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Describe the ultrastructure of the nucleus and the nucleolus.
 - The nucleus has a double membrane with pores.
- It contains chromatin and a nucleolus.
- Nucleolus makes ribosomes.
- Chromatin contains DNA and controls protein synthesis.
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Describe the ultrastructure of a ribosome.
- Made of RNA and protein.
- They are found free in the cytoplasm or attached to the RER.
- Site of protein synthesis.
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Describe the ultrastructure of SER and RER.
- RER:
- Interconnected sacs embedded with ribosomes.
- Site of protein folding and processing proteins.
- SER:
- Interconnected sacs where synthesis of lipids and steroids occurs.
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Describe the ultrastructure of a mitochondria.
- Enclosed by a double membrane.
- The inner membrane is folded into cristae.
- The inner membrane is filled with the matrix.
- Its the site of aerobic respiration where ATP is produced.
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Describe the ultrastructure of centrioles.
- Animal cells contain a pair.
- The are hollow cylinders containing a ring of microtubules.
- The are involved in spindle formation for cell division.
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Describe the ultrastructure of a lysosome.
- Round organelle surrounded by a membrane containing digestive enzymes.
- Used to digest unwanted structures or old cells.
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Describe the ultrastructure of the golgi apparatus.
- Group of fluid filled flattened sacs, vesicles seen next to it.
- Processes and packages new lipids and proteins, and makes lysosomes.
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Explain the role of the RER and the Golgi apparatus.
- Proteins are synthesised in ribosomes which are held at RER.
- Proteins are folded in the RER.
- These proteins are then released in vesicles and fuse with the Golgi apparatus.
- Proteins are modified in the Golgi, e.g. given receptors (glycoproteins).
- The Golgi also produces lysosomes.
- Vesicles full of enzymes move towards the cell surface membrane and leave the Golgi by exocytosis.
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Distinguish between eukaryotic and prokaryotic cells.
- Eukaryotes:
- Larger.
- Have linear DNA.
- Have a nucleus.
- Either have no cell wall, a cellulose cell wall or a chitin cell wall.
- Contain many organelles, including mitochondria.
- Larger ribosomes.
- Prokaryotes:
- Very small.
- Circular DNA.
- No nucleus. (Has plasmids)
- Cell wall made of polysaccharide.
- Few organelles and no mitochondria.
- Smaller ribosomes.
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Describe the terms tissue, organ and system.
- A tissue is on or more similar cell types working together to execute the same function.
- An organ is a group of similar tissues working together to execute several functions.
- A system is a group of organs working together to execute a function.
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Explain the role of mitosis and the cell cycle.
- Mitosis is used for growth, repair and asexual reproduction.
- Prophase:
- Chromatids become visible.
- Centrioles move to opposite poles.
- Nucleolus disappears and nuclear envelope breaks down.
- Spindle begins to form.
- Metaphase:
- Chromosomes line up at equator and are attached to the spindle by their centromere.
- Anaphase:
- Centromeres divide.
- Spindles contract pulling chromatids to opposite poles.
- Telophase:
- Spindle breaks down.
- Chromosomes de condense.
- Nuclear envelope and nucleolus reform.
- Cytoplasm splits forming two identical cells.
- Cytokinesis:
- Cytoplasmic division of two daughter cells.
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Root tip squash mitosis practical.
- 1. Cut a 5mm tip from a growing root.
- 2. Place root on a watch glass and then add HCl.
- 3. Add a few drops of stain (toluidine blue) to darken the chromosomes and make them visible.
- 4. Warm the watch glass using a Bunsen burner.
- 5. Place the root tip on a microscope slide and squash it with a cover slip.
- 6. Warm gently again to intensify stain.
- 7. Repeat experiment to increase reliability.
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Explain meiosis and how it contributes to genetic variation.
- Cell division by meiosis produces gametes.
- Meiosis produces haploid nuclei which are restored to diploid at fertilisation.
- 1. DNA replicated into two sister chromatids.
- 2. Homologous pairs line up along the equator.
- 3. There is cross-over of chromatids before first division. This recombines genetic material and means chromatids now have different combination of alleles. 4 daughter cells will have different combination of alleles.
- 4. At the first division, the homologous pairs are separated by independent assortment and the chromosome no. is halved.
- 5. In the second (mitotic) division the sister chromatids are separated, resulting in a 4 unique haploid daughter cells.
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Explain how mammalian gametes are specialised for their function.
- Sperm cell:
- Streamlined shape to reduce resistance.
- Acrosome containing digestive enzymes (acrosin) to break down zona pellucida.
- Haploid nucleus, which becomes diploid at fertilisation.
- Many mitochondria to provide ATP for sperm to swim.
- Flagellum to swim.
- Receptors in cell surface membrane to bind to egg cell surface membrane.
- Egg cell:
- Follicle cells form protective coating.
- Much larger cell, containing food supply.
- Contains zona pellucida which hardens after sperm enters.
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Describe the process of fertilisation in mammals and the importance of fertilisation.
- Fertilisation occurs in the oviduct.
- When sperm makes contact with zona pellucida acrosome reaction occurs.
- Acrosin is released from the acrosome and digests the zone pellucida.
- When sperm head reaches the egg cell membrane and it fuses, the cortical reaction is triggered.
- Egg cell releases cortical granules which thicken and harden the zone pellucida making it impenetrable.
- The sperm cell nucleus enters the egg cell and the tail is discarded.
- The two nuclei fuse forming a diploid zygote.
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Describe the process of fertilisation in flowering plants.
- Pollen grain lands on the stigma.
- Grain absorbs water and splits open, releasing the tube cell which digests a pollen tube down the style.
- The pollen tube transports the male generative cell (two haploid male gametes).
- The pollen tube grows through the micropyle and into the embryo sac.
- Tube nucleus disintegrates and the two haploid male nuclei enter the embryo sac.
- One nucleus fuses with the egg cell to form a zygote and the other with the polar nuclei to form a triploid endosperm which will be used as a food source.
- Double fertilisation has occured.
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Explain the terms stem cell, pluripotency, multi potency and totipotency.
- Stem cell:
- Unspecialised cells, found in an embryo or in the bone marrow.
- Pluripotency:
- Are stem cells that have the ability to differentiate into any cell except extra embryonic cells.
- Found in the blastocyst.
- Totipotency:
- Stem cells that have the ability to differentiate into any cell including extra embryonic cells.
- No genes are switched off. Found in 1-3 day old embryos.
- Multipotent:
- Stem cells in adult human tissue which can differentiate into a very few select number of cells, e.g. fetal tissue, cord blood, adult stem cells.
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How do cells become specialised?
- Stem cells all contain the same genes but not all of them are expressed/active.
- Under the right conditions some genes in stem cells are activated.
- mRNA is only transcribed form the active genes.
- mRNA from active genes is then translated into proteins.
- Proteins modify the cell - control cell structure and processes.
- These modifications cause the cell to become differentiated/specialised.
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What are the benefits of using stem cell therapies?
- You can replaced damaged tissue, e.g. nerve tissue for spinal chord diseases, cardiac tissue for heart diseases.
- Save lives, e.g. growing organs for transplants.
- Quality of life is improved, e.g. replacing damaged cells in blind people.
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How do we obtain stem cells?
- Adult stem cells:
- Obtained from the bone marrow.
- Operation can cause discomfort.
- A needle is inserted into the centre of the bone and bone marrow removed.
- Adult stem cells are very limited, therefore not very useful.
- Embryonic stem cells:
- Obtained from unused early embryos from IVF.
- Stem cells are removed after 4-5 days and the embryo is then destroyed.
- These stem cells are totipotent until blastocyst phase and after are pluripotent - much more useful.
- But they are more likely to cause immune rejection than adult stem cells.
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What are the issues with stem cells therapies and who makes the decisions?
- ☒It is unethical to destroy and embryo which could give rise to a human life. (Right to life)
- ☑Less objections to stem cells from unfertilised embryos.
- ☑Adult stem cell use requires no destruction of embryos.
- ☒Adult stem cells are limited in use.
- The regulatory authority looks at proposals of stem cell research and decides if it should be allowed.
- Licensing/monitoring centres ensure only fully trained staff are carrying out procedures.
- Guidelines and codes of practice to ensure scientists are working in a similar manner.
- Developments in research and advances are monitored to ensure everything is up-to-date.
- Information and advice is provided to help society understand how embryonic research works.
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Describe how totipotency can be demonstrated practically using plant tissue culture techniques.
- 1. A single cell is taken from a growing area e.g. root.
- 2. Place cell in a sterile growth medium (agar).
- 3. The growth medium should contain nutrients and growth hormones.
- 4. The plant cell will grow into unspecialised cells which will then differentiate into specialised cells in optimum conditions.
- 5. Eventually cells will grow and differentiate into an entire plant.
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Explain how phenotype is the result of an interaction between genotype and the environment.
- Animal hair colour:
- Some animals have fur colour that is product of the environment.
- E.g. Siamese cats should have dark fur.
- Genotype codes for dark fur.
- Enzymes denatured by body heat so phenotype is affected by environment.
- MAOA:
- Enzyme that breaks down monoamines.
- MAOA is monogenic (controlled by single gene).
- But environmental factors like antidepressants and tobacco can reduce MAOA production.
- Low levels of MAOA can lead to mental health problems.
- Cancer:
- Tumour occurs by mutation in cell division controllers (oncogenes, tumour suppressor genes).
- Tumours can also occur by natural mutations caused by radiation, free radicals and carcinogen chemicals.
- Human height:
- Controlled by many genes (polygenic inheritance).
- Also largely affected by environment (diet, sports).
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Explain continuous and discontinuous variation, epistasis and locus.
- Continuous variation:
- Phenotypes appear in a range of values (height).
- Caused by many genes at different loci (polygenic) where environment has large effect.
- Discontinuous variation:
- Phenotypes appear in discrete categories (sex, blood group).
- Controlled by one gene where environment has little effect.
- Epistasis:
- In polygenic traits some genes sometimes suppress others.
- E.g. bald gene suppresses blonde or red hair gene.
- Locus:
- Different alleles for the same gene are formed in the same position on chromosomes which are called the locus.
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