chromosomes

• chromatin is a given region of DNA with its associated proteins on a chromosome
• prokaryotes have one circular chromosome as well as other small independent circular DNA called plasmids in the cytoplasm
• eukaryotic chromosomes are linear and enclosed in a nucleus
• origins of replication are DNA sequences along a chromosome that initiate DNA replication
• centromeres are DNA sequences required for correct segregations of chromosomes
• telomeres are DNA sequences located at the ends of chromosomes that prevent degradation and allow proper replication of chromosomal ends
• most eukaryotic cells are diploid so they have two copies of each chromosome called homologous chromosomes
• only sexually-reproductive cells have a haploid gene
• few eukaryotic chromosomes are polyploid because large cells need lots of protein to sustain it
• chromosomes compact DNA so it can fit into the cell/nucleus as well as protect DNA from damage
• histones are basic positively charged proteins that DNA wind around
• a nucleosome is a histone octomer with lots of DNA wrapped around it
• histone H1 binds linker DNA to nucleosomes to form the 30nm chromatin fibre
• only eukaryotes have histones
• euchromatin are chromosomal regions that have low DNA compaction so genes are actively expressed
• heterochromatin are chromosomal regions that have high DNA compaction so gene expression is silenced
• constitutive heterochromatin are regions where DNA is always highly compacted
• facultative heterochromatin are regions that can switch to euchromatin depending on the cell type and development
• eukaryotic chromosomes have multiple origins
• n represents the haploid number of chromosomes
• 2n represents the diploid number of chromosomes