01 Understanding Genetics and Your Body
Section 4
Overview of Chromosomes
Learning Goal
To understand the components and function of chromosomes and the role they play in cell division.
Chromosomes are thread-like structures found in the nucleus of each cell. Each chromosome consists of a single DNA molecule that has been tightly and repeatedly coiled around spool-like proteins called histones, which support the chromosome’s structure.1,2
Chromosomes are not visible in the cell’s nucleus when the cell is not dividing. It is during cell division when the DNA that makes up chromosomes becomes more tightly packed, making it visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.1
The word chromosome comes from the Greek words for color (chroma) and body (soma). Scientists gave chromosomes this name because they are cell structures, or bodies, that become stained when they come into contact with colorful dyes used in research.2
Chromosomes play a key role in cell division. Cell division is the process by which cells are made or multiply. During cell division a cell divides into two daughter cells containing the same number and kind of chromosomes as the original cell.3 Chromosomes ensure that DNA is accurately copied and distributed during cell division. On rare occasions, changes to the DNA sequence can occur,2 and when these changes affect key cellular functions they can lead to genetic conditions.
Chromosomes1,2
Humans have 23 pairs of chromosomes, for a total of 46 chromosomes.2 These consist of one pair of sex chromosomes and 22 pairs of autosomes. Each parent contributes one chromosome to each pair so that their child inherits half of their chromosomes from their mother and half from their father.2,4
- Sex chromosomes: determine the sex of an individual. Most individuals have two sex chromosomes. Typically, females have two X chromosomes in their cells, while males have both an X chromosome and a Y chromosome in their cells.5
- Autosome: any chromosome that is not a sex chromosome.5
Additional Interesting Fact
The packaging and coiling of DNA molecules around proteins called histones allows DNA to fit inside cells. If all the DNA molecules in a single human cell were unwound from their histones and placed end-to-end, they would stretch to 6 feet long.2
Key Learnings
Chromosomes ensure that DNA is accurately copied and distributed during cell division.
When changes to chromosomes affect key cellular functions they can give rise to genetic conditions.
Humans have 23 pairs of chromosomes, consisting of one pair of sex chromosomes and 22 pairs of autosomes (non-sex chromosomes). An individual typically inherits one set of chromosomes from each parent.
Continue learning about genetics and your body in the next section
Section 5
What is a Gene Mutation?
To understand the different types of gene mutations and how they occur.
References
Help me understand genetics: cells and DNA. MedlinePlus. U.S. National Library of Medicine, National Institutes of Health, U.S. Department of Health and Human Services; 2021. https://medlineplus.gov/genetics/understanding/basics/chromosome/. Accessed December 21, 2021.
Chromosomes fact sheet. U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute; 2020. https://www.genome.gov/about-genomics/fact-sheets/Chromosomes-Fact-Sheet. Accessed December 21, 2021.
- Help me understand genetics: how genes work – how do cells divide? MedlinePlus. U.S. National Library of Medicine, National Institutes of Health, U.S. Department of Health and Human Services; 2021. https://medlineplus.gov/genetics/understanding/howgeneswork/cellsdivide/. Accessed December 21, 2021.
- Chromosome. Talking Glossary of Genetic Terms. National Human Genome Research Institute, National Institutes of Health, U.S. Department of Health and Human Services; 2021. https://www.genome.gov/genetics-glossary/Chromosome. Accessed December 21, 2021.
- X chromosome fact sheet. National Human Genome Research Institute, National Institutes of Health, U.S. Department of Health and Human Services; 2021. https://www.genome.gov/sites/default/files/media/files/2020-03/X_Chromosome_fact_sheet.pdf. Accessed December 21, 2021.
