People have 23 pairs of chromosomes for a total of 46 chromosomes. Males and Females differ in a pair of chromosomes called sex chromosomes. Females have two X chromosomes in their cells and males have one X and one Y chromosome.
Inheriting too many or not enough sex chromosomes can lead to serious problems. For example, females who have extra copies of the X chromosomes are usually taller than average and have mental retardation. Males with more than one X chromosome have Klinefelter syndrome which is a condition characterized by tall stature and often infertility.
Another syndrome caused by imbalance in the number of sex chromosomes is Turner syndrome. Women with Turner syndrome have only one X chromosome. They are usually very short, do not undergo puberty and some have kidney and heart problems.
Preimplantation genetic screening PGS of a cell biopsied from an embryo can improve the chance of a successful pregnancy by determining which embryo has the correct number of chromosomes enabling the transfer of that embryo instead of an embryo that does not have the correct number of chromosomes and would be thus less likely to make a healthy baby. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid DNA.
Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique. The term chromosome comes from the Greek words for color chroma and body soma. Scientists gave this name to chromosomes because they are cell structures, or bodies, that are strongly stained by some colorful dyes used in research. The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones. Without such packaging, DNA molecules would be too long to fit inside cells.
For example, if all of the DNA molecules in a single human cell were unwound from their histones and placed end-to-end, they would stretch 6 feet. For an organism to grow and function properly, cells must constantly divide to produce new cells to replace old, worn-out cells. During cell division, it is essential that DNA remains intact and evenly distributed among cells. Chromosomes are a key part of the process that ensures DNA is accurately copied and distributed in the vast majority of cell divisions.
Still, mistakes do occur on rare occasions. Changes in the number or structure of chromosomes in new cells may lead to serious problems.
For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes.
It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure. If not, the resulting offspring may fail to develop properly. For example, people with Down syndrome have three copies of chromosome 21, instead of the two copies found in other people.
Chromosomes vary in number and shape among living things. Most bacteria have one or two circular chromosomes. Humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell. The only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome.
When two reproductive cells unite, they become a single cell that contains two copies of each chromosome. This cell then divides and its successors divide numerous times, eventually producing a mature individual with a full set of paired chromosomes in virtually all of its cells. Besides the linear chromosomes found in the nucleus, the cells of humans and other complex organisms carry a much smaller type of chromosome similar to those seen in bacteria.
This circular chromosome is found in mitochondria, which are structures located outside the nucleus that serve as the cell's powerhouses. 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.
Source: National Human Genome Research Institute At the end of each chromosome is a repetitive nucleotide sequence cap called a telomere. 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 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.
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.
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