Chromosome homologous pair

Homologous chromosomes The pair of chromosomes with the homologous linear gene sequence derived from father and mother. 

This reduction is achieved through meiosis, a process in which two successive ceU divisions occur without intervening DNA duplication. As with the cell cycle, DNA duphcation wiU have aheady taken place prior to the cell division, so that during the initial prophase each pair of identical DNA molecules forms a pair of chromatids attached to each other at a centrosome. In contrast to mitosis, however, homologous chromosomes now pair up (metaphase I). Note that in a pair of homologous chromosomes each wiU contain the same genes (one of the pair having come from the mother and one from the father) but many of the alleles wiU be different. 

Figure 26-11 Mitosis. Illustrated for a cell with one homologous pair of chromosomes. After Mazia.221...

During the prophase of the first meiotic division (meiosis I) two homologous pairs of partially "condensed" chromosomes must find each other and pair with appropriate orientation. A protein in the telomeres of the chromosomes seems to be involved.269 277 The key structure in meiotic crossing-over is the ribbonlike synaptonemal complex formed by the pairs of homologous chroma fids.271/278 2791 This complex, in which a proteinaceous core or axial element separates the greatly extended chromatid pairs (Fig. 26-13), is fully formed in the pachytene stage of meiosis. Formation of the synaptonemal complex is preceded by development of a few double-stranded breaks in... 

A somewhat similar structure appears to be present in the specialized eukaryotic lampbrush chromosomes (Fig. 27-6), which are observed during the meiotic prophase of oocytes. They have been studied intensively in amphibians such as Xenopus. A lampbrush chromosome is actually a homologous pair of chromosomes, each one in turn consisting of two closely associated chromatids. The chromosomes are highly expanded, and about 5% of the DNA is extended in the form of -4000 perfectly paired loops visible with an electron microscope. Each loop consists of - 50 pm or -150 kb of extended DNA. No evidence of any breaks in the DNA is seen, a fact that supports the... 

When two homologous (i.e., bearing the same kinds ol genes) chromosomes are paired in synapsis (as in early meiosis in the nucleated organisms) recombination may occur. Recombinntion is the exchange, usually equal, of segments of chromosomes. Thus, if u chromosome marked ... 

Heteromorphic sex chromosomes were hypothesized to have evolved from a homologous pair of chromosomes by the suppression of crossing-over between the ancestral pair of sex chromosomes and subsequent loss of gene function on the ancestral Y (or W) chromosome.2 One prediction of this hypothesis is that recombination between the X and Y chromosomes (or between the Z and W chromosomes) will be reduced or nonexistent. This prediction is well supported for eutherian mammals with rare exception, recombination between the X and Y chromosomes is reduced to a small region referred to as the pseudoautosomal region. As a consequence, most of the mammalian Y chromosome is isolated in the genome and is thus clonally inherited from father to son. 

Chromosomes. DNA polymers that carry genes in linear arrangement. Except for specialized cells formed for sexual reproduction, chromosomes occur as matched ( homologous ) pairs in each cell of an organism. 

Recombination takes place before the first melotic cell division in germ cells when the replicated chromosomes of each homologous pair align with each other, an act called synapsis (see Figure 9-3). At this time, homologous DNA sequences on maternally and paternally derived chromatids... 

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