Lecture 10 (nucleus) Flashcards
Nucleus
The most prominent organelle (5-10 microns in diameter)
One nucleus per cell, in most cases (for example no nucleus in RBC in blood but before it was in blood it had one and it abandoned it before it became a part of the circulatory system
Contains most of the cell’s genes (genetic information is inherited therefore preserving DNA is very important)
Serves as repository of inherited genetic information
Some additional genes are found in the mitochondria and chloroplasts
The cell’s control centre
What is the nucleus surrounded by?
The nucleus is surrounded by the nuclear envelope (membrane that surrounds the nucleus) which is composed of two membranes (two seperate phospholipid bilayers). Each of the membranes is a phospholipid bilayer. There are two seperate phospholipid bilayers because what is inside the nucleus is incredibly valuable (holds the genes for future generations) therefore they are there as safety measures
The nuclear pore complex
The function of the nuclear pore complex is to help transport molecules back and forth between the nucleus and the cytoplasm. In addition, the nuclear pore complex also serves as a barrier between the cytoplasm and nucleus to prevent harm to genetic material housed in the nucleus.
Lots of proteins that come together to form a complex
Controls the movements of molecules out of, or into the nucleus (it is a regulated process)
OUT (into the cytoplasm and the surrounding cell) - mRNA, tRNA and ribosomal units
mRNA (messenger RNA) carries information from a gene
tRNA (transfer RNA) and ribosomal subunits are needed to build proteins
IN - controls signals (when to turn a gene on or off), building materials (the building ‘blocks’ for RNA) and energy (energy for chemical synthesis)
Nuclear lamina
Inner surface of the nuclear envelope is lined by the nuclear lamina. It is a meshwork of fibres that are supporting the nucleus. The nuclear lamina is composed of intermediate filaments (which differ from microfilaments/microtubules as they are made to last and be very stable). This helps the nucleus maintain it’s spherical shape. It helps organise the packing of the DNA within the nucleus (spherical structure helps to maintain the organisation of DNA within the nucleus)
What happens if the nuclear lamina is defective?
This occurs in certain genetic disorders resulting in a mutated form of lamin protein (this protein makes intermediate filaments which makes the nuclear lamina)
Cells cannot produce normal nuclear lamina
Thus their nuclei are unable to retain a spherical shape
This limits the ability of affected cells to divide
This results in abnormal features (and premature ageing as the reason you go wrinkly is because cells are not dividing properly)
The nucleolus
The nucleolus is a round body located inside the nucleus of a eukaryotic cell. It is not surrounded by a membrane but sits in the nucleus. The nucleolus makes ribosomal subunits from proteins and ribosomal RNA
A prominent nuclear structure within non-dividing cell
May be two or more nucleoi per cell depending on species and stage of cell cycle
The nucleolus is responsible for making ribosomal RNA which combines with proteins to produce ribosomes
DNA organisation within the nucleus
DNA is a nucleotide polymer (made up of nucleotides strung together)
Each human cell contains about 2.5m of DNA. 250,000 times the diameter of the nucleus therefore DNA must be very well packed but must also be accessible.
The DNA double helix is about 2nm in diameter
The helix interacts (twists around) with specific proteins known as histones (H2-H4) - This process allows the DNA to be packed in but also accessible. This interaction forms a 10nm diameter fibre (takes the appearance of a chain with beads on a microscope)
Each ‘bead’ is called a nucleosome
Further interactions between the DNA and another histone (H1) cause the 10nm fibre to coil to form the 30nm fibre (forms a fibre with a greater diameter)
This 30nm fibre then loops to form a 300nm fibre
During cell division (a time when the DNA needs to be packed more tightly) the 300 nm fibres coil to form metaphase chromosomes
These can be displayed as a karyotype which can be used to screen for chromosomal defects (disorders) as well as sex determinance
Diameter of DNA double helix
2nm
Histone
A small protein with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in chromatin structure.
How much DNA is within the nucleus?
Each human cell contains about 2.5m of DNA. 250,000 times the diameter of the nucleus therefore DNA must be very well packed but must also be accessible.
Euchromatin
Less dense, contains genes being used by that cell
Heterochromatin
More dense, contains genes not being used by that cell. (genes are paced more tightly as they do not need to be used/they are not related to the cells function)
Euchromatin and heterochromatin are in a
Dynamic relationship between euchromatin and heterochromatin. Things can change therefore dynamic, for example if the cell needs to do something different
Cancer cells can have …
Very disorganised chromosomes (chromosomes have been broken up and joined incorrectly which is one of the main reasons why cancer cells find it so difficult to control what’s happening to them)
Karyotype
A karyotype is the number and appearance of chromosomes, and includes their length, banding pattern, and centromere position.
