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Criado por Niamh Webster
mais de 6 anos atrás
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DNA - Deoxyribonucleic Acid The chemical that all genetic material is made from Contains coded information - instructions to put organism together and make it work Determines what inherited characteristics you have Found in the nucleus - in chromosomes Chromosomes come in pairs DNA is a polymer - two strand coiled together in double helix Gene - small section of DNA found in a chromosome Each gene codes for particular sequence of amino acids - make specific proteins Tell cells in what order to put amino acids together DNA determines what proteins the cell produces That in turn determines what type of cell it is Genome - entire set of genetic material in an organism Understanding human genome is important for science and medicine: Allows scientists to identify genes in genome that are linked to different types of disease Knowing which genes are linked to inherited diseases could help to understand them better and develop effective treatment Can look at genomes to trace migration of populations Genome is mostly identical in all humans but gradually developed as people moved around the world By investigating changes, scientists can work out when new populations developed
Sexual Reproduction Sexual reproduction - where genetic information from two organisms id combined to produce offspring Genetically different to either parent Mother and father produce gametes by meiosis Humans - each gamete contains 23 chromosomes - half number of chromosomes in a normal cell Instead of having two of each chromosome, gamete has just one of each Egg and sperm fuse together (fertilisation) to form a cell with full number of chromosomes Sexual reproduction - fusion of 2 male and female gametes 2 parents, offspring contain a mixture of their parents' genes Mixture of genetic information produces variation in offspring Flowering plants can reproduce in this way - also have egg cells Asexual Reproduction Only one parent Offspring are genetically identical to parent Happens by mitosis Ordinary cell makes new cell by dividing in two New cell has identical genetic information - called a clone No fusion of gametes, no mixing of chromosomes and no genetic variation Bacteria, some plants and some animals reproduce asexually
Gametes - one copy of each chromosome, so when fusion takes place there's the right amount of chromosomes To make gametes which only have half original number of chromosome, cells divide by meiosis Involves two cell divisions Humans - only happens in reproductive organs Before cell divides, it duplicates genetic information Forming two armed chromosomes - one is an exact copy of the other After replication chromosomes arrange themselves into pairs In the first division, chromosome pairs line up in the centre of the cell Pairs are then pulled apart - each new cell only has one of each In second division, chromosomes line up again in the centre of cell Arms of chromosomes are pulled apart Get four gametes, each with only a single set of chromosomes Each of the gamete is genetically different from others Chromosomes get all mixed up during meiosis and each gamete gains half of them After 2 gametes have together, resulting new cell divides by mitosis to copy itself Mitosis repeats to produce lots of new cells in an embryo As the embryo develops, cells then start to differentiate into different types of specialised cell that make up whole organism
23 pairs of chromosomes in every human body cell Of these 22 are matched pairs of chromosomes - just control characteristics 23rd pair - labelled XY or XX - control the sex When making sperm, X and Y chromosomes are drawn apart in first division of meiosis 50% chance of each sperm cell gets an X chromosome and 50% chance it gets a Y-chromosome Similar thing happens when making eggs Original cell has two X-chromosomes, so all eggs have one X chromosome
Inherited genes control characteristics you develop Different genes control different characteristics Some are controlled by single but most characteristics are controlled by several genes interacting All genes exist in different versions called alleles 2 alleles for gene that are the same - Homozygous 2 alleles for gene that are different - Heterozygous If 2 alleles are different, only one can determine what characteristics is present Allele for characteristics is called the dominant allele (capital letter) Other is recessive (shown with small letters) To display a recessive characteristic, both alleles must be recessive To display a dominant characteristic, organism can be either both dominant or one dominant and one recessive Dominant allele overrules recessive one if organism is heterozygous Genotype is combination of alleles Alleles work at molecular level to determine characteristics you have - phenotype
Cystic Fibrosis Genetic disorder of cell membranes Results in body producing lots of thick mucus in air passages and in pancreas Allele which causes it is recessive - carried by about 1 in 25 People with only one copy of the allele won't have the disorder - known as carriers Both parents must be carriers or have the disorder There's a 1 in 4 chance of a child having the disorder if both parents are carriers Polydactyly Genetic disorder where baby is born with extra fingers or toes Isn't life threatening Caused by a dominant allele - can be inherited if just one parent carries dominant allele Parent that has the defective allele will have the condition too since allele is dominant There's a 50% chance of a child having the disorder if one parent has one dominant allele
Embryonic Testing During IVF, embryos are fertilised in a lab and then implanted into the mother's womb It's possible, before implantation, to remove a cell from each embryo and analyse its genes Many genetic disorders can be detected in this way Possible to get DNA from an embryo in the womb and test for disorders Lots of ethical, social and economical concerns surrounding embryo screening For embryos produced by IVF - after screening, embryos with "bad" alleles would be destroyed For embryos in the womb - screening could lead to decision to terminate the pregnancy Against Implies people with genetic problems are "undesirable" - increase prejudice May come a point where everyone wants to screen embryos for desirable qualities Screening is expensive For Help stop people from suffering Treating disorders costs the government (& taxpayers) lots of money Laws to stop it going too far Parents can't select sex of their baby
Variation - change or slight difference in something Can be genetic - means it's caused by differences in genotype Genotype is all of the genes and alleles that an organism has An organism's genotype affects its phenotype - characteristics it displays Genes are inherited from parents Not only genotype that can affect an organism's phenotype Interactions with environment can also Most variation in phenotype is determined by a mixture of genetic and environmental factors
Sometimes a gene can mutate Rare, random change in an organism's DNA that can be inherited Occur continuously Mutations mean gene is altered Produces a genetic variant (different form of the gene) As genes code the sequence of amino acids that make up a protein, gene mutations can lead to changes in the protein Most genetic variants have very little or no effect on the protein the gene codes for Some will change it so little that its function is unaffected - means most mutations have no effect on phenotype Some variants have a small influence on phenotype - alter individual's characteristics but only slightly Some characteristics are controlled by more than one gene Mutation in one of the genes may change eye colour - difference may not be huge Very occasionally, variants can have a dramatic effect that they determine phenotype Cystic fibrosis is caused by a mutation that has a huge effect on phenotype Gene codes for a protein that controls movement of salt and water into/out of cells However, protein produced by the mutated gene doesn't work properly - leads to excess mucus production in lungs and digestive system If environment changes, and the new phenotype makes an individual more suited to a new environment Can become more common throughout species relatively quickly by natural selection
Theory of evolution - all of today's species have evolved from simple life forms that first started to develop over three billion years ago Survival of the Fittest Darwin came up with theory of evolution, called natural selection Knew that organisms in a species show wide variation in their characteristics (phenotypic variation) Also knew organisms must compete for limited resources Concluded that organisms with most suitable characteristics for environment would survive - survival of the fittest Successful organisms that survive are more likely to reproduce and pass on the genes for the characteristics that made them successful to their offspring Organisms that are less well adapted would be less likely to survive and reproduce So are less likely to pass on their genes to the next generation Beneficial characteristics become more common in the population and species change - it evolves
Darwin's theory wasn't perfect Relevant scientific knowledge wasn't available wasn't available and couldn't give a good explanation for why new characteristics appeared or how organisms passed on characteristics Discovery of genetics supported Darwin's idea Provided an explanation of how organisms born with beneficial characteristics can pass them on and shows their phenotypes are suited to the environment Fossils also supported the idea Allows to see how changes in organisms developed over time Discovery of how bacteria can evolve to become resistant to antibiotics also further supports evolution by natural selection
Over a long time, phenotype of organisms can change so much because of natural selection that a new species is formed - speciation This happens when populations of the same species change enough to become reproductively isolated - means they can't interbreed to produce fertile offspring
Species become extinct for these reasons: Environmental changes are too quick New predator kills them all New disease kills them all Can't compete with another species for food Catastrophic event that kills them all
When humans artificially select plants or animals that are going to breed so that genes for specific characteristics remain in the population Organisms are selectively bred to develop features that are useful/attractive Animals that produce more milk/meat Crops with disease resistance Dogs with a good, gentle temperament Decorative plants with bug/unusual flowers Selective Breeding process: From existing livestock, select ones with desirable characteristics Breed them Select the best of the offspring, and breed them together Continue this process over several generations, and the desirable trait gets stronger and stronger Eventually all offspring will have the characteristics In farming, selective breeding can be used to improve yields Selective breeding isn't new How we ended up with edible crops from wild plants How we got domesticated animals like cows and dogs
Reduces gene pool - number of different alleles in a population Farmers keep breeding from the "best" animals or plants - which are all closely related Inbreeding can cause health problems because there's more chance of inheriting harmful genetic defects Can also be problems is a new disease appears - not much variation in the population All the stock are closely related so if one of them is killed by a disease it's likely that they will all be
Basic idea is to transfer a gene responsible for a desirable characteristic from one organism's genome into another organism - so that it has desired characteristics Useful gene is isolated from one organism's genome using enzymes and is inserted into a vector Vector's usually a virus or a bacterial plasmid depending on type of organism that the gene is being transferred to When vector is introduced to target organism, useful gene is inserted into its cells Bacteria have been genetically modified to produce human insulin that can be used to treat diabetes Genetically modified crops have had their genes modified, to improve size and quality of their fruit, or make them resistant to disease, insects and herbicides Sheep have been genetically engineered to produce 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 In some cases, the transfer of the gene is carried out when the organism receiving the gene is at an early stage of development Means organism develops with the characteristic coded for by the gene
It's an exciting area of science - has the potential for solving many problems Worries about long-term effects of genetic engineering Changing an organism's genes might accidentally create unplanned problems - could get passed on to future generations Pros GM crops could be developed to contain nutrients that are missing in people's diets GM crops are already grown in some places - without problem Characteristics chosen for GM crops can increase yield, making more food Cons Transplanted genes could get into the natural environment Some say that growing GM crops will affect number of wild flowers that live in and around the crops - reducing farmland biodiversity GM crops might not be safe and people might not fully understand effects
Remains of plants and animals Provide evidence that organisms lived years ago Can tell how much or how little organisms have changed They form in three ways: Gradual replacement by minerals Things like teeth, shells and bones - don't decay easily Eventually replaced by minerals as they decay - forms a rock-like substance shaped like the original Surrounding sediments also turn to rock - fossil stays distinct inside the rock Casts and impressions Fossils can be formed when an organism is buried in a soft material like clay Clay later hardens around it and organism decays, leaving a cast Things like footprints can also be pressed into these materials when soft, leaving an impression when it hardens Preservation in places where no decay happens In amber and tar pits - no oxygen or moisture so decay microbes can't survive In glaciers - too cold for decay microbes to work Peat bogs - too acidic for decay microbes
Bacteria sometime develop random mutations Can lead to changes in bacteria's characteristics Can lead to resistant strains forming as gene become more common Bacteria are rapid at reproducing - can evolve quickly Problem has increased because of overuse and appropriate use of antibiotics More antibiotics are used, the more they develop a resistance In farming, antibiotics can be given to animals to prevent them becoming ill and to make them grow faster Drug companies are trying to develop antibiotics that are effective against resistant strains Rate of development is slow - unlikely to keep up with the demand Very costly process
Traditionally, organisms have been classified according to a system Proposed by Carl Linnaeus in the 1700s In which living things are grouped according to characteristics and structures Living things are first divided into kingdoms Kingdoms are then subdivided into smaller and smaller groups Phylum, class, order, family, genus and species
In 1990, Carl Woese proposed the three domain system Using evidence gathered from new chemical analysis techniques Species thought to be closely related weren't actually as closely related as thought Organisms are split into three large groups called domains: Archaea - primitive bacteria. Often found in extreme places such as hot springs and salt lakes Bacteria - true bacteria. Often look similar to Archaea - biochemical differences Eukaryota - broad range of organisms including fung, plants, animals and protists Subdivided into smaller groups - kingdom, phylum, class, order, family, genus and species
Every organism is given its own two part Latin name First part refers to genus - information of organism's ancestry Second part refers to species Used worldwide - means scientists in different countries all refer to species by the same name
Evolutionary trees Show how scientists think different species are related Show common ancestors and relationships More recent common ancestor, the more closely related the two species And the more characteristics they're likely to share Analyse lots of different types of data to work out relationships Use current classification data - living organisms Use information from fossil records - extinct species
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