Learning Objective

In this lesson we will learn about the arrangement of chromosomes in diploid and haploid cells, and how sex chromosomes determine biological sex.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe the arrangement of chromosomes in diploid cells (body cells) and haploid cells (sex cells), with examples.

  • Distinguish between autosomal chromosomes and sex chromosomes.

  • Describe how sex chromosomes determine biological sex in humans.

 

Chromosomes

  • Cells contain threadlike structures called chromosomes which are located in the cell’s nucleus.
    Chromosomes carry the cell’s genetic information. They are composed of a substance called chromatin, which is a combination of DNA and protein.
    The DNA within chromosomes carries the cell’s genetic information, while the protein is mainly involved in the packaging of the DNA within the cell.
  • Most of the time, chromosomes are not in the condensed form shown in most diagrams. Instead, they are loosely packed so that processes such as gene expression and DNA replication can take place.
    It is only at a certain part of a cell’s life cycle, leading up to cell division, that chromosomes become condensed, making them visible under a microscope.

 
chromosome-structure-chromatin-protein-DNA

Chromosomes consist of a material called chromatin, which contains DNA and protein.

(Image: blueringmedia, Adobe Stock)

 

Chromosomes in Body Cells

  • In most cells, chromosomes exist in pairs, known as homologous pairs.
    In each pair, one chromosome is inherited from the father and one chromosome is inherited from the mother.

      • Chromosomes inherited from the father are called paternal chromosomes.
      • Chromosomes inherited from the mother are called maternal chromosomes.

    Homologous chromosomes have a similar size and shape.

  • Cells that contain chromosomes in pairs (two sets of chromosomes) are described as diploid.
    Diploid cells include most somatic cells of an organism. (Some cells, such as mature red blood cells and cells in hair, nails and the outer layer of skin, do not contain nuclei and therefore do not contain any chromosomes.)
  • A body cell (somatic cell) is any cell that is not a sex cell.
    • In animals, body cells include skin cells, muscle cells and nerve cells.
    • In plants, body cells include cells found in leaves and roots.

 
paired chromosomes body somatic cells

Body cells are diploid – they contain two sets of chromosomes, which consist of homologous pairs.

 

Chromosomes in Sex Cells

  • In some cells, chromosomes exist individually; in other words, they are unpaired.
    Each chromosome contains some sections inherited from the father and some sections inherited from the mother.
  • Cells that contain individual chromosomes (one set of chromosomes) are described as haploid.
    Haploid cells include all mature sex cells.
  • A sex cell (gamete) is a reproductive cell, which can potentially form a new individual, usually by fusing with an opposite sex cell.
    • In animals, sperm cells are the male gametes and egg cells (ova) are the female gametes.
    • In plants, pollen grains are the male gametes and ovules are the female gametes.

 
unpaired chromosomes sex cells gametes

Sex cells are haploid – they contain one set of unpaired chromosomes.

 

Number of Chromosomes in a Set

  • The number of chromosomes in haploid and diploid cells varies between species.
    In humans, there are 23 chromosomes in a set. Therefore, sex cells contain 23 chromosomes and somatic cells contain 46 chromosomes (23 pairs).
    Other Examples
    In dogs, there are 39 chromosomes in a set. Therefore, sex cells contain 39 chromosomes and somatic cells contain 78 chromosomes (39 pairs).
    In peas, there are 7 chromosomes in a set. Therefore, sex cells contain 7 chromosomes and somatic cells contain 14 chromosomes (7 pairs).
  • The haploid number of chromosomes for a species is often written as n and the diploid number of chromosomes written as 2n.
    Therefore, for humans, n = 23 and 2n = 46.

 
human chromosome set karyotype microscope

A diploid set of human chromosomes contains 23 pairs (46 individual chromosomes).

(Image: Josef Reischig, Wikimedia Commons)

 

Autosomal Chromosomes and Sex Chromosomes

  • In animals, each set of chromosomes usually involves one sex chromosome, which is involved in biological sex determination and sex-linked traits.
    All other chromosomes are referred to as autosomal chromosomes (autosomes).
    In humans, each set of chromosomes contains 22 autosomes, numbered 1-22, and one sex chromosome, either an X chromosome or a Y chromosome.

human karyotype autosomal sex chromosomes

A diploid set of human chromosomes contains 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes.

(Image: Alila Medical Media, Adobe Stock)

 

Sex Determination in Humans

  • Since body cells contain two sets of chromosomes, they also contain two sex chromosomes.
    This pair of sex chromosomes determines the biological sex of an individual.
    • If cells contain two X chromosomes (XX), the individual is female.
    • If cells contain one X chromosome and one Y chromosome (XY), the individual is male.

 
biological sex chromosomes xx xy

Sex chromosomes determine the biological sex of an individual.

(Image: MaskaRad, Adobe Stock)

 

  • Since females are XX, egg cells can only contain an X chromosome as the sex chromosome.
    Since males are XY, sperm cells can contain either an X chromosome or a Y chromosome as the sex chromosome.
    Therefore, upon fertilisation, the sperm cell determines the sex of offspring.
    • If the sperm cell contains an X chromosome, the offspring will be female.
    • If the sperm cell contains a Y chromosome, the offspring will be male.

 
biological sex determination sperm egg

The sex of offspring is determined by the sex chromosome carried by the sperm.

(Image: designua, Adobe Stock)

 

Summary

  • Chromosomes are structures located in the nuclei of cells that carry the cell’s genetic information.
    They are composed of DNA (genetic material) and protein (packaging material).
    Chromosomes are loosely packed during most of a cell’s life, but condense during cell division.
  • Body cells (somatic cells) are diploid, which means they contain two sets of chromosomes, which exist in pairs known as homologous pairs.
    One chromosome in each pair is inherited from the mother and the other is inherited from the father.
  • Sex cells (gametes) are haploid, which means they contain one set of chromosomes.
    Chromosomes contain sections inherited from both the mother and the father.
  • The number of chromosomes in a cell varies between species.
    The haploid number is written as n and the diploid number is written as 2n.
    In humans, n = 23. Therefore, human sex cells contain 23 chromosomes and human body cells contain 46 chromosomes.
  • Each set of chromosomes contains one sex chromosome.
    In humans, this is either an X chromosome or a Y chromosome.
    All other chromosomes are referred to as autosomal chromosomes (autosomes).
  • Sex chromosomes determine the biological sex of an individual.
    • Females have two X chromosomes (XX) in their body cells.
    • Males have one X chromosome and one Y chromosome (XY) in their body cells.
    The sex chromosome carried by sperm cells determines the sex of offspring as egg cells always carry an X chromosome.

 
sex determination drosophila fruit flies

Sex determination in fruit flies (Drosophila) occurs in the same way as in humans.

(Image: YassineMrabet, Wikimedia Commons)

 

(Header image: ustas, Adobe Stock)