Biology B3

Some basic facts about DNA:

• DNA stands for deoxyribose nucleic acid.
• It contains all the instructions needed to put an organism together and make it work.
• It's found in the nucleus of animal and plant cells in long molecules called chromosomes.

DNA is a double helix of paired bases

• A DNA molecule has two strands coiled together in the shape of a double helix.
• Each strand is made up of lots of small group called nucleotides.
• Each nucleotide contains a small molecule called a base.
• The bases are paired in the DNA molecule, and they always pair up in the same way- it's always A-T and C-G. This is called complementary base-pairing.
• Adenine Thymine
• Guanine Cytosine

DNA can replicate itself

• DNA copies itself every time a cell divides, so that each new cell still has the full amount of DNA.
• In order to copy itself teh DNA double helix first 'unzips' to form two single strands.
• As the DNA unwinds itself, new nucleotides join on only where the bases fit, making an exact copy of the DNA on the other strand.
• The result is two molecules of DNA identical to the orginal molecule of DNA.

Everyone has unique DNA

DNA fingerprinting is a way of cutting up a person's DNA into small bits and then seperating them. Every person's genetic fingerprint has a unique pattern. This means you can tell people apart by comparing samples of their DNA.

DNA controls the prodution of proteins in a cell

• A gene is a section of DNA that 'codes' for a particular protein.
• Proteins are made up of chains of molecules called amino acids. Eah different type of protein has its own particular number and order of amino acids.
• This gives each protein a different shape, which means each protein can have a different function.
• It's the order of the bases in a strand of DNA that decides the order of amino acids in a protein.
• Each amino acid is coded for by a sequence of three bases in teh strand of DNA.
• Proteins are made from 20 different amino acids, all found in the cytoplasm of cells. They're stuck together to make proteins, following the order of the code in teh DNA.
• You get amino acids from your diet, but if you don't take in all the amino acids you need in the right amounts, your body can change some of them into others. This is called transamintation and it happens in ther liver.
• DNA also determines which genes are switched on or off- and so which proteins the cell produces. That in turn determines what type of cell it is.

Proteins are made by ribosomes

• Proteins are made in the cell by organelles called ribosomes which are found in the cytoplasm. 
• DNA is found in the cell nucleus and can't move out of it because it's too big. 
• So the cell needs to get the information from the DNA to the ribosome in the cell cytoplasm, and it does this using a molecule called RNA. RNA is very similar to DNA, but it's much shorter and only a single strand. 
• RNA is like a messenger between the DNA in the nucleus and the ribosome. 

Humans and great apes share common ancestors

Pygmy chimps are our closest living relatives. DNA studies suggest that humans and chimps shared a common ancestor that lived in teh African rainforest 5-6 million years ago. This ancestral species evolved into two groups- one of which gave rise to modern chimpanzees, the other to humans: 

• The oldest human ancestor is thought to be Australoithecus afarensis from 3.5 million years ago. It could walk on two legs but still spent a lot of time in trees.
• About 2 million years ago, Homoerectus had appeared in Africa- the first species that could be regarded as really human. Homoerectus was a hunter gatherer living in small family groups. About 1.9 million years ago, they began to leave Africa and migrated to Asia and then into Europe.
• Homoerectus evolved into Homoheidelbergensis and lived in Europe about 800,00 years ago. The discovery of spears and other artefacts show that these humans hunted animals. Hunting requires cooperation, so there must have been some social organisation.

• Meanwhile Homoerectus survived for a long time in the Far East. Java Man was an example found in Indonesia.
• About 200,00 years ago, human evolution branched, giving rise to Neanderthal man, or Homoneanderthalensis, in Europe, and Homosapiens in Africa.
• Homosapiens were the more successful species, and when tehy arrived in Europe about 40,00 years ago, the neanderthals gradually became extinct, dying out about 28,00 years ago.

Classification is organising living organisms into groups

• Nowadays scientists classify organisms into groups based on genetic similarities. For example, bats, whales and humans might seem different, but they have a similar bone structure in the for-limbs, and they're all genetically related.
• Living things are divided into kingdoms. Kingdoms are then subdivided into smaller and smaller groups.
  • Domain KingdomPhylumClassOrderFamilyGenusSpecies
• A genus is a group of closely related species- and a species is a group of closely related organisms that can breed to produce fertile offspring.
• The binomial system that's used to name organisms uses the Latin names of the genus and the species that belong to.  

Living things can be plants, animals or something else

• To be a member of the plant kingdom, organisms must contains chloroplasts and therefore be able to make their own food using photosynthesis. 
• Members of the animal kindom move about from place to place and have compact bodies. Animals can't make their own food so they have to find things to eat, such as plants or other animals.
• Other organisms suchas as fungi and bacteria, don't have animal or plant features and are put in other kingdoms.
• Some single-celled organisms have features of both plants and animals.
  • Euglena can move from plave to place by thrashing its flagellum, but also has chloroplasts which allow it to make itw own food.
  • It's put into a kingdom called Protoctista, along with some other single celled organisms.

Vertebrates have backbones

The animal kingdom is divided into vertebrates and invertebrates. Vertebrates are animals with a back bone and an internal skeleton. Invertebrates don't have these structures- some have an external skeleton. Vertebrates are divided into five groups, called classes- fish, amphibians, reptiles, birds and mammals.

• Fish live in water. They have scales and gills for gas exchange.
• Amphibians exchange gas partly through their skin, so gases must be able to move in and out- their skin's go to be permeable and moist.
• Reptiles are more adapted to live on land. They've got a dry scaly skin which stops them losing too much water.
• Most Birds can fly and they've got feathers to help them do this. They also have a beak which is useful for cracking seeds or catching prey.
• Mammalshave fur covering their bodies to keep them warm. They give brith to their young rather than laying eggs and produce milk to feed them.

Mutations are changes to the genetic code

• Occasionally a gene may mutat. Mutations change the sequenceof the DNA bases. This could stop the production of a protein, or it might mean a different protein is produced instead. This can lead to new characteristics, increasing variations.
• Mutations can happen spontaneously- when a chromosome doesn't quite copy itself properly. However, the chance of mutation is increased by exposing yourself to:
  • Nuclear radiation, X-rays or UV light.
  • Chemicals called mutagens. Cigarette smoke contains mutagens. If the mutations lead to cancer, the chemicals are called carcinogens.
• Mutations are usually harmful.
  • If a mutation occurs in reproductive cells, the offspring might develop abnormally or die.
  • If a mutation occurs in body cells, the mutant cells may start to multiply in an uncontrolled way and invade other parts of the body. This is cancer.

Selective breeding is when humans select the plants or animals that are going to breed and flourish, according to what we want from them. It's also called artificial selection. 

• From the existing stock, the organisms which have the best characteristics are selected.
• They're bred with each other.
• The best of teh offspring are selected and bred.
• This process is repeated over several generations to develop the desired traits.

There are disadvantages to selective breeding

• Only someof the original population is bred from- so there's less variety in the gene pool of the organisms. All the organisms in a crop or herd will be closely related and have simliar characteristics- including their level of disease-resistance. Some diseases might be able to wipe out the whole lot.
• Some of the characteristics encouraged by selective breeding are beneficial for humans, but not for the organisms themselves.