MCB 1: Cells and Their Structural Organisation Flashcards
Describe the basic structure of the nucleus
- the nucleus consists of the nucleoplasm and the nucleolus
- enclosed by a double lipid bilayer, the nuclear envelope
- chromatin (unravelled chromosomes) are suspended in the nucleoplasm
Observe the diagram and learn the different structures of the nucleus
Look at the cross-section of the nucleus and its associated organelles
Label the missing parts
What is the function of the nucleolus?
Describe its structure
- it is the site of production of the subunits of the ribosomes
- it is an aggregate of the clusters of rRNA genes that are present at the ends of different pairs of chromosomes
- 5 pairs in humans: around 300 copies of the rRNA genes
- also contains the synthesised rRNA and proteins being assembled to make the ribosome subunits
Observe the TEM micrograph of the nucleus
What is the darker region?
- darker regions are heterochromatin
- these are regions of condensed DNA
- the DNA in these regions are generally less active
Where can ribosomes be present?
How does their function differ depending on where they are situated?
- free in the cytoplasm:
- synthesise cytoplasmic proteins
- attached to the outer membrane of the nuclear envelope and the endoplasmic reticulum:
- make membrane proteins and proteins to be packaged in membranous organelles
What are polyribosomes?
- multiple ribosomes binding to a single mRNA
- numerous copies of the polypeptide are produced from a single mRNA molecule
- translation starts at the 5’ to 3’ end
- when a stop codon is reached, the ribosome and amino acids dissociated from the mRNA
- see diagram
What are ribosomes composed of?
- two distinct subunits
- each of which contains rRNA (ribosomal) and protein
What is the composition and organisation of ribosomes in these organisms / structures :
- prokaryote
- eukaryote
- mammalian mitochondrial
What does the S unit stand for when talking about ribosomes
What is the relationship between the endoplasmic reticulum and Golgi apparatus?
- as seen on the diagram, there is a physical continuity between the nuclear envelope and the endoplasmic reticulum
What is this green mesh-like structure in the diagram?
Describe its function
- the nuclear lamina
- a specialised type of cytoskeleton formed on the internal surface of the nuclear envelope
- important in controlling the assembly/disassembly of the nuclear envelope in cell division
Describe the rough (granular) ER
- structure
- function
- generally occurs as stacks of flattened membrane leaflets (cisternae)
- studded on its outer face with ribosomes
- site of synthesis of membrane proteins and proteins to be packaged into membrane-bound structure e.g. secretory vesicles, lysosomes
Describe smooth (agranular) ER
- generally more tubular
- lacks attacked ribosomes
- specialises in detoxification and lipid metabolism
- enzymes involved are typically associated with the membranes
- the site of Ca2+ storage in cells
- important for cell-signalling contexts
Describe the golgi apparatus
- structure
- function
- a system of stacked, membrane-bounded flattened sacs
- cis face is aligned to the endoplasmic reticulum
- to receive vesicles from the ER
- trans face toward the cell periphery
- to direct vesicles to the rest of the cell and cell surface
Function:
- involved in modifying, sorting and packaging macromolecules for secretion or for delivery to other organelles
Describe the structure of the mitochondrion
Observe the micrograph: what is this?
- they do not always have the sausage-shape
- they can have elongated, complex, branched morphologies seen in the TEM micrograph
- note the spherical organelles called peroxisomes
What are peroxisomes?
Structure
Function
- present in most eukaryotic cells
Structure:
- enclosed by a single membrane
- contain enzymes involved in lipid and oxygen metabolism
- oxidases, catalases, peroxidases
- the enzymes can be present at such high concentrations that they ‘crystallise’ to form the cores observed in the TEM micrograph
Function:
- important in oxidation reactions
- by-product of the reactions is peroxide
What is the cytoskeleton?
What are the three main types?
- a system of filaments formed by the polymerisation of protein monomers
- three main types:
- microtubules
- intermediate filaments
- microfilaments
Observe these fluorescence micrographs of cytoskeleton organisation in animals cells
Where can I find more basic info on the cytoskeleton?
- check MCB 1 E-modules
Describe how life came to be
- watch the video in MCB 1
What is the thickness of
- human hair
- animal cells
- bacterial cells
What is the thickness of ribosomes and the cellular bilayer?
Compare the eukaryote and prokaryote micrograph
Compare these characteristics of prokaryotes and eukaryotes:
- cell size
- nucleus
- DNA organisation
- membrane-bound organelles
- ribosomes
- flagellae and cilia
What are the two types of prokaryotes?
Briefly describe its structure
- two types:
- bacteria
- archaea
- relatively small, simple-structure cells
- do not contain a nucleus
- DNA arranged as a circular chromosome
- no membranous compartmentalisation of the cytoplasm
Why do bacteria get classified as gram +ve or -ve ?
- depends on their cell wall staining properties
Describe gram-positive bacteria
- have a single lipid bilayer membrane
- have a thick peptidoglycan layer
- they are able to retain the dye during the Gram stain procedure
Describe Gram negative bacteria
- they have two lipid bilayer membranes
- which sandwich a thin peptidoglycan layer
- this fails to retain the Gram stain
What is one of the ways in which bacteria is identified?
- through their microscopic shape and size
Describe the cell wall composition of Archaeon cells
- even though archaeon cells appear very similar in structure to bacteria, the composition of their cells is very different
- except one with a double membrane, like Gram negative bacteria
- their cells walls do not contain peptidoglycan
- they contain other large macromolecules
- archaea membrane phospholipids are very different to those in bacteria and eukaryotes
How did cells evolve?
What factor drove cell evolution?
- watch video on MCB 1
What is endosymbiosis?
- a type of symbiosis in which one organism lives inside the other
- the two typically behave as a single organism
What similarities do mitochondria and chloroplasts in eukaryotic cells have to prokaryotes?
What is the significance of these similarities?
Describe the endosymbiotic origin of mitochondria in eukaryotes
- mitochondria are thought to gave evolved from bacteria that were internalised by primitive eukaryotic cells
- forming a mutualistic symbiosis
- see image for the proposed process
Describe the endosymbiotic origin of chloroplasts in eukaryotes
Why are the circular chromosomes of mitochondria and chloroplasts considerably smaller than those of bacteria?
- they also only contain a fraction of the genes that mitochondria need
- this is believed to be the endosymbiotic bacterial genes being transferred to the cell nucleus and lost from the circular chromosome
- mitochondria and chloroplasts have to import proteins coded by genes in the nucleus
- these genes are descendent of bacterial endosymbiotic genes
Compare the size of the mitochondrial chromosomes of various species (red) to a bacterial chromosome (orange)
Observe this evolutionary diagram of when mitochondria came to be
What evidence supports the theory that eukaryotes have an evolutionary connection to both Archaea and Bacteria?
What is a complication regarding the accepted mechanism of evolution of eukaryotes?
