when do chromosomes appear in the cells ? what role do they play?

Answer 1

They appear at the prophase and disappear at telophase.
They are the vehicles of genes. The genes are linearly arranged in chromosomes.

Answer 2

Because when a cell divides it need to produce e a copy of the DNA in order for the new cell to have the information it needs to carry out its processes. Besides, it's to ensure that there are enough chromosomes to be divided evenly between the two daughter cells during mitosis.

Answer 3

Chromosomes become most clearly visible during cell division. During cell division, the chromosome coil tightly and hence are visible via the microscope.
The main function of chromosomes is that they contain DNA and hence contain all the genetic infromation of the organism.

Answer 4

Cell division. They....um, I'm not sure. A chromosome is built by two strands of chromatids,which contains the DNA so...do you get the point? It's like, they contain the synthesized DNA so they're needed or else the daughter cells won't have DNA...right?

Answer 5

a chromosome is a deeply staining rodlike structure with a part which is usually constricted.
during cell division spindle fibres are attached to kinetochore located in the centromeres.

Answer 6

chromosomes appear in cell division mainly in metaphase of mitosis. they carry genes which are the basis for generations and for carry forward of certain traits.

Answer 7

Chromosomes are very important in Human Life. They contain Genes and DNA which contain genetic information.

Chromosome
Any of the organized components of each cell which carry the individual's hereditary material, deoxyribonucleic acid (DNA). Chromosomes are found in all organisms with a cell nucleus (eukaryotes) and are located within the nucleus. Each chromosome contains a single extremely long DNA molecule that is packaged by various proteins into a compact domain. A full set, or complement, of chromosomes is carried by each sperm or ovum in animals and each pollen grain or ovule in plants. This constitutes the haploid (n) genome of that organism and contains a complete set of the genes characteristic of that organism. Sexually reproducing organisms in both the plant and animal kingdoms begin their development by the fusion of two haploid germ cells and are thus diploid (2n), with two sets of chromosomes in each body cell. These two sets of chromosomes carry virtually all the thousands of genes of each cell, with the exception of the tiny number in the mitochrondria (in animal), and a few plant chloroplasts. See also Deoxyribonucleic acid (DNA); Gene.

Chromosomes can change their conformation and degree of compaction throughout the cell cycle. During interphase, the major portion of the cycle, chromosomes are not visible under the light microscope because, although they are very long, they are extremely thin. However, during cell division (mitosis or meiosis), the chromosomes become compacted into shorter and thicker structures that can be seen under the microscope. At this time they appear as paired rods with defined ends, called telomeres, and they remain joined at a constricted region, the centromere, until the beginning of anaphase of cell division. See also Cell cycle; Meiosis; Mitosis.

Chromosomes are distinguished from one another by length and position of the centromere. They are metacentric (centromere in the middle of the chromosome), acrocentric (centromere close to one end), or telocentric (centromere at the end, or telomere). The centromere thus usually lies between two chromosome arms, which contain the genes and their regulatory regions, as well as other DNA sequences that have no known function. In many species, regional differences in base composition and in the time at which the DNA is replicated serve as the basis for special staining techniques that make visible a series of distinctive bands on each arm, and these can be used to identify the chromosome.

Compaction

Each nucleus in the cell of a human or other mammal contains some 6 billion base pairs of DNA which, if stretched out, would form a very thin thread about 6 ft (2 m) long. This DNA has to be packaged into the chromosome within a nucleus that is much smaller than a printed dot. Each chromosome contains a single length of DNA comprising a specific portion of the genetic material of the organism. Tiny stretches of DNA, about 140 base pairs long and containing acidic phosphate groups, are individually wrapped around an octamer consisting of two molecules of each of the four basic histone proteins H2a, H2b, H3, and H4. This arrangement produces small structures called nucleosomes and results in a sevenfold compaction of the DNA strand. Further compaction is achieved by binding the histone protein H1 and several nonhistone proteins, resulting in a supercoiled structure in which the chromosome is shortened by about 1600-fold in the interphase nucleus and by about 8000-fold during metaphase and anaphase, where the genetic material must be fully compacted for transport to the two daughter cells. At the point of maximum compaction, human chromosomes range in size from about 2 to 10 micrometers in length, that is, less than 0.0004 in. See also Nucleosome.

Number and size

Each diploid (2n) organism has a characteristic number of chromosomes in each body (somatic) cell, which can vary from two in a nematode worm and one species of ant, to hundreds in some butterflies, crustaceans, and plants. The diploid number of chromosomes includes a haploid (n) set from each parent. Many one-celled organisms are haploid throughout most of their life cycle. The human diploid number is 46.

There is some relationship between the number of chromosomes and their size. Some of the chromosomes in certain classes of organisms with large numbers of chromosomes are very tiny, and have been called microchromosomes. In birds and some reptiles, there are about 30–40 pairs of microchromosomes in addition to 5–7 or so pairs of regular-sized macrochromosomes. The number of microchromosomes is constant in any species carrying them, and only their size distinguishes them from the widespread macrochromosomes. At least seven microchromosomes in birds have been shown to contain genes, and all are thought to.

In some species of insects, plants, flatworms, snails, and rarely vertebrates (such as the fox), the number of chromosomes can vary because of the presence of a variable number of accessory chromosomes, called B chromosomes. It is not clear what role, if any, B chromosomes play, but they appear to be made primarily of DNA that neither contains functional genes nor has much effect on the animal or plant even when present in multiple copies.

Structure

A telomere caps each end of every chromosome and binds specific proteins that protect it from being digested by enzymes (exonucleases) present in the same cell. Most important, the telomere permits DNA replication to continue to the very end of the chromosome, thus assuring its stability. The telomere is also involved in attachment of the chromosome ends to the nuclear membrane and in pairing of homologous chromosomes during meiosis. The structure of telomeric DNA is very similar in virtually all eukaryotic organisms except the fruit fly (Drosophila). One strand of the DNA is rich in guanine and is oriented toward the end of the chromosome, and the other strand is rich in cytosine and is oriented toward the centromere. In most organisms, the telomere consists of multiple copies of a very short DNA repeat.

The centromere is responsible for proper segregation of each chromosome pair during cell division. The chromatids in mitosis and each pair of homologous chromosomes in meiosis are held together at the centromere until anaphase, when they separate and move to the spindle poles, thus being distributed to the two daughter cells. The kinetochore, which is the attachment site for the microtubules that guide the movement of the chromosomes to the poles, is organized around the centromere. The molecular structures of centromeres in most species are still unclear. The repetitive DNA making up and surrounding the centromere is called heterochromatin because it remains condensed throughout the cell cycle and hence stains intensely.

One or more pairs of chromosomes in each species have a region called a secondary constriction which does not stain well. This region contains multiple copies of the genes that transcribe, within the nucleolus, the ribosomal RNA (rRNA). The number of active rRNA genes may be regulated, and an organism that has too few copies of the rRNA genes may develop abnormally or not survive. See also Ribosomes.

Staining

Staining with quinacrine mustard produces consistent, bright and less bright fluorescence bands (Q bands) along the chromosome arms because of differences in the relative amounts of CG (cytosine-guanine) or AT (adenine-thymine) base pairs. The distinctive Q-band pattern of each chromosome makes it possible to identify every chromosome in the human genome. Quinacrine fluorescence can also reveal a difference in the amount or type of heterochromatin on the two members of a homologous pair of chromosomes, called heteromorphism or polymorphism. Such differences can be used to identify the parental origin of a specific chromosome, such as the extra chromosome in individuals who have trisomy 21. Two other methods involve treating chromosomes in various ways before staining with Giemsa. Giemsa or G-band patterns are essentially identical to Q-band patterns; reverse Giemsa or R-band patterns are the reverse, or reciprocal, of those seen with Q or G banding. In humans, most other mammals, and birds (macrochromosomes only), the Q-, G-, and R-banding patterns are so distinctive that each chromosome pair can be individually identified, making it possible to construct a karyotype, or organized array of the chromosome pairs from a single cell (Fig. 1). The chromosomes are identified on the basis of the banding patterns, and the pairs are arranged and numbered in some order, often based on length. In the human karyotype, the autosomes are numbered 1 through 22, and the sex chromosomes are called X and Y. The short arm of a chromosome is called the p arm, and the long arm is called the q arm; a number is assigned to each band on the arm. Thus, band 1q23 refers to band 23 on the long arm of human chromosome 1.

Imprinting

A chromosome carries the same complement of genes whether it is transmitted from the father or the mother, and most of these genes appear to be functionally the same. However, a small number of mammalian genes are functionally different depending on whether they were transmitted by the egg or by the sperm. This phenomenon is known as imprinting. It appears to be caused by the inactivation of certain genes in sperm or ova, probably by methylation of cytosine residues within the regulatory (promotor) region of the imprinted gene. As a result of imprinting, normal development of the mammalian embryo requires the presence of both a maternal and a paternal set of chromosomes. Parthenogenesis, the formation of a normal individual from two sets of maternal chromosomes, is therefore not possible in mammals.

Sex chromosomes

In most mammals, the sex of an individual is determined by whether or not a Y chromosome is present because the Y chromosome carries the male-determining SRY gene. Thus XX and the rare XO individuals are female, while XY and the uncommon XXY individuals are male. In contrast, sex in the fruit fly depends on the balance of autosomes (non-sex chromosomes) and X chromosomes. Thus, in diploids, XX and the rare XXY flies are female, while XY and the rare XO flies are male. In both mammals and fruit flies, males are the heterogametic sex, producing gametes that contain either an X or a Y chromosome; and females are the homogametic sex, producing only gametes containing an X. In birds and butterflies, however, females are the heterogametic sex and males the homogametic sex. Other sex-determining systems are used by some classes of organisms, while sex in some species is determined by a single gene or even by environmental factors such as temperature (some turtles and alligators) or the presence of a nearby female (Bonellia, a marine worm) rather than by a chromosome-mediated mechanism.

More than 900 gene loci have been mapped to the human X chromosome. If the genes on both X chromosomes were fully expressed in female mammalian cells, then male cells, which have only one X, would exhibit only half as much gene product as female cells. However, dosage compensation is achieved, because genes on only one X chromosome are expressed, and genes on any additional X chromosomes are inactivated. This X inactivation randomly occurs during an early stage in embryonic development, and is transmitted unchanged to each of the daughter cells. Mammalian females are therefore mosaics of two types of cells, those with an active maternally derived X and those with an active paternally derived X. Species other than mammals do not show this type of dosage compensation mechanism for sex-linked genes, and some show none at all.

The Y chromosome is one of the smallest chromosomes in the genome in most mammalian species. Usually the mammalian Y chromosome has a very high proportion of heterochromatin, as does the large Y chromosome in Drosophila. Very few genes are located on the Y chromosome in mammals or in Drosophila, and most of these genes are concerned with either sex determination or the production of sperm. In some species of insects and other invertebrates, no Y chromosome is present, and sex in these species is determined by the X:autosome balance (XX female, XO male). See also Cell nucleus; Genetics; Human genetics; Sex determination; Sex-linked inheritance.