10 Facts about Chromosomes

Chromosomes. Credit: Sergey Panteleev/E+

Chromosomes are cell components that are composed of DNA and located within the nucleus of our cells. The DNA of a chromosome is so long, that it must be wrapped around proteins called histones and coiled into loops of chromatin in order for them to be able to fit within our cells. The DNA comprising chromosomes consists of thousands of genes that determined everything about an individual. This includes sex determination and inherited traits such as eye color, dimples, and freckles.

Discover ten interesting facts about chromosomes.

1: Bacteria Have Circular Chromosomes

Unlike the thread-like linear strands of chromosomes found in eukaryotic cells, chromosomes in prokaryotic cells, such as bacteria, typically consist of a single circular chromosome. Since prokaryotic cells do not have a nucleus, this circular chromosome is found in the cell cytoplasm.

2: Chromosome Numbers Vary Among Organisms

Organisms have a set number of chromosomes per cell. That number varies across different species and is on average between 10 to 50 total chromosomes per cell. Diploid human cells have a total of 46 chromosomes (44 autosomes, 2 sex chromosomes). A cat has 38, lily 24, gorilla 48, cheetah 38, starfish 36, king crab 208, shrimp 254, mosquito 6, turkey 82, frog 26, and E.coli bacterium 1. In orchids, chromosome numbers vary from 10 to 250 across species. The adder's-tongue fern (Ophioglossum reticulatum) has the most number of total chromosomes with 1260.

3: Chromosomes Determine Whether You are Male or Female

Male gametes or sperm cells in humans and other mammals contain one of two types of sex chromosomes: X or Y. Female gametes or eggs, however, contain only the X sex chromosome. If a sperm cell containing an X chromosome fertilizes

4: X Chromosomes Are Bigger Than Y Chromosomes

Y chromosomes are about one-third the size of X chromosomes.

The X chromosome represents about 5 percent of the total DNA in cells, while the Y chromosome represents about 2 percent of a cell's total DNA.

5: Not All Organisms Have Sex Chromosomes

Did you know that not all organisms have sex chromosomes? Organisms such as wasps, bees, and ants do not have sex chromosomes. Sex is therefore determined by fertilization. If an egg becomes fertilized, it will develop into a male. Unfertilized eggs develop into females. This type of asexual reproduction is a form of parthenogenesis.

6: Human Chromosomes Contain Viral DNA

Did you know that about 8 percent of your DNA comes from a virus? According to researchers, this percentage of DNA is derived from viruses known as borna viruses. These viruses infect the neurons of humans, birds and other mammals, leading to infection of the brain. Borna virus reproduction occurs in the nucleus of infected cells.

Viral genes that are replicated in infected cells can become integrated into chromosomes of sex cells. When this occurs, the viral DNA is passed from parent to offspring. It is thought that borna virus could be responsible for certain psychiatric and neurological illness in humans.

7: Chromosome Telomeres are Linked to Aging and Cancer

Telomeres are areas of DNA located at the ends of chromosomes.

They are protective caps that stabilize DNA during cell replication. Over time, telomeres wear down and become shortened. When they become too short, the cell can no longer divide. Telomere shortening is linked to the aging process as it can trigger apoptosis or programmed cell death. Telomere shortening is also associated with cancer cell development.

8: Cells Don't Repair Chromosome Damage During Mitosis

Cells shut off DNA repair processes during cell division. This is because a dividing cell does not recognize the difference between damaged DNA stands and telomeres. Repairing DNA during mitosis could cause telomere fusion, which may result in cell death or chromosome abnormalities.

9: Males Have Increased X Chromosome Activity

Because males have a single X chromosome, it is necessary for cells at times to increase gene activity on the X chromosome.

The protein complex MSL helps to up-regulate or increase gene expression on the X chromosome by helping the enzyme RNA polymerase II to transcribe DNA and express more of the X chromosome genes. With the help of the MSL complex, RNA polymerase II is able to travel further along the DNA strand during transcription, thereby causing more genes to be expressed.

10: There Are Two Main Types of Chromosome Mutations

Chromosome mutations sometimes occur and can be categorized into two main types: mutations that cause structural changes and mutations that cause changes in chromosome numbers. Chromosome breakage and duplications can cause several types of chromosome structural changes including gene deletions (loss of genes), gene duplications (extra genes), and gene inversions (broken chromosome segment is reversed and inserted back into the chromosome). Mutations can also cause an individual to have an abnormal number of chromosomes. This type of mutation occurs during meiosis and causes cells to have either too many or not enough chromosomes. Down syndrome or Trisomy 21 results from the presence of an additional chromosome on autosomal chromosome 21.

_Sources:

• _{"Chromosome." UXL Encyclopedia of Science. 2002. Encyclopedia.com. 16 Dec. 2015 (http://www.encyclopedia.com).}
• _{"Chromosome Numbers For Living Organisms." Alchemipedia. Accessed 16 Dec. 2015 (http://alchemipedia.blogspot.com/2009/10/chromosome-numbers-for-living-organisms.html).}
• _{"X chromosome." Genetics Home Reference. Reviewed January 2012. (http://ghr.nlm.nih.gov/chromosome/X).}

• _{"Y chromosome." Genetics Home Reference. Reviewed January 2010. (http://ghr.nlm.nih.gov/chromosome/Y).}