Chromosomes & Cells (Cambridge O Level Biology)

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Chromosomes

  • 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
  • Think about zooming into the nucleus of a cell, as shown in the diagram below
    • On the right is the zoomed-out view
    • Which zooms in as we read across the diagram right-to-left

The Organisation of DNA into Chromosomes

DNA, genes and chromosomes, IGCSE & GCSE Biology revision notes

 Genes are short lengths of DNA that code for a protein. They are found on chromosomes

  • Alleles are different versions of a particular gene. The ABO gene for blood group type has three alleles, IA, IB and IO
  • Alleles give all organisms their characteristics

Examiner Tip

The Zoom model is always useful when trying to viualise 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

Haploid & Diploid Cells

  • The nuclei of gametes are haploid
    • They contain half the number of chromosomes of a normal body cell
    • In humans, this is 23 chromosomes
  • The nucleus of a zygote is diploid
    • It contains the same number of chromosomes as a normal body cell
    • In humans, this is 23 pairs of chromosomes
    • The zygote continues to stay diploid as it grows into a fetus and embryo during pregnancy

Haploid & Diploid CellsHaploid _ Diploid cells

A haploid cell has one copy of each chromosome while a diploid cell has two copies of each chromosome.

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.