9 Геном, плазмиды, вирусы (Лекции), страница 5

These "hopping genes" were first observedin maize in the 1950s by Barbara McClintock. In addition to thesewell-characterized classes, there is a wide range of unusual rearrangements for which no mechanism or purpose has been proposed. We willfocus only on the first three classes noted above.Any discussion of the mechanics of recombination must always include unusual DNA structures. In homologous genetic recombination,the two DNA molecules interact and align their similar sequences atsome stage in the reaction. This alignment process may involve theformation of novel DNA intermediates in which three or possibly evenfour strands are interwound. (Recall the three-stranded structure ofH-DNA; see Fig. 12-22.) Branched DNA structures are also found asrecombination intermediates.

The exchange of information betweentwo large, helical macromolecules often involves a complex interweaving of strands.The functions of genetic recombination systems are as varied astheir mechanisms. The maintenance of genetic diversity, specializedDNA repair systems, the regulation of expression of certain genes, andprogrammed genetic rearrangements during development representsome of the recognized roles for genetic recombination events. To illustrate these functions, we must first describe the recombination reactions themselves.Barbara McClintock1902-1992840Part IV Information PathwaysFigure 24-25 Meiosis in eukaryotic germ-linecells, (a) The chromosomes of a germ-line cell (sixchromosomes; three homologous pairs) are replicated, except for centromeres. While the productDNA molecules remain attached at their centromeres, they are called chromatids (sometimes,"sister chromatids").

(b) In prophase I, just prior tothe first meiotic division, the three homologous setsof chromatids are aligned to form tetrads, held together by covalent links at homologous junctions(chiasmata). Crossovers occur within the chiasmata(see Fig. 24-26). (c) Homologous pairs separatetoward opposite poles of the cell, (d) The first meiotic division produces two daughter cells, each withthree pairs of chromatids. (e) The homologous pairsalign in the center of the cell in preparation forseparation of the chromatids (now chromosomes),(f) The second meiotic division produces daughtercells with three chromosomes, half the number ofthe germ-line cell.

The chromosomes have resortedand recombined.Homologous Genetic Recombination Has Multiple FunctionsHomologous genetic recombination (also called general recombination)is tightly linked to cell division in eukaryotes. The process occurs withthe highest frequency during meiosis, the process in which a germline cell with two matching sets of chromosomes (a diploid cell) dividesto produce a set of gametes—sperm cells or ova in higher eukaryotes—each gamete having only one member of each chromosome pair (haploid cells).

The process of meiosis is illustrated in Figure 24-25. Inoutline, meiosis begins with replication of the DNA in the germ-linecell so that each DNA molecule is present in four copies. The cell thengoes through two meiotic cell divisions that reduce the DNA content tothe haploid level in each of four daughter cells.After the DNA is replicated during prophase I (prophase of the firstmeiotic division), the resulting DNA copies remain associated at theircentromeres and are referred to as sister chromatids.

Each set of fourhomologous DNA molecules is therefore arranged as two pairs of chromatids. Genetic information is exchanged between the closely associated homologous chromatids at this stage of meiosis by means of homologous genetic recombination. This process involves a breakage andrejoining of DNA. The exchange is also called crossing over, and can beobserved cytologically (Fig.

24-26). Crossing over links the two pairs ofsister chromatids together at points called chiasmata (singular, chiasma). This effectively links together all four homologous chromatids,and this linkage is essential to the proper segregation of chromosomesin the subsequent meiotic cell divisions. To a first approximation, recombination, or crossing over, can occur with equal probability at almost any point along the length of two homologous chromosomes. Thefrequency of recombination in a region separating two points on a chromosome is therefore proportional to the distance between the points.Chapter 24 DNA Metabolism841CentromereHomologouspairChromatidsCrossoverpoint(chiasma)Tetrad(b)(a)2 ju-mThis fact has been used by geneticists for many decades to map therelative positions and distances between genes; homologous recombination is therefore the molecular process that underpins much of theclassical application of the science of genetics.In bacteria, which do not of course undergo meiosis, homologousgenetic recombination occurs in processes such as conjugation, a mating in which chromosomal DNA is transferred between two closelylinked bacterial cells, or it can occur within a single cell between thetwo homologous chromosomes present during or immediately afterreplication.This type of recombination serves at least three identifiable functions: (1) it contributes to genetic diversity in a population; (2) it provides in eukaryotes a transient physical link between chromatids thatis apparently critical to the orderly segregation of chromosomes to thedaughter cells in the first meiotic cell division; and (3) it contributes tothe repair of several types of DNA damage.The first and second functions are often of most interest to scientists studying genes, and homologous recombination is often describedas a source of genetic diversity.

However, the DNA repair function isalmost certainly the most important role in the cell. DNA repair asdescribed thus far is predicated on the fact that a DNA lesion in onestrand can be accurately repaired because the genetic information ispreserved in an undamaged complementary strand. In certain types oflesions, such as double-strand breaks, double-strand cross-links, or lesions left behind in single strands during replication (Fig. 24-27), thecomplementary strand is itself damaged or absent. When this occurs,the information required for accurate DNA repair must come from aseparate, homologous chromosome, and the repair involves homologous recombination. These kinds of lesions commonly result from ionizing radiation and oxidative reactions, and their repair is critical tothe production of viable gametes in eukaryotes and to the everydayexistence of bacteria.

Repair that is mediated by homologous geneticrecombination is simply called recombinational repair; it is discussed in detail later in this chapter.Figure 24-26 Crossing over, (a) The homologouschromosomes of a grasshopper are shown duringprophase I of meiosis. Multiple points of joining(chiasmata) are evident between the two homologous pairs of chromatids. These chiasmata are thephysical manifestation of prior homologous recombination (crossing over) events, (b) Crossing overoften results in an exchange of genetic material.Double-strand breakJ"Double-strand cross-linkLesion in single strandFigure 24-27 Types of DNA damage that requirerecombinational repair.

In each case the damage toone strand cannot be repaired by mechanisms described earlier in this chapter because the complementary strand required to direct accurate repair isdamaged or absent.Part IV Information Pathways842BDNA with strandbreak is alignedb ^ with a second- homologous DNA.Reciprocal strandswitch producesb a Holliday, -.'••» intermediate.iIDThe crossover pointmoves by branch_ migration and strand,-—. .^^ -,.-^-«*«, ,-.- breaks are repaired.The Holliday intermediate can be cleaved(or resolved) in two ways, producing two possiblesets of products.

Below, the orientation of the Hollidayintermediate is changed to clarify differencesin the two cleavage patterns:CleavagebBa___ bHorizontal cleavageresults innonrecombinant ends.LBVertical cleavage resultsin recombinant ends(i.e., b is now attachedto A, and a isattached to B).(a)Figure 24-28 (a) The Holliday model for homologous genetic recombination. Two genes on the homologous chromosomes are indicated by the regionsin red and yellow. Each chromosome has differentalleles of these genes, as indicated by uppercaseand lowercase letters. Note which alleles are linkedin the four final products, (b) A Holliday intermediate formed between two bacterial plasmids in vivo,as seen with the electron microscope.Figure 24—29 Branch migration occurs within abranched DNA structure in which at least onestrand is partially paired with each of two complementary strands.