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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Chromosome Numbers
Chromosomes are thread-like structures made of DNA that carry our genetic information. Understanding how many chromosomes are in our cells and how these numbers change during cell division, is key to understanding inheritance and reproduction.
Key Definitions:
People often mix these up! A chromosome is a single DNA molecule with associated proteins. After DNA replication, a chromosome consists of two identical chromatids joined at the centromere. These sister chromatids separate during cell division to become individual chromosomes in the new cells.
Humans have 46 chromosomes in most cells (diploid), arranged as 23 pairs. One chromosome from each pair comes from your mum and one from your dad. Gametes (sperm and egg cells) have 23 chromosomes (haploid) - half the normal number.
Different species have different numbers of chromosomes and this number isn't related to how complex the organism is!
A tiny fern called Ophioglossum reticulatum has about 1,260 chromosomes - the highest known number! Meanwhile, Jack jumper ants have just 2 chromosomes and a species of roundworm has only 1 chromosome.
Mitosis is the type of cell division that produces two identical daughter cells. It's used for growth and repair.
Throughout mitosis, the chromosome number stays the same from parent to daughter cells:
A human body cell has 46 chromosomes (23 pairs). DNA replicates, creating sister chromatids.
Sister chromatids separate and move to opposite ends of the cell.
Each daughter cell has 46 chromosomes - the same as the parent cell.
Meiosis is the type of cell division that produces gametes (sex cells). It reduces the chromosome number by half.
If sperm and egg cells had the full 46 chromosomes, then when they joined during fertilisation, the resulting embryo would have 92 chromosomes! By halving the chromosome number in gametes, we maintain a consistent chromosome number from generation to generation.
In the first division of meiosis, homologous chromosomes pair up and then separate. This reduces the chromosome number from diploid (2n) to haploid (n). So human cells go from having 46 chromosomes to having 23 chromosomes.
The second division is similar to mitosis. Sister chromatids separate, but the chromosome number stays the same (haploid). Each of the four resulting cells has 23 chromosomes.
Down syndrome occurs when there's an extra copy of chromosome 21 (so three copies instead of the usual two). This happens when chromosomes don't separate properly during meiosis - a process called nondisjunction. The person ends up with 47 chromosomes instead of 46. This shows how important correct chromosome numbers are for normal development.
Meiosis doesn't just halve the chromosome number - it also creates genetic variation through two key processes:
During meiosis I, homologous chromosomes exchange genetic material. This means the chromosomes that end up in gametes are a mix of maternal and paternal DNA.
The way homologous pairs line up during meiosis I is random. For humans with 23 pairs, this creates 2²³ (over 8 million) possible combinations of chromosomes in each gamete!
Most cells in your body are somatic (body) cells with the diploid number of chromosomes. Only your gametes (sperm or egg cells) have the haploid number.
Diploid (2n)
46 chromosomes in humans
Examples: skin cells, muscle cells, neurons
Haploid (n)
23 chromosomes in humans
Produced in ovaries
Haploid (n)
23 chromosomes in humans
Produced in testes
Having the correct number of chromosomes is crucial for normal development and function. Changes in chromosome numbers can lead to genetic disorders:
Having an abnormal number of chromosomes. Examples include:
Having more than two complete sets of chromosomes. This is often fatal in animals but common in plants. Many crops we eat are polyploid, including wheat, strawberries and bananas. Polyploidy often results in larger fruits and higher yields.
Scientists can study chromosome numbers and structures by creating a karyotype - a visual representation of a person's chromosomes arranged in pairs. Karyotyping can identify chromosomal abnormalities and is often used in prenatal testing to check for conditions like Down syndrome.
Chromosome numbers are fundamental to genetics and inheritance. They ensure:
Understanding how chromosome numbers change during cell division helps us make sense of inheritance patterns, genetic disorders and the amazing diversity of life on Earth.
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