Chromosomes

To get a male you need a x and y chromosome. To get a female you need to have two x chromosomes. The SRY region or sex determining Y region exsists on the p or short arm of the y chromosome. This region determines if you have a male because it turns "on" the events that make the embryo turn into a male. Without SRY you get the female, which is the default sex. There are some exceptions like males born with xxy or xxxy and even with all the extra x chromosomes the individual is still male. The extra x chromosomes will show up as barr bodies on the cell. Eggs with two x chromosomes have been injected with the SRY gene and the result shows a male phenotype with a female genotype as in labratory mice. Even with testes, the mice are sterile. Some females are born with a translocated SRY gene on one of the x chromosomes and have testicular tissue.

THe x chromosome carries a lot of different genes, but not many have to do with determining sex. Because males only have one x chromosome and there isn't a match on the y chromosome, this is why a gene on the x chromosome that is recessive in females show up in the males also called X linked. An example is hemophillia A in humans. Males who will inherit the gene from their mother will always show the disease. Heterozygous females will have one normal type copy and one mutant type copy and will show the normal phenotype. This is why they are known as carriers for the disease. There is barely any females with this disease because they would have to inherit a mutant copy from their mother and father.

Picture at left show xx mice with injected SRY gene and have the male phenotype. (Sex chromosomes. 2009. Dec 6 2009< []>.)  Chromosomes are thread-like structures in the nucleus of animal or plant cells that contain DNA and protein. They are found in cells. 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. Structure of chromosomes varies widely for different organisms, they can be either circular or linear. They can be made of up to 1,000,000,000 nucleotides. 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. (__National Humane Genome Research Institute__. 2009. 24 Nov 2009 <[]>)

Genetics had its official 'birth' from Gregor Mendel, and Augustinian monk---who was not a renowned scientist, but merely interested in the natural order of things, both organic and inorganic. Lamarck's work on evolution during the 1800s influenced Mendel to design an experiment in the church yard with ornamental plants. Concluding the experiment, Mendel discovered that not only were the traits from parents passed to progeny, but Lamarck's assumption that environment was the sole force behind phenotypic expression was thus rejected. Mendel now had a new, intriguing problem: How are traits passed from parent to offspring? This problem lead Mendel to one of the greatest discoveries in science history, and, although not appreciated at the time, Mendel's work gained enough recognition to push science into a newer and unknown field: Genetics.

In eukaryotics chromosomes consist primarily of DNA attached to a protein core. They also contain ribonucleic acid (RNA). It is the arrangement of components in the DNA molecules determines the genetic information.Each chromosome actually consists of a set of duplicate chromatids that are held together by the centromere. These sister chromatids separate during mitosis. ( **chromosome**." __Encyclopædia Britannica__. 2009. Encyclopædia Britannica Online. 1 Oct. 2009 < []>)

The human genetic make-up consists of 23 pairs of chromosomes. The first 22 are called autosomes which help to carry and define the genetic make-up of an individual in regards to non-sex defining traits such as eye color or hair color. The last set of chromosomes are the sex defining chromosomes. A last chromosome of X X would depict that the individual is a female and the chromosome combination X Y would depict that the individual is a male. In humans chromosomes are replicated through a process called Meiosis in which gametes are formed. Gametes are haploid units of reproductive information containing half (23) of the chromosomes needed to complete and individuals genetic make-up. The female, or mother, contributes 23 chromosomes and the male,or father,contributes 23 chromosomes to reproduction. Gametes then during reproduction would combine, one from the male (sperm) and one from the female (egg). Combining these two haploid gametes complete the necessary chromosomal # of 46 (23 pairs), forming a diploid, to creat a new individual genetic make-up.

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. Scientists think that, in the past, mitochondria were free-living bacteria with the ability to convert oxygen into energy. When these bacteria invaded cells lacking the power to tap into oxygen's power, the cells retained them, and, over time, the bacteria evolved into modern-day mitochondria. (__National Human Genome Research Institute.__ 2009. 24 Nov 2009. [])

Some chromosomal mutations may occur through deletions, duplications, and inversions. Deletion is when a chromosome breaks in one or more places and a portion of it is lost. The piece that is lost is called the deletion or deficiency. A duplication is a repeated segment of a chromosome. An inversion is a mutation in which a piece of a chromosome is turned around so the sequence is reversed within the chromosome. //Information taken from "Essentials of Genetics", Sixth Edition. Written by Klug, Cummings, and Spencer. Copyright 2007//

Other chromosome mutations are results of variations in the chromosome number. Aneuploidy= change in the number of single chromosomes (but not in the number of sets). Aneuploidy can be notated as 2n(+or-) x chromosomes. Through Aneuploidy, there are three, as well as more, distinct variations in chromosome numbers. Monosomy is the loss of one chromosome and is notated as 2n-1. Trisomy is the addition of a chromosome and is notated as 2n+1. Tetrasomy is an addition of two chromosomes and is notated as 2n+2. More chromosomes can be added, but tend to be rare. Euploidy = change in the number of chromosome sets. Through Euploidy, there are many variations in chromsome sets. A diploid will have two of each chromosome. Each additional set is referred to as polyploids. Organisms with three sets are specifically triploid, four sets are tetraploid, and so on. Gaining and losing an X chromosome can lead to Klinefelter syndrome or Turner syndrome. Klinefelter syndrome occurs in males and about 2/3 of the cases are notated as 47,XXY. This extra X chromosome creates underdeveloped primary and secondary male sexual characteristics, a tall stature and minor learning problems. Sometimes the development of secondary female characteristics happens. This syndrome affects about 1 in 700 males, with life expectancy as normal. Turner syndrome occurs in females and cases are notated as 45,X. The loss of an X chromosome leads to underdeveloped primary and secondary female sexual characteristics and a short stature usually no taller than 4'8". This syndrome affects about 1 in 2,500 females, with life expectancy as normal if taken care of properly. Both Klinefelter and Turner syndromes usually lead to infertility. //Information taken from "Essentials of Genetics", Sixth Edition. Written by Klug, Cummings, and Spencer. Copyright 2007//

Mutations can result in either genetic disorders or cancer. An example of a genetic disorder resulting from the deletion of part of a chromosome is a syndrome called Cri-du-chat. Research has been done and is still being done that also links chromosomal mutations to different cancers. Some mutations result in the absence of particular neccessary proteins. Without the required proteins several types of canc ers can occur, including the following: lung, breast, esophagial, cervix, kidney, liver, pancreatic, colon, and stomach. The loss of a specific protein on a particular gene, called FHIT, is what scientists believe may be one of the possible sources of the cancers previously mentioned. //Information taken from "Essentials of Genetics", Sixth Edition. Written by Klug, Cummings, and Spencer. Copyright 2007//

In mitosis and meiosis chromosomes replicate and seperate into new cells that are either referred to as sister cells or haploid gamete cells depending on meiotic or mitotic division. When these chromosomes replicate but fail to seperate during the divisions that would normally take place in mitosis or meiosis it is called non-disjunction. Non-disjunction would lead to an outcome where some cells have more than necessary genetic information (chromosomes)in them while others do not have the full amount. This is called anuploidy (having more or less than the normal genetic makeup caued by non-disjunction). Nondisjunction can lead to genetic probles such as Down Syndrome and Patau Syndrome which are Trisomic anuploids, meaning they have 3 copies of a particular chromosome in that gamete vs the normal 2. The most common genetic cause of mental retardation is due to nondisjunction leading to trisomy. Trisomy on the 21st chromosome results in Down Syndrome. This form of Down syndrome, known as Trisomy 21, accounts for 95% of Down Syndrome cases. It is denoted by a notation of (47,21+) indicating that there are three copies of chromosome 21.

Translocation which is the movement of a chromosomal segment to a new location in the genome. Reciprocal translation is the exchange of segments between two nonhomologous chromosomes. If the exchange includes internal chromosomes segments four breaks are required two on each chromosome. The risks of reciprocal translocation are very similar to the consequences of inversions. In the sense that genetic information is not lost or gained, but rather there is only rearrangement of genetic material. The presence of a translocation doesn't directly alter the viability of individuals bearing it. //Information taken from "Essentials of Genetics", Sixth Edition. Written by Klug, Cummings, and Spencer. Copyright 2007//

One example of mutation by translocation is Familial Down Syndrome. It accounts for only 5% of Down syndrome cases but is more commonly inherited than Trisomy 21, the more common form of Down syndrome. Familial Down syndrome is due to a translocation involving chromosome 21 and usually chromosome 14. The affected individual has a karyotype of 46,t(14;21). The important indication for Familial DOwn syndrome in this translocation is on the q arm of chromosome 21. It is also possible that during translocation a chromosome is lost resulting in 45 chromosomes. This indicates a translocation carrier and although they are not affected by the disease, they can pass it on. A translocation carrier for Familial Down syndrome is signified by the 45,t(14;21) notation.

Other translocations can cause other disorders, such as a translocation between chromosomes 8 and 14 can cause Burkitt's lymphoma, or a translocation between chromosomes 18 and 14 can caues B-cell leukemia, and a translocation between chromosomes 9 and 22 can cause Chronic Myelogenous leukemia. "Biology" John W. Kimball. Copyright ©2009

The chromosomes of an individual can be cataloged and compared through a karyotype in which they are arranged based on their length, position of the centromere, and their banding pattern. Homologous chromosomes (chromosomes that are the same) are arranged together in a karyotype. They are usually arranged from longest to shorest and numbered consecutively with the longest chromosomes, in humans, being number one and the shortest chromosome, in humans, being the sex chromosomes.

Karyotypes can be made either by a process known as G-banding or a process that uses MFISH. In G-banding chromosomes are arranged and compared using their banding pattern and centromere position which has been stained with dye to produce light areas and dark areas. The dark areas of a chromosomes are areas that are associated with being rich in genes, whereas areas that are light are areas that are associated with having few genes present. MFISH, which stands for Multiple Flourescent In Situ Hybridization, uses a probe that is complementary to the DNA sequences on a chromosome. This probe, which is a bright flourescent color, aligns itself to the chromosome giving it that color as well and allows the chromosome to be easily seen. By using multiple probes each with a different color the chromosomes can be easily distinguished and arranged in a karyotype. The MFISH process is much more preferred than G-banding in the creation of karyotypes.

Here is an example of chromosomes being organized into a karyotype by G-banding

Karyotypes are useful in that they allow us to see what chromosomes are missing, extra, deleted, duplicated, etc. in an indivual and are the cause of the genetic disorder that they have. Karyotypes from different individuals can be compared to see differences that might be the cause of a disease.

//Information taken from "Essentials of Genetics," Sixth Edition. Written by Klug, Cummings, and Spencer. Copyright 2007.//

Chromosomes are also very important in the study of evolution. By comparing the 3 largest chromosomes in Humans and Apes it is evident that there is a close chromosomal relationship between the two species. //Information taken from "Biology"//, R. J. Brooker, E.P. Widmaier, L.E. Graham, and P.D. Stilling. , R

Chromosomes are labeled according to arms, regions, bands, and subbands. The P arm is the shorter arm and the Q arm is the longer arm. The centromere is in the middle. Regions are labeled in both the P and Q arms starting at the top. The bands are labeled within each region, and the subbands are labeled within each band. There can never be more than 9 regions or 9 bands. The notation for Cri-du-chat is 5p15.2. The mutation is on chromosome 5, on the p arm, region 1, band 5, and subband 2.

The advancement of chromosome counting and arrangement lead to the discovery of colchicine, a drug that enlarges chromosomes but inducing polyploidy. Colchicine is usually applied as a cream on a growth point of a plant, such as strawberries, to increase fruit size. Chromosome knowledge and advancement with synthetic drugs (although derived from plants) to increase chromosome size will progress the genetic modification of other crops. One day, maybe for fuels, the next, maybe to increase yield-to-surface area ratio, etc. Assessing chromosomes and their interactions with direct contact with drugs is extremely important as the human population increases, and land for agriculture decreases. Colchicine is a great first step into the process, although others are present.

Chromosomes are characterized and organized by length using a karyotype. This is a list of all chromosomes from longest to shortest. This is done through a process called Electrophoresis, A process that uses electricity to establish the size of the chromosomes. This is done by putting the DNA in a gel and then running a current through the gel and the DNA will flow from the negative to the positive side of the gel. It won’t all flow at the same rate, the shorter the chromosome the further it will flow through the gel. There is a dye added to the DNA so that we can locate all of the chromosomes through the gel after the current is run through it.

Some human chromosome disorders are caused by structural changes as in deletions, duplications, and translocations made after a chromosome breaks. In duplications, the fragment joins the homologous chromosome and that region is repeated, leading to disorders such Fragile X. This is considered to be the most common form of mental retardation. Most individuals have 29 repeats at the end of their X-chromosome while someone with Fragile X has more than 700 repeats. In males this leads to significant intellectual disabilities, connective tissue issues, and ADD among other impacts. Females suffer from the same symptoms but usually at a lesser degree. http://www.biology.iupui.edu/biocourses/n100/2k2humancsomaldisorders.html http://www.nfxf.org/html/summary.htm