Chromosomes | Richard Crooks's Website

The information required to construct an organism is stored in its genetic sequence. The genetic sequence is a sequence of nucleotides contained in nucleic acids. How this information is stored and accessed by the organism varies between organisms, but broadly speaking, complex organisms, known as eukaryotes, which includes plants and animals have common architecture for how they store their genetic sequences.

Diagram showing the nucleus in the cell, and how it responds to signals. — Figure 1: The nucleus is where the cell's genetic material is stored. The nucleus responds to signalling pathways that originate inside or outside the cell, and in response transcribes mRNA. This is then transported to the ribosomes, where proteins are translated from the mRNA sequence and either exported out of the cell, or remain in the cell as intracellular proteins.

Inside the cell there is a nucleus (Figure 1), and it is in this nucleus that DNA is packaged. Genes are organized into structures called chromosomes (Figure 2). These condense into distinct visible chromosome structures during the process of cell division. Different species have different numbers of chromosomes, humans have forty six for example, which are organized into twenty three pairs. Many organisms use chromasomes to determine the sex of an individual, so chromosomes can be distinguished as autosomes which are those not involved in determining the sex of the individual, and sex chromosomes which are. In humans and other mammals, XX chromosomes are typically found in females, while XY chromosomes are found in males [note 1].

Diagram of a chromosome — Figure 2: A chromosome is a condensed fragment of DNA sequence which is used to package the DNA in a compact format for storage in the chromosome. Chromosomes are built from DNA that wraps around histones to produce nucleosomes, which in turn coil into chromatin fibres, which in turn form the structure of the chromosomes. This structure allows incredibly long DNA sequences to be condensed into the small space of the cell nucleus. The centromere, which is the central point of the X shape of the chromosome, is where the sister chromatids meet. These sister chromatids are identical to one another, and during cell division they separate into the daughter cells before reforming the chromosomes. The telomeres, which are the ends of the arms of the chromosomes, whose damage over time has been linked to aging. And the p and q arms, which are the short and the long arms of the chromosomes respectively.

Chromosomes have broadly the same features (Figure 2): the centromere, where the long (q) and short (p) arms meet in the middle, and the telomeres at the ends of each arm. At the centromere, sister chromatids meet. Sister chromatids are identical to one another on the same chromosome, and are vital in cell division, allowing the cells to divide and retain their genetic material.

Chromosomes store genes and for an organism to access a gene, the chromosome provides a way of regulating that access, as well as containing those genes in a compact manner. Because of the role of the chromosomes in storing genes, it has been recognized that changes to the physical structure of the chromosomes, as distinct from changes to the DNA sequences may be responsible for disease.

Entire chromosomes, or sections of them can be removed or duplicated and this can cause disease (Figure 3). Some well known examples of this are Kleinfelter Syndrome (extra X chromosome along with X and Y chromosomes), Turner syndrome (only a single X chromosome with no accompanying X or Y chromosomes), or Down's Syndrome (three copies of chromosome 21). These syndromes are well characterized and cause noticable health problems for those affected, as a large number of genes may be affected. Smaller parts of chromosomes being duplicated, inverted or removed can also cause problems (Figure 3), but these may be only involving the genes which have been affected by the changes, and are thus far less noticeable. These small changes may require expert investigation to link them to any patient symptoms.

Diagram of chromosome abnormalities — Figure 3: Structural changes that can happen to a pair of chromosomes. In a trisomy or a monosomy, an entire chromosome can be added or deleted respectively. This notably happens when an error occurs during meiosis and gametes which are missing, or have an extra copy of a chromosome are produced, and if they are fertilized can give rise to offspring with trisomies or monosomies. In deletions a part of a chromosome has been deleted, depending on what genes have been affected by the deletion, this can have the effect of a loss of function mutation in those genes. In duplications, part of the chromosome has been copied. These are less likely to be harmful than deletions, and there are some common structural features found in all humans that are the result of such duplications that happened in the distant past. Inversions are where the orientation of part of a chromosome swaps. Although the same DNA sequence should be present after a complete inversion, an inversion that bisects a gene would be catastrophic to that gene function. Furthermore, an inversion can change the location of a gene relative to other regulatory elements within a chromosome, which can produce complex effects.

Parts of chromosomes can be rearranged, known as translocations. So that the content of the chromosomes remains the same, but the location of that content changes. These are especially important in cancer, and one notable example is the Philadelphia translocation, where parts of chromosomes 9 and 22 are swapped (Figure 4). This translocation is found in chronic myeloid leukemia, which is a type of blood cancer. But there are other such chromosome changes that appear to be important in other forms of cancer.

Diagram of the Philadelphia chromosome — Figure 4: The Philadelphia chromosome is a significant translocation that is involved in chronic myeloid leukemia, a type of blood cancer. Swapping these sections of chromosomes 9 and 22 causes disruption of the ABL and BCR and creates a ABL-BCR fusion gene, as these genes are involved in regulating the cell cycle, disruption of these genes is a key step in how normal blood cells can start proliferating uncontrollably. Other translocations are found in other forms of cancer. Not all translocations are harmful, some translocations in the past were part of how species diverged from one another. The phenomenon of how different parts of chromosomes can be mapped onto one another across species is known as synteny.

Lastly the ends of the chromosomes known as telomeres can become damaged. The telomeres act as caps that protect the genes found deeper in the chromosomes from damage. However like any other protective cover it can wear out, and it believed that as these shorter, it is responsible for many of the effects of aging.

Despite this being generally true, this is not a absolute rule. Sex development actually involves many genes, some of which can fail to function correctly giving rise to exceptions. Furthermore gender identity and gender expression add a vast amount of complexity on top of this.