Week 3: The Cell Structure and Organelles

Nuclear Envelope

• The nuclear envelope is perforated by pore structures that are about 100 nm in diameter.
• The inner and outer membranes of the nuclear envelope are continuous at the lip of each pore.
• An intricate protein structure called a pore complex lines each pore and plays a role in regulating the entry and exit of proteins and RNAs, and large complexes of macromolecules.
• The nuclear side of the envelope is lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus.

Chromatin

• The genetic material in the nucleus is called chromatin.
• DNA and proteins form chromatin.
• Chromatin condenses to form chromosomes.
• Some proteins help coil the DNA in each chromosome, reducing the length and allowing it to fit into the nucleus.
• Histones are the name of these proteins.
• A typical human cell has 46 chromosomes in its nucleus.
• Exceptions are the sex cells (eggs and sperm), which have only 23 chromosomes in humans.

Heterochromatin vs. Euchromatin

• Chromatin does not have a homogeneous appearance in most cells.
• Clumps of densely staining chromatin can be found in a more lightly staining background.
• The densely staining material is called heterochromatin and the lightly staining material is called euchromatin.

Nucleolus

• The nucleolus is a small area within the nucleus.
• The nucleolus is a site of rRNA synthesis.
• The nucleolus contains DNA in the form of rRNA genes, RNA and proteins.
• The nucleolus contains regulatory cell cycle proteins.

Endoplasmic Reticulum

• The ER membrane is continuous with the nuclear envelope.
• The ER is divided into two distinct regions: rough ER and smooth ER.

Rough ER

• Rough ER has bound ribosomes, which produce secretory proteins.
• Secretory proteins are mostly glycoproteins, or proteins bonded with carbohydrates.
• Rough ER distributes transport vesicles.
• The rough ER is a membrane factory for the cell.

Smooth ER

• Smooth ER synthesizes lipids.
• Smooth ER metabolizes carbohydrates.
• Smooth ER stores calcium.
• The smooth ER detoxifies poisons and drugs.

Golgi Apparatus

• The Golgi apparatus consists of flattened membranous sacs called cisternae.
• The Golgi apparatus modifies, stores and sends products from the ER.
• The Golgi apparatus manufactures certain macromolecules.
• The Golgi apparatus sorts and packages materials into transport vesicles.
• The Golgi apparatus adds molecular identification tags as address in mailing labels.

Lysosome

• Lysosomes are membranous sacs containing hydrolytic enzymes.
• Lysosomes digest macromolecules.
• Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids.
• Lysosomal enzymes work best in the acidic environment of lysosomes.

Mitochondria

• Mitochondria are autonomous organelles that grow and reproduce within the cell.
• Mitochondria have their own DNA.

Peroxisomes

• Peroxisomes are specialized metabolic compartments.
• Peroxisomes produce hydrogen peroxide and convert it to water.
• Peroxisomes contain enzymes that break down fatty acids.
• Peroxisomes in the liver detoxify alcohol and other harmful compounds.

Centrosome – Centrioles

• In animal cells, microtubules grow out from a centrosome, a region near the nucleus considered a “microtubule-organizing center.”
• Centrosomes function as compression-resisting girders.
• Before an animal cell divides, the centrioles replicate.
• Each centriole is composed of nine sets of triplet microtubules arranged in a ring.
• Centrioles are not essential for microtubule assembly in all eukaryotes (not present in fungi and plant cells).

Explore the structures and functions of the nuclear envelope and chromatin in this quiz. Understand the role of pore complexes, the composition of chromatin, and how DNA is organized within the nucleus. Test your knowledge on these fundamental concepts in cell biology.