chromosome

Examples of chromosome in the following topics:

• Chromosomal Theory of Inheritance

  • The Chromosomal Theory of Inheritance identified chromosomes as the genetic material responsible for Mendelian inheritance.
  • During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
  • The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.
  • Despite compelling correlations between the behavior of chromosomes during meiosis and Mendel's abstract laws, the Chromosomal Theory of Inheritance was proposed long before there was any direct evidence that traits were carried on chromosomes.
  • They have three pair of autosomes and a pair of sex chromosomes.

• Identification of Chromosomes and Karyotypes

  • A karyotype is the number and appearance of chromosomes.
  • In a human karyotype, autosomes or "body chromosomes" (all of the non–sex chromosomes) are generally organized in approximate order of size from largest (chromosome 1) to smallest (chromosome 22).
  • However, chromosome 21 is actually shorter than chromosome 22.
  • The X and Y chromosomes are not autosomes and are referred to as the sex chromosomes.
  • Finally, the karyotype can pinpoint translocations, which occur when a segment of genetic material breaks from one chromosome and reattaches to another chromosome or to a different part of the same chromosome.

• Disorders in Chromosome Number

  • Aneuploidy, an abnormal number of chromosomes in a cell, is caused by nondisjunction, or the failure of chromosomes to separate at meiosis.
  • Of all of the chromosomal disorders, abnormalities in chromosome number are the most obviously identifiable from a karyotype and are referred to as aneuploidy.
  • If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that particular chromosome and two gametes with two copies of the chromosome.
  • If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome .
  • Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number.

• Chromosomal Structural Rearrangements

  • Cytologists have characterized numerous structural rearrangements in chromosomes, but chromosome inversions and translocations are the most common.
  • A chromosome inversion is the detachment, 180° rotation, and reinsertion of part of a chromosome.
  • Humans and chimpanzees differ cytogenetically by pericentric inversions on several chromosomes and by the fusion of two separate chromosomes in chimpanzees that correspond to chromosome two in humans.
  • A translocation occurs when a segment of a chromosome dissociates and reattaches to a different, nonhomologous chromosome.
  • A reciprocal translocation occurs when a segment of DNA is transferred from one chromosome to another, nonhomologous chromosome.

• X-Inactivation

  • The presence of extra X chromosomes in a cell is compensated for by X-inactivation in which all but one X chromosome are silenced.
  • Females possess two X chromosomes, while males have only one; therefore, if both X chromosomes remained active in the female, they would produce twice as much product from the genes on the X chromosomes as males.
  • An individual carrying an abnormal number of X chromosomes will inactivate all but one X chromosome in each of her cells.
  • In addition to the loss or gain of an entire chromosome, a chromosomal segment may be duplicated or lost.
  • Duplicated chromosomal segments may fuse to existing chromosomes or may be free in the nucleus.

• Meiosis I

  • The S phase is next, during which the DNA of the chromosomes is replicated.
  • The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations, spreading to cover the entire length of the chromosomes .
  • The tight pairing of the homologous chromosomes is called synapsis.
  • In telophase I, the separated chromosomes arrive at opposite poles.
  • Therefore, only one full set of the chromosomes is present.

• Genomic DNA and Chromosomes

  • Human body cells have 46 chromosomes, while human gametes (sperm or eggs) have 23 chromosomes each.
  • Matched pairs of chromosomes in a diploid organism are called homologous ("same knowledge") chromosomes.
  • The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity.
  • In this image, the chromosomes were exposed to fluorescent stains for differentiation of the different chromosomes.
  • A method of staining called "chromosome painting" employs fluorescent dyes that highlight chromosomes in different colors.

• Genetic Linkage and Distances

  • Morgan identified a 1:1 ratio between a segregating trait and the X chromosome, suggesting that the random segregation of chromosomes was the physical basis of Mendel's model.
  • The fact that each chromosome can carry many linked genes explains how individuals can have many more traits than they have chromosomes.
  • He suggested that alleles become unlinked when chromosomes physically exchange segments.
  • In 1913, Alfred Sturtevant, a student in Morgan's laboratory, created the first "chromosome map," a linear representation of gene order and relative distance on a chromosome .To construct a chromosome map, Sturtevant assumed that genes were ordered serially on threadlike chromosomes.
  • He also assumed that the incidence of recombination between two homologous chromosomes could occur with equal likelihood anywhere along the length of the chromosome.

• Genetic Linkage and Violation of the Law of Independent Assortment

  • Genes that are located on separate non-homologous chromosomes will always sort independently .
  • However, each chromosome contains hundreds or thousands of genes organized linearly on chromosomes like beads on a string.
  • Homologous chromosomes possess the same genes in the same linear order.
  • In preparation for the first division of meiosis, homologous chromosomes replicate and synapse.
  • The result is two recombinant and two non-recombinant chromosomes.

• Eukaryotic Chromosomal Structure and Compaction

  • Chromosomes must coil to pack DNA into the cell during cell division, a process involving 3 levels of compaction.
  • During some stages of the cell cycle, the long strands of DNA are condensed into compact chromosomes.
  • These fibrous proteins also ensure that each chromosome in a non-dividing cell occupies a particular area of the nucleus that does not overlap with that of any other chromosome.
  • After replication, the chromosomes are composed of two linked sister chromatids.
  • When a cell undergoes mitosis, the chromosomes condense even further.