Bio Exam Revision

🧬 Genetics & DNA:

• Structure of DNA:

  • DNA is structured as a double helix; the idea for this model was created by Watson and Crick.

  • The DNA strands are created by repeating units of nucleotides, which are made up of phosphate, sugar and nitrogenous bases (A, T, C, G)
    
    

• Complementary base pairing:

  • The elements of the complementary base pair are Adenine (A), Thymine (T), Cytosine (C) and Guanine (G).

  • The pairs are: A-T’s and C-G’s

  • This ensures accurate DNA replication because, as the DNA is unwound and replicated, each strand will line up the same, creating exact duplicates.
    
    

• Genes, chromosomes, and DNA:

  • Genes: Sections of DNA made up of numerous base sequences, and are for specific traits.

  • Chromosomes: Entire strands of DNA wound up and found in the nucleus of cells; there are 23 pairs in humans.

  • DNA: A molecule that contains all of the genetic information of an organism.
    
    

• Inheritance:

  • The 23 chromosomes that you get from each parent make up the 46 in their offspring. How the parents’ genes are expressed on the chromosomes gives the child/individual their traits.
    
    

• Mitosis and meiosis:

  • Mitosis: The role of mitosis is growth and repair, it happens in body cells all across the body and creates 2, new but identical cells, each with 46 chromosomes.

  • Meiosis: The role of meiosis is reproduction, it happens in the reproductive cells (ovaries and testes) and creates 4 new and non-identical cells, each with 23 chromosomes.
    
    

• Haploid and diploid number:

  • Diploid number: The total number of chromosomes in an organism (46 in humans).

  • Haploid number: Half the number of chromosomes, found in gametes (sperm and eggs) (23 in humans).

• Genetic variation & sex-linked genes:

  • Genetic variation occurs from sexual reproduction because either parent is contributing 23 chromosomes to make up the 46 in their child; this combination makes a genetically unique child.

  • Sex linked genes are found on one of the sex chromosomes (X or Y), examples are colour blindness and muscular dystrophy

  • Chromosomal abnormalities, such as down syndrome, occur when chromosomes do not divide properly during meiosis.
    
    

• Genotype and phenotype:

  • Genotype: The genetic information that is being carried by a person, e.g Bb for eye colour.

  • Phenotype: The physical expression of a person's genes, e.g Brown eyes.
    
    

• Homozygous and heterozygous:

  • Homozygous means that both of the genes a person carries are the same; this could be represented by BB or bb.

  • Heterozygous means that both of the genes a person carries are different; this could be represented by Bb.
    
    

• Dominant and recessive traits:

  • Dominant genes (represented by a capital letter) will always show if they are part of a person's genotype, for example, brown hair.

  • Recessive genes (represented by a lowercase letter) will only show if they are homozygous in a person, for example, blonde hair.
    
    

• Mutations:

  • A mutation is a change in the base sequence of an organisms DNA, which may give that organism an advantage (thick fur) or a disadvantage (cancer).

  • Mutations can be inherited through genes, an example is colour blindness, which is passed through on the X chromosome.

  • Mutations can be caused by environmental factors, an example is cancer, which can be caused by factors like exposure to UV radiation (the sun). 

  • There are 4 types of mutations:

    1. Silent mutations: changes in genetic code that don’t affect the individual, occur when a single base on the DNA strand is changed.

    2. Missense mutations: changes that mean genes produce different protein.

    3. Nonsense mutations: cells stop reading the gene's information before the end, meaning the protein created is incomplete/can’t function.

    4. Frameshift mutations: insertion or deletion of a single base, causing the information after to be jumbled so it can not be read to make a protein.

🧪 Biotechnology & Genetic Engineering:

• Recombinant DNA technology:

  • The steps in the creation of recombinant DNA technology are:

    1. Plasmids are removed from bacterium

    2. Plasmids are cut out using an enzyme

    3. DNA is removed from a human cell

    4. DNA is cut out using an enzyme to isolate a gene

    5. Human gene is inserted into the plasmid to form recombinant DNA

    6. The recombinant DNA is put into a bacterium

    7. Bacterial cells grow and divide ro produce many copies of the introduced gene.
       
       

• Restriction enzymes and plasmids in gene transfer:

  • Restriction enzymes cut the DNA out.

  • Plasmids are used to insert the gene into a bacterium
    
    

• Bacteria used to produce genetically modified products:

  • Bacteria can be used in the production because they can replicate a gene, such as insulin, over and over again.
    
    

• The Human Genome Project, genome sequencing and genetic engineering:

  • A project that mapped/sequenced/identified the 20,000 to 25,000 genes that make up the human genome and the 3 million base pairs.

  • This can benefit medicine and science through the development of new biotechnologies and personalised medicine, as people can get their genome sequenced.

  • This genetic engineering and genome research raises the ethical concern of genetic superiority in ways of people editing their or even their child's genes to their preference and how, in the future, value could be placed on a person's genetics and any modifications that may or may not include.
    
    

🌱 Stem Cells:

• Embryonic stem cells and adult stem cells:

  • Embryonic stem cells are pluripotent, meaning that they can become any of the 220 cell types in the human body, though there is a large ethical debate surrounding them.

  • Adult stem cells are for regenerating and repairing your tissues, they are found in bone marrow and other organs that need constant regeneration (skin, bones). These cells can only become and be used for certain types of cells, and there is less of an ethical debate surrounding them.
    
    

• Stem cell research guidelines and stem cell therapy:

  • Embryonic stem cells can only be used from discarded, assisted reproductive technology (IVF)

  • One condition they hope to treat using stem cell therapy is Leukemia.
    
    

• Arguments surrounding embryonic stem cells:

  • For: They are very useful for the advancement of science and technology, and are helpful in treating certain diseases and conditions because of what they can do.

  • Against: Harvesting the stem cells destroys the embryos, which raises the debate of whether using them is seen as taking a life, as it goes against people's morals and religions.
    
    

🦴 Fossils & Dating:

• The formation & examples of fossils:

  • Original - formed when part or all of an organism has been preserved, e.g teeth and shells.

  • Replacement - when part of the organism is chemically changed into another mineral, e.g shells that have been changed to opal.

  • Carbon film - formed when organisms decay, are decomposed and leave behind a thin deposit of carbon, e.g a leaf.

  • Indirect (cast/mould) - not part of an organism but evidence left behind such as an imprint in a rock, footprints, burrows and poo. There are two types:

    1. Mould = 2D/negative, an imprint left behind in the rock (footprint), space left behind after material has dissolved (common in deep ocean).

    2. Cast = 3D/positive, when an organism in rock dissolves, leaving a space that fill with soil and turns to rock.
       
       
       

• Environmental conditions that improve fossil preservation:

  • Permafrost: Cold conditions prevent decay as it’s too cold for bacteria to thrive.

  • Amber: Plant sap traps invertibrates.

  • Tar: Tar pits associated with oil reserves at the ground surface.

  • Peat: Partly decomposed plant remains in swamp areas. Low oxygen and no bacteria are good for soft tissue preservation.

  • Dry air: Bacteria and fungi need moisture to survive. Dry air dehydrates soft tissue, which fossilises.

• Index fossils and fossil dating:

  • Index fossils are fossils that can be used to compare the ages of strata (layers of sedimentary rock) in different locations, for example, trilobites.

  • Fossils useful for dating fit into four categories: 

    1. They must be widespread.

    2. There must be a lot of them.

    3. They must have lived in a narrow time period.

    4. They must be easy to identify
       
       

• Relative and absolute dating methods:

  • Relative: a technique that compares the age of one fossil or rock with another to determine which is older. An example is fluorine analysis, which compares the amount of fluorine in different bones found in the same rock.

  • Absolute: dating methods that give you the exact age of rocks or fossils. Examples are radioactive dating and tree rings.

🌍 Geological Time & Fossil Record:

• Using fossils to trace evolutionary change:

  • Scientists use fossils to trace evolutionary change over time through the changes and development of features of different organisms as the environments which they would have been in changed.
    
    

• Fossil horses and how they show evolutionary progression:

  • Key changes in fossil horses are: The increase in the size of their body, the length of their legs and the decrease in the number of toes.

  • These changes show evolutionary progress because specific traits of horses changed over millions of years through specific traits being more suited to the changes in their environment, which were happening. The change of 4 toes to 1 hoof may be because of the longer distances they were running and on harder ground

🧬 Natural Selection & Evolution:

• Darwin’s theory of natural selection:

  • An environmental factor acts on a population and results in some organisms surviving longer or having more offspring than others.

  • The change in proportion of a particular genotype of a species over many generations due to environmental selection of a particular characteristic.
    
    

• Factors and their roles in natural selection:

  • Variation: There must be differences in characteristics due to different genes.

  • Selection pressure: Something that causes death/change in the population.

  • Survival: A group survives over the other.
    
    

• Real-life example of natural selection:

  • Natural selection could apply to an environment where there are two different coloured mice (one white and one brown) living in a dark, rocky environment. This means that the brown mice blend in, but the white do not, so when they are being hunted by another animal, such as an owl, the owl will see the white coloured one and eat it over the brown. Because of this, over time, the gene for the white mouse will disappear in favour of the brown gene.
    
    

• Antibiotic resistance:

  • When a person takes antibiotics to fight a disease or illness, it will kill off most of the bacteria, but there might be a few that are not affected by the antibiotic, and because bacteria can replicate so many times and so quickly, over time, it can become completely resistant to that antibiotic.

  • This is a concern because if the bacteria in a person cannot be killed with antibiotics, people may not have the ability to fight specific illnesses.
    
    

• Speciation and isolation:

  • Speciation is the process by which one species splits into two or more separate species. It happens in three steps:

    1. Variation: there must be variation in the population (in genes)

    2. Isolation: Different groups of the population are split apart and prevented from interbreeding.

    3. Selection: natural selection affects the genotype of the isolated populations, eventually causing different species. This is because they each have to adapt to different environments
       
       

• Homologous and analogous structures:

  • Homologous characteristics are ones that have the same basic structure but different functions, which supports common ancestry because it suggests that organisms had a common ancestor who gave them these characteristics, but over time, they have evolved to suit their specific environments and needs.

  • An example of a homologous structure is the pentadactyl limb, which can be found in humans, whales, bats, rabbits, moles and more.

  • Analogous structures look similar on genetically very different organisms, because of how genetically different the organisms are, although they look similar, this does not support common ancestry.

  • An example of an analogous structure is the visual similarities between sharks and dolphins, though they are genetically very different. 

• The theory of evolution and supporting factors:

  • Evolution is a genetic change in the characteristics of a species over many generations, resulting in the formation of a new species.

  • Comparative anatomy: Homologous structures provide evidence that a group of organisms have a single common ancestor.

  • Fossils: Earlier fossils show simple organisms, while later fossils show more complex ones.

  • DNA & embryology: Through the comparison of DNA and embryos and seeing how similar many species are through their structure and DNA, supports that organisms have evolved.