Continuity of species and the sources of variation

Bio3B

Organisms show variationOrganisms 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

DNA controls protein synthesis

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

Events of meiosis

Comparing mitosis and meiosisMitosis 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 2

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

Sources of variation• Mutations

• Random assortment of chromosomes during meiosis

• Crossing over during meiosis

• Non-disjunction during meiosis

• Chance combination of gametes during fertilization

MutationsNew 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 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.

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.

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.