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Chromosomes are highly organized structures that store genetic information in living organisms.The genome is the entirety of an organism's hereditary information.
Chromosomes and Genes

Chromosomes are organized structures of DNA and proteins that are found in cells. Chromosomes contain a single continuous piece of DNA, which contains many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA–bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek 'chroma' (color) and 'soma' (body) due to their property of being stained very strongly by some dyes.

Variation of chromosomes
Chromosomes vary extensively between different organisms. The DNA molecule may be circular or linear and can contain anything from tens of kilobase pairs to hundreds of megabase pairs. Typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome; for example mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes. In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the massively–long DNA molecules to fit into the cell nucleus. The structure of chromatin varies through the cell cycle, and is responsible for the organization of chromosomes into the classic four–arm structure during mitosis and meiosis.

"Chromosome" is a rather loosely defined term. In prokaryotes, a small circular DNA molecule may be called either a plasmid or a small chromosome. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest chromosomes are found in viruses: these DNA or RNA molecules are short linear or circular chromosomes that often lack any structural proteins.

Magnifying eukaryotic chromosome structure Eukaryotic chromosomes are condensed with specialized proteins called as histones
Structure of eukaryotic chromosomes

The total complement of genes in an organism or cell is known as its genome, which may be stored on one or more chromosomes; the region of the chromosome at which a particular gene is located is called its locus. A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded. Prokaryotes – bacteria and archaea – typically store their genomes on a single large, circular chromosome, sometimes supplemented by additional small circles of DNA called plasmids, which usually encode only a few genes and are easily transferable between individuals.

For example, the genes for antibiotic resistance are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via horizontal gene transfer. Although some simple eukaryotes also possess plasmids with small numbers of genes, the majority of eukaryotic genes are stored on multiple linear chromosomes, which are packed within the nucleus in complex with storage proteins called histones.

The manner in which DNA is stored on the histone, as well as chemical modifications of the histone itself, are regulatory mechanisms governing whether a particular region of DNA is accessible for gene expression. The ends of eukaryotic chromosomes are capped by long stretches of repetitive sequences called telomeres, which do not code for any gene product but are present to prevent degradation of coding and regulatory regions during DNA replication. The length of the telomeres tends to decrease each time the genome is replicated in preparation for cell division; the loss of telomeres has been proposed as an explanation for cellular senescence, or the loss of the ability to divide, and by extension for the aging process in organisms.

Bacterial chromosome In contrast to the linear chromosomes found in eukaryotic cells, the strains of bacteria initially studied were found to have single, covalently closed, circular chromosomes. Bacterial plasmids were circular. In fact, the experiments were so beautiful and the evidence was so convincing that the idea that bacterial chromosomes are circular and eukaryotic chromosomes are linear was quickly accepted as a definitive distinction between prokaryotic and eukaryotic cells.
Prokaryotic chromosomes

While the chromosomes of prokaryotes are relatively gene–dense, those of eukaryotes often contain so–called "junk DNA", or regions of DNA that serve no obvious function. Simple single–celled eukaryotes have relatively small amounts of such DNA, while the genomes of complex multicellular organisms, including humans, contain an absolute majority of DNA without an identified function.

However it now appears that, although protein–coding DNA makes up barely 2% of the human genome, about 80% of the bases in the genome may be being expressed, so the term "junk DNA" may be a misnomer.

Eukaryotes (cells with nuclei such as plants, yeast, and animals) possess multiple large linear chromosomes contained in the cell’s nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere, although under most circumstances these arms are not visible as such.

In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic chromosomes. In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi–ordered structure, where it is wrapped around histones (structural proteins), forming a composite material called chromatin.

Illustration of a chromatid A chromatid is one of two identical copies of DNA making up a chromosome that are joined at their centromeres, for the process of nuclear division (mitosis or meiosis).
Chromatin and Chromatid

Chromatin is the complex of DNA and protein found in the eukaryotic nucleus which packages chromosomes. The structure of chromatin varies significantly between different stages of the cell cycle, according to the requirements of the DNA.

During interphase (the period of the cell cycle where the cell is not dividing) two types of chromatin can be distinguished: Euchromatin, which consists of DNA that is active, e.g., expressed as protein and Heterochromatin, which consists of mostly inactive DNA. Heterochromatin seems to serve structural purposes during the chromosomal stages.

A chromatid is one of two identical copies of DNA making up a chromosome that are joined at their centromeres, for the process of nuclear division (mitosis or meiosis). A chromatid is "one–half of a replicated chromosome" and term is used so long as the centromeres remain in contact. When they separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands are called daughter–chromosomes. The tips of the chromatid are called telomeres. They are there to prevent the ends of the chromosome from attaching to other chromosomes.

It has been said that after repeated cell replication, the telomeres get shorter resulting in cell death. This means that the way telomeres work could determine the lifespan of a cell. There are normally 23 pairs of homologous chromosomes in each cell (N=23). However, the quantity of chromatids will be a multiple of 23. In a human cell, there are 23 chromosome pairs (46 chromosomes), and each chromosome has 2 chromatids. Thus, there are 92 chromatids in each cell.

Karyotype of human chromosomes Human cells are diploid and have 22 different types of autosomes, each present as two copies, and two sex chromosomes. This gives 46 chromosomes in total.
Functions of chromatin

In the early stages of mitosis or meiosis (cell division), the chromatin strands become more and more condensed. They cease to function as accessible genetic material (transcription stops) and become a compact transportable form.

This compact form makes the individual chromosomes visible, and they form the classic four arm structure, a pair of sister chromatids attached to each other at the centromere.

During divisions, long microtubules attach to the centromere and to the two opposite ends of the cell. The microtubules then pull the chromatids apart, so that each daughter cell inherits one set of chromatids. Once the cells divide, the chromatids are uncoiled and can function again as chromatin.

Human cells are diploid and have 22 different types of autosomes, each present as two copies, and two sex chromosomes. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosomes, such as Bread wheat which is hexaploid and has six copies of 6 different chromosomes – 42 chromosomes in total.