- Prokaryotic cells- Do not have nuclei or other membrane bound organelles. Bacteria and cynobacteria are prokaryotic cells. Small diameters- 0.5- 5 μm. DNA lies free in the cytoplasm (not associated with a protein). Cell wall always present
- Eukaryotic cells- Have discrete, membrane bound organelles (such as nuclei, mitochondria and chloroplasts). Bigger diameters- 20μm or more. Not all eukaryotic cells have a cell wall
- After protein synthesis, the proteins must be moved through the cell to where they are needed. This movement involves the endoplasmic reticulum, Golgi apparatus and vesicles-
- 1. Transcription of DNA to mRNA
- 2. mRNA leaves the nucleus via the nuclear envelope
- 3. Protein is made on a ribosome, and enters the rough endoplasmic reticulum (RER)
- 4. Protein moves through the RER, assuming it's 3D shape en route
- 5. Vesicles pinch off the end of the RER, containing the protein
- 6. Vesicles from the RER fuse to form the flattened sacs of the Golgi apparatus
- 7. Proteins are modified within the Golgi apparatus
- 8. Vesicles are pinched off the Golgi, containing the protein
- 9. Vesicles fuse with cell surface membrane, releasing proteins such as extracellular enzymes
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How are gametes specialised?
- Ovum- A large cell incapable of independent movement- it is wafted along the oviducts, from the ovary to the uterus (via cilia and muscular contractions)
- Cytoplasm contains massive protein and lipid food reserves for embryo. Surrounding the cell is a jelly-like coating, called zona pellucida
- Sperm- Much smaller, and is capable of movement. Enter the vagina, swim through the uterus (assisted by muscular contractions. If a ovum is met, the ovum attracts the sperm by releasing chemicals
- To enable them to swim, the sperm has a flagellum (long tail) powered by energy released by mitochondria
- To penetrate the ovum, the acrosome in the head of the sperm releases digestive enzymes, which breaks down the zona pellucida.
- Normal human cells have 46 chromosomes (23 pairs). Gametes have 23 individual chromosomes, so when two gametes combine, the resulting zygote has the full 46.
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- There are two types of cell division- one involves mitosis and creates diploid number (full) body cells, and the other involves meiosis and creates gametes with the hapliod number of chromosomes (half)
- Meiosis only occurs in ovaries and testes of animals, and ovaries and anthers of animals. It has two important roles- 1. to result in haploid cells 2. to create genetic diversity among offspring
- Genetic material is shuffled into new combinations during meiosis through independent assortment and crossing over
- Independent assortment- only one chromosome from a pair ends up in each gamete. The process of choosing gametes is random, and produces genetically variable gametes. In organisms with many chromosomes, the number of possible combinations is so large than it is unlikely that any two siblings will be genetically identical (unless twins)
- Crossing over- During 1st meiotic division,chromosomes come together as pairs and all 4 chromatids come into contact. At contact points, the chromatids break and rejoin, exchanging sections of DNA. The break point is a chiasmata, and many occur, leading to large amount of variation
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- Mammals- Nuclei from gametes fuse (in the process of fertilisation). The nucelus from a sperm enters the ovum and the genetic materials fuses, forming a zygote
- Fertilisation is important as it restores the diploid number of chromosomes in the cells, and it increases the variety of chromosomes by combining two sets of genetic info
- Plants- The nuclei also have to fuse, fertilisation takes place in the embryo sac within the ovule.
- 1. The pollen grain germinates (on the style) and a pollen tube grows down to the ovary.
- 2. The pollen grain contains two nuclei (tube nucleus and generative nucleus). On germination, the generative nucleus divides to form two haploid nulcei, which move down the pollen tube
- The two haploid nuclei reach the embryo sac, and two male gamete nuclei enter the sac
- One fuses with the egg cell- diploid zygote, and the second fuses with two nuclei in the embryo sac called polar nuclei- triploid cell
- The diploid zygote forms the embryo, and the triploid cell forms the endosperm (storage tissue)
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The Cell Cycle
- The cell cycle can be split into two parts- interphase and division
- Interphase- Time of intense activity, cell synthesises new cell components (organelles and DNA)
- Proteins are made throughout interphase, but DNA synthesis occurs in S (Synthesis). S phase separates the 1st G1 phases from the 2nd G2 phase.
- Length of interphase depends on the role of the cell (eg. no interphase in early embryo- has all materials for 1st 16 divisions). S and G2 stay constant, G2 varies (liver cells take 1-2 years, nerve and muscle never)
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- By end of interphase, the cell contains enough cytoplasm, organelles and DNA to form two new cells. Next step, share out DNA and contents of cytoplasm so each new cell can function independently. Nuclear division (mitosis) then cytoplasmic division
- 1. Prophase- Chromosomes condense, each chromosome visible as two strands called chromatida. Both identical- joined at the centromere. Spindles form and move to poles of the cell. Nuclear envelope breaks down
- 2. Metaphase- Centromeres attach to spindle fibres
- 3. Anaphase- Centromeres split. Spindles shorten, pulling the two halves of each centromere in opposite direction. Chromatids reach the poles, and spindles break down
- 4. Telophase- Chromosomes unravel, and nuclear envelope reforms- two sets of genetic info enclosed in separate nuclei
- 5. Cytoplasmic division- Cell surface membrane constricts around centre of cell. Protein filaments inside cell contract and cell splits into two new cells
- Mitosis is needed for
- Growth and repair- increases the number of cells, genetic consistency. Can regenerate new body parts, or allow old/damaged cells to be replaced
- Asexual reproduction- Organisms grow copies of themselves through mitosis- genetically identical
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- Stem cells- unspecialised cells
- Totipotent- cel develop into a complete human being (any tissue). Totipotent cells occur in early zygotes and when identical twins form
- 5 days after conception, a hollow ball of cells (blastocyte) forms (becomes placenta). Inner cells form tissues (not all)- called pluripotent embryonic stem cells.
- As the embryo develops, cell become more differentiated. Lose capacity to become a wide range of cells, become increasingly specialised
- Some adult cells can become a small range of different cells- known as multipotent.
- Plants remain totipotent throughout life- plant can be reproduced using plant tissue culture.
- Stem cells have the scope to be used in medicine- by creating tissue/organs for transplant. Embryonic are the most useful, need to be isolated from 'spare embryos' from IVF clinics, and cultured before stem cells are removed (and the embryo destroyed).
- There is a problem with rejection of grown transplants- need tissue typing, immunsupressors and therapeutic cloning (patients diploid cell removed and fused with an ovum- somatic cell nuclear transfer- then stimulated to divide by mitosis)
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Ethical Concerns with Stem Cells
- Stem cells could be used to explore gene expression, how cancer cells develop, how birth defects occur and for screening drugs
- No ethical objections against using stem cells from adult bone marrow- but scientists agree that these are less valuable than pluripotent cells which are only from embryos
- Some people object to using embryos for research, as an embryo is a life and it is unethical to destroy it
- Health regulatory authorities are needed when new research raises ethical concerns
- In the UK, the Human Fertilisation and Embryology Authoritt (HFEA) regulastes research on human embryos.
- In 2001, the House of Lords allowed human embryos to be used in research. In 2004, permission was given for use of TC in research. In 2007, research using animal-human embryo allowed
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Gene Expression and Controlling Development
- As an embryo develops, the cells differentiate and become specialised. Different specialised cells only express some of their genes
- Genes in uncoiled regions of DNA are transcribed into mRNA.
- The enzyme RNA polymerase binds to a section of DNA adjacent to the target gene- promoter region.When the enzyme attaches- transcription begins
- Genes remain switched off until the enzyme attaches correctly. Regulator proteins are also normally needed to start the process
- Transcription can be prevented by repressor molecule attaching to the promoter region- blocking it. They can also attach to the regulator protein- preventing attachment- The gene is switched off
- Cell- In multicellular organisms, are specialised for a particular function
- Tissue- A group of specialised cells working together to carry out a one function
- Organ- A group of tissues working together to carry out one function
- Organ system- A group of organs working together to carry out a particular function
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Genes and Environment
- The characteristics of an organism are known as the phenotype. Differences in phenotype are caused by genotype and environment
- Some characteristics controlled only by genoptype eg. Blood group and eye colour. Such characteristics are controlled by a gene at a single locus and show discontinuous variation. The phenotype fall into discrete groups
- Characteristics controlled by environment and genotype show continuous variation eg. human height. They are controlled by genes at many loci- polygenic inheritance and by environment- directly or by influencing gene expression
- When a number of gene involved in inheritance- polygenic. Conditions where several genetic and 1 or more environmental factors are involved- multifactorial.
- Height- genotype and environment
- Hair colour- genotype and environment. Melanin (darkens skin/fur) is activated by MSH (melanocyte stimulating hormone) when MSH binds to receptor sites on cells. UV light increases MSH and receptors, so more melanin is produced to protect the skin.
- Himalayan rabbits use a enzyme (tryosinase) to produce melanin. It is inactive at normal body temperature, but at colder extremities (ears, tails,paws) it is activated- darker fur
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Genes and Environment 2
- MAOA- An enzyme which breaks down neurotransmitters involved in behaviour. Some individuals do not produce this, leading to a build up of neurotransmitter and more aggressive behaviour. Maltreatment as a child is associated with becoming an anti-social adult.
- However, maltreated children with high MAOA levels were less likely to show violent behaviour than those who have low levels
- Cancer- occurs when cell multiplication is faster than death, causes a tumour to grow. Caused by DNA damage
- Two types of gene affect cell cycle, and trigger cancer- oncogenes and tumour supressor genes
- Oncogenes- code for proteins that stimulate the transition of cells between stages of the cell cycle. Mutations cause the cells to be continually active
- Tumour supressor genes- Produce proteins to stop the cycle. Mutations inactivate these genes
- Many gene mutations have been found to predispose people to cancer- inherited faulty allele
- Damage from the environment (physical or chemical) cause cancer eg. smoking, uv exposure, virus infection, carcinogens
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