Chromosomes are thread-like structures in which DNA is tightly packaged within the nucleus. DNA is coiled around proteins called histones, which provide the structural support. Chromosomes help ensure that DNA is replicated and distributed appropriately during cell division. Each chromosome has a centromere, which divides the chromosome into two sections – the p (short) arm and the q (long) arm. The centromere is located at the cell’s constriction point, which may or may not be the center of the chromosome.
At the end of each chromosome is a repetitive nucleotide sequence cap called a telomere. In vertebrates, the telomere is a TTAGGG sequence repeated to approximately 15,000 base pairs. These DNA regions serve a critical role of preserving the genomic sequence by protecting the genome from degradation, and inhibiting chromosomal fusion and recombination. These regions are also involved in chromosome organization within the nucleus.
This image shows the ends of chromosomes with the telomeres visualized in red.
Source: NCI Center for Cancer Research
In humans, 46 chromosomes are arranged in 23 pairs, including 22 pairs of chromosomes called autosomes. Autosomes are labeled 1-22 for reference. Each chromosome pair consists of one chromosome inherited from the mother and one from the father.
In addition to the 22 numbered autosomes, humans also have one pair of sex chromosomes called an allosome. Instead of labeling these chromosome pairs with numbers, allosomes are labeled with letters such as XX and XY. Females have two copies of the X chromosome (one inherited from the mother and one from the father). Males have one copy of the X chromosome (inherited from the mother) and one copy of the Y chromosome (inherited from the father).
Arranged on the chromosomes are genes. Genes are made of DNA and contain the instructions for building proteins and are integral in making and maintaining the human body.
Locating a gene
The cytogenetic location, a standardized way of describing a gene’s location on the chromosome, consists of a combination of numbers and letters and is made up of three components:
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Number or letter of the chromosome (1-23, X or Y)
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Arm of the chromosome (p or q)
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Position of the gene on the arm (cytogenetic bands). The position is dependent on the light and dark bands that appear on the chromosome when stained and is expressed as a two-digit number (one digit represents region and one represents band). Sometimes the digits are followed by a decimal point and one or more digits. These additional digits represent the distance from the centromere (increasing numeric value indicates farther distance from centromere). “Cen”, “ter”, and “tel” are also used to describe the position of the gene on the arm.
Cen – close to the centromere
Ter (terminus) – close to end of either the p or q arms
Tel (telomere) – close to end of either the p or q arms
Example
Gene: Anaplastic lymphoma kinase receptor
Chromosomal location: 2p23
Location description: chromosome 2, p arm, position 23
Chromosome vs. molecular locations
Chromosome location, or cytogenetic location, is one way to describe the location of gene on a chromosome. Another way to identify the location of a gene is by using the molecular location. The sequencing of the base pairs describes the molecular location of the gene on a chromosome. The molecular location is more precise; however, small variations in the address may occur between research groups as a result of varying genome sequencing methods.
For example, in humans, the chromosomal location of the EGFR gene is 7p12, while the molecular location is Chromosome 7, NC_000007.14 (base pairs 55,019,032 to 55,207,338).
Mitosis vs. meiosis
Cells divide through two processes: mitosis and meiosis. In both processes, diploid cells (containing two sets of chromosomes, or 46 chromosomes) divide. In mitosis, the diploid “parent” cell divides and produces two diploid “daughter” cells. However, in meiosis, the parent cell produces four haploid daughter cells (each containing half of the parent cells chromosomes, or 23 chromosomes).
The critical difference between mitosis and meiosis is that mitosis produces two genetically identical daughter cells, whereas meiosis produces four genetically different daughter cells.
The phases of cell division are similar for both mitosis and meiosis, and both processes result in cytokinesis (cytoplasmic division of the daughter cells). However, in meiosis, the cycle occurs twice (meiosis I and meiosis II) before the four haploid daughter cells are produced.
Another difference between the stages of mitosis and meiosis is that in meiosis, homologous chromosomes pair up during metaphase instead of chromatids. In a homologous pair, one chromosome comes from the mother, and one chromosome comes from the father. Homologous chromosomes are very similar, but they are not identical. They carry the same genes (eg, hair or eye color), but they may not code for the same trait (eg, blonde hair or brown eyes).
Stages of cell division
Interphase – Replication of DNA. Most of a cell’s time is spent in interphase. Occurs before cell division. Consists of three stages: Gap 1 (growth), S phase (DNA replication) and Gap 2 (continues growth, prepares for cell division).
Prophase – Chromosomes condense. Mitotic spindle forms.
Metaphase – Chromosomes line up in the middle of the cell.
Anaphase – Chromatids separate from one another and are pull toward opposite sides of the cell.
Telophase – Division of the cell contents into two new cells occurs (cytokinesis).
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