• Active DNA is transcribed (copied into RNA)
• RNA is processed in the nucleus prior to export to the cytoplasm, where it is translated into protein

RNA and DNA

• Both are nucleic acids
• DNA is double-stranded and holds the genetic information, while RNA is a single-stranded copy of parts of the DNA
• RNA has a 2' OH on the ribose ring, which DNA lacks, and instead of thymine, RNA has the base uracil