Unlike prokaryotic cells, eukaryotic cells have many chromosomes in which they package their DNA. This allows eukaryotes to store much more genetic information.
Most eukaryotic organisms reproduce through sexual reproduction – meaning that each individual has two copies of each chromosome. One copy is inherited from one parent, while the other is inherited from the other parent.
This system enhances genetic diversity and protects against some diseases, since it enables individuals to inherit immune system genes from two different parents, and having two copies of a gene often enables a healthy copy inherited from one parent to “cover for” a copy of a gene that has been corrupted through harmful mutation.
It’s normal for diploid eukaryotic organisms (those which have a full set of chromosomes inherited through sexual reproduction) to have two copies of each autosome.
Sex chromosomes are considered separately from autosomes, since their inheritance pattern works differently. In humans, the sex chromosomes are referred to as the X chromosome and the Y chromosome. Other animals, like birds, use a different system of sex chromosomes.
During the process of meiosis which creates eukaryotic sex cells, the sex cells “remix” DNA between their two copies of each autosome in the process of crossing over. The result is a unique set of chromosomes which has a mix of material from both of the individual’s parents. This process is illustrated below.
When two gametes combine, they produce a cell which will grow into a new individual which will possess a copy of each chromosome from each parent. The individual’s unique genetic profile will include DNA from each of its four grandparents.
When errors are made in distributing chromosomes during meiosis or early in embryonic development, serious diseases can result due to many cells in an individual’s body having the wrong number of chromosomes.
Because each chromosome contains thousands of genes, having too many or too few chromosomes can result in serious imbalances in gene expression. In humans, many pregnancies that do not survive the first trimester are cases where the embryo inherited the wrong number of chromosomes and was not able to survive.
Other errors in chromosome replication can cause more mild syndromes such as Down syndrome, which is caused by inheriting an extra copy of chromosome 21 from one parent.
Function of Autosomes
Each autosome stores many thousands genes, each of which performs a unique function in the organism’s cells.
Under normal circumstances, each chromosome follows a “map” that is shared across individuals in the species. This allows cells to “know” where to start gene expression when they want to express a certain gene. It is thought that factors which effect gene expression use this “map” to accurately respond to a cell’s needs.
When autosomes are healthy, this enables cells to perform an awesome array of functions. Each of hundreds of subtly differing cell types in a eukaryotic organism express a different combination of genes in the right place at the right time, enabling the huge array of cellular functions we see in eukaryotic organisms like ourselves.
Each of our cells contain the necessary compliment of genes to reproduce our whole bodies. Differences between brain cells, skin cells, and muscle cells are made by cells transcribing the right genes in the right places at the right times.
Our bodies get it right almost all the time! But biologists often learn how something works by watching cases where it breaks, and seeing what happens when the mechanism is not working properly.
In the case of autosomes and their carefully arranged “map” that allows for the complexity of our bodies, problems can arise when chromosomes break and their pieces end up in the wrong place.
This event, called “translocation,” can cause genes to the expression of the wrong genes at the wrong times. Some types of cancer may be caused by translocations leading to errors in cell development and reproduction.
Examples of Autosomal Disorders
Trisomy 21 (Down Syndrome)
Down syndrome occurs when a person inherits all or part of an extra copy of chromosome 21 from one parent. This usually occurs due to a one-time error in meiosis and is not passed down through the generations.
People with Down syndrome have a variety of unusual traits and symptoms related to skeletal tissue (unusual skeletal shape, weak ligaments), nerve tissue (cognitive disabilities, poor muscle tone), and have a higher risk of some diseases due to extra expression of material from chromosome 21.
Due to the range of symptoms seen in Down syndrome, some people with Down syndrome can complete regular schooling and have independent careers, while others may need special education classes and may not be able to function independently in the workplace.
The only known risk factor for Down syndrome is having older parents, which can increase the chance that parents’ bodies will incorrectly sort chromosomes during meiosis.
Cri du Chat
Cri du chat, also known as “chromosome 5p deletion syndrome” or “Lejeune’s syndrome,” happens when a person inherits just one copy of part of chromosome 5. Some people with cri du chat also have extra copies of other parts of chromosome 5.
As with Down syndrome, cri du chat usually occurs due to an error in the sorting of the parents’ chromosomes during meiosis.
The syndrome’s name comes from French for “cry of the cat,” in reference to the unusual catlike cry that babies with cri du chat have due to their unusual skeletal and neurological traits.
Like people with Down syndrome, people with cri du chat can have unusual skeletons, weak muscles, and cognitive impairment due to the under-expression of the 5p chromosomal section.
The Philadelphia chromosome is a chromosome found in many leukemia cancer cells, which may give a clue as to how the cancer gets started.
In the Philadelphia chromosome, chromosome 9 and chromosome 22 have swapped some genetic material. The specific place where the two are joined creates a fusion protein – that is, a protein coded for by a fusion of two different genes, one from chromosome 9 and one from chromosome 22.
This gene turns cell replication to “always on,” and as a result leads to uncontrolled replication of cells which never mature and become properly functional. Leukemia occurs when these non-functioning cells multiply out of control and destroy healthy, functioning tissue.
Some scientists believe that chromosomal translocations are a common cause of cancer. At least fifteen different kinds of cancer have been found to frequently involve chromosomal translocation, often resulting in the creation of fusion proteins.
Related Biology Terms
- Gametes – The sex cells used by sexually reproducing species to produce offspring with new gene combinations, created by remixing and combining genetic material from each parent.
- Gene balance – The theory that genes need to be expressed in the right amount within cells. In the theory of gene balance, too much or too little expression of a given gene compared to others results in cellular problems.
- Gene expression – The process by which genes are turned into proteins, which perform important functions in a cell.
1. Which of the following is NOT an autosome?
A. Chromosome 21
B. Chromosome 9
C. X Chromosome
D. Chromosome 22
2. Which of the following is NOT a function of autosomes?
A. To store large amounts of genetic information needed to make a complex multicellular organism.
B. To ensure that genes are expressed in the proper amounts to create healthy tissues.
C. To pass on genetic information to daughter cells.
D. To determine an individual’s sex.
3. Which of the following is not true of gene balance?
A. Expressing too many copies of the same gene can cause problems for a cell.
B. Expressing too few copies of the same gene can cause problems for a cell.
C. Even if the right number of copies of a gene are present, it can still be expressed too much or too little if it is in the wrong place on the chromosomal “map.”
D. A cell cannot survive with the wrong number of copies of a gene.