页数：40页文件大小：1.73 MB来源： 分子生物学基因Geneppt课件
本课件为《分子生物学》第18章染色体（Chromosomes），依据 Gene VII 教材编排，沿Condensing viral、The bacterial等小节推进，讲解本章核心概念与分子机制，配逐页图示与文字版。
Chapter 18 Chromosomes
Figure 18.1 The length of nucleic acid is much gre a ter than the dimensions of the surrounding compartment.
Capsid is the external protein coat of a virus particle.
Figure 18.2 A helical path for TMV RNA is created by the stacking of protein subunits in the virion.
Figure 18.3 Matur at ion of phage lambda passes through several stages. The empty head changes shape and expands when it becomes filled with DNA. The electron micrographs show the particles at the start and end of the matur at ion pathway. Photographs kindly provided by A. F. Howatson.
Figure 18.4 A thin section shows the bacterial nucleoid as a compact mass in the center of the cell. Photograph kindly provided by Jack Griffith.
Figure 18.5 The nucleoid spills out of a lysed E. coli cell in the form of loops of a fiber. Photograph kindly provided by Jack Griffith.
Figure 18.6 The bacterial genome consists of a large number of loops of duplex DNA (in the form of a fiber), each secured at the base to form an independent structural domain.
MAR (matrix attachment site; also known as SAR for scaffold attachment site) is a region of DNA that attaches to the nuclear matrix.
Figure 18.7 Histone-depleted chromosomes consist of a protein scaffold to which loops of DNA are anchored. Photograph kindly provided by Ulrich K. Laemmli.
Figure 18.8 Matrix-associated regions may be identified by characterizing the DNA retained by the matrix isolated in vivo or by identifying the fragments that can bind to the matrix from which all DNA has been removed.
Euchrom a tin comprises all of the genome in the interphase nucleus except for the heterochromatin.
Figure 18.9 The sister chromatids of a mitotic pair each consist of a fiber (~30 nm in diameter) compactly folded into the chromosome. Photograph kindly provided by E. J. Du Praw.
Figure 18.10 A thin section through a nucleus stained with Feulgen shows heterochromatin as compact regions clustered near the nucleolus and nuclear membrane. Photograph kindly provided by Edmund Puvion.
Figure 18.11 G-banding generates a characteristic lateral series of bands in each member of the chromosome set. Photograph kindly provided by Lisa Shaffer.
Figure 18.12 The human X chromosome can be divided into distinct regions by its banding pattern. The short arm is p and the long arm is q; each arm is divided into larger regions that are further subdivided. This map shows a low resolution structure; at higher resolution, some bands are further subdivided into smaller bands and interbands, e.g. p21 is divided into p21.1, p21.2, and p21.3.
Figure 18.13 A lampbrush chromosome is a meiotic bivalent in which the two pairs of sister chromatids are held together at chi as mata (indicated by arrows). Photograph kindly provided by Joe Gall.
Figure 18.14 A lampbrush chromosome loop is surrounded by a matrix of ribonucleoprotein. Photograph kindly provided by Oscar Miller.
Figure 18.15 The polytene chromosomes of D. melanogaster form an alternating series of bands and interbands. Photograph kindly provided by Jose Bonner.
Figure 18.16 Individual bands containing particular genes can be identified by in situ hybridization.
Figure 18.17 A magnified view of bands 87A and 87C shows their hybridization in situ with labeled RNA extracted from heat-shocked cells. Photograph kindly provided by Jose Bonner.
Figure 18.18 Chromosome IV of the insect C. tentans has three Balbiani rings in the salivary gland. Photograph kindly provided by Bertil Daneholt.
Acentric fragmentof a chromosome (generated by breakage) lacks a centromere and is lost at cell division.
Figure 18.19 Chromosomes are pulled to the poles via microtubules that attach to their kinetochores. They sister chromatids are held together until anaphase by glue proteins (cohesins). The centromere is shown here in the middle of the chromosome, but can be located any where along its length, including close to the end (acrocentric) and at the end (telocentric).
Figure 18.9 The sister chromatids of a mitotic pair each consist of a fiber (~30 nm in diameter) compactly folded into the chromosome. Photograph kindly provided by E. J. Du Praw.
Figure 18.20 C-banding generates intense staining at the centromeres of all chromosomes. Photograph kindly provided by Lisa Shaffer.
Figure 18.21 The centromere is identified by a DNA sequence that binds specific proteins. These proteins do not themselves bind to microtubules, but establish the site a which the microtubule-binding proteins in turn bind.
Figure 18.22 Three conserved regions can be identified by the sequence homologies between yeast CEN elements.
Telomere is the natural end of a chromosome; the DNA sequence consists of a simple repeating unit with a protruding single-stranded end that may fold into a hairpin.
Figure 18.24 Telomerase positions itself by base pairing between the RNA template and the protruding single-stranded DNA primer. It adds G and T bases one at a time to the primer, as directed by the template. The cycle starts again when one repeating unit has been added.
Figure 18.23 The unusual behavi or of telomeric fractions may be explained by G-G interactions. In the upper model a duplex hairpin is formed by G-G pairing. In the lower model, a G quartet is formed when 1 G is contributed by each of 4 repeating units.
Telomerase is the ribonucleoprotein enzyme that creates repeating units of one strand at the telomere, by adding individual bases to the DNA 3end, as directed by an RNA sequence in the RNA component of the enzyme.
Figure 18.25 A loop forms at the end of chromosomal DNA. Photograph kindly provided by Jack Griffith.
Figure 18.26 The 3’ single-stranded end of the telomere (TTAGGG)n displaces the homologous repeats from duplex DNA to form a t-loop. The reaction is catalyzed by TRF2.
The genetic material of all organisms and viruses takes the form of tightly packaged nucleoprote in. Transcriptionally active sequences reside within the euchrom a tin that comprises the majority of interphase chromatin. Lampbrush chromosomes of amphibians and polytene chromosomes of insects have unusually extended structures, with packing ratios <100. The centromeric region contains the kinetochore, which is responsible for attaching a chromosome to the mitotic spindle. Telomeres make the ends of chromosomes stable.
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