topic 6- inheritance, variation and evolution
dna
chromosomes are long molecules of dna
dna is the chemical that genetic material in a cell is made up from
it contains code instructions that make an organism work
dna determines the inherited characteristics you have
dna is found in the nucleus of animal and plant cells in chromosomes
chromosomes come in pairs
dna is a polymer- made of two strands in a double helix
gene codes for specific protein
a gene is a small section of dna found on a chromosome
each gene codes for a particular sequence of amino acids which are put together to form a specific protein
only 20 amino acids are used, but they make thousands of different proteins
genes simply tell cells in what order to put amino acids together
dna determines what proteins the cell produces and what type of cell it is
every organism has a genome
genome= the entire set of genetic material in an organism
it is important to understand:
-allows scientists to identify genes linked to diseases
-knowing what genes causes inherited diseases may help us find cures for them
-scientists can use genome to trace migration of people around world
the structure of dna and protein synthesis
dna is made up of nucleotides/ bases
dna strands are polymers made up of repeating units called nucleotides
each nucleotide consists of one sugar molecule, one phosphate molecule and one 'base'
the sugar and phosphate molecules form backbone of dna strands as they alternate. one of 4 bases (A,T,C,G) join to each sugar
each base links to another base on opposite strand of helix
A with T and C with G- complementary base pairing
its the order of amino acids in a gene that decides the order of amino acids in protein
the amino acids are joined to make various proteins, depending on order of genes bases
some parts of dna don't code for protein, they can control whether other genes are expressed
RNA carries code for ribosomes
proteins are made in ribosomes in cytoplasm
to make proteins ribosomes use code in dna - dna cant move out of nucleus so rna copies form of dna
rna acts as a messenger between dna and ribosome- carries code between the two
the correct amino acids are bought to ribosomes in correct order by carrier molecule
proteins have many different functions
when a chain of amino acids is formed it folds into a unique shape which allows the protein to perform the task its meant to
enzymes= act as biological catalysts (speed up reactions)
hormones= used to carry messages around body
structural proteins= are physically strong e.g. collagen strengthens connective tissue (ligaments)
mutations
mutations are changes to genetic code
mutation is random change in an organisms dna (can be inherited)
they occur continuously but chance can be increased by exposure to radiation and certain substances
mutations change sequence of dna bases in a gene which produces a genetic variant, as dna codes for a sequence of amino acids it can change the protein coded for
most mutations have very little/no effect on protein
some mutations can seriously effect protein, shape can be altered, could effect ability to perform function
bad change= shape of active site might be changed so substrate will no longer bind, structural proteins will lose strength
different forms of mutation
insertions
= when a new base is inserted into dna base sequence where it shouldn't be
an insertion changes the way groups of 3 bases(which code for a particular amino acid) are read which can change what amino acids code for
insertions can change more than one amino acid as have knock on effect on bases further on in sequence
deletions
=when a random base is deleted from dna base sequence
they change the way that the base sequence (3 bases) is read and have knock on effects further on in sequence
substitutions
= when a random base in the dna base sequence is changed to a different base
reproduction
sexual reproduction produces genetically different cells
sexual reproduction= when genetic information from 2 different organisms is combined to produce offspring that are genetically different to either parent
the mother and father produce gametes by meiosis (egg and sperm in animal s
in humans each gamete has 23 chromosomes- half normal number
the egg and sperm fuse together by fertilization to form a cell with full number of chromosomes
sexual reproduction involves the fusion of male and female gametes because there are 2 parents, offspring has a mixture of parents genes
the mixture of genetic material creates variation in offspring
asexual reproduction produces genetically identical cells
there is only 1 parent so offspring are genetically identical
happens by mitosis - and ordinary cell makes new cell by dividing in 2
clone of parent
bacteria, some plants and animals produce asexually
meiosis
gametes are produced by meiosis
to make a gamete which has half normal number of chromosomes, cells divide by meiosis
this process involves 2 cell divisions
in humans it only happens in reproductive organs (ovaries, testes)
meiosis steps
1) before cell divides it duplicates genetic info forming two armed chromosomes which rearrange themselves into pairs
2) in first division chromosomes line up down centre of cell
3) the pairs are pulled apart so each new cell has only 1 copy of each chromosome, some of fathers some of mothers in new cell
4) in second division the chromosomes line up again down centre of cell, the arms of chromosome are pulled apart
5) you get 4 gametes each with only a single set of chromosomes in it, each gamete is genetically different from others, random which chromosomes they get
more on reproduction
sexual reproduction has advantages over asexual
offspring of sexual have mixture of two sets of chromosomes, which produces variation in offspring
variation increases chance of species surviving a change in environment
natural selection occurs best in sexual reproduction
we can use selective breeding to speed up natural selection and get desired characteristics
asexual over sexual advantages
only needs one parent
uses less energy, as organisms don't have to find mate
faster
many identical offspring can be produced in favorable conditions
some organisms can do both
malaria - the parasite produces sexually while in mosquito and asexually while in human host
fungus - spores can be spread asexually and sexually
x and y chromosomes
control whether your male or female
the 23rd pair are labelled XY or XX
these chromosomes decide your sex
males= XY
female= XX
there is a 50% during the first division of meiosis each sperm cell gets an x or y chromosome
genetic diagrams
some characteristics are controlled by single genes
what genes you inherit control what characteristics you develop
different genes control different characteristics, some by a single gene e.g. mouse fur colour
however most are controlled by several genes interacting
all genes exist in different versions called alleles
you have two alleles of each genes in the body (one on each chromosome pair)
homozygous= when organism has 2 alleles that are the same
heterozygous= when two alleles for a particular gene are different
dominant= allele expressed if one copy is present
recessive= allele expressed if two copies of allele are present
genotype= genetic alleles present, combination of alleles that can be expressed as phenotype
phenotype= physical characteristic, physical appearance resulting from inherited alleles
genetic diagrams show possible alleles of offspring
PUNNET SQUARES
inherited disorders
cystic fibrosis is caused by recessive allele
people with 1 copy of the allele cant get it- they are carriers
for a child to have the disorder, the parents must both be carriers or have the disease themselves
polydactyly is caused by a dominant allele
caused by a dominant allele, can be inherited if just one parent carries defective allele
the parent that has defective allele will also have the disease as its dominant
theres a 50% chance of child getting it
embryos can be screened for genetic disorders
during IVF embryos are fertilized and implanted in womb
before being implanted it is possible to remove a cell from each embryo and analyze its genes
many genetic disorders can be detected in this way
many ethical, economic and social concerns about it
all embryos with bad alleles are destroyed
for= help stop people suffering, treating disorders is expensive for government, there is a lot of laws stopping it from going too far
against= implies that people with genetic problems are undesirable- causes prejudice, its expensive, cause playing god
the work of mendel
did genetic experiments with pea plants
Mendel had shown that the height characteristic was determined by separately inherited 'hereditary units' passed on from each parent
the ratio showed that tall T was dominant to short t
3 important conclusions
characteristics in plants were determined by 'hereditary units'
'hereditary units' are passed onto offspring unchanged from both parents, one unit from each parent
'hereditary units' are dominant or recessive
understanding his work
wasn't understood by people of his time
in 1800's scientists became familiar with chromosomes and understood cell division
in the 20th century realized 'hereditary units' were chromosomes
in 1953 the structure of dna was determined
variation
organisms of the same species have differences
these differences are called variation within a species
2 types of variation
genetic variation
environmental variation
different genes cause genetic variation
all organisms have characteristics similar to their parents
an organisms characteristics are determined by the genes inherited from their parents
the genes are passed on in gametes, from which the offspring develop
most organisms get some genes from their father and some from their mother
this combining of genes from 2 parents causes genetic variation- no two of the same species are genetically identical
some characteristics are only inherited by genes (blood group, inherited disorders)
characteristics are also influenced by the environment
environmental variation
environmental variation covers a wide range of differences (suntan)
most characteristics are due to genes and environment
e.g. the maximum height someone grows to depends on their genes but whether they grow to that height depends on environment
mutations introduce variation
mutations are changes to the sequences of bases in DNA which can change the protein the gene codes for. most mutations have no effect so don't effect the phenotype
if the environment changes and the new phenotype makes an individual more suited to the new environment it can become common throughout a species by natural selection
some mutations have a small influence and alter the phenotype slightly. however some mutations result in a new phenotype
evolution
THEORY OF EVOLUTION= all of today's species have evolved from simple life forms that started to develop over 3 billion years ago
survival of the fittest
created by Charles Darwin
he used observations from his trip around the world to suggest the theory of evolution by natural selection
Darwin knew that organisms in a species have a wide phenotype variation. he also knew that organisms compete for limited resources in an ecosystem
Darwin realized that organisms which have characteristics most suited to their environment would be more likely to survive
the successful organisms that survive are more likely to reproduce so they will pass on their genes to next gem
the organisms that are less well adapted are less likely to survive and reproduce so they are less likely to pass on their genes to the next gen
overtime beneficial characteristics become more common in the population and the species evolves
new discoveries helped develop this theory
Darwin's theory could not explain why new characteristics appeared or exactly how organisms passed on beneficial adaptations to their offspring
we now know the phenotype is controlled by genes. new phenotypic variations arise from genetic variants caused by mutations
the development of new species is called SPECIATION
Over a long time the phenotype of organisms can change so much because of natural selection that a new species is formed
speciation happens when populations of the same species change enough to become reproductively isolated - they cant interbreed to produce fertile offspring
extinction is when no individuals of a species remain
reasons for it
environment changes too quickly
a new predator kills them all
a new disease kills them all
they cant compete with another new species for food
a catastrophic event happens that kills them all
more about evolution
not everyone agreed with Darwin
it went against common religious beliefs about how the earth developed
Darwin could not explain why these new useful characteristics appeared or how they were passed on from individuals to their offspring
there wasn't enough evidence to convince many scientists
Lamarck's ideas
he argued that changes in an organism will be passed on to its offspring
he thought that if a characteristic was used a lot by an organism then it would become more developed during its lifetime and its offspring would inherit it
scientists developed different hypotheses from observations
Lamarck's hypothesis was rejected because experiments did not support his hypothesis
the discovery of genetics supported Darwin's idea because it showed how organisms with a beneficial characteristic could pass it on
other evidence was found by looking at fossil record which showed how organisms changed overtime
selective breeding
its simple
selective breeding= when humans artificially select the plants or animals that are going to be bred so that the gene's for a particular characteristic remain
reasons
to develop more useful features
to make an organism more attractive
the process
1) from existing stock select organisms that have desired charcteristics
2) breed them with eachother
3) select the best of the offspring and breed them together
4) continue process over several generations and the desirable trait gets stronger- eventually all offspring will have that characteristic
the main drawback is reduction in gene pool
reduces number of alleles in a population- this is because the best are bred together (which are closely related) this is inbreeding
inbreeding can cause health problems because there's more chance of organism inheriting genetic defects when the gene pool is limited
there can also be problems if a new disease appears because there's not much variation in a population. increases likelihood of all population dying
genetic engineering
genetic engineering transfers genes between organisms
genetic engineering= to transfer a gene responsible for a desirable characteristic from one organisms genome into another organism so it has that desired trait
1) a useful gene is isolated (cut) from one organism's genome using enzymes and is inserted into a vector
2) the vector is usually a virus or a bacterial plasmid depending on the type of organism that the gene is being transferred into
3) when the vector is introduced to the target organism, the useful gene is inserted into its cells
4) in some cases the transfer of the gene is carried out when the organism receiving the gene is at an early stage of development . this means the organism develops with the characteristic coded for by the gene
controversial
concerns about long term effects
side effects on human beings
pros
increase the yield of food
people who live in developing countries who are not getting required nutrients. GM crops could be engineered to contain them
GM crops are already being grown in some places and have had no problems
cons
GM crops will effect number of wild flowers- reducing biodiversity
GM crops might not be safe and effects on human health long term have not been investigated
transplanted gene's might get out into natrual environment
cloning
plants can be cloned from cuttings or tissue culture
tissue culture
where a few plant cells are put in a growth medium with hormones
they grown into new plants (clones of the parent)
they can be grown quickly in very little space
cuttings
gardeners can take cuttings from good parent plants and then plant them to produce genetically identical copies of the parent plant
these plants can be produced quickly and cheaply
you can make animal clones using embryo transplants
sperm cells are taken from a male organism and an egg is taken from female organism of same species
the sperm is used to artificially fertilize the egg. the embryo then develops and splits many times to form many clones before any cells are specialized
these cloned embryos can be implanted into lots of other female organisms from that species where they produce genetically identical offspring
adult cell cloning
adult cell cloning involves taking an unfertilized egg and removing its nucleus. this nucleus is them removed from an adult body cell and is inserted into the empty egg
the egg cell is then stimulated with an electric shock which makes it divide
when the embryo is a ball of cells its implanted into the womb of an adult female . it grows into an identical clone of the original adult body cell as it has the same genetic info
issues surrounding cloning
cons
pros
cloning could be used to preserve an endangered species
however study of animal clones could improve understanding of the embryo and aging related disorders
it can reduce the gene pool which could result in a species being wiped out if a disease was introduced
cloned animals could be less healthy than normal animals
concerns that humans could be cloned in the future
fossils
fossils= remains of organisms from thousands of years ago found in rocks
fossil formation
gradual replacement by minerals
things like teeth, bones don't decay easily so last a long time when buried
they're eventually replaced by minerals as they decay forming rock like substances shaped like the original part
the surrounding sediments also turn to rock but the fossil stays distinct inside the rock and eventually someone digs it up
from casts and impressions
sometimes fossils are formed when an organism is buried in a soft material like clay. the clay later hardens around it and the organism decays leaving a cast of its self
from preservation in places where no decay happens
in amber and tar pits there is no oxygen or moisture so decay organisms can't survive
in glaciers its too cold for decay microbes to work
peat bogs are too acid for decay microbes
no one knows how life began
lots of hypotheses suggesting how life came along
no support for hypotheses because of lack of evidence
many early forms of life were soft bodied which means they decay completely- fossil record is incomplete
fossils that did form millions of years ago may have been destroyed by geological activity
speciation
speciation is the development of a new species
a species is a group of similar organisms that can reproduce to produce fertile offspring
speciation occurs when populations of the same species become so different that they can no longer successfully interbreed to produce fertile offspring
isolation and natural selection may lead to speciation
isolation= where populations of a species are separated. this can happen due to a physical barrier (flood)
conditions on either side of barrier might be slightly different
because of the environments are different, different characteristics will become common in each population due to natural selection
1) each population shows genetic variation because of range of alleles
2) in each population individuals with traits that make them more adapted to their environment are more likely to survive and breed
3) so the alleles that control the beneficial characteristics are more likely to be passed on to next gen
wallace
Wallace was a scientist who contributed to idea of speciation
together with Darwin they published work on natural selection in 1858. which prompted Darwin to write 'on the origin of species'
observations made by Wallace as he travelled the world
he did work on warning colours of prey which is used to deter predators
antibiotic resistant bacteria
bacteria can evolve and become antibiotic resistant
bacteria can develop random mutations in their dna which can lead to changes in the bacteria's characteristics. this can lead to antibiotic resistant strains forming as the gene for antibiotic resistance becomes more common in the population
bacteria reproduce and evolve quickly
for bacteria antibiotic resistance is and advantage as its better able to survive and reproduce
the bacteria easily spread as there is no treatment
resistant bacteria are becoming more common
M R S A
antibiotic resistance is becoming more common
the problems of antibiotic resistance is getting worse
caused by
over use/ prescription of antibiotics
people not finishing their courses
classification
kingdom, phylum, class, order, family, genus, species
Linnaean system