2015

The continuity of life from one cell to another has its foundation in the reproduction of cells by way of the cell cycle. The cell cycle is an orderly sequence of events in the life of a cell from the division of a single parent cell to produce two new daughter cells, to the subsequent division of those daughter cells. The mechanisms involved in the cell cycle are highly conserved across eukaryotes. Organisms as diverse as protists, plants, and animals employ similar steps. Genomic DNA Before discussing the steps a cell undertakes to replicate, a deeper understanding of the structure and function of a cell's genetic information is necessary. A cell's complete complement of DNA is called its genome. In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle. The region in the cell containing this genetic material is called a nucleoid. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. In eukaryotes, the genome comprises several double-stranded, linear DNA molecules (Figure \(\PageIndex{1}\)) bound with proteins to form complexes called chromosomes. Each species of eukaryote has a characteristic number of chromosomes in the nuclei of its cells. Human body cells (somatic cells) have 46 chromosomes. A somatic cell contains two matched sets of chromosomes, a configuration known as diploid. The letter n is used to represent a single set of chromosomes; therefore a diploid organism is designated 2n. Human cells that contain one set of 23 chromosomes are called gametes, or sex cells; these eggs and sperm are designated n, or haploid.

Chromosome research : an international journal on the molecular, supramolecular and evolutionary aspects of chromosome biology, 2001

The recently available DNA sequences from chromosomes 21 and 22 enabled us to define the relationships of different band types with isochores and with gene concentration and to compare these relationships with previous results. We showed that chromosomal bands appear as Giemsa or Reverse bands depending not on their absolute GC level, but on the composition GC level relative to those of adjacent contiguous bands. We also demonstrated that the GC-richest, and gene-richest H3+ bands are characterized by a lower DNA compaction compared with the GC-poorest, gene-poorest L1+ bands. Moreover, our results indicate that the human genome contains about 30,000 genes.

2000

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Human Genetics, 1978

Our working hypothesis is that the Q-darker human chromosome segments have higher gene densities than the bright regions. Especially prominent in thisrespect are six. hot spots, the short Q-dark regions in 3p, 6p, 1 lq, 12q, 17q, and 19 (p or q), which have been chosen because their density of mitotic chiasmata is above 5. Chromosomes with gene-rich segments would act as trisomy lethals in very early embryos, whose spontaneous abortions would not be recognized. Containing active genes, the regions would be looped out in interphase and thus be more easily available for mitotic pairing and crossing-over. To test this hypothesis, correlations and partial correlations of the following parameters have been determined: the density of mitotic chiasmata, the number and density of localized genes, the incidence of trisomic abortions, the length of chromosomes, and their Q-brightness. Overall, the correlations and partial correlations agree with, but do not prove, the working hypothesis. Far stronger evidence for our hypothesis comes from the highly significant negative effect of hot spots on trisomic abortions which would act as a kind of trisomy lethal. The gene numbers on the hot-spot chromosomes as compared with the controls, on the other hand, are in the right direction, but the difference is not significant.

Chromosome Research, 2007

The first draft of the human genome has revealed enormous variability in the global distribution of Alu repeat elements. There are regions such as the four homeobox gene clusters, which are nearly devoid of these repeats that contrast to repeat dense regions in other transcriptionally active regions of the genome. Our analysis of the completely sequenced chromosomes 21 and 22 reveal a striking bias in Alu distribution. These elements are more clustered in genes which are involved in metabolism, transport and signalling processes. In contrast, they are significantly lower in genes coding for information pathway components as well as structural proteins. This bias in Alu distribution, is independent of the effect of Alu density of the flanking genomic region and is also not affected by the GC content of the gene and its upstream and downstream regions. The relative proportions of Alu subfamilies (Alu J, Alu S and Alu Y) are not significantly different in genes with high Alu density belonging to the functional categories of transport, metabolism and signalling. However, in the structural proteins and information genes these proportions are lower than other three categories. We suggest that Alus might be involved in regulatory mechanisms and are therefore differentially selected in primate genomes. by guest on December 15, 2014

Experimental Cell Research, 1976

The karyotypes of 100 males and 100 females, each assembled by the trypsin banding method, are examined in a study designed to investigate sex differences among autosomes. It is shown that female autosomes are consistently longer than those of the males, with respect to both the short and long arm measurements. In addition, discriminant analysis is used to distinguish between the male and female karyotypes. We find that, using autosomal measurements alone, this can be done with a high probability of success.

Recent investigations have shown that isochores are characterized by a 3-D structure which is primarily responsible for the topology of chromatin domains. More precisely, an analysis of human chromosome 21 demonstrated that GC-poor isochores are low-heterogeneity sequences characterized by the presence of oligo-Adenines that are intrinsically stiff, curved and unfavorable for nucleosome binding. This leads to a structure of the corresponding chromatin domains, the Lamina Associated Domains, or LADs, which is well suited for interaction with lamina. In contrast, the high-heteorogeneity GC-rich isochores are in the form of compositional peaks characterized by gradients of oligo-Guanines that lead to increasing nucleosome depletions in the corresponding chromatin domains, the Topological Associating Domains, or TADs. These results encouraged us to investigate in detail the di- and tri-nucleotide profiles of 100Kb segments of chromosome 21, as well as those of the di- to octa-Adenines a...

Humangenetik, 1974

Several statistical models were used to investigate differences in the measurements of chromosomes from normal people, and in particular, to gain evidence for either the genetic or non-genetic origin of these differences. Computer-aided measurements of size and eentromeric index were made on the chromosomes of 1. five sets of monozygotic twins, 2. five sets of dizygotic twins, 3. three sets of repeat cultures from three different persons, and 4. a group of 15 normal unrelated people. Every chromosome had been identified by Q or G banding.

Journal of Theoretical Biology, 2011

There are mathematical constants that describe universal relationship between variables, and physical/ chemical constants that are invariant measurements of physical quantities. In a similar spirit, we have collected a set of parameters that characterize the human genome. Some parameters have a constant value for everybody's genome, others vary within a limited range. The following nine human genome parameters are discussed here, number of bases (genome size), number of chromosomes (karyotype), number of protein-coding gene loci, number of transcription factors, guanine-cytosine (GC) content, number of GC-rich gene-rich isochores, density of polymorphic sites, number of newly generated deleterious mutations in one generation, and number of meiotic crossovers. Comparative genomics and theoretical predictions of some parameters are discussed and reviewed. This collection only represents a beginning of compiling a more comprehensive list of human genome parameters, and knowing these parameter values is an important part in understanding human evolution.