Chromosome

A chromosome is a threadlike structure found in the nucleus of most cells that carries the genetic material in the form of a linear sequence of deoxyribonucleic acid (DNA). In prokaryotes, or cells without a nucleus, the chromosome represents circular DNA containing the entire genome. In eukaryotes, or cells with a distinct nucleus, chromosomes are much more complex in structure. The function of chromosomes is to package the extremely long DNA sequence. A single uncoiled chromosome could be as long as 3 inches (7.6 centimeters). If DNA were not coiled within chromosomes, the total DNA in a typical eukaryotic cell would extend thousands of times the length of the cell nucleus.

DNA is the genetic material of all cells and contains information necessary for the synthesis of proteins. DNA is composed of two strands of nucleic acids arranged in a double helix. The nucleic acid strands are composed of a sequence of nucleotides. The nucleotides in DNA have four kinds of nitrogen-containing bases: adenine, guanine, cytosine, and thymine. Within DNA, each strand of nucleic acid is partnered with the other strand by bonds that form between these nucleotides. Complementary base pairing dictates that adenine pairs only with thymine, and guanine pairs only with cytosine (and vice versa). Thus, by knowing the sequence of bases in one strand of the DNA helix, the sequence on the other strand is determined. For instance, if the sequence in one strand of DNA were ATTCG, the other strand's sequence would be TAAGC.

DNA functions in the cell by providing a template by which another nucleic acid, called ribonucleic acid (RNA), is formed. Like DNA, RNA is also composed of nucleotides. Unlike DNA, RNA is single stranded and does not form a helix. In addition, the RNA bases are the same as in DNA, except that uracil replaces thymine. RNA is transcribed from DNA in the nucleus of the cell. Genes are expressed when the chromosome uncoils with the help of enzymes called helicases and specific DNA binding proteins. DNA is transcribed into RNA.

Newly transcribed RNA is called messenger RNA (mRNA). Messenger RNA leaves the nucleus through the nuclear pore and enters the cytoplasm. There, the mRNA molecule binds to a ribosome (also composed of RNA) and initiates protein synthesis. Each block of three nucleotides, called codons, in the mRNA sequence encodes for a specific amino acid, the building blocks of a protein.

Genes are part of the DNA sequence called coding DNA. Noncoding DNA represents sequences that do not have genes and only recently have been found to have many new important functions. Out of the 3 billion base pairs that exist in human DNA, the sequence obtained from the Human Genome Project has revealed that there are only about 20,000 protein-coding genes. The noncoding sections of DNA within a gene are called introns, while the coding sections of DNA are called exons. After transcription of DNA to RNA, the RNA is processed. Introns from the mRNA are excised out of the newly formed mRNA molecule before it leaves the nucleus.

The human genome (which represents the total amount of DNA in a typical human cell) has approximately 3 × 10⁹ base pairs. If these nucleotide pairs were letters, the genome book would number over a million pages. There are 23 pairs of chromosomes, for a total number of 46 chromosomes in a diploid cell, or a cell having all the genetic material. In a haploid cell, there is only half the genetic material. For example, sex cells (the sperm or the egg) are haploid, while other cells in the body are diploid. One of the chromosomes in the set of 23 is X or Y (sex chromosomes), while the rest are assigned numbers 1 through 22. In a diploid cell, males have both an X and a Y chromosome, while females have two X chromosomes. During fertilization, the sex cell of the father combines with the sex cell of the mother to form a new cell, the zygote, which eventually develops into an embryo. If one of the sex cells has the full complement of chromosomes (diploidy), then the zygote would have an extra set of chromosomes. This is called triploidy and represents an anomaly that usually results in a miscarriage. Sex cells are formed in a special kind of cell division called meiosis. During meiosis, two rounds of cell division ensure that the sex cells receive the haploid number of chromosomes.

Other species have different numbers of chromosomes in their nuclei. As some examples, mosquitoes have 6 chromosomes, lilies have 24 chromosomes, earthworms have 36 chromosomes, chimps have 48 chromosomes, and horses have 64 chromosomes.

Chromosomes can be visualized using a microscope just prior to cell division, when the DNA within the nucleus uncoils as it replicates. By visualizing a cell during metaphase, a stage of cell division or mitosis, researchers can take pictures of the duplicated chromosome and match the pairs of chromosomes using the characteristic patterns of bands that appear on the chromosomes when they are specially stained. The resulting arrangement is called a karyotype.

The ends of the chromosome are referred to as telomeres, which are required to maintain stability and recently have been associated with aging. An enzyme called telomerase maintains the length of the telomere. Older cells tend to have shorter telomeres. The telomere has a repeated sequence (TTAGGG), and intact telomeres are important for proper DNA replication processes.

Karyotypes are useful in diagnosing some genetic conditions, because the karyotype can reveal an aberration in chromosome number or large alterations in structure. For example, Down syndrome—the most common chromosome abnormality in human—is caused by an extra chromosome 21, a condition called trisomy 21. A karyotype of a child with Down syndrome would reveal this extra chromosome.

A chromosome usually appears to be a long, slender rod of DNA. Pairs of chromosomes are called homologues. Each separate chromosome within the duplicate is called a sister chromatid. The sister chromatids are attached to each other by a structure called the centromere. Chromosomes appear to be in the shape of an X after the material is duplicated. The bottom, longer portion of the X is called the long arm of the chromosome (q-arm), and the top, shorter portion is called the short arm of the chromosome (p-arm).

Several kinds of proteins are important for maintaining chromosomes, in terms of organization and gene expression. Some proteins initiate DNA replication when the cell prepares to divide. Other proteins control gene transcription in the preliminary stages of protein synthesis. Structural proteins help the DNA fold into the intricate configurations within the packaged chromosome. DNA in chromosomes is associated with proteins and this complex of DNA and proteins is called chromatin. Euchromatin refers to parts of the chromosome that have coding regions or genes, while heterchromatin refers to regions that are devoid of genes or regions where gene transcription is turned off. DNA binding proteins can attach to specific regions of chromatin. These proteins mediate DNA replication, gene expression, or represent structural proteins important in packaging the chromosomes. Histones are structural proteins of chromatin and are the most abundant protein in the nucleus. In fact, the mass of histones in a chromosome is almost equal to that of DNA. Chromosomes contain five types of these small proteins, which participate in organizing DNA within the chromosome.

A histone complex functions as a spool from which DNA is wound two times. Each histone-DNA spool is called a nucleosome. Nucleosomes occur at intervals of every 200 base pairs of the DNA helix. In photographs taken with the help of powerful microscopes, DNA wrapped around nucleosomes resembles beads (the nucleosomes) threaded on a string (the DNA molecule). The DNA that exists between nucleosomes is called linker DNA. Chromosomes can contain some very long stretches of linker DNA. Often, these long linker DNA sequences are the regulatory portions of genes. These regulatory portions switch genes on when certain molecules bind to them.

Chromosomes in eukaryotes perform a useful function during mitosis, the process in which cells replicate their genetic material and then divide into two new cells (also called daughter cells). Because the DNA is packaged within chromosomes, the distribution of the correct amount of genetic material to the daughter cells is maintained during the complex process of cell division.

Before a cell divides, the chromosomes are replicated within the nucleus. In a human cell, the nucleus just prior to cell division contains 46 pairs of chromosomes. When the cell divides, the sister chromatids from each duplicated chromosome separate. Each daughter cell ends up with 23 pairs of chromosomes, and after DNA replication the daughter cells will have a diploid number of chromosomes.

In meiosis, the type of cell division that leads to the production of sex cells, the division process is more complicated. Two rounds of cell division occur in meiosis. Before meiosis, the chromosomes replicate, and the nucleus has 46 pairs of chromosomes. In the first round of meiotic cell division, the homologous chromosomes pairs separate as in mitosis (a stage called meiosis I). In the second round of cell division (meisosis II), the sister chromatids of each chromosome separate at the centromere, so that each of the four daughter cells receives the haploid number of chromosomes.

DNA is bound up within chromatids, which serve as storage units for the DNA. In order for an mRNA molecule to be transcribed from a DNA template, the DNA needs to be freed from its tightly bound and condensed conformation so that the RNA molecule can form on its exposed strands during transcription. Some evidence exists that transcription can take place through histones. However, most often the genes on the DNA are activated after a DNA binding protein unwinds the chromatid structure. Thus loosened, transcriptionally active regions of DNA then resemble microscopic “puffs” on the chromosomes. When RNA transcription concludes, the puffs recede, and the chromosome is thought to resume its original unwound conformation.