Nitrogen cycle

Nitrogen gas in the atmosphere is turned into nitrogen compounds that a plant can use. The plant is eaten or decayed by decomposers forming ammonia compounds either from decay or the urea of animals. The ammonia is then turned into nitrates by nitrifying bacteria. Nitrates are then converted back into nitrogen gas in the atmosphere by de-nitrifying bacteria.

Human influences on the environment

CO2 and other greenhouse gases: CO2, Methane, water vapour

1. Released from deforestation
2. Respiration
3. Burning of fossil fuels

The greenhouse effect is the trapping of heat within the atmosphere which increases global warming. It then radiates the heat back down towards earth.

SO2 is released from the burning of fossil fuels and released from sewage treatment plants. Can cause acid rain and erosion of rock i.e. limestone.

Carbon monoxide is released from incomplete combustion. It can bind irreversibly to haemoglobin thus preventing oxygen from being transported around the body.

Ammonia [from fertilisers] can be leached into rivers and streams.

Leaching is when excess dissolved nutrients and minerals from fertilisers run into streams and rivers and pollute water. This causes algae to bloom in immense quantities. Algae prevents sunlight from reaching plants on the river bed hence prevents plants from photosynthesising. The plants would die and the quantity of dead matter in the water. This pollutes the water even more. Bacteria respire aerobically to decompose the matter. This uses up all the oxygen in the water hence the organisms cannot respire. Nitrates and phosphates.

Natural selection

Natural selection occurs when a mutation arises in members of a species.

If the mutation is beneficial to the organism, it will live longer and reproduce more.
More of its offspring would have inherited the characteristic and too will survive.
This will continue over generations until the mutation becomes a common gene in the species in which those without will not be able to compete and will die.

Mutations

Occur when a gene has been copied incorrectly. This is not done on purpose and this faulty gene can be passed down through generations.

Stages of mitosis

1. Prophase is when the dna begins to condense and become more visible. The nucleus has also disappeared.
2. Metaphase is when the spindle of fibres have formed across the length of the cell and are prominent.
3. Anaphase is when the fibres have split in half and are pulled to the opposite sides of the cell.
4. Telophase is when the chromosomes have reached the opposite sides of the cell and the nuclear membrane has begun to reform. Cytokinesis has occurred and two identical daughter cells have been created.

Cytokinesis is the split of the cell physically.

Meiosis and Mitosis [haploid and diploid]

Meiosis occurs in the ovaries and in the testes. It is when one cell divides to produce four haploid cells i.e. gametes. A haploid cell is one with one chromosome i.e. half the genetic makeup. As a result, gametes are genetically different.

Mitosis occurs during growth and development. It is when one cell divides to produce two diploid cells. Diploid cells are those with a pair of chromosomes i.e. the full genetic makeup.

Offspring are genetically different to their parents as they have different genotypes as a result of the receipt of different chromosomes.

Note that it is at the stage of a foetus where cells begin to specialise.

DNA structure and purpose

DNA is found in the nucleus of any cell [except red blood cells which have none]. They must be able to do two things:

1. Contain instructions which control protein production hence which control the characteristics of an organism.
2. Be able to replicate in a way that preserves all the genetic information required. It should be able to be transferred from a parent to a child.

DNA is in the form of a double helix, with two strands forming the shape of a twisted ladder. Each 'rung' of the ladder consists of a base, Adenine, Thymine, Cytosine and Guanine. Bases are held together by intermolecular forces.

A sequence of bases contains a code that informs the cell of what protein to make. There are millions of sequences and a vast number of proteins that can be produced. A section of DNA that contains the instructions to create on complete protein is called a Gene.

DNA = sentence
Gene= Paragraph
Chromosome= Chapter

We have a pair of each chromosome so that there we have two copies of each gene which results in more variation within our species. A gene can come in different forms such as alleles. Alleles can be either dominant or recessive. We may have a chromosome from our mother who has the gene for brown hair and a chromosome from our father who has a gene for blue eyes. We have characteristics from both parents. A child who looks very much like their parent means their alleles are most likely dominant.

Source: BBC Bitesize

A pair of chromosomes carry the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called alleles. These alleles may be the same (homozygous) on each pair of chromosomes, or different (heterozygous), for example, to give blue eyes or brown eyes.

Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.

Which chromosome we get from each pair is completely random. This means different children in the same family will each get a different combination. This is why children in the same family look a little like each other and a little like each parent, but are not identical to them.

A dominant allele is the one that will be made.
A recessive allele is one that will be masked.
A homozygous genotype is one that contains two of the same allele.
A heterozygous genotype is one that contains two different alleles, one dominant and one recessive.
Phenotype: A set of characteristics in an organism that originate from a combination of a genotype and interaction with the environment.
Genotype: Set of genes in DNA responsible for a particular trait.