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Title: BIOLOGY OCR F215
Description: OCR Board A2 Level Biology F215 SECTION 1: CELLULAR CONTROL AND VARIATION
Description: OCR Board A2 Level Biology F215 SECTION 1: CELLULAR CONTROL AND VARIATION
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BIOLOGY
F215: Control, Genomes and Environment
1) Cellular Control and Variation
a) Cellular Control
i) State that genes code for polypeptides, including enzymes
ii) Explain the meaning of the term genetic code
iii) Describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the
amino acid sequence in a polypeptide
(1) Protein – a large polypeptide, usually composed of 100 or more amino acids and one
or more polypeptide chains
(2) Polypeptide – polymer of a specific sequence of amino acids joined by peptide bonds
(3) Allele – alternative version of a gene, still at the same locus on the chromosome but
the alteration to the DNA base sequence may alter the proteins structure
(4) DNA – a polynucleotide or a long sequence of DNA nucleotides (genetic material)
that codes for protein synthesis/a sequence of amino acids in a polypeptide
(5) Gene – short section of a length of DNA with up to 2000 in one chromosome that
codes for one or more polypeptides (a unit of heredity) eg…
(a) Structural proteins eg
...
TCT codes for serine in any organism but in
ciliated protoctists, two of the standard stop codes code for glutamic acid
(8) Genome (of an organism) – the entire DNA sequence of that organism
(9) Nucleotide – monomer of all nucleic acids
(a) Pentose sugar (C₅H₁₀O₄) – either ribose in RNA or deoxyribose in DNA
(i) Forms covalent bonds with the nitrogenous base and the phosphate groups
via condensation reactions
(ii) Nitrogenous base bonds with hydroxyl group on first carbon
(iii) Phosphate group bonds with hydroxyl group on fifth carbon
(b) Organic nitrogenous bases (at right angles to the sugars)
(i) Purines – double rings of carbon and nitrogen, adenine and guanine
(ii) Pyrimidines – single rings of carbon and nitrogen (fairly stable as it is a single
molecule), thymine cytosine and uracil
(c) Phosphoric acid (inorganic molecule abbreviated to Pi)
(i) Negatively charged as the hydrogens in the hydroxyl groups are lost
(ii) Molecule attracts water forming a protective coat around the final molecule
(10) Nucleic acids – nucleotides joined together by the Pi joining with 3rd carbon of
pentose sugar by a condensation reaction to form a phosphodiester bridge (resulting
in a repeating sugar-‐phosphate chain) between the third and fifth carbons
(11) Structure of DNA – polymer of 2 chains of nucleic acids running antiparallel to one
another and twisted into a double helix
(a) The two strands are linked by paired bases which project towards each other and
are held together by hydrogen bonds (making DNA a very stable structure)
(b) Pyrimidine always bonds with a purine – keeping constant width between strands
(c) Nucleotides with adenine can make two hydrogen bonds with nucleotides with
thymine (or uracil in the case of RNA) as the base
(d) Nucleotides with guanine can make three hydrogen bonds with nucleotides with
cytosine as the base
(e) Chains run antiparallel so one end finishes on the 3rd carbon and the other on the
5th – the strands run and the sugars point in opposite directions to each other
(f) The antiparallel chains twist like a rope ladder to form a final, double helix
iv) Describe, with the aid of diagrams, how the sequence of nucleotides within a gene is
used to construct a polypeptide, including the roles of messenger RNA, transfer RNA and
ribosomes
(1) Messenger RNA – single strand of nucleotides where uracil replaces thymine and
ribose sugar replaces deoxyribose sugar (copy of the DNA coding strand), responsible
for bringing the information contained in DNA out of the nucleus to the ribosome
(2) Transfer RNA – carries the anti codon and the specific amino acids to the ribosomes
where they are bonded together to form polypeptides
(a) Lengths of RNA fold into hairpin shapes
(b) Have three exposed bases at one end where a particular amino acid can bind
(c) At the other end, there are three unpaired nucleotide bases – anticodon
(3) Ribosomal RNA – spherical with a small sub-‐unit on left and large sub-‐unit on the
right (found in ribosomes), forming part of the structure of a ribosome and
accounting for 80% of the total RNA in a cell
(4) Protein synthesis
(a) DNA à mRNA à tRNA à polypeptide
(b) Transcription – creation of a single-‐stranded mRNA copy of the DNA coding
strand (which is small enough to leave the nucleus)
(i) Genetic code in nucleus of cell contains the information for making
polypeptides
(ii) The sequences of bases on DNA make up codes for particular protein
molecules, acting as a template strand – coding for the sequence of amino
acids in the protein
(iii) The gene to be transcribed unwinds and unzips by being dipped into the
nucleus so hydrogen bonds between complementary bases break
(iv) The sequence for a particular gene is exposed
(v) Free floating DNA nucleotides are found in the nucleoplasm and free RNA
nucleotides are found in the nucleolus
(vi) The nucleotides are activated to ATP, GTP, CTP and UTP – they have two extra
phosphoryl groups attached
(vii) Free activated RNA nucleotides pair up and bind temporarily with hydrogen
bonds to their exposed complementary bases (G to C and A to U) on the
template strand of unwound DNA
(viii) The two extra phosphoryl groups are released, releasing energy for bonding
adjacent nucleotides
(ix) Their sugar phosphate groups are bounded together to form the sugar-‐
phosphate backbone via condensation reactions
(x) A single-‐stranded piece of mRNA thus forms, catalysed by RNA polymerase
(xi) The mRNA is released from the DNA and leaves the nucleus, through a pore in
the nuclear envelope, to a ribosome
(c) Translation – the formation of a sequence of amino acids to form a polypeptide
on a ribosome using mRNA as the code, whereby the amino acids are assembled
into the sequence dictated by the sequence of codons on the mRNA
(i) NB: in prokaryotes, translation starts as soon as some mRNA has been made
(ii) Ribosomes are assembled in the nucleolus of eukaryote cells from ribosomal
RNA and protein – 2 sub-‐units and groove into which a length of mRNA can fit
(iii) The ribosome has two sites for amino acids to bind on the ribosomal groove
(iv) mRNA peels away from the DNA and passes from the nucleus to the
cytoplasm by a nuclear pore, carrying the code to ribosomes in the cytoplasm
(v) Ribosome assembles around the mRNA and moves along three bases at a time
(vi) tRNA is made in the nucleus and passes into the cytoplasm
(vii) The end of the tRNA molecule has an anticodon (three exposed bases) – 61
different tRNA molecules exist (a codon – a triplet of three bases on mRNA)
(viii) Activation – each tRNA molecule binds to a specific amino acid and brings it
to the ribosome according to the base sequence on the mRNA
1
...
The tRNA molecule is then said to be charged
(ix) The first exposed mRNA codon is always AUG
(x) Using ATP and an enzyme, a tRNA with methionine and the complimentary
anticodon UAC forms hydrogen bonds with this codon in the ribosome’s
binding site
(xi) The second site is now filled by the appropriate tRNA, bringing the next amino
acid to the ribosome as specified by the mRNA codon
(xii) The adjacent amino acids attached to the tRNA molecules are joined
together by a peptide bond, catalysed by an enzyme in the small ribosomal
sub-‐unit
(xiii) The first tRNA molecule is now ‘empty’ and leaves the ribosome to return to
the cytoplasm to pick up a further amino acid
(xiv) The ribosome then moves along the mRNA by a distance of one codon
(xv) This leaves the second site for amino acid binding empty and so a third
amino acid can be added to the growing polypeptide chain
(xvi) Elongation of the polypeptide chain continues until a STOP codon is reached
(UAA, UAG, UGA) – no corresponding tRNAs for these three codons
(xvii) Polypeptide chain separates from ribosome – protein synthesis is complete
(xviii) Sequence of amino acids in a protein is critical because…
1
...
The specific primary structure gives rise to the secondary and tertiary
structure of the protein appropriate to its function
3
...
The tertiary structure of a protein is what allows a protein to function
5
...
changing the shape of the active site of an enzyme
v) State that mutations cause changes to the sequence of nucleotides in DNA molecules
(1) Mutation – any random change in the structure or amount of DNA in an organism ie
...
tobacco, UV light, X-‐rays, gamma rays
(2) Gene Mutation – a change in the structure of DNA ie
...
substitutions – a single base pair is replaced by another,
substituted for another base
(i) Missense mutation – arises if another amino acid is coded for as a result
1
...
UAA, UAG, UGA
1
...
This can yield a truncated/abbreviated protein product
3
...
Eg
...
a base is removed
(iv) Results in a frame shift – might appear to make sense, but in fact it can result
in the production of an entirely different polypeptide
(v) Eg
...
Sickle cell anaemia – point mutation
(i) On codon 6 of the gene for the beta polypeptide chains of haemoglobin
(ii) The replacement of A by T in the 17th nucleotide (ie
...
Cystic Fibrosis – deletion
(i) 70% of cases is due to the mutation of a triplet of base pairs
(ii) 1 amino acid from the sequence of 1480 amino acids in the normal
polypeptide chain is deleted
(f) Eg
...
Huntington disease – stutter
(i) Results from an expanded triple nucleotide repeat
(ii) Normal gene has repeating CAG sequences
(iii) Protein is altered sufficiently when CAG sequences are expanded to above a
threshold number
(iv) Symptoms manifest later in life eg
...
In the number or structure of chromosomes
(b) Includes a deletion, inversion or translocation
(c) The most common chromosome mutation is trisomy 21 aka
...
although the base triplet has changed, it still codes
for the same or similar amino acid so the protein is unchanged
(c) The mutation may cause a change to the structure of the protein, and therefore a
different characteristic, but the changed characteristic gives no particular
advantage or disadvantage to the organism
(i) Eg
...
Ability to roll your tongue (genetic trait)
(iii) Eg
...
Melanin in skin
(b) Early humans in Africa had dark skin – pigment melanin protected them from
harmful effects of UV light whilst still being able to synthesise vitamin D
(c) Any humans with mutations to some of the genes determining skin colour,
producing paler skin, would have burned and suffered from skin cancer
(d) As humans migrated to more temperate climes, the sunlight was not intense
enough to cause enough vitamin D to be made by those with dark skin
(e) Humans with a lack of melanin could synthesise more vitamin D
(f) Those without a lack of melanin and a deficiency in vitamin D would suffer from
rickets and, in females, a narrow pelvis producing difficulties in childbirth
(3) Depending on the environment, the same mutation for paler skin can be beneficial or
harmful – the environment is never static
(4) Natural selection – the mechanism for evolution
(a) Individuals within a population who have a certain characteristic may be better
adapted to the new environment
(b) The well-‐adapted organisms with the advantageous allele can out-‐compete those
in the population that do not have the advantageous characteristic
(c) Thus, without genetic mutations there would be no evolution
vii) State that cyclic AMP activates proteins by altering their three-‐dimensional structure
(1) Cyclic AMP (from ATP) – some proteins have to be activated by chemicals, which
change their 3D shapes so they are a better fit to their complementary molecules
(2) The presence or absence of glucose affects the concentration of cyclic AMP in the
cell, and therefore the lac operon
(3) Concentration of cyclic AMP in E
...
code for proteins eg
...
Coli bacteria cells – converted into glucose and galactose
(h) Sugars can then be used for respiration to produce energy from the lactose
(i) As the concentration of lactose decreases, there are less repressor proteins with
bound lactose
(j) Transcription of structural genes therefore eventually stops
(7) In absence of lactose, with the agar containing glucose as the only energy source
(a) The regulator gene is expressed (transcribed and translated)
(b) The repressor protein is synthesised with two binding sites (one that binds to
lactose and one that binds to the operator region)
(c) The repressor protein binds to the operator region
(d) Part of promoter region is covered – where RNA polymerase normally attaches
(e) This prevents RNA polymerase from binding to the promoter region
(f) Structural genes cannot be transcribed into mRNA
(g) Translation of proteins/enzymes required to break down lactose is blocked
ix) Explain that the genes that control development of body plans are similar in plants,
animals and fungi, with reference to homeobox sequences (HSW1)
(1) Process of cell differentiation
(a) Much of what is known about the molecular basis of cell differentiation has come
from the study of the fruit fly – Drosophila melanogaster
(b) DNA replication in the Drosophila eggs is faster than is known for any other
organism (one every 6-‐10 minutes)
(c) A series of mitotic divisions, which are not followed by cell division, begin after
the eggs are laid
(d) A syncytium is formed – a multinucleate structure
(e) After the 8th division, the 256 nuclei migrate from the middle of the cells to the
outer layer
(f) By the 11th division, the nuclei form a single layer around a yolk filled core
(g) The division rate slows (the 14th division takes 60 minutes)
(h) The nuclear genes switch from replicating to transcribing
(i) Plasma membranes invaginates (folds inwards) from the cell surface and
surround each individual nucleus to form about 6000 cells
(j) About 15 nuclei, found at the extreme posterior of the cell, form the pole cells –
primordial germ cells that will give rise to the eggs or sperm
(k) After another 2-‐3 hours, the embryo divides to form a series of segments –
corresponding to the organism’s organisation/body plan
(l) The three Md, Mx and Lb segments merge to produce the head
(m) T1-‐3 are the thorax segments
(n) A1-‐8 are the abdominal segments
(o) Metamorphosis – larval form becomes the adult as the legs, wings and antenna
develop
(2) Three main stages of development
(a) Development of axes – anterior, posterior, dorsal and ventral
(b) Segmentation – division of body into head, abdomen and thorax
(c) Segment identity – structures specific for each segment developed
(3) Controlling cell development and an organism’s body plan using homeobox genes
(a) Homeobox genes – code for polypeptides that act as transcription factors, turning
transcription of other genes on or off in the order that they are expressed
(i) Found in all segmented animals, plants and fungi
(ii) Contain a sequence of 180 base pairs, therefore coding for 60-‐amino acid long
polypeptide chains
(iii) Arranged in Hox clusters – as the complexity of the organism increases, the
number of Hox clusters the organism has also increases
(b) Maternal effect genes – controls development of axes/the embryo’s polarity
(i) Transcribes mRNA
of genes that allow
axes to be
established
(ii) Come from the
mother who puts
mRNA into each egg
before fertilisation
(iii) mRNA codes for
proteins eg
...
Kruppel gene – responsible for development of embryo’s middle region
(c) Homeotic selector genes – specify the identity of each, individual segment and
direct their development including the formation of appendages
(i) The master genes in the control networks of regulatory genes
(ii) Some regulate the development of the thorax and abdomen segments
(iii) Others regulate the development of the head and thorax segments
(d) Antennapedia – mutation in the homeotic genes can change one body part into
another eg
...
CNS
(b) Retinoic acid is a morphogen – substance that governs the pattern of tissue
development ie
...
cranial deformities
x) Outline how apoptosis (programmed cell death) can act as a mechanism to change body
plans
(1) Apoptosis – a series of biochemical events that together are a fast process of
programmed cell death that occurs in multicellular organisms
(2) Hayflick constant – cells should undergo about 50 mitotic divisions and then undergo
a series of biochemical events that leads to an orderly and tidy cell death
(a) Cancer cells, on the other hand, are immortal
(b) The rate of cells dying should
balance the rate of cells
produced by mitosis
(c) Not enough apoptosis leads to
the formation of tumours
(d) Too much apoptosis leads to
cell loss and degeneration
(3) Necrosis – an untidy and damaging
cell death that occurs after trauma
and releases hydrolytic enzymes
(4) Mechanism for apoptosis
(a) Enzymes break down the cell
cytoskeleton
(b) Cell surface membrane changes
(c) Cytoplasm becomes dense, with
organelles tightly packed
(d) Chromatin condenses
(i) Nuclear envelope breaks
(ii) DNA breaks into fragments
(e) Blebs – cell breaks up into small
vesicles
(f) Vesicles are taken up by
phagocytosis
(5) Controlling apoptosis
(a) Events are controlled by a
diverse range of internal and
external cell signals
(b) Eg
...
(Names of the main stages are expected, but not the subdivisions of
prophase)
(1) Binary Fission – reproduction in prokaryotes producing genetically identical offspring
(a) Single DNA molecule replicates
(b) Both copies attach to the cell membrane
(c) Cell membrane begins to grow between the two DNA molecules
(d) Bacterium doubles in size
(e) Cell membrane pinches inward
(f) Cell wall forms between two DNA molecules dividing the original cell into two
identical cells
(g) Genetic variation will only be introduced by mutation
(2) Mitosis – asexual reproduction in eukaryotes
(a) Produces two genetically identical cells
(b) Each daughter cell contains the same number and types of chromosomes as the
original parent cell
(3) Meiosis is a reduction division – sexual reproduction in eukaryotes
(a) Occurs in the sex organs ie
...
centrioles and mitochondria, replicate
(v) ATP is produced
(b) Meiosis I – homologous chromosomes pair up and swap portions of chromatids
(crossing over) before separating (the cell splits in two)
(i) Prophase I
1
...
Chromosomes condense (shorten and thicken) by supercoiling and
become visible – can take up stains and be seen with a light microscope
3
...
Non-‐sister chromatids wrap around each other and attach at crossover
points (chiasmata)
5
...
Centrioles migrate to opposite poles of the cell and start to produce
spindle fibres made of protein microtubules
(ii) Metaphase I
1
...
Members of the homologous pairs face opposite poles – so chromosomes
can independently segregate when pulled apart in anaphase I
3
...
Chiasmata are still present
(iii) Anaphase I
1
...
Chromosomes move to opposite poles of the cell
3
...
Chiasmata separate
5
...
Plant cells skip telophase I and go from anaphase I to meiosis II
2
...
Chromosomes arrive at opposite poles of the cell
4
...
May or may not be followed by cell division (cytokinesis)
6
...
If a nuclear membrane has reformed, it breaks down again
2
...
Chromosomes relax and then recondense
4
...
Chromosomes line up along the equator of the spindle
2
...
Chromosomes are attached to the spindle fibres at the centromeres
(iv) Anaphase II
1
...
Centromeres splits
3
...
Once pulled apart, chromatids are known as chromosomes
(v) Telophase II
1
...
A new nuclear membrane reforms around each set of chromosomes
3
...
In plants, a tetrad (group of four) haploid cells is formed
ii) Explain the terms allele, locus, phenotype, genotype, dominant, codominant and
recessive
(1) Gene – length of DNA that codes for one or more polypeptides
(2) Allele – alternative versions of a particular gene eg
...
red allele and
white allele will be both expressed in flowers and produce a pink flower
(a) In horses – chestnut and white colours are codominant so heterozygous horses
have a blend of both colours and are golden, tan palominos
(10) Recessive – allele only expressed in the phenotype when it is present in the
homozygous state (with another identical allele)/in the absence of the dominant
allele for the same gene
(11) Autosomes – chromosomes not concerned with determining sex
iii) Explain the terms linkage and crossing-‐ over
(1) Linkage – two or more genes that are located on the same chromosome
(a) Reduces the number of phenotypes resulting from a cross
(b) Linked alleles with no chiasmata formed between them are inherited together
because they do not segregate independently during meiosis
(c) The closer together genes are on a chromosome, the less chance there is of them
being separated by crossing over
(d) Crossing over would otherwise produce recombinants
(2) Sex linkage – characteristic which is coded for/determined by a gene that is found on
the sex (either the X or Y) chromosomes
(a) In most animals, the small Y chromosome has few genes
(b) Most sex-‐linked genes are likely to be found on the X chromosome
(c) Eg
...
human blood groups IA, IB and IO
(e) In the case of codominance, the genes is also given an upper case letter and the
alleles are superscripted
(3) Pedigree diagrams
(a) Pedigree analysis – one of the most powerful tools in human genetic studies
(4) Characteristics of genetic diseases controlled by a single recessive gene
(a) Children born to two carriers have a 25% chance of being homozygous recessive
therefore have the disorder
(b) Carriers will not have the disorder
(c) Eg
...
Mutation disrupts the transport of chloride ions and water across the
membranes of cells lining the airways, gut and reproductive tracts
2
...
Cilia are not properly hydrated and cannot shift mucus
(ii) Effect/symptoms
1
...
More likely to get respiratory, bacterial infections
3
...
Affected gut and pancreas so food is not digested efficiently
5
...
Homozygous dominant – all chloride ion channels function
2
...
Heterozygous – no symptoms but do have some abnormal chloride ion
channels yet enough normal channels for their lungs to functions
(d) Eg2
...
Factor VIII – blood clotting protein coded for by a gene on the X
chromosome
2
...
Hemizygous – males have only one X chromosome, if it has the allele for
haemophilia A, the male will suffer from haemophilia
(ii) Effect
1
...
Internal bleeding from knocks and bleeding into joints are particularly
harmful
(e) Eg3
...
Dystrophin – DMD gene for a muscle protein on the X human chromosome
2
...
Mutations of the gene usually result in a severely truncated dystrophin
protein or no dystrophin at all (deletion mutation)
(ii) Effect
1
...
Usually wheelchair-‐bound by 10 years old
3
...
Hypercholesterolemia – heterozygote has a reduced number of functional low
density lipoprotein receptors
(b) Individuals possessing one copy of the affected allele will have the disease
(c) One carrier parent and one normal parent have a 50% change of producing an
affected child
(d) If even one affected parent is homozygous, all offspring will be affected
(6) Characteristics of traits carried on the X chromosome – sex linked eg
...
Sickle cell anaemia
(i) Cause
1
...
On codon 6 of the gene for the beta polypeptide chains of haemoglobin
3
...
6th amino acid)
changes GAG to GTG
4
...
Deoxygenated, abnormal haemoglobin is not soluble and becomes
crystalline – aggregating into more linear and less globular structures
2
...
Sticky, sickle cells are not adapted to carrying oxygen or squeezing through
the capillaries
4
...
Eventually organs (especially heart, lungs and kidneys) become damaged
6
...
HAHA – normal haemoglobin
2
...
HAHS – half the haemoglobin is normal and half is sickled when red blood
cells are made in the bone marrow
4
...
At whole organism level this condition could be considered to be recessive
6
...
Shorthorn cattle and one of the genes for coat colour
(i) When complete dominance is lacking, upper and lower case are not used to
represent the genes
(ii) Homozygous individuals with genotype CRCR have red (chestnut) coats
(iii) Homozygous individuals with genotype CWCW have white coats
(iv) Heterozygous individuals with genotype CRCW have red and white hairs and
the coat is roan – roan cattle
(c) Eg3
...
The ABO blood system classifies red blood cells according to antigens on the
cell surface membranes
(f) The antigens are determined by gene ‘I’ which has three alleles (IO, IA, IB)
(g) IA and IB are codominant but are both dominant to IO
(9) Linkage – when both genes are carried on the same chromosome so they are
segregated together
(a) Monohybrid – gene that codes for one characteristic
(b) Dihybrid – two genes controlling two different characteristics
(i) Eg
...
Completely random, independent assortment
(c) Linkage group – group of genes that are normally inherited together because they
are found on the same chromosome
(i) Alleles are linked on the same chromosome
(ii) Not independently assorted during meiosis I
(iii) Cross two heterozygotes for two genes
(iv) Only two gametes are produced – YG and yg (not YG, Yg, yG, yg)
(v) Recombinants can occur in low frequencies due to crossing over – can
separate two linked genes
1
...
Genes that are close together rarely get separated by crossing over
(vi) Not the expected 9:3:3:1 dihybrid and unlinked ratio
vi) Describe the interactions between loci (epistasis)
...
Salvia
1
...
aa – epistatic to both alleles of the gene B/b
3
...
Leghorn and White Wyandotte chickens
1
...
C – coloured feathers
3
...
Sweet peas – purple colour is produced by the action of two enzymes (C + P)
(c) If either gene is homozygous recessive, the purple colour will not be produced
viii) Use the chi-‐squared (χ2) test to test the significance of the difference between observed
and expected results
...
height, weight, arm length
(a) Describes quantitative differences in phenotypes
(b) No distinguishable categories
(c) Wide variation in the population
(d) Frequency distribution = normal distribution
(e) Mode (most frequently occurring value) falls in the middle of the range (median)
which is the same as the mean
(f) Most of the organisms in the population fall in the middle of the range with
approximately equal numbers showing the two extremes
(4) Discontinuous variation – limited number of distinct forms within a population with
no intermediates in between eg
...
Only one gene is involved in the inheritance of the CFTR gene, inheritance of
two faulty genes results in cystic fibrosis
(d) If there is more than one gene involved – genes interact in an epistatic way
(e) Different alleles at a single gene locus have large effects on the phenotype
(f) Different gene loci have quite different effects on the phenotype
xi) Explain that both genotype and environment contribute to phenotypic variation
...
tall genes won’t
be fully expressed if you are undernourished at crucial stages in your development
xii) Explain why variation is essential in selection
(1) Variation – raw material for both natural and artificial selection (a plausible
mechanism for evolution)
(a) Individuals within a species differ from each other – variation
(b) Offspring resemble their parents – inherit
(c) Far more offspring are generally produced than survive to maturity – they may
suffer from predation, disease or competition
(d) Populations are usually fairly constant in size
(2) Process of natural selection
(a) The environment changes
(b) Selection pressure – an environmental factor that confers greater chances of
surviving to reproductive age on some members of the population
(i) Eg
...
If mutation does occur – forward and backward mutations are equal
(vi) No genetic drift – no changes in allele frequencies resulting from random,
chance processes (more likely to affect smaller populations)
(vii) Characteristic being studied is not sex-‐linked – only for traits with dominant
and recessive alleles
xiv) Explain, with examples, how environmental factors can act as stabilising or evolutionary
forces of natural selection
(1) Population growth
(a) All organisms can reproduce – have the potential to increase their population size
(b) Over time, the population size will fluctuate around a mean level
(c) Great environmental resistance – population size will shrink, competition will be
reduce and the population will grow
(d) As the population increases – more intraspecific competition for resources and
the population size will fall again
(e) Carrying capacity – maximum size that the environment can sustain and then
remain stable
(f) In this case, not all the young produced survive to adulthood – otherwise they
would produce young too and the population would continue to expand
(g) Natural selection can affect allele frequencies in several different ways
(h) For continuously varying characteristics, these selection methods modify their
frequency distributions in different ways
(2) Stabilising selection – natural selection keeps things the way they are
(a) Acts against the extremes within a range of phenotypic variation
(b) Leads to a reduction in the range of variation within the population without any
change in the mode
(c) Operates in an unchanging environment to maintain the best adapted genotypes
(d) Eg
...
Agouti coat can camouflage rabbits and prevent predation, white coat
rabbits will be advantageous if the environment changes and snow then falls
(f) Environmental factors – caused by abiotic or biotic components
(i) Space – for plants to grow, for animals to defend a feeding territory, for
animals to rear young
(ii) Availability of food, light, minerals or water
(iii) Predation
(iv) Infection by pathogens
(3) Directional selection – evolutionary force of natural selection that leads to an
evolutionary change
(a) Operates in changing environments
(b) Acts for or against extremes of phenotypes within the variable population
(c) Main type of selection practised by man when selecting domesticated plants and
animals for the improvement of stocks
(d) Reduction in the range of variation within a population
(e) Progressive shift in the mode
(f) Eg
...
Copper tolerance in grasses
(i) Copper tolerance is a genetically-‐determined quantitative character,
displaying many degrees of tolerance (polygenic inheritance)
(ii) Soil in and around old copper mines contains high concentrations of copper –
normally toxic to most species of plants and animals
(iii) Agrostis tenuis – copper-‐tolerant plant found growing in areas of land where
the soil is contaminated with copper
(iv) Two varieties of Agrostis tenuis are found growing around old copper mines
(v) Non-‐tolerant plants are unable to grow in the copper-‐rich soil, but tolerant
plants are able to grow in normal soil
1
...
Samples of mature plants taken from the copper-‐rich soil of the
abandoned mine show a high level of tolerance to copper and a small
range of variation
(vi) When seeds were collected from the sampled copper-‐tolerant plants and
grown in ordinary soil, variation in tolerance is much greater – no selection
pressure/forces for copper tolerance
1
...
due to a
natural disaster or disease pandemic
(ii) Eg
...
016
(vi) As 5% currently suffer from the disorder, current allele frequency = 0
...
as they may not recognise each other’s courtship behaviour
(i) Eg
...
When the finches reproduced, some of the alleles would be lost as a
natural part of how gametes randomly fuse – down to chance
2
...
Not all the offspring will survive – also reducing the number of alleles
4
...
Rising sea level isolates a population on an island into three sub-‐populations
(i) The separated islands become different environments as climatic and other
factors influence the physical nature of the environment
(ii) Natural selection acts in different ways on the variable population
(iii) Differences in their genetic composition are promoted so they can adapt
independently to each environment
(iv) Eventually the sub-‐populations diverge to such an extent that they become
reproductively isolated from one another and become different species
(d) Egs
...
incompatible genitals
xvii) Explain the significance of the various concepts of the species, with reference to the
biological species concept and the phylogenetic (cladistic/evolutionary) species concept
(HSW1)
(1) Species can be regarded as the starting point in classification of living organisms –
group of similar organisms that can interbreed and produce fertile offspring
(reproductively isolated from other such groups)
(2) Biological species concept – a group of similar organisms that interbreed to produce
fertile offspring and are reproductively isolated from other such groups
(a) Problematic when deciding which species an organism belongs to/new species
(b) Biologists want to classify living organisms that do not reproduce sexually – never
reproduce together even if they belong to the same species eg
...
males
and female ants have very different castes
(d) Might be extinct – can’t study their reproductive behaviour
(e) Practical/ethical issues – geography not allowing you to see if organisms
reproduce successfully in the wild, can’t study them in a lab
(3) Phylogenetic/cladistic/evolutionary species concept – hierarchical classification of
species based on the study of the evolutionary ancestry of groups of organisms
(what is related to what and how closely)
(a) Basics
(i) Share similar morphology (shape), physiology (biochemistry), embryology
(stages of development) and behaviour, and occupy the same ecological niche
(ii) All evolved from shared, common ancestors
(iii) The more closely related two species are, the more recent their last common
ancestor will be
(iv) Which species or new species? – close relations to members of another
species, then its probably the same species
(v) Difficulty – no cut off point to say how different two organisms have to be to
be different species eg
...
Differences are caused by base substitutions – expressed as % divergence
2
...
of substitutions / no
...
Grouping of reptiles excludes birds which are descendants of reptiles
(iv) Eg2
...
Taxonomic classification systems
(i) Focus on evolution vs
...
less emphasis
(iii) Uses DNA and RNA sequencing vs
...
Manual creations of such diagrams would be difficult with many species
(v) No distinction between extinct and extant (surviving) species (both may be
included in cladograms) vs
...
monkeys, apes and
humans have opposable thumbs = primate
(c) Unique features may determine a species but useless for determining ancestry
(i) Only shared by virtue of having long histories
(ii) Evolutionary ‘hangers on’ that have persisted in several linages and continued
to diverge so are no longer closely related eg
...
In agricultural system
(c) Advantageous to humans
(d) Relatively fast
(e) Costs money
(3) Both
(a) Involve selection of parents for advantageous traits
(b) Involve inheritance of alleles
(c) Change allele frequencies
(d) Takes several generations
(4) Process of selectively breeding – artificial selection
(a) Choose a male and female with the desired characteristic(s) and mate them
(b) Inspect the offspring
(i) Any traits with a high level of heritability will come out in the offspring
(ii) Very few characteristics are the result of one gene and are usually a result of
polygenes/epistasis/linkage
(iii) Easy to identify characteristics that affect appearance eg
...
tameness
(c) Allow the best offspring to breed
(d) Repeat this over several generations
(e) Until all offspring show homozygosity for the desired characteristic
xix) Describe how artificial selection has been used to produce the modern dairy cow and to
produce bread wheat (Triticum aestivum) (HSW6a, 6b)
(1) Characteristics artificially selected for in cows
(a) Modern dairy cow Chillingham White is thought to be the breed that looks the
most like original wild cattle
(b) Also Holstein-‐Friesian, Brown Swiss, Guernsey, Ayrshire, Jersey, Milky Shorthorn
(c) Some breeds of dairy cattle have thick coats and can live in the Scottish Highlands
(d) High milk yield
(e) High meat yield
(f) Ability to survive in particular environments
(g) Docility (tameness)
(2) Principles of selecting breeding bulls practiced today
(a) Progenies of the bulls are tested to determine which ones produce high milk-‐
yielding daughter cows (only a few need to be kept to inseminate many cows)
(b) Milk yield from each cow is measured and recorded
(c) Best cows can be given hormones to encourage prolific egg production
(d) Cows are artificially inseminated with semen from a good quality bull OR eggs are
fertilised in vitro
(e) Embryos are implanted into surrogate mothers
(f) Embryos can also be cloned
(g) à A few elite cows can give rise to more offspring than they would naturally
(3) Wheat
(a) Can grow in large areas of the world
(b) Makes up 33% of all cereal crops
(c) Breeders continue to carry out selection programmes to produce improved
varieties
(d) Characteristics artificially selected for in wheat
(i) Increased yield
(ii) High protein content
(iii) Stiff straw (stem) – to support large shaft
(iv) Resistance to lodging – prevent stems falling over in wind and rain which
would make harvest more difficult
(v) Resistance to disease eg
...
wild wheat
(a) Large, plump seeds vs
...
seeds can’t disperse themselves
(d) Non-‐shattering rachis (stem) when grape is ripe vs
...
hulls (outer coating/shell of seed)
(i) Hull around seed comes off during threshing
(ii) Grain only has to be winnowed to blow the hull away from the seeds
(iii) Wild varieties are hulled instead – hull stays around the seed even after it has
been threshed, and so needs to be pounded
(iv) Laborious and breaks the grain so it is less suitable for storage
(f) Seeds retained and germinated together vs
Title: BIOLOGY OCR F215
Description: OCR Board A2 Level Biology F215 SECTION 1: CELLULAR CONTROL AND VARIATION
Description: OCR Board A2 Level Biology F215 SECTION 1: CELLULAR CONTROL AND VARIATION