Chromosomes (Leaving Cert Biology): Revision Notes
📚 Revision Notes
Chromosomes
What are Chromosomes?
- Chromosomes are found in the nucleus of a cell.
- They are made up of DNA (deoxyribonucleic acid) and proteins.
- DNA contains a sugar called deoxyribose.
- When the cell is not dividing (in interphase), the chromosomes are stretched out into long, thin threads called chromatin.
- During cell division, chromatin contract to form visible chromosomes.
Functions of Chromosomes
Chromosomes
- Control cell activity.
- Are passed on from cell to cell when cells divide.
- Are also passed to offspring when gametes (sex cells) fuse during sexual reproduction.
Genes on Chromosomes
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A gene is a section of DNA that contains the instructions for the formation of a protein.
- Genes are sections of DNA located along the length of each chromosome.
- Genes are known as the units of inheritance because they carry the traits passed from parents to offspring.
- In humans, genes control features like eye colour, hair colour, and face shape.
- In plants, genes control traits like flower colour, height, and leaf shape.
- All the genes in an organism make up its genome.
Chromosome Numbers
- In somatic cells (non-reproductive cells), the number of chromosomes is constant for each species.
- Each species has a specific number of chromosomes.
- In humans:
- Most cells have 46 chromosomes, arranged in 23 pairs.
- Sex cells (sperm in males and eggs in females) have 23 chromosomes.
- The first 22 pairs of chromosomes are called autosomes and control everyday cell functions.
- The 23rd pair are the sex chromosomes, which determine an individual's gender.
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Genome: the collection of all the genes in an organism.
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Somatic cells are any cells in the body that are not involved in reproduction. E.g. skin cells.
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Why do sex cells only have 23 chromosomes each? When a sperm and egg come together during fertilisation, they combine to form 46 chromosomes, giving the offspring a complete set.
