o Sections of DNA that aren’t in use are wrapped tightly around globular proteins called histones.
o Eight histones wrapped in DNA form a nucleosome.
o Nucleosomes, in turn, wrap into coils called solenoids, which wrap into supercoils.
o The entire DNA/protein complex (including a small amount of RNA) is called chromatin.
o The basicity of histones gives them a net positive charge at the normal pH of a cell.
Types of Chromatin
Chromatin is found in two varieties: euchromatin and heterochromatin.
o Euchromatin is chromatin that can be uncoiled and transcribed.
o Euchromatin is only coiled during nuclear division.
o Heterochromatin is a tightly packed form of DNA.
o Its major characteristic is that transcription is limited.
o Some chromatin called constitutive heterochromatin, is permanently coiled.
o In the nucleus of a human somatic cell, there are 46 double stranded DNA molecules.
o The chromatin associated with each one of these molecules is called a chromosome.
o In human cells, each chromosome possesses a partner that codes for the same traits as itself.
o Two such chromosomes are called homologues.
o Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis.
o The pair are non-identical chromosomes that both contain information for the same biological features and contain the same genes at the same loci but possibly each have different alleles (that is, different genetic information) at those genes.
o Any cell that contains homologous chromosomes are called diploid.
o Any cell that doesn’t contain homologous chromosomes are called haploid.
o In the nucleus of human cells, there are 46 chromosomes before replication, and 46 chromosomes after replication.
o The duplicates can be referred to separately as sister chromatids.
"Prokaryotic chromosomes exist in the cytoplasm"
o Eukaryotes have multiple pairs of linear chromosomes.
o The gene-coding sequences are interspersed with non-coding regions that may control gene regulation.
o Within a gene sequence, there are exons (coding regions) and introns (non-coding regions that will be removed from the mRNA).
o Eukaryotic chromosomes are contained in a nucleus.
o Prokaryotes have 1 circular chromosome, and may have smaller extrachromasomal DNA in the form of plasmids.
o Prokaryotic DNA does not have the non-coding regions or introns.
o Bacteria do NOT have histones, but archaea have histone-like structures associated with their chromosomes.
o Prokaryotic chromosomes exist in the cytoplasm.
o Reality about Bacterial Chromosomes: Although many have only the 1 circular chromosome, there are many exceptions to this rule.