Presentation on theme: "Continuity of species and the sources of variation Bio3B."— Presentation transcript:
Continuity of species and the sources of variation Bio3B
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.
Inheritance Organisms inherit characteristics from their parents Characteristics are controlled by DNA In asexual reproduction, organisms inherit DNA from 1 parent In sexual reproduction, organisms inherit DNA from both parents
DNA DNA is found in the nucleus of cells It is organised into segments called chromosomes Chromosomes are only visible when the cell is dividing
Genes and chromosomes The segment of DNA that controls one characteristic is called a gene Genes are found on structures called chromosomes The location of the gene on a chromosome is called its locus
Why are proteins important? Roles of proteins in the body include Structural proteins eg collagen, keratin Enzymes (organic catalysts) eg digestive enzymes Transport proteins eg haemoglobin Regulatory proteins eg hormones Protective proteins eg antibodies, clotting factors Therefore proteins determine what you will look like, and how your body functions
Asexual reproduction One parent No variation in offspring Advantages - low complexity, rapid division Disadvantages – as all offspring identical, have smaller ability to survive change
Sexual reproduction 2 parents Variation in offspring Advantages – variation gives better chance of species survival if change occurs Disadvantages – more complex, takes longer to produce offspring
Cell division for reproduction The process of cell division for asexual reproduction is called mitosis Offspring will have the same number of chromosomes as their parent The process of cell division for sexual reproduction is called meiosis It is necessary in order to reduce the number of chromosomes Gametes will contain half the number of chromosomes – this is called the haploid number
Comparing mitosis and meiosis MitosisMeiosis Where it occursBody cellsGonads (reproductive organs) Why it occursCell repair, growth, asexual division Sexual reproduction Number of cells produced24 Number of divisions12 Number of chromosomes in daughter cells Same as parent (diploid)Half that of parent (haploid) Amount of variation in daughter cells NoneLots Advantages for reproductionSimple Rapid division Allows variation Disadvantages for reproduction No variationMore complex Slower reproduction
Sources of variation Mutations Random assortment of chromosomes during meiosis Crossing over during meiosis Non-disjunction during meiosis Chance combination of gametes during fertilization
Mutations New genes can appear due to mutations or changes in DNA – usually due to mistakes in the copying of DNA during meiosis. These can be Beneficial eg disease resistance Harmful eg haemophilia Neutral eg tongue rolling
Sexual reproduction Crossing over – swapping of genetic material between homologous chromosomes Random segregation during meiosis – its random which combination of alleles ends up in each gamete Random combination of gametes at fertilisation – its random which gametes end up together
Crossing over This 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.
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 2 23 possible chromosome combinations (which is about 8.4 million) for each gamete.
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.