Topic 3 SNAB Biology

Some questions with bullet pointed answers on some of the key topic within topic 3 of the SNAB BIOLOGY A-LEVEL course

  • Created by: VicC
  • Created on: 20-05-10 17:18


  • the sperm reaches the ovum
  • cells surrounding the ovum release chemicals, triggering the acrosome reaction
  • the acrosome swells and fuses with the sperm cell surface membrane
  • digestive enzymes in the acrosome are released
  • the enzymes digest through the follicle cells and zona pellucida surrounding the ovum
  • the sperm fuses with its cell surface membrane
  • the sperm nucleus enters the ovum
  • enzymes released from the lysosomes cause the zona pellucida to thicken, preventing the entry of other sperm
  • nuclei of the ovum and sperm fuse: fertilisation
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Explain the process of PLANT FERTILISATION.

  • pollen frain germinates on style
  • a pollen tube grows down style towards ovary (growth controlled by tube nucleus)
  • pollen grain contains 2 nuclei:
    • tube nucleus
    • generative nucleus
  • on germination of pollen, generative nucleus divides by mitosis to form 2 halpoid male gamete nuclei
  • 2 male haploid gamete nuclei move down pollen tube
  • pollen tube grows through small pore (micropyle) into embryo sac
  • both male haploid male gamete nuclei enter the embryo sac
    • 1 of the male gamete nuclei fuses with the egg nuclei --> forms dipliod zygote (divides to form the embryo)
    • 1 of the male gamete nuclei fuses with the 2 polar nuclei in the embryo sac, --> forms triploid zygote (divides to form the seed's storage tissue = endosperm)
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What are the uses of MITOSIS?

- growth of multicellular organisms

- repairing damaged tissues

- some organisms: important for reproducing asexually producing offspring that is genetically identical to parent offspring

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Describe the CORE PRACTICAL on how to observe MITO

1. Cut 5mm onion root tip.

2. Place on watch glass.

3. Add a few drops of HCl.

4. Add a few drops of the stain: ACETIC ORSEIN.

5. Heat by passing over a bunsen burner flame.

6. Put root tip on slide.

7. Place cover slip on top and squah down gently.

8. Observe under microscope.

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  • undifferentiated/ unspecialised cells
  • which can keep dividing
  • and give rise to other types of cell
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Describe the different stages of Mitosis.

  • INTERPHASE: G1 phase/ S phase (DNA replication)/ G2 phase
    • so the cell has enough organelles, DNA and cytoplasm for 2 new cells
    • chromosomes shorten and thicken
    • centrioles move to opposite ends, forming spindle across cell between 2 poles
    • nuclear envelope breakdown (nucleolus disappears)
    • chromosomes' centromeres attach to spindle fibres at the equator
    • spindle fibres contract, pulling chromosomes apart - one chromatid of each chromosome is pulled to each pole of the spindle
    • spindle breaks down
    • chromosomes unravel
    • nuclear envelope reforms around 2 groups of chromosomes
    • nucelolus reappears
    • cytoplasm divides
    • 2 identical daughter cells are produced
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How would you demonstrate TOTIPOTENCY by PLANT TIS

1. Sterilise surface

2. Take a few plant cells of one type (explant) from a growing area of a plant (e.g. root/ shoot)

3. Place the cells on a sterile agar medium with nutrients and growth regulators.

4. Cover with tin foil.

5. Let the plant cells grow and divide by mitosis. Mitosis is shown as: unspecialised cells differentiate into specialised cells that grow and differentiate into an entire plant.


  • same batch agar
  • same light level
  • same temperature
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  • differentiational gene expression is caused by turning different genes on and off, by controlling transciption
  • which determines differentiation
  • under the right conditions, some genes are activated and some are not
  • the active genes make active mRNA
  • which is translated into proteins within cell (by ribosomes)
  • this controls cell processes and determines cell structure
  • --> forming a SPECIALISED CELL
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How can the transcription of genes be prevented?

Protein repressor molecules attach to: DNA PROMOTER REGION-

  • blocking the attachment site
  • preventing the attachment of the TRANSCRIPTION INITIATION COMPLEX
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characteristics of an organism, defined by:

  • GENOTYPE: genetic make-up
  • ENVIRONMENT: in which the individual develops
10 of 15

Describe 3 differences between Eukaryotes and Prok

1. SIZE: Eukaryotes are larger; Prokaryotes are smaller.

2. DNA: Eukaryotes have linear DNA (in nucleus); Prokaryotes have circular DNA (free in cytoplasm)

3. ORGANELLES: Eukaryotes has a nucleus/ membrane-bound organelles such as mitochondria; Prokaryotes has no nucleus or membrane-bound organelles (apart from small ribosomes)

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How could DNA damage in the embryo result in inher

- when the embryo divides by mitosis into an adult

- cells giving rise to testes/ ovaries may have DNA error

- therefore a gamete with faulty DNA can form

- cancer causing error could be passed on in these gametes to next generation

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Explain the importance of fertilisation in sexual

- restores the diploid number of chromosomes

- combines the genome from 2 cells, which is important for GENETIC VARIATION

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What is the function of the GOLGI APPARATUS, SMOOT


  • modifies and packages lipids and proteins in vesicles for transport
  • makes lysosomes


  • makes lipids and steroids


  • site of protein synthesis
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Explain the TRANSPORT of PROTEINS within the cell

- nucleus

- ribosome on rough endoplasmic reticulum

- vesicle

- golgi apparatus

- vesicle

- cell surface membrane

- out of cell by EXOCYTOSIS

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This was very helpful, thankyou! :)

Dean Shanks


thanks :)



Very good example questions! Topics are discussed well, thanks!



thanks :D



This is great! Thank you! :) 

Elisha-Marie Edwin


Very Good thanks x



Awesome! 5 Stars :)

komal maqsood


ThankYou So Muchhhh :) 



its good but there are spelling errors, just fix them up :)

e.g. page two first line - frain = grain

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