What is a Chromosome? Simple definitions for understanding human genetics

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From cancer risk to eye color, human genetics determines who you are, based on the DNA you inherit from your parents. But what is a chromosome, what is DNA, and what are genes?

Here’s some simple definitions for how human genetics works.

What is DNA?

Every cell in the human body needs to know how to work and how to grow. These instructions are called DNA (deoxyribonucleic acid).

Like letters in an alphabet, DNA is the language that cells use to know what to do.

If a cell has too much or too little DNA, it will either die, or grow wrong, which can lead to cancer.  Every cell needs the right instructions to function normally.

What is DNA graphic


What is a Gene?

Like a word in a book, a gene is the basic unit of DNA instruction. Each gene has specific job to do.

Some genes carry the instructions for making building blocks for cells, called proteins, or for turning DNA off or on, or instructions for how to modify other proteins or DNA within a cell.

For example, the KRT genes provide the instructions for how to make hair proteins, called keratins.  Depending on which KRT genes you have, your hair will be either curly or straight, or blonde, red, or brown.

What are genes graphic


What is a Chromosome?

A chromosome is how DNA is stored, and transferred from one cell to another. One chromosome contains hundreds or thousands of genes.

Each chromosome is like a book, carrying a specific group of genes. Each chromosome carries a different set of biological instructions. For example, the Y chromosome carries all the instructions for making a boy.

Human DNA is stored on 46 chromosomes.  Every cell needs all 46 chromosomes to live and function normally.  If a cell loses a chromosome, it no longer has all the instructions it needs to function and grow correctly.

What is a chromosome graphic

Other Facts About Human Chromosomes

  • Humans have 23 pairs of chromosomes (46 chromosomes total): one set comes from your father and the other set comes from your mother.
    • 22 pairs are autosomal chromosomes, meaning they are non-sex chromosomes
    • 1 pair is sex-related;  either XX for a female, or XY for a male
  • A chromosome has a short arm (p arm), and a long arms (q arm).
  • When a cell divides, chromosomes are held together at the center, called a centrosome.  This helps keep the right number of chromosomes in each cell after cell division.

What is a chromosome made out of?

  • Chromosomes are made out of chromatin, which is a a collection of DNA, protein, and other large molecules.  The chromatin helps change the shape of a chromosomes to help during replication (copying the DNA), and transcription (using the DNA).
    • Structures called histones and nucleosomes are part of chromatin. Together, they act like a spool of thread, helping coil up the chromosomes in order to protect or expose DNA when needed by the cell.
  • The ends of chromosomes are called telomeres which help protect the DNA during replication.  As people get older, the ends of their chromosomes naturally get shorter, much like the fraying off of a shoelace.

What is a Genome?

A genome is the collection of all the DNA needed to make a particular living creature.

All 46 human chromosomes together make the complete human genome.

Other living things may have a different number of chromosomes and genes. For example, pigs have only 38 chromosomes in their genome, while hermit crabs have 254 chromosomes, which is the most chromosomes currently known in any animal species.  Potatoes have 48 chromosomes, so that’s two more than in humans!!!

What is the human genome graphic


A simple way to think about human genetics:  DNA are the letters, genes are the words, and chromosomes are the books, to make life work correctly.  A genome is the entire library for each living thing.

If any letters are changed in the DNA, the words no longer have the correct meaning. And without all the books, a cell does not have all the necessary instructions to survive.

References

Genetics Home Reference provides information about each human chromosome written in lay language.

basic introduction to chromosomes is available from the National Human Genome Research Institute.