Cellular and Molecular Biology Topics
Chromatin is a complex of DNA, histone proteins and
non-histone proteins. Histones package DNA into nucleosomes: 146 base pairs
wrapped in 1.75 left handed coils around a histone octamer which contains two of
each core histone. Linker DNA is unwrapped DNA between nucleosomes. Nucleosomes
are packed into a spiral or solenoid arrangement, forming 30nm fibers.
Histones are small, very basic proteins, highly conserved. One fourth of the histone residues are basic (Arg or Lys). They have a central globular domain involved in protein-protein interactions that stabilize the nucleosome, and a more flexible N-terminus that binds to DNA. Histones are often acetylated on this N-terminus domain, in order to regulate how tightly the nucleosomes are packed. There are 5 types of histones: H1 is a linker histone, while H2A, H2B, H3 and H4 are core histones. H1 binds to the outside of the wrapped DNA (20-80 base pairs).
The eukaryotic chromosome is attached to the mitotic apparatus by its centromere. Each end of the chromosome is known as a telomere. Each chromosome contains one linear DNA molecule which codes for a number of genes.
Genes contain control regions called exons and introns. Exons are the sequences ultimately present in RNA, while introns are noncoding sequences spliced from RNA before transcription.
Chromosomes also contain intergenic regions that do not encode for any RNA. Repetitive DNA refers to many copies of related sequences per genome. Some have structural roles (telomeres and centromeres), others have no known role. The most common are Short Interspersed Nuclear Elements (SINEs), others are Long Interspersed Nuclear Elements (LINEs), both of which can affect regulation. Other sequences called microsatellites are simpler repeats (like GTGTGT...etc). Microsatellites can serve as markers, but their expansion can lead to disease. There are also Middle Repetitive Sequences (some are genes or regulatory elements) and unique (single copy) sequences which comprise most of the structural genes.
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