Chromosomes: Storing every living being's genetic setup in one molecule

Chromosomes come in different types and numbers according to the species.

Here's all you need to know about them.

What are chromosomes made of?

Chromosomes are made of a single DNA molecule that contains a certain number of genes and several types of proteins that comply with different functions.

The mixture of DNA and proteins is called chromatin. This complex helps compact DNA strands, which would otherwise be too long to fit in the cell. For example, human beings have about 5.9 feet (1.8 meters) of DNA in each cell. These DNA molecules end up fitting in just 30 nanometers of chromatin fibers, thanks to the presence of histones — a protein in the chromosome that aid in packaging the DNA into a highly compact form.

Mainly located in the nucleus of the cell, histones support structural units of DNA packaging called nucleosomes and play important roles in DNA replication, repair, and recombination. In other words, they not only give shape to the chromosomes but also protect the DNA in them and control gene activity.

DNA molecules wrap around the histone proteins, forming nucleosomes that look a bit like beads on a string. Nucleosomes further condense to form fibrous material called chromatin. When cells replicate, duplicated chromatins condense further and are separated into daughter cells during cell division.

Non-histone proteins in the chromosome include scaffold proteins, which help package DNA by binding two or more proteins together in a stable way; and DNA polymerases, which add new building blocks of DNA (nucleotides) to the DNA strands, “proofread” the integrity of the genome at the end of the process, and repair the strand if necessary —which makes them essential for DNA replication.

Types of human chromosomes

Humans, along with other animals, 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. These are termed allosomes, as opposed to autosomes (nonsex chromosomes).

The cells of humans and other complex organisms also include a much smaller circular chromosome, similar to those found in bacteria. Circular chromosomes are found in the mitochondria — structures located outside the nucleus which serve as the cell's "powerhouses."

Allosomes

Allosomes are the chromosomes that contain the genes that determine biological sex. In humans (as well as other primates and many other animal species), our sex chromosomes are the X and Y chromosomes. These pass on traits that are related to biological sex, such as the external and internal reproductive organs, and the secondary sexual characteristics that appear due to a “programmed” elevation of sex hormone levels in puberty.

Like other mammals with the XY system, human females have two X chromosomes (XX), and human males have an X chromosome and a Y chromosome (XY). This means that a woman’s reproductive cell (an ovum) will always contain a single X chromosome, but a man’s reproductive cell (a spermatozoon) can have either an X or a Y chromosome.

During sexual reproduction, a male’s individual sperm fertilizes an ovum. If the sperm has an X chromosome, the resulting offspring will be female (X from the mother and X from the father). If the sperm has a Y chromosome, the resulting offspring will be male (X from the mother and Y from the father).

There are a number of differences between the X and Y chromosomes. The Y chromosome is significantly smaller than the X chromosome and contains far fewer functional genes. The end of the Y chromosome contains a critical gene known as SRY (sex-determining region of Y). This gene acts as a "switch" for activating other genes that lead to the development of the testis.

Autosomes

The term “autosomes” refers to nonsex chromosomes, which control all nonsex traits in the human body. They contain DNA that is often termed autosomal DNA (atDNA or auDNA) to distinguish their DNA from the one in sex chromosomes.

Autosomes are homologous chromosomes, meaning that they come in pairs with all their genes in the same exact position in each copy. Sex chromosomes are homologous in females (XX is a homologous pair of sex chromosomes) but not in males.

Autosomes are of different sizes and contain different numbers of genes. For example, Chromosome 1 contains almost 3,000 genes, and Chromosome 22 has about 750 genes.

The mutation of genes in autosomes can lead to medical conditions called genetic autosomal disorders. The specific diagnoses, as well as the symptoms, are determined by which chromosome is affected and how.

How many chromosomes do humans have?

Humans have 46 chromosomes distributed in 23 pairs. We have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY).

Our chromosomes are organized in pairs because, as sexually reproducing organisms, we have diploid cells. A diploid cell is one that has two complete sets of chromosomes —one from each parent.

Occasionally, a baby receives two copies of a chromosome, or part of a chromosome, from one parent and none from the other parent. This is called uniparental disomy and can lead to certain medical conditions, such as Prader-Willi Syndrome (PWS), which in 25% of cases is due to the newborn inheriting two copies of chromosome 15 from the mother.

Conversely, other people can lack a copy of a chromosome partially or totally. For example, it is known that the partial deletion of chromosome 5 can lead to Cri du Chat syndrome, a condition that makes babies cry like meowing kittens due to changes in development in the larynx and the nervous system.

When an individual inherits an additional copy of a chromosome, it’s called a trisomy. Down’s syndrome, one of the most common trisomies, occurs when the infant inherits an extra copy of chromosome 21.

Trisomies in sex chromosomes can lead to other genetic conditions such as:

• Triple X syndrome (XXX) does not always show symptoms, and therefore, it is believed to be common but rarely diagnosed.
• Klinefelter syndrome (XXY) may cause infertility, breast growth, and/or speech problems in men (among other issues).
• Jacobs syndrome (XYY) may cause acne, increased growth rate, and learning difficulties in men.

Chromosomes in other species

Chromosomes are pretty much the same in eukaryotes; they only vary in number. For example, domestic cats have 38 chromosomes, while the wheat plant has 42.

The main difference here is that wheat is a hexaploid organism —its 42 chromosomes are not organized in pairs but in groups of six.

In some species that employ the XY system, it is the ratio of X chromosomes to Y chromosomes that determines sex. For example, in the specie Drosophila melanogaster, XX, XXY, and XXYY individuals are all females, while XY and XO individuals are males.

Other animals, including many birds, reptiles, and many insects, use a ZW system for sex determination, in which males are ZZ, whereas females are ZW. In these organisms, it is the males that have two copies of the same kind of sex chromosome.

Asexually reproducing species have only one set of chromosomes that can repeat itself (in diploid species) or not (in haploid species).

Prokaryotes, or single-celled organisms, tend to have only one circular or linear chromosome containing only a few genes. This allows them to replicate faster. In fact, some species can have hundreds or thousands of copies of their chromosomes. This is the case with the bacteria Epulonipiscium, which has up to 100,000 copies of its chromosome. Even in species such as Vibrio cholera, which has two chromosomes, the chromosomes are unique from one another and are not found in pairs.