What is a chromosome?

Image: Chromosome Terminology image, Image by Christinelmiller, Sourced Under a Creative Commons 4.0 License from Wiki Commons (Note: this isn't a process in particular but IS crucial theoretical and contextual knowledge that you will be expected to know.) A chromosome is a structure that is composed of DNA and proteins that contain linear arrays of genes carrying genetic information. Prokaryotes generally have one circular chromosome, whilst eukaryotes have a number of chromosomes. There are key differences in the chromosomes of eukaryotes and prokaryotes which you'll be expected to know of.

Eukaryotic Chromosomes

Image: Chromosomes, Image by Facts in Brief A eukaryote is a complex cell that consists of many membrane-bound organelles, including a nucleus (the control centre of the cell). Chromosomes are composed of a complex of DNA and protein called chromatin that condenses during cell division. DNA exists as a single, long double-stranded fibre extending the chromosome’s entire length. Each unduplicated chromosome contains one DNA molecule – duplicated chromosome = 2 chromatids. Every 200 nucleotide pairs, the DNA wraps twice around a group of 8 histone proteins to form a nucleosome. Higher-order coiling and supercoiling also help condense and package the chromosome inside the nucleus. A karyotype (seen in the image above) can be created which is an ordered, visual representation of the chromosomes found in the cell. Chromosomes are photographed when they are highly condensed, and then photos of the individual chromosomes are arranged in order of decreasing size. In humans, each somatic cell has 46 chromosomes, made up of two sets, one set of chromosomes comes from each parent.

Key Terminology

Heredity: The study of inheritance, the genetic transmission of characteristics from one generation to another

Genetics: A branch of biology concerned with the study of genes, genetic variation and heredity in organisms

Gene: A unit of heredity that transmits information from one generation to the next: a segment of DNA that codes for a polypeptide

Alleles: One of the various versions of the same gene (at the same locus) distinguished by small differences in the DNA sequence

Trait: An inheritable characteristic; phenotype

Genome: All of the genetic material contained in an organism or a cell; it includes the chromosomes within the nucleus and the DNA in mitochondria and chloroplasts Genomics: The study of the genome – how genes interact with one another, the environment and the resultant proteins produced; knowledge of an organism’s entire DNA sequence Histone: A protein around which DNA winds in eukaryotic cells to form a nucleosome Chromosome: A structure composed of DNA and protein that contains linear arrays of genes carrying genetic information; prokaryotes generally have one circular chromosome, whereas eukaryotes have a number of linear chromosomes Chromatin: An organised, loosely coiled complex of DNA and its proteins that is found in eukaryotic non-dividing cells; it is more compact than the DNA of prokaryotes; chromatin supercoils to become the chromosomes observable during eukaryotic cell division Chromatid: Daughter strand of a duplicated chromosome that is joined to another chromatid by a centromere Karyotype: A display of the number and appearance of the chromosomes of an organism or cell as observed at metaphase Homologous chromosomes: A pair of chromosomes of the same size and shape and that has the same genes at the same locations Locus: The position a gene occupies on a chromosome Autosomes: A chromosome that is not a sex chromosome Sex chromosomes: A chromosome that determines the sex of an organism and affects sexual traits

Prokaryotic Chromosomes

A key feature of prokaryotes is their lack of membrane-bound organelles. The DNA found within prokaryotes usually forms a circular chromosome (single) which falls within direct contact with the cytosol. There are also plasmids, small circular strands of DNA, which are found in prokaryotes.

Plasmids are utilised in biotechnology and genetic engineering (which is covered in the biotechnology: processes section of the content!).

Prokaryotes have less DNA in comparison to eukaryotes as they are usually haploid and they also have less non-coding DNA which is repetitive.