Chromosome: a unit of genetic material. May be thought of as a package of genes. (Indge, 60) The dark structures inside each cell’s nucleus. Each chromosome contains both protein and one long, thin molecule of DNA. Although DNA is packed into these dark rods with the help of proteins, experiments showed that DNA, not the proteins, are the blueprints for making more proteins. (Batiza, 9)

Chromosomes are the structures in cells that package "genes" and ensure their safe transfer into new cells. A person has 46 chromosomes, half of which were inherited from each parent. (GNN) Chromosomes are paired. One comes from the mother, the other from the father. The offspring thus receives one copy of each gene from each of its two parents. When a paired chromosome separates, as it does during “cell division,” the genes on each chromosome must be copied exactly into genes on the new chromosome. Life’s key processes—the storing and passing on of biological information from one "generation" to the next—are carried out through the “replication” of chromosomes and the “expression” of genes. (Kandel, 242) (A chromosome) is duplicated and transmitted, via “mitosis” or “meiosis,” to the next cell generation. (Lewis, 236) Except for the sex chromosomes, ‘X’ and ‘Y,’ the human chromosomes are numbered according to size. In chromosome 1 there resides 8% of each cell’s total DNA, about a quarter of a billion “base pairs.” (In contrast), chromosomes 21 and 22 contain some 40 and 45 million base pairs respectively. (Watson, 87)

Autosomes: non-sex chromosomes. (Batiza, 43) Same in both sexes. Paired chromosomes. (Brooker, 337) In the human body cell, there are 23 pairs of chromosomes. One pair of these are sex chromosomes, the other 22 pairs are autosomes. (Indge, 26) There are 22 pairs of these ”homologues” in humans, which equals 44 chromosomes. (Norman, 7/21/09) Adjective - 'autosomal.'

Chromosome Structure: consists of tightly coiled DNA which is wound around “histones.” (Norman, 6/17/09) Composed of twisted parallel strands of DNA. Has thousands of DNA (sequences) called genes that are strung like beads along its length. (Hockenbury, 352-353) Includes small amounts of “RNA.” (Lewis, 236)

Centromere: an indentation on a chromosome where its identical "sister chromatids" are connected and where 'spindle' fibers attach. (Batiza, 166) A constricted region of a chromosome that separates it into a short arm and a long arm. Serves as an attachment site for a group of proteins that form the “kinetochore.” (Brooker, 308)

Chromatid: one of two identical halves of a replicated chromosome. During “cell division,” the chromosomes first replicate so that each "daughter cell" receives a complete set of chromosomes. Following DNA replication, the chromosome consists of two identical structures called "sister chromatids," which are joined at the centromere. (NHGRI) One of the two side by side replicas produced by chromosome replication in mitosis or meiosis. (NCIt)

Sister Chromatids: when DNA is replicated, two identical copies of the original double helix are created. These copies, along with associated proteins, lie side by side and are termed “sister chromatids.” When a cell prepares to divide, they become highly compacted at a region called the centromere. (Brooker, 308)

Chromatin: the material of chromosomes. It is a complex of DNA, histones, and non-histone proteins found within the nucleus of a cell. (MeSH) The biochemical composition of chromosomes. (Brooker, G-7) The "genetic material" of the nucleus occurs in two forms during the phase between “mitotic divisions.” (Occurs) as ‘heterochromatin,’ (which is) seen as a condensed (form), (and as) ‘euchromatin’ which is dispersed. During mitotic division the chromatin condenses into chromosomes. (NCIt) Designed to help to compact... chromosome to fit inside the nucleus. (Brooker, 76) A substance within a chromosome consisting of DNA and protein. The major proteins in chromatin are histones, which help package the DNA (into) a compact form that fits in the cell nucleus. Changes in chromatin structure are associated with DNA replication and "gene expression." (NHGRI) It is about 30% histones, 30% DNA, 30% DNA scaffold, and 10% RNA. Specific points along the chromatin attach it to the inner face of the “nuclear membrane.” (Lewis, 171)

Histone(s): a type of protein which is found with DNA in chromosomes. In “eukaryotic” cells, half of the total mass of each chromosome is due to the DNA it contains and half is due to protein. The commonest type of protein. Important in the packaging of DNA to form chromosomes. (Indge, 137) During cell division, DNA winds tightly around histones so that it is visible. (Norman, 6/17/09) Chemical modification of histones controls when particular DNA sequences unwind and become accessible. (Lewis, 171)

Nucleosome: repeating structural units of chromatin, each consisting of approximately 200 base pairs of DNA wound around a protein core. This core is composed of the histones. (MeSH) The basic repeating unit of chromatin. In a human cell, about six feet of DNA must be packaged into a nucleus with a diameter less than a human hair. Nucleosomes are arranged like beads on a string. They are repeatedly folded in on themselves to form a chromosome. (NHGRI)

Locus: a position on a chromosome on which a particular gene is located. (Oxford) The physical location of a gene on a chromosome. (Brooker, G-21) The position on a chromosome of a gene or other chromosome “marker.’ Also, the DNA at that position. (HGPIA) The locus of a gene is the same for each member of a "homologous" pair, whether the individual is "homozygous" or "heterozygous" for that gene. (Brooker 333) Plural - ‘loci.’

Telomere: the end of a chromosome. Made of repetitive sequences of “non-coding DNA” that protect the chromosome from damage. Each time a cell divides, the telomeres become shorter. Eventually, the telomeres become so short that the cell can no longer divide. (NHGRI) Located at the ends of chromosomes and composed of a series of repeat sequences. (For example, in humans ‘TTAGGG’). Has a region at the '3-prime end' that is termed a '3-prime overhang.' “Primase” makes an RNA "primer” near the end of the telomere, and “DNA polymerase” synthesizes a complementary strand that is sealed by “ligase.” (Brooker, 226) All cells eventually loose their telomeres. (Norman, 7/2/09) Shortens with each “mitotic” cell division. (Lewis, 236)

Subtelomers: part of the chromosome. Located between telomeres and protein-rich areas. Region extends from 8,000 to 300,000 bases. 500 protein encoding genes lie in the subtelomere regions of all chromosomes. (Lewis, 236-237) The chromosomal region (next) to the telomere composed of highly “polymorphic” repetitive DNA sequences that are typically situated adjacent to gene-rich areas. Subtle rearrangements that disrupt genes in the subtelomeric regions can cause (“neurological disorders”). Use of “fluorescent in situ hybridization (FISH)” to evaluate subtelomeric regions is usually required for detection of these (disorders). (GeneReviews)

Chromosome Territory: each chromosome is located in a distinct, non-overlapping region within the cell nucleus which is visible when cells are exposed to dyes that label each type of chromosome. (Brooker, 76)

Chromosome Theory of Inheritance: chromosomes carry the genes that determine an organism’s traits. (Indge, 337) The theory stating that patterns of inheritance can be explained by assuming that chromosomes are the carriers of genetic information and that a given gene is located at a particular site on a chromosome. (Lawrence)

Genomic Imprinting: the process by which one chromosome of a pair is chemically modified, depending on whether the chromosome comes from the father or the mother. These modifications lead to differential expression of a gene or genes on a maternally derived chromosome versus a paternally derived chromosome. (GeneReviews) For certain genes, parental origin does influence the “phenotype.” These genes are said to be 'imprinted.' "Methyl groups" cover a gene or several linked genes and prevent them from being accessed to synthesize protein. The result of this gene 'cloaking' is that a disease may be more severe, or different, depending upon which parent transmitted the (mutation). A particular gene might function if it came from the father, but not if it came from the mother, or vice versa. (Lewis, 124) A phenomenon in which the disease phenotype depends on which parent passed on the disease gene. For instance, both ‘Prader-Willi’ and ‘Angelman' syndromes are inherited when the same part of chromosome 15 is missing. When the father's complement of 15 is missing, the child has Prader-Willi, but when the mother's complement of 15 is missing, the child has Angelman syndrome. (NCI3) (This) variable phenotypic expression of a gene depends on whether it is of paternal or maternal origin. (Caused by) the DNA 'methylation' pattern. Imprinted regions are observed to be more methylated and less active. (MeSH) Also referred to as ‘imprinting.’

Sex Chromosome(s): the X or Y chromosome in human beings. Determines the sex of an individual. Females have two X chromosomes in diploid cells; males have an X and a Y chromosome. The sex chromosomes comprise the 23rd chromosome pair in a “karyotype.” (HGPIA) In humans and other mammals females have two “X chromosomes” while the male has one X chromosome and one “Y’ chromosome.” Although X and Y chromosomes differ from each other in appearance, they can pair with each other during meiosis. This is because part of each sex chromosome is identical. (Indge, 247) These genes have their own characteristic “inheritance patterns.” The study of sex chromosomes proved pivotal in confirming the chromosome theory of inheritance. (Brooker, 337) In Genetics, the ‘XX’ notation indicates female and the ‘XY’ notation indicates male. (Norman, 7/21/09)

X chromosome: a human female has two X chromosomes and zero Y chromosomes. In female mammals, the paternally inherited X chromosome is shut off in some cells. (Lewis, 5)

Y chromosome: a human male has one X chromosome and one Y chromosome. (Includes) genes that determine maleness. (Lewis, 5) Egg cells all contain an X chromosome, while sperm cells contain an X or a Y chromosome. This arrangement means that during fertilization, it is the male that determines the sex of the offspring. (NHGRI)