Nucleus

• Ribosomes are molecular machines found in living cells which are important for protein synthesis, translating genetic material encoded in the messenger RNA (mRNA) into proteins.
• Ribosomes are found in two locations:

  1. Bound ribosomes attached to the rough ER
  2. Free ribosomes in the cytoplasm
• The nucleus is a bound organelle found in the eukaryotic cells. it serves as a control centre for cellular activities. It contains genetic material, conducts gene expression and cellular functions.
• Nucleus

  • 5-10nm diameter
  • one nucleus per cell
• Nucleus Structure

  • Surrounded by nuclear envelope
  • has channels called nuclear pores
  • contains tightly packaged DNA
  • has prominent area called nucleolus
• The nucleus is surrounded by a double membrane layer called nuclear envelope. It consists of 2 lipid bilayers with narrow space between them called perinuclear space.
• The nuclear envelope is perforated by nuclear pores, which allow selective exchange of molecules between nucleus and cytoplasm. Each membrane is a phospholipid bilayer and the outer membrane is continuous with ER.
• The nuclear lamina is inside the nuclear envelope, containing a network of fibrous proteins. The nuclear lamina is composed of intermediate filaments. They provide structural support and helps maintain its shape and help organise packaging of DNA.
• Nuclear pores are channels made of proteins that form the nuclear pore complex. They span nuclear envelope and about 1000 per cell. they control the movement of molecules out or in the nucleus.
• Nucleus to Cytoplasm

  The mRNA and tRNA and ribosomal subunits moves from the nucleus to the cytoplasm.
• mRNA

  It is synthesised through the nucleus by transcription, where the RNA strand is produced from the DNA structure. They are transported using nuclear pores. Once in the cytoplasm, mRNA is translated by ribosomes to produce proteins.
• tRNA

  It is responsible for delivering amino acids to the ribosome during protein synthesis. They are synthesised in the nucleus by RNA polymerase.

  In the cytoplasm, tRNA binds to their corresponding amino acids and deliver them to the ribosome.
• Cytoplasm to nucleus

  It controls signals, building materials and energy from cytoplasm to nucleus. This occurs:

  • when to turn gene on or off
  • for building blocks of RNA
  • energy for chemical synthesis
• The nucleolus is found within the nucleus of eukaryotic cells. It is responsible fro the synthesis an assembly for RNA and formation of ribosomal units, which are transported out of the nucleus and assembled as ribosomes in the cytoplasm.
• DNA double helix is 2nm in diameter
• The helix in dna interacts with specific proteins and histones and forms a 10nm fibre. Each bead is a nucleosome. Further interactions cause the 10nm fibre to coil to form a 30nm fibre, then loops to a 300nm fibre. During cell division, they form metaphase chromosomes.
• Euchromatin is less dense and loosely packed and contains genes being used by the cell. They are associated by active gene expression and regulation of cellular processes.
• Heterochromatin are more dense and contains genes that are not being used by the cell. They are involved with genome stability, chromosome segregation and silencing the repetitive DNA.