Chromosomes

This lesson covers:

  1. What chromosomes and chromatids are
  2. What homologous chromosomes are
  3. The structure of chromosomes after replication
  4. The terms haploid and diploid

DNA is stored as chromosomes

Chromosomes are thread-like structures located inside the nucleus of animal and plant cells.


As we saw in the lesson on the genetic code, they consist of DNA tightly coiled around proteins called histones.

Diagram showing the structure of DNA molecule, chromatin, histones, and chromatids.

The human genome

Humans have a total of 46 chromosomes, divided into 23 pairs, within each cell.


The first 22 pairs are called autosomes, which are identical in both males and females. The 23rd pair, known as the sex chromosomes, differs between males and females. Males have one X and one Y chromosome (XY), whereas females have two X chromosomes (XX).


We can see this in the photograph below, which shows the chromosomes in a typical human male cell.  

Photograph showing the chromosomes in a typical human male cell, including 22 pairs of autosomes and one pair of sex chromosomes XY.

Homologous pairs

As you can see in the image above, each pair of chromosomes consists of two homologous chromosomes. These are referred to as homologous pairs. One chromosome in the pair is a paternal chromosome inherited from the individual's father, and the other is the maternal chromosome inherited from the mother.


Although the chromosomes are identical in terms of the genes they carry, they may contain different alleles.

Photograph showing a pair of human chromosomes.

Chromosomes may contain one or two chromatids

Chromosomes may contain one or two chromatids depending on the stage in the cell cycle.


During interphase, DNA is replicated, which means that each chromosome goes from having one chromatid to having two.

Diagram showing homologous chromosomes before and after replication, highlighting centromere and sister chromatids.

The two chromatids in each chromosome are held together by the centromere. The two chromatids within a chromosome are known as sister chromatids because they are genetically identical. 

Diploid and haploid cells

In most organisms the body cells are diploid and the sex cells (or gametes) are haploid. 

Illustration of a diploid cell with 2n chromosomes.

Diploid cells

Diploid cells contain two copies of each chromosome, one copy from each parent. 


The total number of chromosomes in diploid cells is described as '2n' with 'n' representing one set of chromosomes. 


For example, humans have 46 chromosomes in each body cell. 23 are maternal chromosomes (from the mother) and 23 are paternal chromosomes (from the father). 


The diploid number for human cells is 2n = 46. 


Note that the diagram on the right shows only 8 chromosomes rather than the 46 chromosomes found in human diploid cells. 

Illustration of a haploid cell with chromosomes inside.

Haploid cells

Haploid cells only contain one copy of each chromosome. 


The total number of chromosomes in haploid cells is described as 'n'. 


For example, human sex cells have 23 chromosomes, so n = 23. 


Note that the diagram on the right shows only 4 chromosomes rather than the 23 chromosomes found in human haploid cells.