---
title: "2.3 Structure and composition of chromosomes"
canonical: "https://wiki.groenkennisnet.nl/space/TAB/270893173/2.3%20Structure%20and%20composition%20of%20chromosomes"
format: markdown
---
The different number of chromosomes impedes the production of crosses between species. In the formation of a zygote out of a sperm and an oocyte their single chromosomes are regrouped into pairs. With a different number of chromosomes this process fails when sperm cells and oocytes originate from different species.

Chromosomes are [double-stranded helices](http://en.wikipedia.org/wiki/Nucleic_acid_double_helix), consisting of two long [biopolymers](http://en.wikipedia.org/wiki/Biopolymer) made of [simpler units](http://en.wikipedia.org/wiki/Monomer) called [nucleotides](http://en.wikipedia.org/wiki/Nucleotide)—each nucleotide is composed of a [nucleobase](http://en.wikipedia.org/wiki/Nucleobase) ([guanine](http://en.wikipedia.org/wiki/Guanine), [adenine](http://en.wikipedia.org/wiki/Adenine), [thymine](http://en.wikipedia.org/wiki/Thymine), and [cytosine](http://en.wikipedia.org/wiki/Cytosine)), recorded using the letters G, A, T, and C, as well as a [backbone](http://en.wikipedia.org/wiki/Backbone_chain) made of alternating [sugars](http://en.wikipedia.org/wiki/Monosaccharide) ([deoxyribose](http://en.wikipedia.org/wiki/Deoxyribose)) and [phosphate](http://en.wikipedia.org/wiki/Phosphate) groups (related to [phosphoric acid](http://en.wikipedia.org/wiki/Phosphoric_acid)), with the nucleobases (G, A, T, C) attached to the sugars (Wikipedia). Within a species and within a breed the nucleotides and nucleobases are situated in a fixed order.

![image](media://78441611-caed-4797-a61c-d352ca03558b)