inheritance, variation and evolution Flashcards
meiosis vs mitosis in terms of identicality
meiosis leads to non-identical cells being formed while mitosis leads to identical cells being formed
what does sexual reproduction involve
the fusion of male and female gametes
- sperm and egg cells in animals
- pollen and egg cells in flowering plants
how does sexual reproduction work
there is a mixing of genetic information which leads to variety in the offspring. the formation of gametes involves meiosis
how does asexual reproduction work
only one parent and no fusion of gametes, resulting in no mixing of genetic information. this leads to genetically identical offspring (clones).
- mitosis in eukaryotes
- binary fission in prokaryotes
what does gametes being haploid mean
they have half the genetic material of a somatic (normal) body cell. i.e., in humans, gametes have 23 chromosomes, while a normal body cell has 46 chromosomes
where does meiosis occur
in the reproductive organs, i.e., the testes and ovaries, to form gametes
what happens when a cell divides to form gametes
- copies of the genetic information are made
- the cell divides twice to form four gametes, each with a single set of chromosomes
- all gametes are genetically different from eachother
how does fertilisation restore the full number of chromosomes
the fusion of a male and female gamete during fertilisation forms a zygote, which is a diploid cell with 2 sets of chromosomes
what happens after fertilisation
the fertilised egg (zygote) will divide by mitosis to produce genetically identical cells, forming an embryo. as the embryo develops, cells differentiate
advantages of sexual reproduction
- produces variation in the offspring
- if the environment changes, variation gives a survival advantage by natural selection
- natural selection can be sped up by humans in selective breeding to increase food production
advantages of asexual reproduction
- only one parent needed
- more time and energy efficient as do not need to find a mate
- faster than sexual reproduction
- many identical offspring can be produced when conditions are favourable
disadvantages of asexual reproduction
- no variation
- susceptible to environmental change
disadvantages of sexual reproduction
- reliant on sperm and egg coming into contact
- slower than asexual reproduction
how do malarial parasites reproduce
asexually in the human host, but sexually in the mosquito
how do fungi reproduce
asexually by spores but also reproduce sexually to give variation
how do plants reproduce
they produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils
what is DNA
a polymer made up of two strands forming a double helix. it’s contained in structures called chromosomes in the nucleus
define a gene
a small section of DNA on a chromosome
what does each gene code for
a particular sequence of amino acids to make a specific protein
define a genome
the genome of an organism is the entire genetic material of that organism
use of the human genome
the whole human genome has now been studied and this will have great importance for medicine in the future
importance of understanding the human genome
1) helps us search for and identify genes that are linked to certain diseases, e.g., genes that increase risk of cancer
2) helps us understand and treat lots of inherited disorders like CF
3) it can be used to trace human migration patterns from the past by comparing genomes from people all around the world in order to help people discover their ancestry
define a nucleotide
monomers made of phosphate, a common sugar and a base that make up DNA
four DNA bases
A, C, G and T
what is the code for a particular amino acid
a sequence of three bases; the order of bases controls the order in which amino acids are assembled to produce a particular protein
what is the DNA polymer made up of
repeating nucleotide units
what do the long strands of DNA consist of
alternating sugar and phosphate sections. attached to each sugar is one of the four bases
what results in a change in DNA structure
it may result in a change in the protein synthesised by a gene
define a mutation
a random change in the DNA base sequence
what happens if a mutation has an effect
a few mutations code for an altered protein with a different shape. this means an enzyme may no longer fit the substrate binding site or a structural protein may lose its strength
describe the effect of mutations
they occur continuously, but most do not alter the protein, or only alter it slightly so that its appearance or function is not changed
what are non-coding parts of DNA
the parts of DNA that do not code for specific proteins
what do non-coding areas do
they can switch genes on and off, therefore if a mutation occurs in a non-coding section, this may affect how genes are expressed. this means cells could produce wrong proteins at wrong times, which could cause uncontrolled mitosis leading to cancer
define an allele
a different version of the same gene
define a chromosome
a strand of DNA that contains many genes
define a gamete
a sex cell
define dominant
the allele that is always expressed in the phenotype as long as there is one copy present
define recessive
the allele that is only expressed in the phenotype if there are two recessive alleles present
define homozygous
when a person has 2 copies of the same allele
define heterozygous
when a person has 2 different alleles
define genotype
a combination of alleles
define phenotype*
the genotype expressed
how do the alleles (or genotype) present operate
they operate at a molecular level to develop characteristics that can be expressed as a phenotype
what are most characteristics a result of
multiple genes interacting, rather than a single gene
what are inherited disorders
disorders caused by the inheritance of certain alleles
what is cystic fibrosis
an inherited disorder of cell membranes caused by a recessive allele (sufferer must be homozygous recessive)
what does cystic fibrosis cause
produces thick, sticky mucus in the respiratory and digestive systems, making breathing difficult and causing digestive problems
symptoms of cystic fibrosis
breathlessness, coughs, repeated chest infections
why does cystic fibrosis lead to low weight gain
the mucus produced clogs the pancreatic duct and blocks enzyme secretion. results in poor digestion, causing low weight gain
genotype of carrier and sufferer for cystic fibrosis
carrier: Cc
sufferer: cc
what is polydactyly
an inherited disorder of genetic sequencing caused by a dominant allele
what does polydactyly cause
in an abnormal number of fingers and toes
genotype of carrier and sufferer for polydactyly
carrier: you can’t be a carrier of polydactyly
sufferer: Pp or PP
what is embryo scanning
taking cells from embryos so that their DNA can be analysed to see if the embryo has the alleles for inherited disorders
disadvantages of embryonic screening
- expensive for the NHS; money could be spent elsewhere
- a large number of embryos are created but only a small number are implanted, meaning that healthy embryos with the potential for life are destroyed
- gives rise to designer babies, where we only screen for desirable features, which is also unethical
- miscarriage risk
- screening can give false positive or negative result
- parents of children with genetic disorders that have a good quality of life may argue that it’s unethical
- can be a painful procedure
fully describe chorionic villus sampling (embryo scanning)
- sample of placental tissue taken through wall of abdomen or vagina
- done at 10-12 weeks
- 1-2% miscarriage risk
fully describe amniocentesis
- needle inserted into the amniotic fluid to collect cells from placenta and foetus
- done at 15-16 weeks
- 0.5-1% miscarriage risk
what is pre-implantation genetic screening used for
for use in IVF before the embryo is implanted
describe how pre-implantation genetic screening works
- eggs are fertilised to produce embryos
- embryos analysed for genetic defects
- only healthy embryos are injected into uterus
- mother gives birth to genetically healthy baby
two purposes of screening for genetic disorders
- to check that they haven’t been passed onto the embryo
- to prepare for aiding the child e.g., buying specialised medical equipment
advantages of embryonic screening
- allows you time to prepare things like specialised medical equipment if the child is a sufferer of a genetic disorder
- allows you to see whether the developing embryo is a sufferer
- allows the option of terminating a pregnancy if the couple doesn’t want to give birth to a sufferer
what do the pairs of chromosomes do
22 pairs control characteristics, but one of the pairs carries the genes that determine sex
sex chromosomes in females and in males
females: XX
males: XY
define variation
differences in the characteristics of individuals in a population
what can variation be due to
- differences in the genes they have inherited (genetic causes)
- differences in the conditions in which they have developed (environmental causes)
- a combination of genes and the environment
what usually occurs within a population of a species
there is usually extensive genetic variation
what do all variants arise from and how does this affect phenotypes
all arise from mutations
- most have no effect on the phenotype
- some influence phenotype
- very few determine phenotype
what happens if a mutation leads to a new phenotype
mutations occur continuously - if the new phenotype is suited to an environmental change, it can lead to a relatively rapid change in the species
define evolution
a change in the inherited characteristics of a population over time through a process of natural selection, which may result in the formation of a new species
define a species
a group of animals that are similar enough to be able to breed successfully together
continuous vs discontinuous variation
continuous, e.g., leg length, fur length, height, weight
discontinuous, e.g., fur type (straight or curly), eye colour, fur colour
what is the theory of evolution by natural selection
a theory by Charles Darwin which states that all species of living things have evolved from simple life forms that first developed more than three billion years ago
structure of natural variation questions
- use example of a giraffe
natural variation/mutation:
mutation led to some [giraffes] having [longer necks]
competition:
when competing for [food], the [giraffes] with [longer necks] have the advantage as they have [greater reach]
survival of the fittest and breeding:
the [giraffes] with [longer necks] are therefore more likely to survive and breed
passing on of genes:
genes for [longer necks] are passed on to offspring
what happens if two populations of one species become different
if they become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species
define selective breeding (artificial selection)
the process by which humans breed plants and animals for particular genetic characteristics
when did selective breeding originate
humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals
selective breeding framework
- choose parents with the desired characteristics from a mixed population; say which traits are looked for in these
- breed them together
- from the offspring, those with the desired characteristic are bred together
- this continues over many generations until all the offspring show the desired characteristic
characteristics for selective breeding to look out for
- disease resistance in food crops
- animals which produce more meat or milk
- domestic dogs with a gentle nature
- large or unusual flowers
what can selective breeding lead to and what is this
inbreeding - where some breeds are particularly prone to disease or inherited defects
problems with selective breeding
- process can be long as there are no guarantees the offspring will have the desired features
- faster in organisms that produce lots of offspring at once
- can lead to inbreeding (increased chance of inherited disorders)
- because a limited number of organisms are used for breeding, this creates less variety in the gene pool, which can lead to inbreeding
what did charles darwin propose
the theory of evolution by natural selection
natural selection framework
- mutation within the population leads to variation
- competition between individuals
- individuals with characteristics most suited to the environment are more likely to survive to breed successfully
- the characteristics that have enabled these individuals to survive are then passed on to the next generation
why was the theory of evolution only gradually accepted
- the theory challenged the idea that God made all the animals and plants on Earth
- there was insufficient evidence at the time the theory was published to convince many scientists
- the mechanism of inheritance and variation was not known until 50 years after the theory was published
what did Lamarck propose
changes that occur in an organism during its lifetime can be inherited - we now know that in the vast majority of cases this type of inheritance cannot occur
how did wallace and darwin work together
wallace independently proposed the theory of evolution by natural selection. he published joint writings with darwin in 1858 which prompted darwin to publish ‘On the Origin of Species’ in 1859
explain the work of alfred russel wallace
worked worldwide gathering evidence for evolutionary theory - he is best known for his work on warning colouration in animals and his theory of speciation
how has the theory of speciation developed over time
alfred wallace did much pioneering work on speciation but more evidence over time has led to our current understanding of the theory of speciation
speciation framework
- geographical separation
- environmental differences e.g., predators, habitats
- random mutation leads to variation; difference between the two populations
- best adapted in each area more likely to survive, breed and pass on alleles and genes
- eventually the different populations can no longer successfully breed with each other
how does a new species arise
- population of organisms separated by some form of barrier e.g., mountain, ocean
- conditions on each side of the barrier are different
- long periods of time elapse
what phrasing in a question will indicate the use of the speciation framework
when ‘common ancestor’ is mentioned
define a species
a group of animals that are similar enough to produce fertile offspring
how can you prove that organisms are of a different species
by trying to breed them together - if they do produce offspring but they’re infertile, then the organisms are of a different species
why is evidence for darwin’s theory now available
as it has been shown that characteristics are passed on to offspring in genes from further scientific research
list of evidences for evolution
- looking at the anatomy of animals
- all cells have a very similar chemistry
- some organisms can be observed evolving e.g., bacteria (antibiotic resistance)
- fossil record
define fossils
the remains of organisms from millions of years ago, which are found in rocks
state three ways fossils may be formed
- from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
- when parts of the organism are replaced by minerals as they decay
- as preserved traces of organisms, such as footprints, burrows and rootlet traces
why can scientists not be certain about how life began on earth + what the first life forms looked like
because the fossil record is incomplete. many early forms of life were soft-bodied, which means that they have left few traces behind. what traces there were have been mainly destroyed by geological activity
what can we learn from fossils
how much or how little different organisms have changed as life developed on earth, and how species may have arisen from common ancestors
how do fossils act as partial evidence for the theory of evolution
fossils of the simplest organisms are found in the oldest rocks, and fossils of more complex organisms are found in newer rocks
what do antibiotics treat
bacterial illnesses
why can we observe the evolution of bacteria
because they reproduce at a fast rate
how have antibiotic resistant bacteria evolved
- a mutation of bacterial pathogens produces new strains
- some strains might be resistant to antibiotics so aren’t killed
- the non-resistant bacteria that has been killed means there is less competition for nutrients/oxygen between the resistant bacteria
- the resistant bacteria survive and reproduce, so the population of the resistant strain rises
- the genes for antibiotic resistance are rapidly passed on to offspring through asexual reproduction via binary fission to their offspring
- the resistant strain will then spread because people are not immune to it and there is no effective treatment
define antibiotic resistance
when a bacteria that used to be killed by an antibiotic isn’t affected by it anymore
what is MRSA
a type of bacteria that is resistant to antibiotics
methods for reducing antibiotic resistant bacteria
- doctors shouldn’t prescribe antibiotics inappropriately, such as treating non-serious or viral infections
- patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains
- agricultural use of antibiotics should be restricted
why are there increasingly new resistant strains of bacteria
because the development of new antibiotics is costly and slow, so it is unlikely to keep up with the emergence of new resistant strains
how have living organisms been traditionally classified
into groups depending on their structure and characteristics in a system developed by Carl Linnaeus
what did carl linnaeus do
classify organisms into a hierarchal structure
linnaeus’ structure
kingdom
phylum
class
order
family
genus
species
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how did improvements in microscopes affect the classification system
improvements in microscopes allowed internal structures and the understanding of biochemical processes to become more developed, meaning new models of classification were proposed
how are organisms named
by the binomial system of genus and species
what has evidence available from chemical analysis allowed
there is now a three-domain system developed by Carl Woese
what are organisms divided into in Woese’s system
- archea: primitive bacteria usually living in extreme environments
- bacteria: true bacteria (no membrane enclosed nucleus)
- eukaryota: includes protists, fungi, plants and animals
what are evolutionary trees
a method used by scientists to show how they believe organisms are related
how do evolutionary trees work
they use current classification data for living organisms and fossil data for extinct organisms
advantages of classifying organisms
- makes them easier to study
- helps us understand how things are related
- helps us recognise biodiversity
- gives scientists a common language to communicate with
- allows us to make sense of the living world
when does extinction occur
when there are no remaining individuals of a species still alive
describe the work of gregor mendel
in the mid 19th century, he carried out breeding experiments on plants
what was one of mendel’s observations
the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged
what was discovered in the late 19th century about genetics
behaviour of chromosomes during cell division was observed
what was discovered in early 20th century about genetics
it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways - this led to the idea that the ‘units’, now called genes, were located on chromosomes
what was discovered in the mid 20th century about genetics
the structure of DNA was determined and the mechanism of gene function worked out
how was the gene theory developed
by the scientific work of many scientists
what does cloning using tissue culture use
small groups of cells from part of a plant to grow identical new plants
describe how clones from tissue culture are produced
- a few cells are taken and placed on nutrient agar with auxin to promote root and shoot development
- then planted normally
- all plants produced are genetically identical to the original plant
what are the cloning methods in plants
taking cuttings and using tissue culture
what is tissue culture important for
preserving rare plant species or commercially in nurseries
why do you bag plants when cloning
to maintain damp and warm conditions
briefly describe taking cuttings for cloning
an older, but simple, method used by gardeners to produce many identical new plants from a parent plant
method for taking cuttings to clone plants
cut off a growing side stem, keep in damp conditions (bag it) until roots grow and then plant in soil
what are the cloning methods in animals
embryo transplants and adult cell cloning
method for adult cell cloning
- nucleus removed form unfertilised egg cell
- nucleus from adult body cell e.g., skin cell, inserted into egg cell
- electric shock stimulates egg cell to divide to form an embryo
- embryo cells contain same genetic info as adult skin cell
- embryo develops into ball of cells
- inserted into uterus of adult female to continue its development
what does embryo cloning involve
splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers
method for embryo cloning
- fertility hormones given to chosen female animal to produce many eggs
- sperm from selected male animal used to fertilise female’s eggs by artificial insemination
- early embryos develop and gently washed out of female’s uterus
- embryo split apart into individual cells (not specialised atp)
- each cell develops into embryo and transplanted into host surrogate mothers; prepared for pregnancy by hormones
- genetically identical clones born. babies not biologically related to host mothers
pros of adult cell cloning
- helps save animals from extinction
- can bring back animals from extinction
- can clone prized animals
- help infertile couples
- clone pets
cons of adult cell cloning
- clones reduce genetic diversity
- fears about cloning human babies
- making clones of yourself
define genetic engineering
modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic
what have plant crops been genetically engineered for
to be resistant to diseases or to produce bigger better fruits
what have bacterial cells been genetically engineered for
to produce useful substances such as human insulin to treat diabetes
process of genetic engineering
- enzymes used to isolate required gene; gene is cut out and inserted into a vector, usually bacterial plasmid or virus
- vector is used to insert gene into required cells
- genes transferred to cells of animals, plants or microorganisms at early stage in their development so that they develop with desired characteristics
where are the genes inserted to in plants and what are they used for
desired genes often inserted into meristem cells which used to produce identical clones of genetically modified plant
pros of genetic engineering
- improved plant growth rates
- increased crop food value
- ability to grow crops in adverse conditions
- ability to produce plants that make own pesticide
- herbicide-resistant crops
how could genetic engineering potentially cure genetic disorders
‘healthy’ genes inserted into affected cells, making them work properly. this changes the affected gene so no longer have disorder
cons of genetic engineering
- relatively new; long-term effects unknown
- insects may develop resistance to pesticides by eating foods gm to produce pesticides
- cost of genetically engineered products too high for farmers
- gm crops previously been made infertile so farmers buy new seed each year
- infertile genes in gm crops may spread into wild plants
- people could start to genetically modify humans or the genes of future children
protein synthesis transcription stage
1) base sequence of gene copied into complementary template molecule (mRNA) - DNA too large to leave nucleus
2) mRNA passes out of nucleus and into cytoplasm
protein synthesis translation stage
1) mRNA attaches to ribosome
2) amino acids brought to ribosome on carrier molecules (tRNA)
3) ribosome reads triplet of bases on mRNA and uses this to join together correct amino acids in correct order
4) when protein chain is complete, folds into unique shape
5) shape determines function of the protein
protein synthesis order of stages
transcription -> translation
