Biology Topic 6: Inheritance, Variation and Evolution

- Genes code for particular sequences of amino acids, which are put together to form particular proteins.

- DNA determines what proteins a cell produces, and therefore the type of cell it will be.

- Genome is the entire set of an organism's genetic material

- Understanding the human genome helps in medicine for many reasons:

• Allows scientists to identify the genomes linked to particular diseases
• If there is more understanding of the genes causing diseases we can develop effective treatments
• Scientists can study human genomes to see the migration patterns of certain populations around the world; as different populations migrated away from Africa, their genomes altered slightly. These alterations can be studied by scientists and can show when new populations split off in a different route.

- DNA strands are polymers, coiled in a double helix, made of many repeating units called nucleotides.

- Nucleotides consist of one sugar molecule, one phosphate molecule, and one 'base'.

• The backbone of DNA strands consists of the alternating sugar and phosphate molecules; the four bases are A, C , G and T, with one of them joining to each sugar
• Each base links to another base on the opposite strand.
• A always pairs with T; C always pairs with G- complementary pairings

- The order of bases determines the order of amino acids in a protein, with each amino acid being coded for by a sequence of three bases in the gene.

- The parts of the DNA that don't code for proteins 'switch' genes on and off- they control whether or not a gene is expressed (used to make a protein).

- Protein synthesis takes place in the ribosomes of cells

- Triplets of bases code for specific proteins; a different sequence of bases codes for a different protein

- The DNA 'unzips' itself and copies to form a single strand, mRNA; this molecule acts as a messenger between the DNA and ribosome, carrying the code between the two: this is called transcription

- The ribosome decodes the sequence of amino acids to know what sequence of proteins to produce in the stage called translation; it does this by matching the tRNA (translation RNA) to the mRNA

- The particular tRNA and mRNA molecules end up in a polypeptide chain, forming a protein.

- A mutation is a random change in an organism's DNA, which is sometimes inherited

- They change the sequence of the DNA bases in a gene, creating a genetic variant

• as the sequence of DNA codes for the sequence of amino acids to make up a protein, gene mutations sometimes lead to changes in the protein coded for

- Most mutations have no effect on the protein made, however sometimes it will code for an altered protein with a change in its shape, which could affect its ability to function

• if the shape of an enzyme's active site is changed, its substrate may no longer be able to bind to it
• structural proteins, such as collagen, could lose their strength if their shape is changed, making them invaluable for providing structure and support

- Sexual reproduction is when the genetic information from two organisms is combined to produce offspring genetically different to either parent

- In humans, each of the parent's gametes contains 23 chromosomes- half the number of chromosomes in a normal cell

• the sperm cell fertilises the egg cell to form a cell with the full number of chromosomes

- The offspring inherits features from both the mother and father because they have half of each parent's genetic information; this mixture of genetic information produces variation in the offspring

- Asexual reproduction is when only one parent reproduces, making its offspring genetically identical to that parent

- It happens by mitosis, with each cell having exactly the same genetic information as the original parent cell; a clone

- There is no fusion of gametes or mixing of chromosomes, so no variation between the parent and the offspring

- Bacteria, some plants and some animals reproduce asexually

- The cell duplicates its genetic information, forming chromosomes with two arms, each identical to the other.

- After this replication, the chromosomes arrange themselves into pairs

- In the first division, the chromosome pairs line up in the middle of the cell

- The pairs are then pulled apart to that each new cell has only one copy of each chromosome

- In the second division, the chromosomes line up again in the centre of the cell; this is when the arms of the chromosomes are pulled apart

- At the end of meiosis, there are four gametes, each with only a single set of chromosomes in it. Each of the gametes is genetically different to the others as the chromosomes have been mixed together and each gamete only gets half of them, at random

SEXUAL REPRODUCTION

• offspring has a mixture of two sets of chromosomes, inheriting genes from both parents, producing variation in the offspring
• variation increases the chance of a species surviving a change in the environment; variation is likely to have led to some of the offspring being able to survive due to a survival advantage
• individuals with characteristics making them better adapted to the environment have a better chance of survival, making them more likely to breed successfully and pass the genes for characteristics on- natural selection
• selective breeding can be used to speed up natural selection, allowing us to produce animal offspring with desirable characteristics; we can increase food production this way by breeding animals that produce a lot of meat

ASEXUAL REPRODUCTION

• only one parent is needed
• less energy is required because organisms don't need to find a mate
• faster than sexual reproduction, meaning many identical offspring can be produced in favourable conditions

- The genes a person inherits determine the characteristics they develop

- All genes exist in different versions called alleles; there are two versions of every gene inside the body, one on each chromosome in a pair

- If an organism has two alleles of a particular gene which are the same, it's homozygous for that trait; if the alleles are different, the trait is heterozygous

- If the two alleles are different, only one can determine what characteristic is present

• the allele for the characteristic that is shown is called dominant (shown by a capital letter- B)
• the allele that is masked by the dominant allele is called recessive (shown in lover case- b)
• for an organism to display a recessive characteristic, both alleles must be recessive

- Genotype is the combination of alleles a person has; alleles work together at a molecular level to determine a person's characteristics- this is called their phenotype

- Cystic fibrosis is a genetic disorder of the cell membranes; it results in the body producing lots of mucus in the air passages and pancreas.

• the allele for cystic fibrosis is recessive, shown by 'f', and is carried by around 1 in 25 people
• people with one copy of the allele aren't affected, only carriers, as the allele is recessive
• for a child to have it, both parents must be either carriers or have the disorder themselves
• if both parents are carriers, there's a 25% chance the child has the disorder

-Polydactyly is a genetic disorder where a baby's born with extra fingers or toes; it doesn't usually cause any other problems so isn't life-threatening.

• the disorder is caused by a dominant allele, 'D', so can be inherited if just one parent carries the defective allele
• the parent with the defective allele will also have the condition since the allele is dominant
• if one parent has the D allele, there's a 50% chance of a child also having the disorder

- Before embryos are implanted into the mother's womb in IVF treatment, it's possible to remove a cell and analyse its genes.

- Many genetic disorders, such as cystic fibrosis, can be detected this way; it's also possible to get DNA from an embryo in the womb and test that for disorders.

- Embryonic screening is controversial because of the decisions it can lead to

• for embryos produced my IVF- after screening, embryos with 'bad' alleles would be destroyed
• for embryos in the womb- screening could lead to the decision to terminate the pregnancy
• it implies people with genetic problems are 'undesirable', which could increase prejudice
• everyone may one day want to screen embryos so they can pick the most 'desirable' one
• screening is expensive

- There are also arguments for embryonic screening

• it will stop people suffering
• treating disorders costs the Government (and taxpayers) a lot of money
• there are currently laws to stop it going too far; parents cannot even select their baby's sex

- Mendel noted how characteristics in plants were passed in from one generation to the next; the results were published in 1866 and eventually became the foundation of modern genetics

- He had shown that the height characteristic in pea plants was determined by separately inherited 'hereditary units' passed on from each parent

• the ratios of tall: dwarf plants in the offspring showed that the unit for tall plants, R, was dominant over the unit for dwarf plants, t.

- He reached three important conclusions about heredity in plants

• characteristics in plants are determined by 'hereditary units'
• hereditary units are passed on to offspring unchanged from both parents, one unit from each
• they can be dominant or recessive; if an individual has both units for a characteristic, the dominant one will be expressed

- Mendel's work was cutting edge and new to scientists in the 19th Century; they didn't have the background knowledge to properly understand his findings

- After his death, people realised how significant his work was; using it as a starting point, the observations of many scientists have contributed to our understanding of genes today.

GENETIC VARIATION

- An organism's characteristics are determined by the genes inherited from their parents; most animals get genes from both the mother and father.

- This combining of genes from two parents leads to genetic variation- no two of the species are genetically identical.

- Some characteristics are determined only by genes; in animals these include eye colour, blood group and inherited diseases.

ENVIRONMENTAL VARIATION

- The environment and conditions organisms live and grow in cause differences between members of the same species.

• eg. a plant grown in plenty of sunlight would be luscious and green, whereas the same plant grown in darkness would grow tall and spindlt with yellow leaves

- Charles Darwin knew that organisms in species had wide phenotypic variation and that they had to compete for limited resources in an ecosystem.

- He concluded that organisms with the most suitable characteristics for the environment would be more successful competitors and more likely to survive; 'survival of the fittest'.

- The surviving organisms are more likely to reproduce and pass on the genes for the characteristics that made them successful to their offspring.

- Tthe organisms that are less adapted would be less likely to survive and reproduce, so they are less likely to pass on their genes to the next generation.

- Over time, the beneficical characteristics become more common in the population and the species evolves.

- Darwin's 1859 book 'On the Origin of Species' was very controversial

• it went against religious beliefs about how life on Earth developed; it was the first plausible explanation for the existence of life on earth without the need of 'God'
• Darwin couldn't explain why these new and useful characteristics appeared or how they were passed on from individual organisms to their offspring
• there wasn;t enough evidence to convince many scientists as not many other studies about how organisms change over time had been conducted

- Jean-Baptiste Lamarck argued that changes that an organism acquires during its lifetime will be passed on to its offspring

• he believed that if a characteristic was used a lot by an organism, it would become more developed during its lifetime, with its offspring inheriting the acquired characteristic
• eg. if a rabbit used its legs to run a lot, its legs would grow longer- the offspring of that rabbit would then be born with longer legs

- Organisms are selectively bred to develop features that are useful or attractive

• animals that produce more meat/ milk
• crops with disease resistance
• dogs with a good, gentle temperament
• decorative plants with big/ unusual flowers

- In selective breeding, the organisms with the desired characteristics are bred together to produce 'desirable' offspring; this process is continued over several generations as the desirable trait becomes stronger- eventually all offspring will have the characteristic.

- Genetic engineering is when a gene responsible for a desirable characteristic is transferred from one organism's genome into another organism, so that it too has the desired characteristic.

- A useful gene is isolated from one organism's genome by enzymes and is inserted into a vector.

- The vector is usually a virus or bacterial plasmid, depending on the type of organism the gene is being transferred to.

- When the vector is introduced to the target organism, the useful gene is inserted into its cells; scientists use this method for many reasons

• bacteria have been genetically modified to produce human insulin which can treat diabetes
• GM crops have had their genes modified to improve the size and quality of their fruit, or make them resistant to disease, insects, and herbicides, or to increase the crop yield
• scientists are researching genetic modification treatments for inherited diseases caused by faulty genes; this is done by inserting working genes into people with the disease- gene therapy

CLONING PLANTS

- Tissue culturing is when a few plant cells are put in a growth medium with hormones and they grow into new plants, clones of the parent plant; it is used by scientists to preserve rare plants that are hard to reproduce naturally, and by plant nurseries to produce lots of stock quickly.

- Gardeners can take cuttings of good parent plants, and then plant them to produce clones of the parent plant; these plants can be produced quickly and cheaply.

EMBRYO TRANSPLANTS

- Sperm and egg cells can be taken from a prize bull and cow and artificially fertilised; the embryo that develops is then split many times, forming clones, before any cells can become specialised.

-The cloned embryos are then implanted into surrogate cows where they grow into genetically identical calves

ADULT CELL CLONING

- Adult cell cloning involves removing the nucleus from an unfertilised egg cell; the nucleus is then removed from an adult body cell and is inserted into the 'empty' egg cell.

- The egg cell is then stimulated by an electric shock, making it divide, just like a normal embryo.

- When the embryo is a ball of cells, it's implanted into the womb of an adult female, where it grows into a clone of the original adult body cell.

FOSSILS

- Speciation occurs when populations of the same species become so different that they can no longer successfully interbreed to produce fertile offspring

- Isolation and natural selection lead to speciation

• isolation is when populations of a species are separated; it can happen due to a physical barrier, such as floods and earthquakes, which makes the conditions on either side of the barrier different, so the species on either side has to develop different characteristics to survive
• each population shows genetic variation because they have a wide range of alleles
• in each population, individuals with characteristics better adapted to their environment have a better chance of survival and so are more likely to breed successfully
• the alleles that control the beneficial characteristics are more likely to be passed on to the next generation

- Eventually, the individuals from the different populations will have changed so much that they won't be able to breed with one another to produce fertile offspring- they have become separate species

- Bacteria can sometimes develop random mutations in their DNA, which can lead to changes in the bacteria's characteristics- this can lead to antibiotic-resistant strains forming as that gene becomes more common in the population.

- For the bacterium, the ability to resist antibiotics is an advantage; it has a better chance of survival so it lives for longer and reproduces many more times, increasing the population size of the strain.

- Antibiotic-resistant strains are a problem for people who become infected with these bacteria as they aren't immune to the new strain and there is no effective treatment- this means the infection easily spreads; MRSA is a relatively common 'superbug' that often affects people in hospitals and can be fatal if it enters their bloodstream.

- The more often antibiotics are used, the bigger the issue of anitibiotic resistance becomes, making it important that doctors only prescribe antibiotics when they are really needed.

- It is also important that patients take the full course of antibiotics so all the bacteria are destroyed, which means there are none left to mutate and develop into antibiotic-resistant strains.

- Organisms have traditionally been classified according to Carl Linnaeus' system, which groups living things according to the characteristics and structuresthst make them up

• in the Linnaean system, living things are first divided into kingdoms
• they are then subdivided into smaller groups - phylum, class, order, family, genus, species

- In 1990, Carl Woese proposed the three-domain system

• ARCHAEA- organisms are primitive bacteria, often found in extreme places, eg. salt lakes
• BACTERIA- containing true bacteria like E. coli; have many biological differences to archaea
• EUKARYOTA- a broad range of organisms including fungi, plants, animals and protists
• these are then subdivided into smaller groups- kingdom, phylum, class, order, family, genus

- In the binomial system, every organism has its own two-part Latin name; the first part refers to the organism's genus (giving information about its ancestry), and the second part is its species.

- Evolutionary trees show how scientists think different species are related; they show common ancestors and relationships between species; the more recent the common ancestor, the more closely related the two species- and the more characteristics they're likely to share.