Inheritance, variation and evolution

• DNA stands for deoxyribonuceic acid and is the chemical that all genetic material in a cell is made up from, containing coded information on how to put an organism together and how to make it work. This determines what inherited characteristics you have.
• DNA if found in chromosomes in the nucleus, these chromosomes usually come in pairs.
• DNA is a polymer and made up of two strands coiled together in the shape of a double helix.
• A gene is a small section of DNA found on a chromosome, each one codes for a particular sequence of amino acids which are out together to make specific proteins, telling cells what order to put amino acids together. Only 20 amino acids are used but this makes up thousands of different proteins. DNA also determines what proteins the cell produce which, in turn, determines what type of cell it is.
• The genome is the entire set of genetic material in an organism, scientists have worked out the entire human genome. This is important for three main reasons:
  * It allows scientists to identify genes in the genome that are linked to different types of disease.
  * Knowing which genes are linked with inherited diseasecan help scientists understand them better and develop a treatment.
  * Scientists can look at genomes to trace migration patterns of poulations around the world.

Sexual reproduction involves genetic information from two organisms combining to make offspring which are genetically different to both parents. This is where both the mother and father produce gametes by meosis. In humans, each gamete contains 23 chromosomes as they are not in pairs. The egg and sperm then fertilise to form a cell with a full set of chromosomes, this is why the offspring inherit features from both parents. This form of reproduction causes variation. Some flowering plants reproduce this way, with the pollen acting as sperm.
Sexual reproduction involves only one parent and so the offspring is gentically identical to the parent (a clone), this happens by mitosis. Bacteria, some plants and some animals reproduce this way.

Meiosis involves two cell devisions:

• Before the cell starts to divide, it duplicates its genetic information, forming two-armed chromosomes, each arm being identical to the other. The chromosomes then arrange themselves into pairs.
• In the first division the chromosomes line up in the centre of the cell.
  The pairs are then pulled apart so each new cellonly has one copy of each chromosome. Some of both parents genetic material go into each new cell
• In the second division, the chromosomes line up agian and are both arms are pulled apart.

Consequently, you get four gamets, each one with a single set of chromosomes, each gamete being genetically different.
After two gametes have fused during fertilisation, the resulting new cell divides by mitosis to replicate itself, producing lots of new cells in an embryo. These then differentiate to make up a whole organism.

The 23rd chromosome in every human cell are either XY (male) or ** (female), and determine your sex.
When making sperm, the X and Y chromosomes are drawn apart in the first division, where there is a 50% chance of getting either an Y or X. This is similar to eggs, exept the orginal cell has two X chromosomes, so all the eggs have 1 X chromosome.
Genetic diagrams are models to show the probability of an outcome when you cross different genes or chromosomes. This can be shown using a punnet square or other genetic diagrams.
(Look at book for more info).

• Differences within a species is called variation.
• This can be genetic (differeneces in genotype). A genotype is all of the genes and alles that an organism has.
• This affects an organisms phenotype - the characteristics it displays.
• Interactions with the enviroments can also affect the phenotype, for example sunlight for a plant. Most variation is a mixture of these two factors.
  Occasionally, a gene can mutate. A mutation is a rare, radom change in an organisms DNA that can be inherited, these occur continuously. Mutations mean that the gene is altered, producing a genetic variant. As the gene codes for the sequence of amino acids that make up a protein, gene mutations sometimes lead to changes in the protein that it codes for. Most genetic variants have little or no effect on the protein the gene codes for, changing to such a little extent thats its function is unaffected, therfore not affecting its phonotype. Some do cause a small change, for example a change in phenotype. Sometimes, they have much larger effects, determing phenotype. For example, cystic fibrosis is caused by the deletion of just three bases. The genes codes for a protein that controls the movement of salt and water into and out of cells, however the protein produced by the mutated gene does not work properly, leading to excess mucus production in the lungs and digestive system, which can make it difficult to breathe and digest foods.

The theory of evolution is that all of today's species have eveolved from simple life forms that first started to develop over three billion years ago.

Darwin came up with the theory of natural selection, he observed that organisms in a specis show wide variation in their characteristicsand that they gad to compete for limited resouces in an ecosytem. He concluded that organisms with the most suitable charcteristics are more likely to suvive, reproduce and pass on their characteristics. Therofore, over time beneficial charteristics become more common in a population and the species changes.

The relevant scientific knowledge as not avalable at the time and so he could not give a good explanation to why new charateristics appeared or how organisms passed on adaptions. The discovery of genetics supported Darwins's theory, showing that it was genetic variants that give rise to phenotypes that are suited to the enviroment. The fossil record also allowed scientists to see hw species change doer time and the discovery how bacteria is able to evolve and become resistant to antibiotics furthers support for natural selection.

Speciafication if when a new species is formed from natural selection. Ths happens when populations of the same species change enough to become reproductivley isolated - can not inbreed to produe fertile offspring.

species become extinct for these reasons:

• An envirment changes too quickly.
• A new predator or disease kills them or makes it so that they can no longer compete for food.
• A catastrophic event happens that kills them all.

Selective breeding is when humans artificially select the plants or animals that are going to breed so that the genes for a particular characteristic stay in the population. For example, animals that produce more milk or meat; crops with disease resistance; dogs with a good, gentle temprement; decrative plants with big, beautiful flowers. Selective breeding has three main steps:

• From your existing stock, select the ones with desirable characteristics and breed them.
• Select the best of the offspring and breed them together.
• Continue this process over several generatons, the desirable trait will get stronger.

This is how we ended up with edible crops from wild crops and domesticated cats and dogs.

The main problem with selective breeding is that it reduces the gene pool - the number of different alleles in a population. This is because the farmer keeps breeding from the "best" which are closely related, this is known as inbreeding. This can cause health problems because theres more chance of an organism inheriting harmfull genetic defects. There can also be problems if a new disease appears, if one of the organisms die it is likely the rest will also.

Genetic engineering is the transfer of a gene responsible for a desirable charcteristic from one organism's genome into another organism, so that it also has the desired charcteristic:

• A useful gene is isolated from one organism's genome using enzymes and is inserted into a vector.
• The vector is usually a virus or a bacterial plasmid (a circular DNA found in bacterial cells).
• When the vector is introduced to the target organsm, the useful gene is inserted into its cells(s).

This can be used in a number of different ways:

• Bacteria have been genetically modified to produce human insulin that can be used to treat diabetes.
•  Genetically modified crops have their genes modified, to have desired qualities such as disease resistant .
• Sheep have been genetically engineered to poduce substances, like drugs, in their milk that can be used to treat human diseases.
• Scientists are researching genetic modification treatments for inherited diseases caused by faulty genes - gene therapy.

The transfer of the gene is carried out when the organism receiving the gene is at an early stage of development, meaning the organism develops with the characteristic coded for by the gene.

Pros

• The characteristics chosen for GM crops can increase the yield, making more food.
• People living in developing nations often lack nutrients in their diets. GM crops could be engineered to contan the nutrient thats missing.
• GM crops are already being grown in some places without any poblems.

Cons

• Some say it will affect the number of wild flowers and so the the population of insects, that live in and around the crops, therefor decreasing biodiversity.
• Some think GM crops may not be safe and are concerned that we do not fully understand the effects of eating them.
• Transported genes may get out into the natural enviroment, creating 'superweeds'.

Fossils are the remains of organisms from thousands of years ago, which are found in rocks. They provide the evidence that organisms have been around for so long and tell us alot about how much an organism has changed over time. Fossils form in rocks in one of three ways:

• From gradual replacement by minerals - things like bones and teeth which dont decay easily can last a long time when burried and theyre eventually replaced by minerals as they decay, forming rock-like substances shaped lke the original hard part. The surrounding sedments also turn to rock, but the fossil stays distinct inside the rock and eventually is dug up.
• From casts and impressions - formed when an organism is buried in a soft layer of material like clay. The clay later hardens around it and the organism decays, leaving a cast of itself. An animal's burrow or a plant's roots can be preserved as casts. Things like footprints can also be pressed into these materals when soft, leaving an impression when it hardens.
• From preservation in places where no dacey happens - in amber and tar pits there's no oxygen or moisture so decay microbes can't survive. For example, in glaciers it's too cold for the decay microbes to work or in peat bogs the conditions are too acidic for decay microbes.

There are various hypothoses suggesting how life first came into being. Perhaps the first life forms came into existane in a primordial swamp (or under the sea) here on earth. Maybe simple organic molecules were brought to earth on comets, which later turned more complicated. However these a lack of good, valid evidence as many early forms of life were soft-bodied and soft tissue tends to decay away completely. Fossils that did form millions of years ago may have been destroyed by geological activity.

Bacteria sometimes revelop random mutations in their DNA, these can lead to changes in the bacteria's characteristics, e.g. being less affected by a particular antibiotic. This can lead to antibiotic-resistant strains froming as the gene for antibiotic resistance becomes more common in a poopulation. Because bacteria are so rapid at reproducing, they can evolve quickly. For the bacterium, the ability to resist antibiotics is a big advantage and so lives for longer and reproduces more times, inceasing the population size of the antibiotic-resistant strain. This means that infections can spread more easily and 'superbugs' are becoming more common, for exmample MRSA.

This problem is getting worse, partly due to overuse or innapropriate use of antibiotics. The more antibiotics are used, the worse the problem gets as they create a situation where naturally resistant bacteria have an advantage and so increase in number. It is also important to take the whole course of antibiotics to make sure all the bacteria are destroyed, otherwise these could develop into antibiotic-resistant strains.

Giving antibotics to animals in farming is also a concern as they could pass on antibiotic-resistant bacteria.

Drug companies are developing new antibiotics that are effective against these new strains, however the rate of development is slow and is a very expensive process.

At first, organisms were classified according to a system propsed in the 1700's by Carl Linnaeus, which groups living things on their charcteristics and the structures that make them up. This system in known as the Linnaean system:

• Kingdom (king)
• Phylum (Philip)
• Class (came)
• Order (over)
• Family (for)
• Genus (great)
• Species (sausages)

As knowledge of the biochemical processes taking place insde organisms developed and microscopes improved, scientists put forward new models of classification. In 1990, Carl Woese proposed the three-domain system, using evidence gathered from new chemical analysis techniques, such as RNA sequence analysis, he found that in some cases species thought to be closely related in traditional classification systems were not as closely related as thought.

• Archaea - primitive bacteria, often found in extreme places.
• Bacteria - true bacteria, e.g. E. coli. There are lots of biochemical differences between them and archaea.
• Eukaryota - a broad range of organisms including fungi, plants, animals and protists.