Edexcel A Level Biology A Topic 3: Voice of the Genome Exam Questions

About Edexcel A Level Biology A Topic 3: Voice of the Genome

Topic 3 of Edexcel A Level Biology A (Salters-Nuffield) explains how a single fertilised egg develops into a complex organism. You study the ultrastructure of eukaryotic and prokaryotic cells, how proteins are processed through the rough endoplasmic reticulum and Golgi apparatus, and the structure of mammalian gametes and fertilisation in mammals and flowering plants. The topic covers the cell cycle and mitosis, meiosis and sources of variation, and stem cells, including the ethical debate around their use. You then learn how differential gene expression and transcription factors lead to cell specialisation, and how genotype and environment interact to produce phenotype. The root tip squash core practical is often examined.

What you need to know

• Cells and organelles Know the structure and function of the nucleus, nucleolus, ribosomes, rough and smooth ER, mitochondria, centrioles, lysosomes and Golgi apparatus, and how prokaryotic cells differ.
• Protein transport Proteins made on ribosomes on the rough ER are folded, packaged into vesicles, modified in the Golgi apparatus and released by exocytosis.
• Gametes and fertilisation In mammals the acrosome reaction releases enzymes to digest the zona pellucida, and the cortical reaction prevents polyspermy. In flowering plants a pollen tube grows to the ovule and double fertilisation forms the zygote and endosperm.
• Mitosis and meiosis Mitosis produces genetically identical cells for growth and repair. Meiosis produces haploid gametes and causes variation by independent assortment and crossing over. Gene linkage and sex linkage affect inheritance patterns.
• Stem cells Totipotent cells can form any cell including placenta; pluripotent cells can form most cell types. Uses in medicine raise ethical issues, regulated by bodies that consider the views of society.
• Gene expression and phenotype Cells specialise because different genes are switched on by transcription factors. Many characteristics are polygenic and influenced by the environment, giving continuous variation.

Common mistakes in Topic 3: Voice of the Genome

• Saying different cells contain different genes; all cells contain the same genes but express different ones.
• Confusing the acrosome reaction with the cortical reaction; the acrosome is in the sperm, cortical granules are in the egg.
• Describing meiosis as producing two cells; it produces four haploid cells.
• Writing that totipotent and pluripotent mean the same thing; only totipotent cells can form extra-embryonic tissues.
• Forgetting to stain and squash root tips properly in the mitosis practical; acid hydrolysis separates cells so they form a single layer.

Try three Topic 3: Voice of the Genome exam-style questions