Lecture 4 Flashcards
The nucleus
-discovered by Robert Brown
-contains most of DNA in a eukaryotic cell
functions of the nucleus (7)
-separate DNA from cytosol
-separate transcription from translation
-store genes on chromosomes
-organise genes into chromosomes to allow cell division
-organise the unwilling of DNA to replicate products via nuclear pores
-produce mRNA that coded for proteins
-produce ribosomal RNA in nucleus
Nucleus structures
-nucleolus
-chromatin
-nuclear envelope
-inner membrane
-outer membrane
-nuclear pore
The nuclear envelope
made up of inner membrane, outer membrane and nuclear pore
Nuclear envelope components
-pores regulate the entry and exit of molecules from the nucleus such as proteins
-the inside surface is lined by nuclear lamina
-this is composed of protein filaments to maintain its shape and structure
-inside the nucleus, the nuclear matrix consists of a framework of fibres
lamina
-internal scaffold and support for nucleus made from intermediate filaments (IF)
a network of intermediate filaments that extend over inner surface of nuclear envelope
Defects in the nuclear lamina
the gene that codes for lamina is LMNA, mutations in this gene have been found to be the cause of a rare premature aging disorder called progeria
- mutations result in major shape changes to the cell nuclei
- these changes get worse with age and affect cell division
Nuclear matrix
- non-chromatin/DNA structures in the nuclei
- nuclear matrix appears as fibres attached to granular material
- 10nm fibre nuclear matrix is composed of internal nuclear matrix and nuclear lamina
Nucleoplasm
the non-staining liquid substance of the interphase nucleus which fills the nuclear space around the chromosomes and the nucleoli
Composition of nucleoplasm
primarily water, dissolved ions and a complex mixture of molecules
Nucleoplasm function
-to act as a suspension medium for the organelle of the nucleus
-to maintain nuclear shape and structure
-to transport ions molecules and other substances important to cell metabolism
Nuclear pore complex (NPC)
-pores regulate the entry and exist of molecules from the nucleus, such as proteins and RNAs
-small proteins and ions can diffuse through pores
-larger proteins must be actively transported through the NPC with the help of transporter molecules
-these bind to the large proteins and interact with nucleoporins as they pass through
NPC structure
-octagonal shape made up of membrane-embedded proteins
-on the cytoplasmic surface proteins called nucleoporins extend out into the cytoplasm
-on the nuclear surface protein filaments extend inwards into the nucleus forming a nuclear basket arrangement
Nucleolus
a region in the nucleus where ribosomal RNA is transcribed and processed
0.1-1 micrometers
Nucleolus compartments
fibrillar centre
dense fibrillar component
granular component
What happens in the nucleolus?
-protein and DNA complexes are found in the FC
-ribosomal subunits are made in the DFC
-ribosomes are assembled in GR
Discovery of chromatin
by Walther Fleming
identified a structure within the cell that strongly absorbed basophilic dyes
Chromatin
a complex of DNA and protein and if found in the nucleus or eukaryotic cells
-uncondensed
Histones
proteins that are responsible for the first level of DNA packing in chromatin
-contain a lot of positively charged amino acids such as arginine and lysine so they can easily bind to the negatively charged DNA
Chromatin composition
50% DNA
50% proteins (mainly histones)
Chromatin and chromosomes
chromatin is condensed during mitosis to become chromosomes which allows a lot of DNA to fit in a much smaller space
Chromosomes
a discrete unit of the genome carrying many genes
-composed of highly compacted chromatin
-chromsomes form during cell division to ensure all DNA is divided correctly and completely
Karyotype
a complete set of chromosomes
Chromatin remodelling factors
convert chromatin from active into inactive and back by addition/removal of acetyl/methyl groups
Histone modifications
-in histone acetylation, larger acetyl groups are attached to positively charged lysines in histone tails
-this loosens chromatin structure thereby promoting the initiation of transcription
-the addition of smaller methyl groups can condense chromatin (found in inactive heterochromatin)
Euchromatin
active chromatin
-less condensed
-lightly packed
-transcriptionally active
-histones in euchromatin have specific modifications
-characteristic for interphase chromosomes
Heterochromatin
inactive chromatin
-highly condensed tightly packed and darkly stained
-so transcriptionally inert
-histones in heterochromatin have specific modifications
