2Section 12.1Viral and Bacterial Chromosomes Are Relatively Simple DNA MoleculesBacterial and viral chromosomes are usually:a single nucleic acid moleculelargely devoid of associated proteinsmuch smaller than eukaryotic chromosomes
3Section 12.1Chromosomes of Viruses consist of single- or double-stranded DNA or RNA.Figure 12-1 Electron micrographs of (a) phage and (b) the DNA isolated from it. The chromosome is 17m long. The phages are magnified about five times more than the DNA.
4Figure 12-2 Electron micrograph of bacteriophage T2, which has had its DNA released by osmotic shock. The chromosome is 52m long.
5Section 12.1Bacterial chromosomes are double-stranded DNA and are compacted into a nucleoid.DNA in bacteria may be associated with HU and H1 DNA-binding proteins.Figure 12-3 Electron micrograph of the bacterium Escherichia coli, which has had its DNA released by osmotic shock. The chromosome is 1200m long.
6Section 12.2Supercoiling Facilitates Compaction of the DNA of Viral and Bacterial ChromosomesMost closed circular DNA molecules in bacteria are slightly underwound and supercoiled.
7Section 12.2Topoisomerases cut one or both DNA strands and wind or unwind the helix before resealing the ends.
8Section 12.3Specialized Chromosomes Reveal Variations in the Organization of DNAPolytene chromosomes and lampbrush chromosomes are very large and can be visualized by light microscopy.
9Section 12.3 Polytene chromosomes: have distinctive banding patterns represent paired homologsare composed of many DNA strands
10Section 12.3Polytene chromosomes have puff regions where the DNA has uncoiled and are visible manifestations of a high level of gene activity.
11Section 12.3Lampbrush chromosomes - large and have extensive DNA looping.Found in oocytes in the diplotene stage of meiosis.
12Section 12.4 DNA Is Organized into Chromatin in Eukaryotes Eukaryotic chromosomes are complexed into a nucleoprotein structure called chromatin.Chromatin is bound up in nucleosomes with histones H2A, H2B, H3, and H4.
13Section 12.4Electron microscopic observations of chromatin have revealed its fibers are composed of a linear array of spherical particle.
14Section 12.4Nucleosomes are condensed several times to form the intact chromatids.
15Section 12.4Chromatin remodeling must occur to allow the DNA to be accessed by DNA binding proteins.Figure The nucleosome core particle derived from X-ray crystal analysis at 2.8 Å resolution. The double-helical DNA surrounds four pairs of histones.
16Section 12.4Histone tails are important for histone modifications such as acetylation, methylation, and phosphorylation.
17Section 12.4Euchromatin is uncoiled and active, whereas heterochromatin remains condensed and is inactive.
18Section 12.5Chromosome Banding Differentiates Regions along the Mitotic Chromosome
19In C-banding, only the centromeres are stained. Figure A human mitotic chromosome preparation processed to demonstrate C-banding. Only the centromeres stain.Figure 12.11
20G-banding is due to differential staining along the length of each chromosome. Figure G-banded karyotype of a normal human male. Chromosomes were derived from cells in metaphase.Figure 12.12
21Section 12.6Eukaryotic Chromosomes Demonstrate Complex Organization Characterized by Repetitive DNARepetitive DNA sequences are repeated many times within eukaryotic chromosomes.
22Figure 12-14 An overview of the various categories of repetitive DNA.
23Section 12.6Satellite DNA is highly repetitive and consists of short repeated sequences.Figure In situ hybridization between a radioactive probe representing mouse satellite DNA and mitotic chromosomes. The grains in the autoradiograph localize the chromosome regions (the centromeres) containing satellite DNA sequences.
24Centromeres are the primary constrictions along eukaryotic chromosomes mediate chromosomal migration during mitosis and meiosis
25Section 12.6Telomeric DNA sequences consist of short tandem repeats that contribute to the stability and integrity of the chromosome.
26Section 12.6 Moderately repetitive DNA includes: variable number tandem repeats (VNTRs)minisatellitesmicrosatellites
27Section 12.6Short interspersed elements (SINES) and long interspersed elements (LINES) are dispersed throughout the genome rather than tandemly repeated, and constitute over 1/3 of the human genome.These transposable elements are generated via an RNA intermediate and are referred to as retrotransposons.
28Section 12.7The Vast Majority of a Eukaryotic Genome Does Not Encode Functional GenesOnly a small portion of the eukaryotic genome (2%–10%) constitute protein-encoding genes.There are also a large number of single-copy noncoding regions, some of which are pseudogenes.