Problem: Measured linearly, the Escherichia coli genome (4.6 Mb) would be 1,000 times longer than the E. coli cell. The human genome (3.4 Gb) would be 2.3 m long if stretched linearly. Solutions: 1.Supercoiling DNA double helix is twisted in space about its own axis, a process is controlled by topoisomerases (enzymes). (can also occur in a linear DNA molecule) 2.Looped domains

Eukaryotic chromosome structure Chromatincomplex of DNA and chomosomal proteins ~ twice as much protein as DNA Two major types of proteins: 1.Histonesabundant, basic proteins with a positive charge that bind to DNA 5 main types: H1, H2A, H2B, H3, H4 ~equal in mass to DNA evolutionarily conserved 2.Non-histonesall the other proteins associated with DNA differ markedly in type and structure amounts vary widely >> 100% DNA mass << 50% DNA mass

Packing of DNA into chromosomes: 1.Level 1Winding of DNA around histones to create a nucleosome structure. 2.Level 2Nucleosomes connected by strands of linker DNA like beads on a string. 3.Level 3Packaging of nucleosomes into 30-nm chromatin fiber. 4.Level 4Formation of looped domains.

More about different types of DNA you should know about: Centromeric DNA (CEN)Specialized sequences that function with the microtubles. Telomeric DNAAt extreme ends of the chromosome, maintain stability, and consist of tandem repeats. Demonstrated to play a role in DNA replication and stability of DNA. Unique-sequence DNAOften referred to as single-copy and usually code for genes. Repetitive-sequence DNAMay be interspersed or clustered and vary in size. SINEsshort interspersed repeated sequences ( bp) LINEslong interspersed repeated sequences (>5,000 bp) microsatellites short tandem repeats (e.g., TTATTATTATTA)

Figure 8.15 MITOSISMEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE I Tetrad formed by synapsis of homologous chromosomes PROPHASE Duplicated chromosome (two sister chromatids) METAPHASE Chromosome replication 2n = 4 ANAPHASE TELOPHASE Chromosomes align at the metaphase plate Tetrads align at the metaphase plate METAPHASE I ANAPHASE I TELOPHASE I Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I ; sister chromatids remain together No further chromosomal replication; sister chromatids separate during anaphase II 2n2n2n2n Daughter cells of mitosis Daughter cells of meiosis II MEIOSIS II Daughter cells of meiosis I Haploid n = 2 nnnn