right-handed twist
forms double helix of DNA; can be over- or underwound
overwound helix
forms a positive supercoil
underwound helix
forms a negative supercoil (form of all circular bacterial chromosomes)
negative supercoiling functions
- facilitates functions that require DNA strand separation such as transcription and translation
- aids in compaction of chromosomes which helps with transmission
- necessary for binding of DNAa to origin of replication
characteristics of prokaryotic chromosomes
- circular
- have single origin of replication
- occupies region of cytoplasm called nucleoid
characteristics of eukaryotic chromosomes
- contain telomeres
- contained within the nuclear envelope
- compacted using histones
- linear
- have multiple origins of replication
histone proteins
small proteins which DNA binds and winds around, and compacts DNA with the nucleus
nucleosomes
formed by a complex of 8 histones
diameter of 11nm or 110A
levels of DNA packaging
double helix (20 A)
- histone proteins
- nucleosomes (110 A)
- tight helical fiber or salemoid structure (300 A)
- supercoil (2400 A)
- Chromosome (40,000 A)
metacentric
centromere found near middle
submetacentric
centromere found near top
acrocentric
centromere found at top but still have 4 arms
telocentric
centromere found at very top, only 2 bottom arms are present
euchromatin
- state of DNA organization
- loosely packed
- accessible to transcription machinery
- found in prokaryotes
- major state of most genes
heterochromatin
- state of DNA organization
- tightly packed
- inaccessible to transcription machinery
- major state of inactive X chromosome
