The Eukaryotic Cell: Genetic Instructions, the Nucleus, and Ribosomes

Concept 4.3: The Eukaryotic Cell’s Genetic Instructions

Overview

Eukaryotic cells contain their genetic instructions within the nucleus, and these instructions are carried out by ribosomes. This section explores the structure and function of the nucleus and ribosomes, which are central to genetic control and protein synthesis in eukaryotic cells.

The Nucleus: Information Central

Structure and Function of the Nucleus

• Nucleus: The nucleus is the most prominent organelle in most eukaryotic cells, typically about 5 μm in diameter. It contains most of the cell’s DNA, which encodes genetic information.

• Location of Genes: While the nucleus contains most genes, some are also found in mitochondria and chloroplasts.

• Nuclear Envelope: The nucleus is enclosed by a double membrane called the nuclear envelope, which separates its contents from the cytoplasm. The envelope consists of two lipid bilayers with a space of 20–40 nm between them.

• Nuclear Pores: The nuclear envelope is perforated by nuclear pores (about 100 nm in diameter), which regulate the entry and exit of molecules such as proteins, RNAs, and large complexes.

• Nuclear Lamina: The inner surface of the nuclear envelope is lined by the nuclear lamina, a network of protein filaments that maintains the shape of the nucleus.

Organization of Genetic Material

• Chromosomes: DNA is organized into discrete units called chromosomes. Each chromosome consists of a single DNA molecule associated with proteins (mainly histones), forming a complex called chromatin.

• Chromatin: Chromatin appears as a diffuse mass when the cell is not dividing, but condenses to form visible chromosomes during cell division.

• Species Variation: Each eukaryotic species has a characteristic number of chromosomes. For example, human somatic cells have 46 chromosomes, while human sex cells (eggs and sperm) have 23.

Nucleolus: Site of Ribosome Production

• Nucleolus: A prominent structure within the nucleus, the nucleolus is the site where ribosomal RNA (rRNA) is synthesized and combined with proteins to form ribosomal subunits.

• Ribosome Assembly: Ribosomal subunits are assembled in the nucleolus and then transported to the cytoplasm, where they combine to form functional ribosomes.

Flow of Genetic Information

• Transcription: The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions in the DNA.

• Translation: The mRNA exits the nucleus through nuclear pores and is translated by ribosomes in the cytoplasm into polypeptides (proteins).

Ribosomes: Protein Factories

Structure and Function of Ribosomes

• Ribosomes: Ribosomes are complexes made of ribosomal RNA (rRNA) and protein. They are the cellular components that carry out protein synthesis.

• Types of Ribosomes: Ribosomes can be free in the cytosol or bound to the endoplasmic reticulum or nuclear envelope. Free and bound ribosomes are structurally identical but differ in their cellular location and the proteins they produce.

• Protein Synthesis: Ribosomes translate the genetic code carried by mRNA into a specific sequence of amino acids, forming a polypeptide chain.

• Ribosome Abundance: Cells with high rates of protein synthesis have large numbers of ribosomes (e.g., pancreas cells).

Summary Table: Nucleus vs. Ribosomes

Feature	Nucleus	Ribosomes
Main Function	Stores genetic information (DNA); site of transcription	Protein synthesis (translation)
Structure	Double membrane with pores; contains nucleolus and chromatin	Complex of rRNA and proteins; two subunits (large and small)
Location	Central organelle in eukaryotic cells	Free in cytosol or bound to ER/nuclear envelope
Key Molecules	DNA, RNA, proteins	rRNA, proteins, mRNA (as template)

Key Terms and Definitions

• Nucleus: Membrane-bound organelle containing most of the genetic material in eukaryotic cells.

• Nuclear Envelope: Double membrane that encloses the nucleus, separating it from the cytoplasm.

• Nuclear Lamina: Protein network that supports the nuclear envelope.

• Chromatin: DNA-protein complex that makes up chromosomes.

• Nucleolus: Region within the nucleus where rRNA is synthesized and ribosomal subunits are assembled.

• Ribosome: Molecular machine that synthesizes proteins by translating mRNA.

• mRNA (Messenger RNA): RNA molecule that carries genetic information from DNA to ribosomes.

• rRNA (Ribosomal RNA): RNA component of ribosomes, essential for protein synthesis.

Relevant Equations

• Central Dogma of Molecular Biology:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Example: Protein Synthesis Pathway

• DNA in the nucleus is transcribed into mRNA.

• mRNA exits the nucleus through nuclear pores.

• Ribosomes in the cytoplasm translate the mRNA into a polypeptide chain, forming a protein.