DNA, Genes & Chromosomes

Forms of DNA

Within the nucleus of a cell, DNA is loosely packaged like wool so it can be easily accessed. This loose form is called chromatin. When a cell prepares to reproduce, the DNA condenses into a tight, visible structure called a chromosome.

Chromosomes

A cell may contain one or two copies of its genetic material depending on where it is in the cell cycle. One copy is a single‑stranded chromosome. Two copies form a double‑stranded chromosome. Each individual copy of the genetic material is called a chromatid. A single‑stranded chromosome has one chromatid, while a double‑stranded chromosome has two chromatids joined at the centromere. The two chromatids are referred to as sister chromatids.

Putting it all together

Unravelled DNA in the nucleus is called chromatin. When it condenses and becomes visible, it is called a chromosome. A chromosome can be single stranded or double stranded depending on whether it has one or two copies of DNA. Each copy is called a chromatid, and chromatids are joined at the centromere.

Gene

A gene is a long stretch of DNA that codes for something, usually a protein or a group of proteins. Human genes can range from about 300 to over 1,000,000 nucleotide pairs in length.

Mutation

A mutation is any change in the nucleotide sequence of a DNA or RNA molecule.

Genes

Genes are small sections of DNA that determine the characteristics of an organism. The order of nitrogenous bases within a gene gives individuals their unique traits. Genes are inherited from parents, which is why family members often share similar characteristics.

Karyotype and Homologous Pairs

Most human body cells contain 46 chromosomes, with 23 inherited from each parent. These chromosomes are arranged in homologous pairs. A karyotype is a pictorial representation of all chromosomes in a cell, showing them in their homologous pairs.

Autosomes and Sex Chromosomes

The first 22 pairs of chromosomes are autosomes and determine most characteristics and functions of the organism. The final pair are sex chromosomes, which determine biological sex. XX indicates female and XY indicates male.