Inheritance, evolution and variation

• Meiosis is the formation of four non-identical cells from one cell.
• Mitosis is the formation of two identical cells from one cell.
• Sexual reproduction involves the joining of male and female gametes, each containing genetic information from the mother or father.
• Gametes are formed by meiosis, as they are non-identical.
• A normal cell has 46 chromosomes.
• There are two sets of chromosomes (i.e. 23 pairs) in each pair, one chromosome is from the father and the second set is from the mother.
• Each gamete has 23 chromosomes and they fuse in fertilisation.
• The genetic information from each parent is mixed, producing variation in the offspring.
• Asexual reproduction involves one parent with no gametes joining, it happens using the process of mitosis, where two identical cells are formed from one cell.
• There is no mixing of genetic information in asexual reproduction, it leads to clones, which are genetically identical to each other and the parent.
• Examples of organisms that reproduce this way are bacteria, some plants and some animals.
• Cells in the reproductive organs divide by meiosis to form gametes.
• Gametes only have one copy of each chromosome.
• The cell makes copies of its chromosomes, so it has double the amount of genetic information.
• The cell divides into two cells, each with half the amount of chromosomes (46).
• The cell divides again producing four cells, each with a quarter the amount of chromosomes (23).
• These cells are called gametes and they are all genetically different from each other because the chromosomes are shuffled during the process, resulting in random chromosomes ending up in each of the four cells.
• Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes, the normal number.
• The three-domain system was developed due to advancements in science such as the improvement of the microscope and increased knowledge of biochemistry, which found that some species were more distantly related than first thought.
• Carl Woese added three large groups called domains above kingdoms: Archaea, Bacteria, and Eukaryota.
• Archaea are primitive bacteria which live in extreme environments such as hot springs.
• Bacteria are true bacteria, despite having similar features to archaea.
• Eukaryota are organisms who have a nucleus enclosed in membranes, including the kingdoms protists, fungi, plants and animals.
• To complete an evolutionary tree, scientists use classification data and fossils for extinct species.
• Evolutionary trees are used to show how closely related organisms are.
• People are concerned about the effects of genetic engineering on human health.
• Genetic engineering is the modification of the genome of an organism by introducing a gene from another organism to give a desired characteristic, a process that involves cutting genes from chromosomes using restriction enzymes, joining them with a gene from a virus or bacterial plasmid, and then placing the combined loop in a vector, such as a bacterial cell, to allow it to multiply as it will now contain the modified gene.
• Speciation is the process where one population of a species becomes so different that they can no longer interbreed to produce fertile offspring, becoming a new species.
• Selective breeding is when humans choose which organisms to breed in order to produce offspring with a certain desirable characteristic, a process that has been happening for many years since animals were domesticated and plants were grown for food.
• Genetic engineering can be useful in medicine to mass produce certain hormones in microorganisms (bacteria and fungi), but it also has potential risks, such as GM crops potentially affecting wild flowers and insects, and the potential for genes from GM crops to spread into wild plants, leading to infertility in other species and affecting the entire environment.
• The problem with selective breeding is that it can lead to inbreeding, as breeding those with similar desirable characteristics means it is likely you are breeding closely related individuals, reducing the gene pool and increasing the chance of genetic defects in offspring.
• In agriculture, genetic engineering can be used to improve yields by improving growth rates, introducing modifications that allow crops to grow in different conditions, and introducing modifications that allow plants to make their own pesticide or herbicide.
• This cell divides by mitosis to produce many copies.
• More and more cells are produced, and an embryo forms.
• The cells begin to take on different roles after this stage (differentiation).
• The advantages of sexual reproduction are the disadvantages of asexual reproduction, and vice versa.
• Produces variation in offspring, meaning that if the environment changes it is likely that an organism in the species will have a characteristic that allows them to survive (called a survival advantage).
• Although some individuals may die, variation decreases the chance of the whole species becoming extinct.
• Only one parent is needed, it allows us to use selective breeding.
• In tissue culture, plant cells are taken, placed in a growth medium with nutrients and hormones, and they grow into new plants, and are clones as they are genetically identical to the parent.