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 DNA
In asexual reproduction, organisms inherit DNA from 1 parent
In sexual reproduction, organisms inherit DNA from both parents

4. 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

5. 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

6. DNA controls protein synthesis

7. 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

8. Asexual reproduction One parent No variation in offspring
Advantages - low complexity, rapid division
Disadvantages – 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 occurs
Disadvantages – more complex, takes longer to produce offspring

10. 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

11. Events of meiosis

12. Comparing mitosis and meiosis
Where it occurs
Body cells
Gonads (reproductive organs)
Why it occurs
Cell repair, growth, asexual division
Sexual reproduction
Number of cells produced 2 4
Number of divisions 1
Number of chromosomes in daughter cells
Same as parent (diploid)
Half that of parent (haploid)
Amount of variation in daughter cells
None
Lots
Advantages for reproduction
Simple
Rapid division
Allows variation
Disadvantages for reproduction
No variation
More complex
Slower reproduction

13. Sources of variation Mutations
Random assortment of chromosomes during meiosis
Crossing over during meiosis
Non-disjunction during meiosis
Chance combination of gametes during fertilization

14. 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

15. 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

16. 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.

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.