Inheritance, Variation and Evolution

DNA

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 

Reproduction

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 

Meiosis

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

X and Y chromosomes

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 

Genetic Diagram

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 

Inherited Disorders

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

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 

Evolution

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 

Selective Breeding

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 

Genetic Engineering

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 

Fossils

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   

Antibiotic Resistant Bacteria

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 

Classification

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