Summary
Thread-like structures of DNA and protein carrying genes; humans have 46 in 23 pairs.
In depth
Chromosomes are made of DNA and proteins (chromatin). Humans have 22 pairs of autosomes and one pair of sex chromosomes. Each chromosome has a centromere and telomeres, and genes lie along it at fixed loci. Homologous chromosomes are similar in size and gene content, one from each parent.
The chromosome number differs among species: fruit flies 8, potatoes 48, some ferns over 1000. Number alone does not reflect complexity.
Chromosome number varies widely and does not reflect complexity: the Indian muntjac deer has 7 in females, humans 46, dogs 78, and some ferns over 1000. Chromosomes are classified by centromere position as metacentric, submetacentric or acrocentric.
Chromatin packs DNA in levels: nucleosomes (beads on a string), 30 nm fibre, loops and the metaphase chromosome, which condenses about 10,000-fold. Genes are mostly in less condensed euchromatin. Prokaryotes usually have a single circular chromosome.
Example: the human Y chromosome is small with about 70 genes, while chromosome 1 is large with over 2000 genes.
Frequently asked questions
What is Chromosomes?
Thread-like structures of DNA and protein carrying genes; humans have 46 in 23 pairs.
Which area of biology covers Chromosomes, and how advanced is it?
Chromosomes is one of the 54 topics in Genetics. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Karyotype, Mitochondrial Inheritance, Crossing Over.
Can you give an example of Chromosomes?
A normal human body cell has 46 chromosomes while gametes have 23.
What should I learn before Chromosomes?
Chromosomes is a Beginner topic, so no earlier topic is required. Other Beginner topics in Genetics that pair well with it include Heredity, Gene, Allele.
What topics are related to Chromosomes?
The closest topics to Chromosomes are Karyotype, Mitochondrial Inheritance, Crossing Over, Law of Independent Assortment. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Chromosomes?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Chromosomes. To practise the whole of Genetics, use the Genetics quiz.
How is Chromosomes different from Karyotype?
Chromosomes: Thread-like structures of DNA and protein carrying genes; humans have 46 in 23 pairs. Karyotype: An organised picture of an individual's chromosomes arranged by size, used to detect abnormalities. Read both side by side and note what each one explains.
How should I study Chromosomes step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Karyotype and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.