4.1. Nucleus and DNA Flashcards
Binary fission…
Prokaryotic cells have one circular chromosome.
As DNA replicates, each of the two resulting DNA molecules attaches to the plasma membrane.
As the cell grows, a new plasma membrane is added between the two attachment points and the DNA molecules are moved apart.
Cytokinesis separates the one cell into two, each with a complete chromosome.
Nuclear pores…
An access point into the nucleus, each one surrounded by 8 large protein granules.
Allows access for proteins, lipids, carbohydrates and signalling molecules.
Allows exit for RNA and ribosomal proteins.
Have a nuclear basket, nuclear ring and nuclear cage which guide entry for molecules into the nucleus.
Chromosomes…
Thread-like structures that DNA is packed in to. Not visible if the cell is not dividing.
Have a constriction point in the middle, called a centromere, that divides into two:
- P arm: short arm.
- Q arm: long arm.
Histones help to compact the chromosomal DNA through coiling. Through DNA associating with this protein, it becomes more compact.
DNA is negatively charged which allows it to be attracted towards the histones (H1, H2A, H2B, H3 and H4).
Nucleosomes…
The basic repeating structural and functional unit of chromatin.
It contains 9 histones and 166 base pairs of DNA.
It is still too large. It must be coiled further into an even shorter 30nm fibre.
It is highly irregular and not uniform.
Chromatin…
Name given to DNA, histones and other proteins that make up the chromosomes.
Split into euchromatins (extended) and heterochromatin (condensed) forms.
Euchromatins are the only part of DNA that is accessible.
Three functions of chromatin…
- To package the DNA into smaller volumes to fit into cells.
- To strength DNA to undergo mitosis and meiosis.
- To regulate gene expression and DNA replication.
Two mechanisms by which chromatin can be made more accessible…
- Histones can be enzymatically modified.
- Histones can be displaced by chromatin remodelling complexes.
