Nucleus, Genes & Ribosomes lecture 9

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Lecture 9 Objectives

• Describe the basic structure and function of the nucleus
• Describe the structure of the nuclear pore complex and its role in nucleo-cytoplasmic exchange
• Describe the organisation of DNA within the nucleus
• Outline the functional differences between euchromatin and heterochromatin

Nucleus and Genes

• Genes in the nucleus code for proteins
• RNA carries genetic information out of the nucleus
• Ribosomes translate RNA information into proteins

Ribosomes Composition and Location

• Ribosomes consist of ribosomal RNAs and proteins
• Ribosomes are found in two locations including bound ribosomes and free ribosomes
• Bound ribosomes are attached to rough ER
• Free ribosomes are in the cytoplasm
• Ribosomes carry out translation
• The more protein synthesis a cell undertakes, the more ribosomes it possesses

Nucleus Facts

• The nucleus is a prominent organelle with a diameter of 5–10 µm
• Most cells contain one nucleus
• The nucleus contains most of the cell's genes
• Additional genes reside within mitochondria and chloroplasts
• The nucleus serves as the repository of inherited genetic information
• The nucleus functions as the cell's control centre

Nucleus Structure

• The nucleus is surrounded by the nuclear envelope
• The nuclear envelope contains channels called nuclear pores
• The nucleus contains tightly packaged DNA and a prominent area known as the nucleolus

Nuclear Envelope Composition

• The nuclear envelope is composed of two membranes: outer and inner
• Perinuclear space is locatedbetween the outer and inner membranes
• Each membrane is a phospholipid bilayer
• The outer membrane is continuous with the ER

Nuclear Lamina Composition and Function

• The inner surface of the nuclear envelope is lined by the nuclear lamina
• Nuclear lamina consists of intermediate filaments that maintains nuclear shape
• Nuclear lamina helps organise DNA packing within the nucleus

Defective Nuclear Lamina

• Certain genetic disorders result from defective nuclear lamina
• Defective nuclear lamina results in an altered form of lamin protein
• Defective nuclear lamina prevents cells from producing a typical nuclear lamina
• The result is a nucleus unable to maintain a spherical shape
• The ability of affected cells to divide is limited
• Atypical features and premature aging can result from limited cell division

Nuclear Pores

• Nuclear pores are channels made of proteins called nucleoporins and form the Nuclear Pore Complex
• Nuclear pores span the nuclear envelope
• There are about 1000 nuclear pores per cell
• Nuclear pores control the nucleo-cytoplasmic exchange
• The movement of molecules out of, or into, the nucleus is controlled by the nucleo-cytoplasmic exchange

Nucleus to Cytoplasm Transportation

• mRNA, tRNA, and ribosomal subunits move from nucleus to cytoplasm
• mRNA (messenger RNA) carries information from a gene
• tRNA (transfer RNA) and ribosomal subunits are needed to build proteins

Cytoplasm to Nucleus Transportation

• Control signals, building materials, and energy move from cytoplasm to nucleus
• This includes signals for when to turn a gene on or off
• This includes the building "blocks" for RNA and energy for chemical synthesis

Nucleolus Facts

• The nucleolus is only present in non-dividing cells
• The nucleolus is a prominent nuclear structure
• Cells can have more than one nucleolus
• The nucleolus is specialises and not bound by a membrane
• The nucleolus is within the nucleus
• The nucleolus is responsible for making ribosomal RNA & ribosomal subunits

Nucleolus Assemby and Cell Count

• Ribosomal RNA & ribosomal subunits are transported out of nucleus to be assembled as ribosomes in cytoplasm
• Cells may have two or more nucleoli depending on species and cell cycle stage

DNA Facts

• DNA is a nucleotide polymer
• Each human cell contains about 2.5 m of DNA
• This is 250,000 times the diameter of the nucleus
• Therefore, DNA must be very well packed, but must also be accessible

DNA Organisation

• The DNA double helix has a diameter of about 2 nm
• The helix interacts with specific proteins called histones (H2-H4)
• This interaction forms a 10-nm diameter fibre
• Each 'bead' is called a nucleosome
• Further interactions between the DNA and another histone (H1) cause the 10-nm fibre to coil to form the 30-nm fibre
• The 30 nm fibre then loops to form 300-nm fibre
• During cell division the 300-nm fibres coil to form metaphase chromosomes
• Chromosomes can be displayed as a karyotype
• Karyotypes can screen for chromosomal variants
• During interphase the location of chromosomes within the nucleus isn't random
• Coloured tags can show the location of each chromosome
• The pairs are generally not located together

DNA and Disease

• Diseases like cancer can have very disorganised chromosomes

Euchromatin and Heterochromatin

• Euchromatin is less electron-opaque
• Euchromatin contains genes being used by that cell
• Heterochromatin is more electron-opaque
• Heterochromatin contains genes not being used by that cell
• There is a dynamic relationship between euchromatin and heterochromatin
• Transcriptional machinery needs to access specific DNA regions to begin transcription
• Euchromatin is less tightly packed, so allows access

Lecture 9 Summary

• The nucleus is the control centre of the cell
• The nucleus is surrounded by a double membrane called the nuclear envelope
• The nuclear lamina supports the nuclear envelope structurally
• The nucleus contains most of a cell's genes
• Nuclear pores are channels made by nucleoporins inserted in the nuclear envelope
• Nuclear pores control the movement of molecules in and out of the nucleus
• The nucleus contains a specialised nucleolus, where ribosomal RNA and ribosomal subunits are made; which are then transported to the cytoplasm for assembly
• DNA is packaged tightly around histone proteins forming 10-nm fibres and nucleosomes
• These are further packaged into larger (30 nm) and larger (300 nm) fibres and, during cell division, into chromosomes
• Euchromatin (less tightly packed) contains genes that are being used by the cell
• Heterochromatin (tightly packed) contains genes that are not being used by the cell