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Cell Biology Revision:
The nucleus and cytoplasm:
The outside boundary of the mammalian cell is its membrane
Organelles are defined by membranes
...

Functions of the nucleus:
• Safe storage of DNA
o Very long polymers encoding the genetic information for the organism
• Site of DNA replication (DNAàRNA)
o Used in cell division
• Site of transcription (DNAàRNA)
o Used to make mRNA (an intermediate in
protein synthesis) and other RNA
molecules
Enclosed by the nuclear envelope (double membrane)
Outer nuclear membrane is continuous with the ER
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What is the consequence of having a nucleus?
DNA is protected for shearing forces of the cytosol and the DNA can increase in size
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The cytosol:
The part of the cytoplasm which is outside the membrane-bound compartments
• Water based gel
• Contains approximately 20% protein
• Site of chemical reactions (protein synthesis)
• Contains a cytoskeleton
The cytoskeleton:
• Mechanical strength of the cell
• Controls the cell shape
• Drives and guides the movement of organelles
• A system of girders, ropes and motors (Alberts et al)

intermediate filaments

actin filaments

The cytoplasm is not static!
• Parts of the cytoplasm can assemble and disappear in minutes
• Organelles move along the cytoplasm
• Proteins buzz around
Endoplasmic Reticulum:
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•
•
•

Internal membrane network throughout the cell
Continuous with outer membrane of the nuclear envelope
Space inside – lumen
Interconnecting tunnels

There are 2 types of ER
• Rough (with ribosomes)
o Ribosomes are associated with the rER
o Synthesises proteins designed for secretion

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During synthesis protein is transported into the ER lumen

Smooth (without ribosomes)
o No ribosomes associated with it
o Specialised for lipid production

Golgi apparatus:
• Collection of flattened disks stacked like dinner plates
• Each eukaryotic cell has one Golgi body
• Molecules travel through the Golgi stack via transport vesicles
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Mitochondria:
The powerhouse of the cell
• Aerobic production of ATP
Approx
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5 - 1µm across
It contains its own DNA and ribosomes that resemble those of bacteria
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05 – 0
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Cells hold together by cell-cell adhesions, extracellular matrix or both
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g
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g
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Both strands of DNA are replicated!
DNA polymerase – enzyme – polymerisation of deoxyribonucleoside triphosphates (dNTPs)
on the growing DNA strand
Only adds dNTPs on the 3’ end of the growing DNA strand
5’ à 3’ direction
The job of the polymerase:
Need to unwind double-stranded DNA to produce Y-shaped junctions where DNA synthesis
can take place – DNA helicase replication fork!
Problem:
How do we synthesise both strands if DNA polymerase only works in the 5’ à 3’ direction
and the strands are anti-parallel

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