Presentation on theme: "Continuity of species and the sources of variation"— Presentation transcript:
1 Continuity of species and the sources of variation Bio3B
2 Organisms show variation Organisms within a species show a great deal of differences in their appearance.Some of this may be due to differences in the environment eg diet, but much of this is due to the presence of different gene combinations or DNA sequences in different members of each species.DNA is inherited (or passed on) from parents to children, so many differences are inherited.We use the term variation to refer to the presence of inheritable differences between individuals due to differences in their genes or DNA.
3 Inheritance Organisms inherit characteristics from their parents Characteristics are controlled by DNAIn asexual reproduction, organisms inherit DNA from 1 parentIn sexual reproduction, organisms inherit DNA from both parents
4 DNA DNA is found in the nucleus of cells It is organised into segments called chromosomesChromosomes are only visible when the cell is dividing
5 Genes and chromosomesThe segment of DNA that controls one characteristic is called a geneGenes are found on structures called chromosomesThe location of the gene on a chromosome is called its locus
7 Why are proteins important? Roles of proteins in the body includeStructural proteins eg collagen, keratinEnzymes (organic catalysts) eg digestive enzymesTransport proteins eg haemoglobinRegulatory proteins eg hormonesProtective proteins eg antibodies, clotting factorsTherefore proteins determine what you will look like, and how your body functions
8 Asexual reproduction One parent No variation in offspring Advantages - low complexity, rapid divisionDisadvantages – as all offspring identical, have smaller ability to survive change
9 Sexual reproduction 2 parents Variation in offspring Advantages – variation gives better chance of species survival if change occursDisadvantages – more complex, takes longer to produce offspring
10 Cell division for reproduction The process of cell division for asexual reproduction is called mitosisOffspring will have the same number of chromosomes as their parentThe process of cell division for sexual reproduction is called meiosisIt is necessary in order to reduce the number of chromosomesGametes will contain half the number of chromosomes – this is called the haploid number
12 Comparing mitosis and meiosis Where it occursBody cellsGonads (reproductive organs)Why it occursCell repair, growth, asexual divisionSexual reproductionNumber of cells produced24Number of divisions1Number of chromosomes in daughter cellsSame as parent (diploid)Half that of parent (haploid)Amount of variation in daughter cellsNoneLotsAdvantages for reproductionSimpleRapid divisionAllows variationDisadvantages for reproductionNo variationMore complexSlower reproduction
13 Sources of variation Mutations Random assortment of chromosomes during meiosisCrossing over during meiosisNon-disjunction during meiosisChance combination of gametes during fertilization
14 MutationsNew genes can appear due to mutations or changes in DNA – usually due to mistakes in the copying of DNA during meiosis. These can beBeneficial eg disease resistanceHarmful eg haemophiliaNeutral eg tongue rolling
15 Sexual reproductionCrossing over – swapping of genetic material between homologous chromosomesRandom segregation during meiosis – its random which combination of alleles ends up in each gameteRandom combination of gametes at fertilisation – its random which gametes end up together
16 Crossing overThis involves the swapping of genes between 2 homologous chromosomes.Sometimes during meiosis, the strands (called chromatids) of two homologous chromosomes get tangled up. The point where they cross is called a chiasma (plural chiasmata).When this happens, the chromatids may break at this point and reattach to the other chromosome.This results in a swap of genes called recombination. In this way new combinations of genes can be formed.Crossing over can occur during prophase.
17 Random assortment of chromosomes When the homologous pairs line up during meiosis and then separate, the order of each pair is totally random, and each will separate independently of all the other pairs. This means that there are 223 possible chromosome combinations (which is about 8.4 million) for each gamete.
18 Chance combination of gametes There are about 8.4 million possible chromosome combinations for each gamete.There is no way of predicting which combination would be present in the single ovum to be fertilised and for each of the millions of sperm trying to reach it.Of these sperm only one can be successful in fertilising the ovum, but there is no way of predicting which one it will be.The new offspring produced will therefore be a random combination of the genes from each parent.
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