Chromosomes
Chromosomes are thread-like structures found in the nucleus of a cell that carry genetic information in the form of genes. They are composed of DNA and proteins, and are visible during cell division. Humans typically have 46 chromosomes, with 23 pairs inherited from each parent. Chromosomes play a crucial role in the inheritance and transmission of genetic traits.
7 Key excerpts on "Chromosomes"
eBook - ePubAdvanced Molecular Biology
A Concise Reference
- Richard Twyman(Author)
- 2018(Publication Date)
- Garland Science(Publisher)
...Chapter 5 Chromosome Structure and Function Fundamental concepts and definitions A chromosome is a discrete DNA molecule which carries essential genetic information, together with any associated proteins which define its structure. The scope of the term can include the bacterial nucleoid, organelle genomes and virus genomes, as well as eukaryotic nuclear Chromosomes, but only the last is considered in this chapter. The original suggestion that genes were carried on Chromosomes was based on the parallel behavior of Chromosomes and Mendelian genes (occurring in pairs, equal segregation, independent assortment). This is the chromosome theory of inheritance. There are three levels of duality in diploid eukaryotes. Chromosomes occur in homologous pairs, and when individual Chromosomes become visible at mitosis, they can be seen to comprise a pair of chromatids, joined at a common centromere, each chromatid containing two DNA strands in a duplex. Despite the presence of eight DNA strands, the cells are still diploid. Only one strand of each DNA duplex actually carries the (transcribed) information, and the presence of two chromatids represents the doubling of information in preparation for segregation into daughter cells — cells are born with only one copy (a chromatid becomes a chromosome when it segregates). Furthermore, the paired chromatids of each mitotic chromosome are identical, whereas homologous Chromosomes are not. Throughout the cell cycle, therefore, each locus is represented by just two alleles. Two types of chromosome can be distinguished at meiosis: autosomes, which have homo-morphic (structurally identical) partners and thus form homologous chromosome pairs, and heterosomes (or allosomes) which have heteromorphic (structurally dissimilar) partners and form pairs which are homologous over only part of their lengths...
eBook - ePub30-Second Genetics
The 50 most revolutionary discoveries in genetics, each explained in half a minute
- Jonathan Weitzman, Matthew Weitzman(Authors)
- 2020(Publication Date)
- Ivy Press(Publisher)
...In bacteria, Chromosomes are usually composed of a small circle of DNA and proteins. In more complex organisms with several Chromosomes, such as ourselves, the DNA is condensed with proteins called histones. DNA, histones and other proteins constitute the chromatin. In eukaryotic cells, such as ours, the Chromosomes are located inside the nucleus. Humans have 23 pairs of Chromosomes, with more than 2 metres (6 ft 6 in.) of DNA packed within each cell. Of these pairs, 22 are called autosomes or homologs. They are similar copies of each other that may serve as a back-up of genetic information. The remaining chromosome pair comprises the X and Y Chromosomes, called ‘sex Chromosomes’, because they determine the sex of an individual. The entire set of Chromosomes is called the karyotype, which is studied during cell division at the moment when DNA has been duplicated. At this instant, each chromosome has two condensed copies of its genetic material known as the chromatids. During cell division, the mother cell transmits one chromatid of each chromosome to the two daughter cells. A failure in this process leads to an abnormal number of Chromosomes, as is often seen in cancer cells. 3-SECOND THRASH Chromosomes contain the DNA of an organism in a highly condensed form. The full set of Chromosomes in each organism is known as the karyotype. 3-MINUTE THOUGHT A chromosome is literally a ‘coloured body’. They were discovered (as bodies reacting to specific colourants) before DNA was first described. Chromosomes can be easily stained when condensed. The staining test creates a pattern of bands so that each chromosome is distinguishable, allowing the creation of a map called an ideogram...
eBook - ePub- Chang-Hui Shen(Author)
- 2019(Publication Date)
- Academic Press(Publisher)
...Chapter 5 The Genome Abstract The most important function of genetic materials is to carry genes—the functional units of heredity. The genetic information specifies all the RNA and protein molecules, and directs when, in what types of cells, and in what quantity each protein is to be made. In eukaryotes, the genes in the nucleus are distributed between a set of different Chromosomes which makes up the genome. Therefore, the genome is the complete set of genes of an organism. Ultimately, it is defined by the complete DNA sequence. For example, the human genome, which contains 3.3 × 10 9 nucleotides, is distributed over 23 pairs of Chromosomes. Each chromosome consists of a single, enormously long, linear DNA molecule associated with histone proteins that fold and pack the fine DNA thread into a more compact structure known as chromatin...
eBook - ePubThe Human Genome in Health and Disease
A Story of Four Letters
- Tore Samuelsson(Author)
- 2019(Publication Date)
- Garland Science(Publisher)
...Eukaryotic cells have characteristic cellular substructures in the cytosol, such as mitochondria, organelles that are specialized energy factories. Like the nucleus, mitochondria have DNA. This DNA is quite different from the nuclear DNA. In a human, it is a molecule about 16,000 nucleotides in length, much smaller than the DNA molecules found in the nucleus, as elaborated on later in this chapter. DNA as Seen under the Microscope What can we learn about DNA by examining human cells under the microscope? By the end of the nineteenth century, Chromosomes —located in the nucleus—were identified using light microscopy. The name chromosome (from the Greek words chroma [“color”] and soma [“body”]) was derived from the fact that these structures were strongly stained with specific dyes. By the early 1900s, Walter Sutton and Theodor Boveri suggested that Chromosomes carry genetic information. The molecular nature of Chromosomes was not known at this time. We now know that Chromosomes are composed of DNA and protein and that DNA is the actual carrier of genetic information. Miescher used white blood cells for isolation of DNA (Chapter 3). However, all human cells contain DNA. The only exception is red blood cells that lack both nuclei and mitochondria. In all cells, the genetic information is distributed among different Chromosomes, each containing a distinct DNA molecule. In humans, there are 22 different Chromosomes (non-sex Chromosomes or autosomes) and, in addition, two different sex-specific Chromosomes, X and Y. As the human is a diploid organism, 1 all individuals have two copies of each of the 22 Chromosomes, where one copy originates from the father and the other from the mother. In each of these chromosome pairs, the two Chromosomes are similar and are said to be homologous to each other. Furthermore, females have two X Chromosomes, whereas males have one X and one Y chromosome (Figure 4.1)...
eBook - ePub- Alexander McLennan, Andy Bates, Phil Turner, Michael White(Authors)
- 2012(Publication Date)
- Taylor & Francis(Publisher)
...The DNA in each chromosome is a single linear molecule, which can be up to several centimeters long (Section C3). All this DNA must be packaged into the nucleus, a space of approximately the same volume as a bacterial cell; in fact, in their most highly condensed forms, the Chromosomes have an enormously high DNA concentration of perhaps 200 mg mL -1 (Section C1). This feat of packing is accomplished by the formation of a highly organized complex of DNA and protein, known as chromatin, a nucleoprotein complex (Section A4). More than 50% of the mass of chromatin is protein. Chromosomes greatly alter their level of compactness as cells progress through the cell cycle (Sections C3 and N1), varying between highly condensed Chromosomes at metaphase (just before cell division), and very much more diffuse structures in interphase. This implies the existence of different levels of organization of chromatin. Histones Most of the protein in eukaryotic chromatin consists of histones, of which there are five families, or classes: H2A, H2B, H3, and H4, known as the core histones, and H1. The core histones are small proteins, with masses between 10 and 20 kDa, and H1 histones are a little larger at around 23 kDa. All histone proteins have a large positive charge; between 20 and 30% of their sequences consist of the basic amino acids, lysine, and arginine (Section A3). This means that histones will bind very strongly to the negatively charged DNA in forming chromatin. Members of the same histone class are very highly conserved between relatively unrelated species, for example between plants and animals, which testifies to their crucial role in chromatin. Within a given species, there is normally a number of closely similar variants of a particular class, which may be expressed in different tissues, and at different stages in development...
eBook - ePubHeredity under the Microscope
Chromosomes and the Study of the Human Genome
- Soraya de Chadarevian(Author)
- 2020(Publication Date)
- University of Chicago Press(Publisher)
...I believe that your book will give them the necessary direction.” 112 However, the story of chromosome structure and mapping told in this chapter also points to the epistemic and historical commonalities between the two contending approaches that developed at the same time. Chromosome researchers and molecular biologists shared an interest in the material basis and the mechanism of heredity as well as in the nature of mutations, a vital concern in the nuclear age. Together they established genetics as a key issue in biology and biomedical research. Both the Chromosomes and the letters composing the DNA code are presented as the “alphabet” of life, the language in which the book of nature is written and that in the end defines “us,” as a species and individually. 113 Thus, when the artist Suzanne Anker and the sociologist Dorothy Nelkin discussed the scriptural metaphors of genetic information and their artistic renderings, they could draw indiscriminately on DNA and Chromosomes as examples. 114 Similarly, the compact disc that DNA sequencing pioneer and enthusiast Walter Gilbert pulled out of his pocket to demonstrate how people will carry their personal genetic information with them is matched by the personal karyotypes exchanged by chromosome researchers as “self-portraits,” visiting and greeting cards thirty years earlier. 115 Furthermore, chromosome researchers and molecular biologists shared the assumption that the study of structure would explain function. Pattern recognition, mapping, and the harnessing of computers as both memory and calculating machines played a role in both endeavors. 116 Indeed, imaging techniques like X-ray diffraction and electron microscopy, including recent advances in cryo-electron microscopy, have been at the center of much research in molecular biology. Similarly, both cytogeneticists and molecular biologists seized first on radioactive markers and later on fluorescent tagging to visualize structures and functions...
eBook - ePub- Thomas D. Pollard, William C. Earnshaw, Jennifer Lippincott-Schwartz, Graham Johnson(Authors)
- 2016(Publication Date)
- Elsevier(Publisher)
...In skin, defects in these attachments cause blistering diseases. Nucleus The nuclear envelope is a double membrane that separates the nucleus from the cytoplasm (Fig. 1.11). All traffic into and out of the nucleus passes through nuclear pores that bridge the double membranes. Inbound traffic includes all nuclear proteins and ribosomal proteins destined for the nucleolus. Outbound traffic includes mRNAs and ribosomal subunits. FIGURE 1.11 ELECTRON MICROGRAPH OF A THIN SECTION OF A NUCLEUS. (Courtesy Don Fawcett, Harvard Medical School, Boston, MA.) The nucleus stores genetic information in extraordinarily long DNA molecules called Chromosomes. Remarkably, portions of genes encoding proteins and structural RNAs make up only a small fraction (<2%) of the 3 billion nucleotide pairs in human DNA, but more than 50% of the 97 million nucleotide pairs in a nematode worm. Regions of DNA called telomeres stabilize the ends of Chromosomes, and other DNA sequences organize centromeres that direct the distribution of Chromosomes to daughter cells when cells divide. Much of the DNA encodes a myriad of RNAs with regulatory activities. The DNA and its associated proteins are called chromatin (Fig. 1.5). Interactions with histones and other proteins fold each chromosome compactly enough to fit into discrete territories inside the nucleus. During mitosis, Chromosomes condense and reorganize into separate structural units suitable for sorting into daughter cells (Fig. 1.5). Regulatory proteins called transcription factors turn specific genes on and off in response to genetic, developmental, and environmental signals. Enzymes called polymerases make RNA copies of active genes, a process called transcription. mRNAs specify the amino acid sequences of proteins. Other RNAs have structural, regulatory, or catalytic functions. Most newly synthesized RNAs are processed extensively before they are ready for use...
