Genetic information, Variation and Relationships between Organisms

Question 1

What is a gene?

Answer 1

A section of DNA that contains a code for making a polypeptide and functional RNA. The code is a specific sequence of bases.

Question 2

What is the locus?

Answer 2

The location of a particular gene on a chromosome.

Question 3

What is an allele?

Answer 3

An alternative form of a gene.
The order of bases in each allele is slightly different, so they code for slightly different versions of the same polypeptide.

Question 4

What is a chromosome?

Answer 4

Where DNA is stored.
Humans have 23 pairs of chromosomes, so 46 in total.
Held together by a centromere.

Question 5

What are homologous pairs?

Answer 5

Pairs of matching chromosomes.
They are chromosomes which are exactly the same size, have exactly the same genes but may have different alleles.

Question 6

How is DNA stored in a eukaryotic cell?

Answer 6

DNA is stored as chromosomes inside of the nucleus.
To tightly coil the DNA to fit in the nucleus as chromosomes, the DNA is tightly wound around proteins called histones.
The complex on DNA wrapped around a histone is called a nucleosome.

Question 7

How is DNA stored in a prokaryotic cell?

Answer 7

Prokaryotes los carry DNA in chromosomes, but the DNA molecules are shorter and circular.
The DNA is not wound around histones. Instead it supercoils to fit in the cell.

Question 8

Where can DNA also be found and how is it similar to the DNA in prokaryotes?

Answer 8

Chloroplasts and mitochondria.
They are both short and circular.

Question 9

What is the function of a histone protein?

Answer 9

To associate with the DNA to assist in the tightly coiling of DNA to fit chromosomes into the nucleus.

Question 10

What is functional RNA?

Answer 10

RNA molecules other than mRNA, which perform special tasks during protein synthesis, which forms part of ribosomes.

Question 11

What are introns?

Answer 11

Sections of DNA where genes that code for polypeptides contains sections that don’t code for amino acids.
There can be several introns within a gene.
Introns are removed during protein synthesis — so they don’t affect the amino acid order.
Their purpose isn’t known for sure.

Question 12

What are exons?

Answer 12

The bits of a gene that do code for amino acids.

Question 13

What are non-coding repeats?

Answer 13

Multiple repeats outside of genes in eukaryotic DNA.
They repeat over and over. E.g CCTTCCTTCCTT
These areas don’t code for amino acids.

Question 14

What is RNA and the 2 out of many types of RNA involved in protein synthesis?

Answer 14

RNA is a single polynucleotide strand and it contains uracil (U) as a base instead of thymine.
Uracil always pairs with adenine during protein synthesis.

2 types:
Messenger RNA (mRNA)
Transfer RNA (tRNA)

Question 15

What is messenger RNA (mRNA)?

Answer 15

mRNA is made during transcription.
It carries the genetic code from the DNA to the ribosomes, where it’s used to make a protein during translation.
mRNA is a single polynucleotide strand.
In mRNA, groups of three adjacent bases are called codons, sometimes called triplets or base triplets.

Question 16

What is transfer RNA (tRNA)?

Answer 16

tRNA is involved in translation.
It carries the amino acids that are used to make proteins to the ribosomes.
tRNA is a single polynucleotide strand that’s folded into a clover shape.
Hydrogen bonds between specific base pairs are able to hold the molecule in its shape.
Every tRNA molecule has a specific sequence of three bases at one end called an anticodon.
They also have an amino acid binding site at the other end.

Question 17

explain the first stage of protein synthesis.

Answer 17

the first stage of protein synthesis is transcription. during transcription, an mRNA copy of a gene is made from DNA.
transcription starts when RNA polymerase attaches to the DNA double-helix at the beginning of a gene.
the hydrogen bonds between the two DNA strands in the gene are broken by a DNA helicase attached to the RNA polymerase. this separates the strands and the DNA molecule uncoils at that point, exposing some of the bases.
one of the strands is then used as a template to make an mRNA copy.
the RNA polymerase lines up free RNA nucleotides alongside the exposed bases. A specific, complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand.
once the RNA nucelotides have paired up with their specific bases on the DNA strand, they’re joined together by RNA polymerase, forming an mRNA molecule.
the RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand.
the hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by and the strands coil back into a double-helix.
when RNA polymerase reaches a particular sequence of DNA called a stop signal, it stops making mRNA and detaches from the DNA.
mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place.

Question 18

explain the second stage of protein synthesis.

Answer 18

the second stage of protein synthesis is translation.
translation occurs at the ribosomes in the cytoplasm.
during translation, amino acids are joined together to make a polypeptide chain following the sequence of codons carried by the mRNA.
the mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to it. ATP provides the energy needed for the bond between the amino acid and the tRNA molecule to form.
a tRNA molecule carrying an amino acid with an anticodon attaches itself to the mRNA by specific base pairing.
a second tRNA molecule attaches itself to the next codon on the mRNA in the same way.
the two amino acids attached to the tRNA molecule are joined by a peptide bond. the first tRNA molecule moves away, leaving its amino acid behind.
a third tRNA molecule binds to the next codon on the mRNA. its amino acid binds to the first two and the second tRNA molecule moves away.
this process continues, producing a chain of linked amino acids until there’s a stop signal on the mRNA molecule.
the polypeptide chain moves away from the ribosome and translation is complete.

Question 19

what products does transcription make in eukaryotes and prokaryotes?

Answer 19

eukaryotes- during transcription, introns and exons are both copied into mRNA. the mRNA strands containing introns and exons are called pre-mRNA. a process called splicing then occurs where introns are removed and the exons are joined together forming the mRNA strands.
prokaryotes- mRNA is produced directly from the DNA; without splicing taking place. splicing doesn’t occur because there are no introns in prokaryotic DNA.

Question 20

what is the genetic code?

Answer 20

this is the sequence of base triplets in mRNA which code for specific amino acids.
in the genetic code, each base triplet is read in sequence, separate from the triplet before it and after it.
the genetic code is non-overlapping, degenerate and universal.

Question 21

how is the genetic code non-overlapping?

Answer 21

where base triplets don’t share their bases.

Question 22

how is the genetic code degenerate?

Answer 22

where there are more possible combinations of triplets than there are amino acids. this means that some amino acids are coded for by more than one base triplet.

Question 23

how is the genetic code universal?

Answer 23

where the same specific base triplets code for the same amino acids in all living things. e.g UAU codes for tyrosine in all organisms.

Question 24

what are start and stop signals?

Answer 24

where some triplets are able to tell the cell when to start and stop the production of the protein.
they are found at the beginning and end of the mRNA. e.g UAG is a stop signal.

Question 25

how are gametes formed by meiosis?

Answer 25

meiosis is a type of cell division that takes place in the reproductive organs.
before meiosis starts, the DNA unravels and replicates so there are two copies of each chromosome called chromatids.
the DNA condenses to form double-armed chromosomes, each made from two sister chromatids. the sister chromatids are joined in the middle by a centromere.
in meiosis I (first division)- the chromosomes arrange themselves into homologous pairs.
these homologous pairs are then separated, halving the chromosome number.
in meiosis II (second division)- the pairs of sister chromatids that make up each chromosome are separated (the centromere is divided).
four haploid cells (gametes) that are genetically different from each other are produced.

Question 26

what happens to the chromatids in meiosis I?

Answer 26

the homologous pairs of chromosomes come together and pair up. the chromatids twist around each other and bits of chromatids swap over. the chromatids still contain the same genes but now have a different combination of alleles.

Question 27

what two main events during meiosis lead to genetic variation?

Answer 27

the crossing over of chromatids.
the independent segregation of chromosomes.

Question 28

what is the crossing over of chromatids?

Answer 28

this occurs in meiosis I where each of the four daughter cells formed from meiosis contain chromatids with different alleles.

Question 29

what is the independent segregation of chromosomes?

Answer 29

each homologous pair has one chromosome from your mum and one chromosome from your dad.
when the homologous pairs are separated in meiosis I, it’s completely at random in deciding which chromosome from each pair ends up in which daughter cell.
the four daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes.
the ‘shuffling’ of chromosomes leads to genetic variation in any potential offspring.

Question 30

what are the different outcomes of both meiosis and mitosis?

Answer 30

meiosis- produces cells with half the number of chromosomes as the parent cell.
mitosis- produces cells with the same number of chromosomes as the parent cell.

meiosis- daughter cells are genetically different from one another and the parent cell.
mitosis- daughter cells are genetically identical to each other and to the parent cell.

meiosis- produces four daughter cells.
mitosis- produces two daughter cells.

Question 31

why are there different outcomes in meiosis and mitosis?

Answer 31

there are different outcomes because mitosis only involves one division (which separates the sister chromatids) whereas meiosis has two divisions (which separate the homologous pairs and then the sister chromatids).
there’s no pairing or separating of homologous chromosomes in mitosis and so no crossing over or independent segregation of chromosomes, which is why they produce genetically identical daughter cells.

Question 32

what are gametes?

Answer 32

gametes are sperm cells in males and egg cells in females. they join together at fertilisation to form a zygote.
gametes have a haploid (n) number of chromosomes; there’s only one copy of each chromosome.

Question 33

what is the diploid number (2n)?

Answer 33

normal body cells have the diploid number (2n) of chromosomes where each cell contains two of each chromosome, one from the mum and one from the dad.

Question 34

what occurs at fertilisation?

Answer 34

a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. half of these chromosomes are from the father and half are from the mother.

Question 35

how does random fertilisation of haploid gametes further increase genetic variation within a species?

Answer 35

random fertilisation produces zygotes with different combinations of chromosomes to both parents, as any sperm can fertilise any egg. this mixing of genetic material in sexual reproduction increases genetic diversity within a species.

Question 36

how do chromosome mutations occur?

Answer 36

when there are errors during meiosis.
the cells produced in meiosis may contain variations in the numbers of whole chromosomes or parts of chromosomes.

Question 37

what can chromosome mutations lead to?

Answer 37

they can lead to inherited conditions because the errors are present in the gametes (the hereditary cells).

Question 38

what is the non-disjunction chromosome mutation?

Answer 38

it’s a failure of the chromosomes to separate properly. in humans, non-disjunction of chromosome 21 during meiosis can lead to Down’s Syndrome.

Question 39

what is gene mutation?

Answer 39

gene mutation involves a change in the DNA base sequence of chromosomes.

Question 40

what type of errors occur in gene mutation?

Answer 40

substitution.
deletion.

Question 41

what is the substitution gene mutation?

Answer 41

where one base is substituted with another. e.g ATGCCT becomes ATTCCT (G is swapped for T).

Question 42

what is the deletion gene mutation??

Answer 42

where one base is deleted. e.g ATGCCT becomes ATCCT (G is deleted).

Question 43

what also happens if a mutation occurs in a gene, other than a change in the DNA base sequence of chromosomes?

Answer 43

the sequence of amino acids the gene codes for and the protein form could be altered.

Question 44

why don’t some substitution mutations affect the order of amino acids?

Answer 44

the degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet. this means that some substitutions will still code for the same amino acid.

Question 45

why do deletion mutations always cause affect the order of amino acids?

Answer 45

the deletion of a base will change the number of bases present, which will cause a shift in all the bases triplets after it.

Question 46

what are mutagenic agents?

Answer 46

these are things which can cause an increase in the rate of mutations.
they include; ultraviolet radiation, ionising radiation, some chemicals and viruses.

Question 47

what is genetic diversity?

Answer 47

this is the number of different alleles of genes in a species or population.
genetic diversity is what allows natural selection to occur.

Question 48

what is a population?

Answer 48

a group of organisms of one species living in a particular habitat.

Question 49

what is genetic diversity within a population increased by?

Answer 49

mutations in the DNA; forming new alleles.
different alleles being introduced into a population when individuals from another population migrate into them and reproduce; gene flow.

Question 50

what are genetic bottlenecks? and how do they reduce genetic diversity?

Answer 50

a genetic bottleneck is an event that causes a big reduction in a population.
this reduces the number of different alleles in the gene pool and so reduces genetic diversity.
the survivors from the event reproduce and a larger population is created from a few individuals.

Question 51

what is the founder effect?

Answer 51

this is a type of genetic bottleneck.
the founder effect describes what happens when just a few organisms from a population start a new colony and there are only a small number of different alleles in the initial gene pool.
the frequency of each allele in the new colony might be very different to the frequency of those alleles in the original population. this may lead to a higher incidence of genetic disease.
the founder effect can occur as a result of migration leading to geographical separation or if a new colony is separated from the original population for another reason.

Question 52

what is natural selection?

Answer 52

randomly-occurring mutations sometimes result in a new allele being formed. where this can be harmful, resulting in said mutation dying out, there are some mutation which produce alleles that are beneficial to an organism, helping the organism to survive in certain environments. when the allele codes for a characteristic that increases the chances of an organism surviving, its frequency within the population can increase.

Question 53

how is natural selection able to increase advantageous alleles in a population?

Answer 53

there’s differential reproductive success in a population where individuals that have an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes with the beneficial allele, than individuals with different alleles.
they, in turn, are more likely to survive, reproduce and pass on their genes.
so the frequency of the beneficial allele increases from generation to generation.
over generations this leads to evolution as the advantageous alleles become more common in the population.

Question 54

what are adaptations?

Answer 54

they help organisms survive in their environment.

Question 55

what are the 3 types of adaptations?

Answer 55

behavioural
physiological
anatomical

Question 56

what are behavioural adaptations? include an example.

Answer 56

ways an organism acts that increases its chances of survival and reproduction.
e.g an animal playing dead if being threatened by a predator.

Question 57

what are physiological adaptations? include an example.

Answer 57

processes inside an organism’s body that increases its chance of survival.
e.g some animals hibernate to lower their rate of metabolism. this is to conserve energy, so they don’t need to look for food in the months where its scarce.

Question 58

what are anatomical adaptations? include an example.

Answer 58

structural features of an organism’s body that increases its chance of survival.
e.g whales have a thick layer of blubber which helps them keep warm in the cold sea.

Question 59

what are the 2 types of natural selection?

Answer 59

directional selection.
stabilising selection.

Question 60

what is directional selection?

Answer 60

directional selection is where individuals with alleles for characteristics of an extreme type, are more likely to survive and reproduce. this could be in response to an environmental change.

Question 61

explain ‘bacteria evolving antibiotic resistance’ as an example of directional selection.

Answer 61

some individuals in a population have alleles that give them resistance to an antibiotic.
the population is exposed to the antibiotic, killing bacteria without the resistant allele.
the resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring.
after some time, most organisms in the population will carry the antibiotic resistance allele.

Question 62

what is stabilising selection?

Answer 62

stabilising selection is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. it occurs when the environment isn’t changing, and it reduces the range of possible characteristics.

Question 63

explain ‘human birth weight’ as an example of stabilising selection.

Answer 63

humans have a range of birth weights.
very small babies are less likely to survive because they find it hard to maintain their body temperature.
giving birth to large babies can be difficult, so large babies are less likely to survive too.
conditions are most favourable for medium-sized babies; so the weight of human babies tend to shift towards the middle of the range.

Question 64

what are aseptic techniques?

Answer 64

these are techniques used to prevent contamination of cultures by unwanted microorganisms.

Question 65

why are aseptic techniques important?

Answer 65

because contamination can affect the growth of the microorganisms you’re working with.

Question 66

when carrying out the agar plates investigation, what aseptic techniques should be used?

Answer 66

regularly disinfect work surfaces to minimise contamination.
contaminated utensils should be placed in a beaker of disinfectant.
use sterile equipment and discard safely after use.
work near bunsen flame, so microbes in the air should be drawn away from your culture.
minimise the time spent with the lid off the agar plate, to reduce the chance of airborne microorganisms contaminating the culture.
briefly flame the neck of the glass container of broth just after it’s opened and just before it’s closed.

Question 67

what is phylogeny?

Answer 67

this is the study of the evolutionary history of groups of organisms.
phylogeny tells us who’s related to whom and how closely related they are.

Question 68

what is a phylogenetic tree?

Answer 68

all organisms have evolved from shared common ancestors and this can be shown on a phylogenetic tree.
the tree shows the relationship between members of a family.
the first branch point represents a common ancestor of all the family members. this ancestor is now extinct.
each of the following branch points represents another common ancestor from which a different group diverged.
closely related species diverged away from each other most recently, you can see this because their branches are close together.

Question 69

what is taxonomy?

Answer 69

this is the science of classification.
it involves naming organisms and organising them into groups. this makes it easier to identify and study them.

Question 70

explain the phylogenetic hierarchy.

Answer 70

there are 8 levels of groups used to classify organisms. these groups are called taxa. each group is called a taxon.
the groups are arranged in a hierarchy, with the largest groups at the top and the smallest groups at the bottom. organisms can only belong to one group at each level in the hierarchy; there’s no overlap.
organisms are first sorted into three large groups called Domains; the Eukarya, bacteria and Archaea.
related organisms in a domain are then sorted into slightly smaller groups called kingdoms. more closley related organisms from that kingdom are then grouped into a phylum, then a class and then it continues down the hierarchy.

Question 71

what are the 8 levels of the phylogenetic hierarchy?

Answer 71

domain
kingdom
phylum
class
order
family
genus
species

Question 72

define species.

Answer 72

a group of similar organisms able to reproduce to give fertile offspring.

Question 73

what nomenclature is used for classification and explain how it works?

Answer 73

the binomial system; all organisms are given one internationally accepted scientific name in Latin that has two parts.
the first part of the name is the genus name and has a capital letter. the second part is the species name and begins with a lowercase letter.
the binomial system helps to avoid the confusion of using common names.

Question 74

how is courtship behaviour used in classification?

Answer 74

courtship behaviour is carried out by organisms to attract a mate of the right species.
it can be fairly simple, such as, releasing chemicals. or it can be quite complex like doing a series of displays.
the more closely related species are, the more similar their courtship behaviour.

Question 75

how does courtship behaviour help to prevent interbreeding?

Answer 75

courtship behaviour is species specific; only members of the same species will do and respond to that courtship behaviour. this allows members of the same species to recognise each other, preventing interbreeding and making reproduction more successful. mating with the wrong species won’t produce fertile offspring.

Question 76

why is courtship behaviour useful in classification?

Answer 76

courtship behaviour can be used to help classify species. courtship behaviour from a species will only be responded to by the same species. this can classify organisms.

Question 77

how can advances in technology clarify evolutionary relationships?

Answer 77

new or improved technologies can result in new discoveries being made and the relationships between organisms being clarified. this can lead to the classification systems being updated.

Question 78

what technologies have been used to help clarify evolutionary relationships?

Answer 78

genome sequencing
comparing amino acid sequence
immunological comparisons

Question 79

explain genome sequencing.

Answer 79

advances in genome sequencing have meant that the entire base sequence of an organism’s DNA can be determined. the DNA base sequence can then be compared to the DNA base sequence of another organism, to see how closely related they are.
closely related species will have a higher percentage of similarity in their DNA base order.

Question 80

explain comparing the amino acid sequence.

Answer 80

proteins are made of amino acids. the sequence of amino acids in a protein is coded for by the base sequence in DNA. related organisms have similar DNA sequences and so similar amino acid sequences in their proteins.

Question 81

explain immunological comparisons.

Answer 81

similar proteins will also bind to the same antibodies.

Question 82

how has gene technology has caused a change in the
methods of investigating genetic diversity?

Answer 82

early estimates of genetic diversity were made by looking at the frequency of measurable or observable characteristics in a population. different alleles determine different characteristics and so a wide variety of each characteristic in a population indicates a high number of alleles and so a high genetic diversity.
but now gene technology has allowed us to compare the DNA base sequences of the same gene in different organisms in a population. this allows scientists to find out how many alleles of that gene there are in that population.
these new technologies can all be used to give a more accurate estimate of genetic diversity within a population or species.

Question 83

define variation.

Answer 83

variation is the differences that exist between individuals. there’s variation between species and within species.

Question 84

what causes variation?

Answer 84

genetic factors
differences in the environment
and a combination of genetic and environmental factors.

Question 85

explain how genetic factors cause variation between and within species.

Answer 85

between species- different species have different genes.
within species- individuals of the same species have the same genes, but different alleles.

Question 86

explain how environmental factors cause variation within species.

Answer 86

differences in the environment such as climate, food and lifestyle may cause changes within species.

Question 87

how can we study variation?

Answer 87

using a sample of a population.
the sample has to be random. because sample data will be used to draw conclusions about the whole population, its important that it accurately represents the whole population and that any patterns observed are tested to make sure they’re not due to chance. we can do this by analysing the results statistically. this allows you to be more confident that the results are true and therefore will reflect the whole population.

Question 88

how can we look for variations between samples?

Answer 88

using the mean.
most samples will be either side of the mean and so we end up with a bell-shaped graph; normal distribution.

Question 89

how can we look for variations within a sample?

Answer 89

using standard deviation.
standard deviation tells you how much the values in a single sample vary. It’s a measure of the spread of values about the mean.
e.g 9 ± 3. this means that the mean is 9 and the standard deviation is 3, so most of the values are spread between 6 to 12.
a large standard deviation means the values in the sample vary a lot. a small standard deviation tells you that most of the sample is around the mean value, so varies a little.

Question 90

how is standard deviation plotted on a graph?

Answer 90

using error bars.
error bars extend one standard deviation above and one below the mean. the mean is in the middle of the error bar.
the longer the bar, the larger the standard deviation and the more spread out the sample data is from the mean.

Question 91

define biodiversity.

Answer 91

the variety of living organisms in an area.

Question 92

define habitat.

Answer 92

the place where an organism lives.

Question 93

define community.

Answer 93

all the populations of different species in a habitat.

Question 94

define local biodiversity.

Answer 94

a variety of different species living in a small habitat local to someone, perhaps in a pond, meadow or your back garden.

Question 95

define global biodiversity.

Answer 95

a variety of different species on earth. the total number of species on earth estimates at 8.7 million.

Question 96

how is biodiversity measured?

Answer 96

species richness.
using an index of diveristy.

Question 97

explain using species richness as a measure of biodiversity.

Answer 97

species richness is a measure of the number of different species in a community. it can be worked out by taking random samples of a community and counting the number of different species. the population sizes of different species also affect biodiversity.

Question 98

explain using an index of diversity as a measure of biodiversity.

Answer 98

it’s calculated using an equation that takes both the number of species in a community and the abundance of each species into account.
the higher the number, the more diverse the area is. if all individuals are of the same species the index is 1.

Question 99

what is the formula for the index of diversity?

Answer 99

N(N-1) divided by ∑ n(n-1)
N = total number of organisms of all species.
n = total number of organisms of one species.
∑ = ‘sum of’

Question 100

what agricultural practices reduce biodiversity? (3)

Answer 100

woodland clearance- this is done to increase the area of farmland. it directly reduces the number of trees. it also destroys habitats, so some species could lose their shelter and food source. species could die or are forced to migrate to another area, further reducing biodiversity.
herbicides- these are chemicals that kill unwanted plants (weeds). this reduces plant diversity and could reduce the number of organisms that feed on the weeds.
monoculture- this is when farmers have fields containing only one type of plant. a single type of plant reduces biodiversity directly and will support fewer organisms which further reduces biodiversity.

Question 101

how is biodiversity protected from agriculture? include an example.

Answer 101

by conservation schemes.
e.g giving legal protection to endangered species.