biology chapter 6 - inheritance, variation and evolution

68 - DNA

• DNA stands for deoxyribonucleic acid

• contains coded information on how to put an organism together and make it work

• what’s in your DNA determines your inherited characteristics

• its a polymer, in a structure called a double helix

• DNA is found in the nucleus in long structures called chromosomes

• genes are small sections of DNA found on a chromosome

• genes code for (tell cells to make) specific amino acids, which create specific proteins

• only 20 amino acids are used, genes simply tell cells in what order to put them together to create a specific protein

• the protein the cells makes determines what type of cell it is - e.g if it makes haemaglobin its a red blood cell

• the genome is the entire set of genetic material in an organism

• understanding the human genome tells us about human migration, as well as find out what genes are connected to certain diseases, so can help develop effective treatments

69 - reproduction

• sexual - uses gametes (sperm and egg), creates genetic variation in offspring, uses meiosis, two parents

• asexual - uses mitosis, clones are all genetically identical, one parent

• sexual reproduction creates genetic variation because each parent’s gamete contains 23 chromosomes, their offspring contains a mixture of their chromosomes, making them have different characteristics to the parents

70 - meiosis

• gametes are produced by meiosis

• meiosis produces 4 gametes, with a single set of chromosomes in it (23)

• meiosis takes place in the ovaries and testes

• before the cell begins to divide, it duplicates its genetic info into two-armed chromosomes, which get themselves into pairs

• they then line up in the center of the cell, and it splits to produce 2 seperate cells so that they each have a one copy of each chromosome

• they line up again and the arms of the chromosomes are pulled apart and create another set, with 2 other cells, making 4, each with a mixture of the mother and father’s chromosomes

• after meeting of 2 gametes/fertilisation, the new cell will divide by mitosis to produce many new cells to become an embryo, these cells will differentiate later on

71 - x and y chromosomes

• there are 23 pairs of chromosomes in every human body cell, 22 of the pairs determine other characteristics whereas your 23rd pair of x and y chromosomes determine your sex

• xx chromosomes is female (the combination of xx chromosomes is what allows female characteristics to develop)

• xy chromosomes is male (the y chromosome is what allows male characteristics to develop)

• there is a 50% chance of offspring being male or female, as sperm cells have a 50% chance of containing an x or y chromosome, as eggs only contain 1 x chromosome, since the original cells contains 2 x chromosomes

72 - genetic diagrams

• the genes you inherit determine what characteristics you have

• most characteristics are determined by many genes interacting together, but some are determined by the presence of just one gene (mouse fur colour or red-green colour blindness)

• each gene has 2 different versions, these are called alleles

• they are either dominant (DD/Dd) or recessive (dd)

• if you have 2 alleles that are the same, it’s homozygous for that trait

• if you have 2 alleles that are different, it’s heterozygous

• genotype is the combination of alleles you have

• phenotype is the characteristics or traits determined by the allele

73 - more genetic diagrams

• just practice, can’t explain!

74 - inherited disorders

• cystic fibrosis is caused by a recessive allele

• if one parent has one allele they are carries, but to have the disease you must have both of the recessive alelles

• the resessive allele for cystic fibrosis is carried by one in 25 people

• cystic fibrosis causes a build up of thick sticky mucus in the air passages and pancreas

• polydactly is caused by a dominant allele

• if you parent has the allele, they will have the disorder, meaning there’s a 50% chance that a parent carrying the allele will have a child with polydactly

• it causes an extra finger or toe to grow, not life threatening

• embryos can be screened in IVF and in the womb before birth for genetic disorders

• positives - can help keep people healthy, laws in place to ensure people don’t use it for other things, e.g selecting gender or eye colour and treating disorders costs taxpayers and the government alot of money

• negatives -people believe that because embryos with genetic diseases are destroyed or women choose to terminate them, is a waste of human life, some believe it makes people with those disorders look ‘undesireable’ increasing predjudice, it’s also very expensive

75 - variation

• variation are the differences of characteristics within a species

• variation can effected by both genotype (the combination of alleles you have, which determines you phenotype which is your physical characteristics) and your environment

• genotype variation - caused by differences in your genotype, e.g your eye colour or your height

• environmental variation - how tall you grow depends on how much food you’re given, if it’s not enough you may not reach the full height that your genotype determines

• for example - a plant grown outside will have green leaves, due to it having lots of sunlight, the same plant’s genotypes will determine it to also be green but if it’s left in a dark room, it’s leaves will be brown not green due to environmental variation

• mutations can occur within genes

• a mutation is a random, rare change in an organism’s DNA that can be inherited

• mutations means that the gene is altered, meaning it can effect the sequence of amino acids the gene codes for, changing the protein the gene codes for

• most genetic variants have little effect on the protein the gene codes for, meaning it won’t effect your phenotype

• some have a little change, for example the genes that work together to determine your eye colour may be changed slightly if one of them is mutated and a genetic variant

• however, in some cases the genetic variant has massive effect on the protein the gene codes for, changing your phenotype for example, cystic fibrosis can be caused by genetic variant, since the gene codes for a protein that moves water and salt out and into cells, but the protein produced by a mutated gene won’t work, which can lead to cystic fibrosis, a condition that causes a build up of thick sticky mucus in the pancreas and air passages, as well as trouble with digestion

• if the environment changes, and a new phenotype makes an individual more suited to it, it may become common throughout the species through natural selection e.g a mouse with white fur when majority have black fur may be caused by a mutation in the mouse’s gene, causing a genetic variant which makes it’s fur white, but white fur may allow the mouse to better hide from predators, so while the black mice die out, the white fur offspring of the pg white mouse swill survive

76 - evolution

• darwin’s theory of evolution is that all of today’s species have evolved from simple life forms that began to develop 3 billion years ago

• survival of the fittest - since organisms have to fight for food, shelter ect, only those best adapted to their environments would survive, as they would breed and have offspring with the same successful characteristics, meaning they evolve!

• darwin’s theory had little proof at the time, due to little scientific development, as he couldn’t explain how organisms passed on the beneficial characteristics or any proof of species changing and adapting over time

• it is now supported by: discovery of genetics, fossils and bacteria resistance to antibiotics

• speciation = when the phenotype of a species changes so drastically due to natural selection that a new species is created

• speciation happens when populations of the same species change enough become reproductivly isolates - meaning they can’t interbreed to produce fertile offspring

• extinction = when no members of a species remain

• the fossil record shows most of the species we know that have gone exctint, e.g the dinosaurs and dodos (as humans hunted them, introduced species that ate their eggs and destroyed the forest that was their habitat)

• species go extinct if: a new disease kills them all, a new predator kills them all, the environment changes too quicky, a catastrophic event occurs that kills them all or they can’t compete with another species for food

77 - selective breeding

• selective breeding is when humans artificially select the plants or animals with the desired characteristics they want and breed them together

• this can be used to create: domesticated animals, animals that produce lots of meat or milk, big and unusual decorative flowers and crops with disease resistance

• method: chose from your existing stock which has the desired characteristics, breed them together so the offspring share the genes with the desired characteristics, keep doing it for generations until the genes become stronger and there is a large population of them (inbreeding)

• the problem: reduction in the gene pool

• since the animals/plants are inbred, they’re all closely related, meaning there is very little variation in the gene pool (the number of different alleles in a population), this can lead to harmful genetic diseases being passed down through generations, also if a disease is able to kill one of the population, it is likely it will kill all of them, since they share the same genes so have little resistance to the same diseases

78 - genetic engineering

• genetic engineering is used to transfer a gene carrying a desirable characteristic from one organism’s genome to another

• method: isolate/cut the gene from the genome using enzymes and insert it into a vector (a virus or bacterial plasmid, depending on the organism the gene is going in to), when the vector is introduced to the organism, the gene will be inserted into its cells

• genetic engineering is used for: making GM crops (giving them characteristics such as resistance to disease, insects and herbicides), modifying bacteria cells to produce human insulin to treat diabetes, gene therapy (inserting healthy working genes into people with inherited disorders), modifying sheep’s milk to produce substances like drugs to treat human diseases

• sometimes the gene is inserted into the cell at an early stage (egg/embryo) so that it develops with the desired characteristic

• pros - higher yields of crops so more food for people, can help people in developing countries get food with nutrients they lack (e.g golden rice contains lots of beta-carotene, which can causes blindness if there’s a lack of it in the diet), the crops are being grown in many countries without any issues

• cons - some people don’t believe it’s safe to eat, thinking they’ll develop alerrgies to the crops or that we don’t understand the effect GM crops could have on human health, due to high resistance to things such as herbicides, if the gene spreads to common plants such as weeds they’ll grow out of control, the growing of Gm crops reduces farmland biodiversity, as less wildflowers will grow, in turn leading to less insects.

79 - fossils

• fossils show the remains of organisms that lived many years ago, they can occurs in 1 of 3 ways:

• replacement by minerals - solid things that don’t decay easily (bones, shells and teeth) can last a long time when buried, gradually minerals replace them, forming rock like structures around the original part, the surrounding sediment turns to rock, preserving the fossil until it’s dug up

• casts and impressions - the fossil can be pressed into a soft surface like clay, leaving an impression of its shape, the clay will harden, leaving a cast of the fossil, happens with animal burrows and plant root (root traces)

• preservation in places where no decay happens - in amber, there is no oxygen or moisture so decay microbes don’t work, this is the same in glaciers (too cold) and peat bogs (too acidic)

• some believe life began in primordial swamps under the sea, or simple organic molecules brought to earth on comets, but these hypothesis have little evidence - many early forms of life were soft bodied, had soft tissues that decayed easily, and were not preserved as fossils, so the fossil record is incomplete and fossils that maybe did exist could’ve been destroyed by geological activity

80 - antibiotic-resistant bacteria

• bacteria can experience random changes to their DNA (mutations) that allow them to become resistant to some antibiotics

• since bacteria can reproduce rapidly, the resistance strain can be passed down quickly

• MRSA is a bacteria resistant to most antibiotics

• to stop the spread of antibiotic-resistant bacteria, doctors have tried to come up with new antidotes that are effective against antibiotic-resistant bacteria, but the process is slow and expensive

• doctors can slow down the spread of antibiotic-resistant bacteria by only prescribing antibiotics when absolutely nessecary, for example, giving antibiotics for something that isn’t serious, or a virus, can increase the amount of antibiotic-resistant bacteria

• antibiotics are also given to animals to make them better and make them grow faster, this can lead to a-r bacteria developing in animals then transferring to humans e.g via eating meat - farmers have had to restrict antibiotic use on their animals

• you have to finish a full course of antibiotics, so that now bacteria are left, even if you feel better, if there’s some left, they could mutate and develop into a-r bacteria

81 - classification

• king philip came over for good soup

• kingdom phylum class order family genus species

• carl linneaus developed this syterm in the 1700s, begining with the kingdom (plant/animal) then splitting into smaller sub-divisions e.g phylum, class, family, order…

• the binomial name is latin and consists of the genus and the species - it is used worldwide to communicate about species and avoid confusion due to language barriers

• due to scientific development, such as microscopes and an understanding of biochemical processes inside organisms, the organisms thought to be closely related were shown to be not so close after all

• carl woese developed the three domain system - arachea (primitive bacteria living in extreme conditions), bacteria (true bacteria such as e.coli that had biochemical differences to the primitive bacteria) and eukaryota (fungi, plants, animals and protists)

• practice evolutionary trees