Chromosomes, Genes & Proteins (Cambridge International IGCSE Biology)

The structure of genes

Spec point — 17.1.1 · State that chromosomes are made of DNA, which contains genetic information in the form of genes Define a gene as a length of DNA that codes for a protein Define an allele as an alternative form of a gene

The structure of genes

Inheritance of chromosomes, genes and DNA

• Inheritance is the transmission of genetic information from generation to generation
• Chromosomes are located in the nucleus of cells
• They are thread-like structures of DNA, carrying genetic information in the form of genes
• A gene is a short length of DNA found on a chromosome that codes for a specific protein
• This could be a structural protein such as collagen found in skin cells, an enzyme or a hormone
• Genes control our characteristics as they code for proteins that play important roles in what our cells do

* Genes are short lengths of DNA that code for a protein; genes are found on chromosomes*

• Alleles are variations of the same gene
• Humans have two copies of each chromosome (one from each parent), so they have two copies of each gene - and therefore two alleles for each gene
• One of the alleles for each gene is inherited from the mother and the other from the father
• The two alleles code for the same characteristic, but may code for different versions of it
• For example, an individual has two copies of a gene for eye colour, but one allele could code for brown eyes and one allele could code for blue eyes

Exam Hint The Zoom model is always useful when trying to visualise where you are in a cell, and what molecules are doing what. Imagine a zoom lens that has the power to zoom right out to look at a whole organism, but also to zoom right into individual molecules. This helps in recognising the structures in cell nuclei visible as chromosomes, then zooming in to picture the individual genes and then, the bases that make up the DNA