3.2 - A - Chromosome Structure & Number
As discussed in Topic 1.2, in most prokaryotes there is one chromosome consisting of a circular DNA molecule that contains all the genes needed for life processes. The DNA is not associated with proteins ad so is sometimes called naked.
Because there is only one chromosome present, typically only a single copy of each gene is present. The cell only has two copies after DNA replication when the cell prepares for cell division. The two copies of the chromosome are moved to opposite poles and then the cell splits via binary fission.
Some prokaryotes also have plasmids, which are small extra DNA molecules. They are usually very small, circular and naked, containing genes that may be useful to the cell but that are not required for life processes. For example, the genes may bestow antibiotic resistance, which are useful when antibiotics are present in the environment.
Copies of plasmids can be transferred from one cell to another, allowing them to spread through a population. They can even cross between species.
Unlike prokaryotes, chromosomes in eukaryotes are composed of DNA associated with proteins (histones). The DNA is a single immensely long, linear molecule. Each DNA-histone group is separated by a short stretch of DNA, which gives the chromosome the appearance of a string of beads.
Eukaryote diploid cells have two sets of chromosomes. Chromosomes that have the same sets of genes (one from each parent gamete) are referred to as homologous pairs. If two organisms are members of the same species, we can expect them to have
When chromsomes supercoil during mitosis and meiosis, they can be visualized using stains that bind to DNA or proteins. When this is done, chromsomes of varying length and centromere location can be seen.
Most species have at least two different 'types' of chromosomes (humans have 23). Every gene is located at a specific place on a chromosome, called the locus. Therefore, each chromosome contains a specific sequence of genes (can be more than a thousand).
Nuclei containing one chromosome of each pair are called haploid, while those that contain both pairs of homologous chromosomes are called diploid.
The number of chromosomes is a fundamental characteristic of a species. Organisms with different chromosome counts are very unlikely to be able to interbreed, so all members of a species have the same count.
The table to the left shows the number of chromosomes found in many species. Note that the total number is not necessarily tied to complexity