This document provides information on various biology topics: - Causes of variation including genetic and environmental sources of intraspecific variation - The processes of mitosis, meiosis, genetic bottlenecks, and the founder effect - The life cycle of a cell and stages of the cell cycle - How DNA is structured and its function in coding for proteins - The processes of DNA replication, transcription, and translation - Plant gas exchange and transport of water and minerals in plants

1. Biology
AQA
Module 2

2. Causes of Variation
Interspecific – variation that exists between different
species
Intraspecific – variation that occurs within a species
Intraspecific Variation:
Genetic:
All members of species have same genes
Individuals within a species have different alleles
Environmental:
e.g. food, health, temperature
Variation is often a combination of both

3. Mitosis
Asexual
No Gametes
Cloning
Single Parent
Chromosomal Number Unchanged – Diploid
No Variation In Genes
Spindle Fibre

4. Meiosis
Sexual
Gametes – Egg & Sperm
No Cloning
Two Parents
Chromosomal Number Halved – Haploid
Variation in Gene Pool
Mutations in the DNA of Chromosomes are
mistakes
Homologous
Pairs
Chiasmata
(Cross-Over) –
Bivalent cross up
to 8x
Cytokinesis (New
Membrane
Formed)
¼ of Original Cell ½ of Original Cell
The assortment of
genes changes the
outcome,
depending on
which side they
stay on

5. Meiosis
Produces four daughter cells each with half the units of DNA
Needed for reproduction
Meiosis 1:
Homologous chromosomes pair up and chromatids wrap around each other.
When chromatids twist around other chromatids, tension is created. Parts of the
chromatids break off. These broken parts rejoin with the other chromatid called
recombination
Independent segregation lines up homologous pairs randomly down the centre of
the cell
Meiosis 2:
Chromatids move apart by the end 4 cells have been produced
Gene – A section of DNA that codes for a polypeptide
Locus – Position of a gene on a chromosome or DNA molecule
Allele – Different form of a gene

6. Genetic Bottleneck
An event causes a large reduction in number of individuals within a
species
Reduces the number of different alleles in the gene pool
This reduces genetic diversity
The few survivors reproduce and a larger population is created from
few individuals
When a few members of a species move away creating a new colony
So only a few individuals contribute to gene pool, so more inbreeding
occurs, therefore there is a higher incidence of genetic disease
Founder Effect

7. Life Cycle of a Cell
P
M
A
T
Cytokinesis
G1
S
G2
Chart Title
Organelles
Copied
DNA Replication Proteins made,
Ribosomes
copied etc…
G1 G2 is Interphase

8. How DNA is made & it’s
Function
It’s a large polymer of repeating Nucleotides.
G & A Large Bases (Purines)
C & T Small Bases (Pyrimidines)
A - T, C - G are the complementary pairs
20 amino acids so we need to use a triplet code e.g. AAA, ATA The
triplet code makes 64 codes so 1 amino acids can have different
codes. (Degenerate Code)
If AAA made badly/mutates and ATA produced won’t affect the
outcome as both are Glycine.
Small % of DNA are genes, the rest is junk (VNTRS) used to create a
DNA fingerprint
A – Adenine
G – Guanine
C – Cytosine
T (U) – Thymine
(Uracil in RNA)
P – Phosphate Group
S – Sugar
N – Nitrogen Base (Varies)
Gen
e
Loci
Exo
n
Intron
A & T have 2 H bonds
G & C have 3 H bonds
Allele: A variant of a gene

9. DNA Replication
During Interphase
DNA helicase breaks H-bonds between strands creating 2 single
strands
Each original strand acts as a template for a new strand
Free floating nucleotides join to nucleotides on the original strands
In the complementary pairing A & T, G & C
The nucleotides on the strand are bonded together by DNA polymerase
creating H-bonds
Each new DNA molecule contains 1 original & 1 new strand
This is called Conservative Replication

10. S-Phase of the Cell Cycle
DNA Replication – Semi - Conservative Replication
Helicase – breaks ‘H’ bond between Nitrogen bases
DNA Polymerase – aligns complementary nucleotides
New Strand
Old Strand
Meselson & Stahl:

11. Creation of Tissue Fluid
It supplies cells with O2, Glucose, H2O, AAs, Fatty Acids, Salts. It
takes away waste products e.g. CO2.
Large molecules stay in blood RBCs & Proteins
Hydrostatic pressure, at the arterial end, is created due to narrowing
of blood vessels.
Blood H2O potential more –ve, due to fluid loss, so some water re-
enters the capillaries at the venule end by osmosis, known as the
osmotic pressure.
Hydrostatic > Osmotic (Pressure)

12. Reabsorption of Tissue
Fluid
Loss of fluid from capillaries reduces hydrostatic pressure
Osmotic pressure increases due to larger H2O potential
gradient, so more H2O enters the capillaries by osmosis.
Excess tissue fluid drains into the lymphatic system (which
puts it back into the circulatory system).
Osmotic > Hydrostatic (Pressure)

13. Plant Gas Exchange
Plants need: CO2 for photosynthesis to produce own food (Autotrophs)
O2 for respiration to produce ATP (energy compound)
Disproportionate cell wall thickness when guard cell is turgid opens the
stoma.
At night no photosynthesis so guard cells become flaccid closing the
stoma
Gas Breathed
In
Breathed
Out
CO2 0.04% 4%
N2 79% 79%
O2 20% 16%
Other
s
<1% <1%
Waxy Cuticle
Upper Epidermis
Palisade Layer
Number of
Chloroplast
s
Airy Cells,
lots of
space Spongy Mesophyll
Lower Epidermis
Substomat
al Cavity
Guard Cell
Contains chloroplasts –
photosynthesis –
glucose produced –
more –ve H20 potential
– more water into cell by
osmosis

14. Blood Vessels
Arteries:
Carry O2 blood away from heart
Pulmonary Arteries carry deO2
blood to lungs
Thick & muscular walls
Folded lining to be able to expand
Capillaries:
Link arterioles to veins
Found near exchange tissues
One cell thick
Lots of them so a high SA created
Veins:
Carry deO2 blood to the heart
Little muscle or elastic
Wide lumen , low pressure
Contain valves to stop counter
flow
Pulmonary veins carry O2 blood
to heart from lungs
Arterioles:
Smaller arteries

15. Gas Exchange in Humans
RBC – Large SA:V ratio, no nucleus, more room for Hb
Hb made from 2 polypeptides with Fe ion in centre (prosthetic group)
Hb has 4 binding sites for O2 to bind onto
Hb high affinity for O2 – oxyhaemoglobin (HbO8 when saturated)
Hb higher affinity for CO2 than O2 – when binded to CO2 won’t release
O2 binds to Hb in high PO2 and unloads when there is a low PO2
Bohr Effect
When cells respire they produce CO2, increasing the PCO2
This increases the rate of O2 unloading
2+
Hb – Quaternary Protein

16. Hb Saturation Graphs
Bohr Effect

17. Gas Exchange in Animals
Frog: moist skin & basic lungs – moist skin to avoid gas bubbling in blood (the
bends)
Arthropods:
Spiracles allow large SA & limited H2O loss.
Tracheoles carry gas to every cell.
Tracheole walls made of Keratin, very strong.
When insect moves it squeezes tubes pushing gases to the cells.
Moist tracheole tip to allow quick gas diffusion.
Contain air sacs in Haemocoel (blood storage).
Fish:
Gill flap (Operculum)
Water forced over gills in opposite direction to blood (counter current multiplier)
Gill filaments stacked closely on the gill bar (high SA). Blood vessels flow
through gill bar.
Large Surface Area : Volume
Ratio in smaller animals

18. Classification
Prokaryotes – No organelles
Eukaryotes – With a nucleus
Kingdoms: Animalia, Plantae, Fungii, Monera (Bacteria), Proctoctista (if
not sure), Viruses
Kingdom, Phylum, Class, Order, Family, Genus, Species
Taxonomy – shared characteristics – reducing group size – each more
similar
A species is a group of similar organisms able to reproduce to give fertile
offspring

19. Phylogenetics
On DNA & Protein Sequences
Protein Sequences:
Look at common protein e.g. Haemoglobin of both
Compare primary structure and count number of differences
e.g. 1 difference between us & gorilla
25 differences between us & cow
DNA Sequencing:
Order of nucleotides, to real any mutations hidden by
degenerate code
DNA-DNA Hybridisation:
Heat mixture to break strands apart. Then
allowed to re-bond with other strands. Heated 1
degree at a time until bonds break. Higher temp
= more related
Α Β
Anneal –
Where there is
complementar
y pairing

20. Behaviour of Organisms
Alarm Signal – higher chances of survival
Territorial Behaviour – males establish territories for breeding
rights and control of food
Courtship – reduces interbreeding and DNA wastage, show they
are ready to mate
Social Behaviour – usually developed from parents

21. Antibiotic Resistance &
Bacterial Interactions
Antibiotic – a chemical that only affects bacteria
Either – Bacteriocidal – kill bacteria
or – Bacteriostatic – holds bacteria, stops them multiplying
This can lead to osmotic lysis when the cell wall is weakened so the
pressure is too great and the cell bursts
Antibiotics tend to be produced by fungi
e.g. Penicillin from Penicillium – weaken cell wall (affects glycoproteins)
aka Lysis
Bacteria can swap DNA so they can pass on resistant DNA
Bacteria have a higher rate of mutation because they multiply more
often.

22. Gene Transfer in Bacteria
Vertical:
Bacteria reproduce asexually, creating an exact copy of the parent
So the parent passes on the resistant DNA
Conjugation – Horizontal:
Plasmid copies itself, it passes into another bacterium
Requires cell contact, creating a Pilus (tube)
Usually occurs between related species
Transposon, is a large piece of DNA other than a plasmid
Transduction:
Using a Phage, which takes up DNA from host
If phage doesn’t kill the host cell then the host receives the phage’s DNA
Quite Large
Bacterium
Phage

23. Passage of Water through a
PlantWater and mineral ions absorbed by root hairs. They have a large SA and thin surfacelayer.
Soil has less –ve water potential, water moves by osmosis into root.
Water moves through root cells by the apoplastic and symplastic pathway
Symplastic Pathway:
Water travels through cytoplasm. Through small gaps, Plasmodesmata. Continuouscolumn of
cytoplasm. Each cell will have a less –ve H2O potential when water enters it. Sowater will
move into the next cell along.
Apoplastic Pathway:
Water travels through the cell walls. Due to cohesive properties of water. Cellulosecell walls
have water-filled spaces, reducing resistance. When water reaches the endodermiscells a
Casparian strip pushes water in cytoplasm. (waterproof barrier)Apoplast
Symplast
Endodermis
Phloem
Xylem
Root Hair
Cortex
Epidermis
Casparian Strip
Plasmodesma

24. Passage of Water through the
XylemEndodermal cells actively transport alts into xylem (using energy and proteins).
Water potential of xylem more –ve, so water moves into xylem by osmosis
This creates root pressure
Evidence for Root Pressure:
Pressure increases with temp rise
Pressure decreases with metabolic inhibitors (e.g. cyanide, stops cellsrespiring)
Pressure decreases with lack of O2 & respiratory substrates
Movement of water up stem:
Water evaporates from stomata pulling more water up xylem (transpiration)
Supported by root pressure & cohesion-tension theory
Cohesion-Tension Theory:
Water evaporates from stomata by transpiration. H-bonds stick H2Omolecules
together. Forming a continuous column. So as some water evaporatesmore water is
pulled up the xylem. Transpiration puts xylem under –ve tension.
Evidence for CT Theory:
Diameter of tree trunks smaller during day (due to –ve tension).
Xylem broken, air enters xylem so continuous column broken.
Xylem broken, water doesn’t leak out as its under tension

25. Factors Affecting Transpiration
Light:
Stomata allows CO2 into leaf needed for photosynthesis, so stoma need to be open,
when open water is lost from leaf. Increased light = increased transpiration.
Temperature:
Increased temp = increased kinetic energy = increased movement = increased evaporation =
increased transpiration
Humidity:
Number of water molecules in air. Increased humidity = decreased concentration gradient =
decreases transpiration
Air Movement:
Water accumulates around stomata once evaporated. Wind blows the water away = increasing
water potential gradient = increased transpiration
Transpiration
Ensures all material are moved around the plant dissolved in water (e.g. sugars, mineral ions
etc…)

26. Xerophytic Plants
Thick Cuticle:
Less water evaporates
Rolled Leaves
Traps moist air, air saturated with water. No water potential gradient
Hairy Leaves
Traps moist air next to leaves
Stomata in Pits:
Traps moist air next to leaf
Reduced SA:Vol
Slower diffusion rate, so water loss reduced, needs to balanced against plant’s need
to PS
Reduction in Air Flow:
Small hairs reducing water loss, by breaking up air flow

27. Size & Surface Area
Exchanged between organisms & environment: Respiratory Gases, Nutrients,
Excretory Products, Heat by Osmosis, Diffusion & Active Transport
Small organisms = Large SA:Vol
Allows efficient exchange across their body
As organisms size increases = SA:Vol decreases
Takes too long to diffuse gases into middle of organism
These organisms have adapted:
A flattened shape so all cells are nearer the surface
Specialised exchange surfaces with large SA:Vol e.g. Lungs
Features of Specialised Exchange Surface:
Large SA:Vol, very thin, partially permeable, movement of internal & environmental
medium – maintaining a concentration gradient
Fick’s Law: Diffusion Rate = (SA x Conc Grad)
Thickness of Exchange Surface

28. Biodiversity
Biodiversity:
The number & variety of living organisms in a particular area
Species Diversity:
Describes a community in terms of the number of different species present & the
number of organisms in each species
Measuring Species Diversity:
Allows us to assess the diversity in a community, using species diversity index
N = Total number of individuals of all species present in the community
n = Total number of individuals of each individual species
D = N(N – 1)
Total n(n – 1)

29. More Biodiversity
Lower Value:
Unfavourable (e.g. desert), fewer species present and in smaller populations.
Abiotic factors affect which species are present e.g. rainfall, temperature
Ecosystems are usually unstable in these conditions
Higher Value:
Favourable (e.g. rainforest), many species present and in larger populations. Biotic
factors affect which species are present e.g. competition
Ecosystems are usually stable
Why don’t we just count the number of species present:
Species diversity measures both number of species & number of individuals
Doesn’t take into account that some species will have very small populations and
be very rare

30. Species Diversity & Human
Activities
Impact of Agriculture:
Chosen by humans and only certain species are chosen to grow. To be economical
large areas of land are need to grow the species. This means less land is available
for other species. Therefore there is more competition. So fewer species survive.
Pesticides also increase competition as the crops grown by farmers will be of better
health etc..
Deforestation:
Deforestation is the permanent removal of forests for land to be used for farming
etc…
This reduces the number of habitats present to organisms. Lowering the species
diversity.

31. Glucose:
Forms a Glycosidic link between 2 glucose units (condensation reaction)
Starch:
Energy storage in plants, polysaccharide of alpha-glucose. Two type of alpha-
glucose:
Amylose: contains 1,4 – glycosidic bonds, unbranched chain
Amylopectin: contains 1,4 &1,6 – glycosidic bonds, branched chain
How is is structure related to its function?
Compact shape – lots can be stored in a small space
Easily Hydrolysed – Glucose rapidly available for respiration
Insoluble – no osmotic effect on cells & stays in cells (for respiration)
Test = add Iodine Brown – Black +ve
Glycogen:
Energy storage in animals, polysaccharide of alpha-glucose, similar to amylopectin
but more branched. More easily hydrolysed than starch due to more branches
Glucose, Starch & Glycogen

32. Cellulose
Found in plant cell walls, providing strength
Polysaccharide of beta-glucose
To bond 2 beta units, alternate units are upside down
Cellulose contains 1,4 – Glycosidic bonds, unbranched chain
H-bonds form between chains, forming Microfibrils
Microfibrils then come together to form Cellulose fibres, overlapping to give
strength
Cellulose is permeable due to gaps between fibres

33. Plant Cell
Palisade Cell:
Absorb light for photosynthesis
Adaptations for it’s function:
Lots of chloroplasts – more light absorbed = more PS
Long & thin – large SA = more PS
Vacuole pushes chloroplasts to edge of cell – less distance for light to travel = more PS
Chloroplasts:
Contain chlorophyll – absorbs light – for PS
Converts light energy into chemical (in the form of glucose)
Inner & Outer Membrane – controls what enters & leaves the cell
Thylakoid – 1st stage of PS takes place here
Stroma – 2nd stage of PS takes place here
Granum – Large SA for 1st stage of PS
Starch Grain – Energy storage of alpha–glucose
Stroma Starch GrainThylakoid
Granum
Inner &
Outer
membran
e

34. More Plant Cell Stuff
Cellulose cell wall:
Middle lamellae cements plant cells together, increasing stability
Vacuole:
Stores cell sap (water, glucose, salts, amino acids)
Vacuole membrane called the Tonoplast
Xylem:
Thick cell walls, when matured they contain lignin
End of cells break down creating a vessel
Xylem made up of dead cells
Plant Cells Animal Cells
Cellulose Cell Wall & Cell –
Surface Membrane
Cell – Surface Membrane
Chloroplasts (in most cells) No Chloroplasts
Large, Central Vacuole filled
with Cell Sap
Small Vacuole Scattered
through Cell
Starch Grains store energy Glycogen Granules store energy
Lignin

35. Variation & Sampling
Variation:
Exists between members of the same species
Similarities & differences are a result of genetics, environmental factors or both
Genetic Variation:
Due to Mutations – changes to DNA
Due to Meiosis – cross-over & independent assortment
Fusion of Gametes – Offspring inherit mixed characteristics from parents
– Which gamete fuses at fertilisation is random
Environmental Variation:
Climatic Changes e.g. temp, rain, sunlight
Soil Conditions e.g. type of soil, nutrient availability
Food Availability
pH
Genetic & Environmental Combined:
Most cases involve both factors
Hard to distinguish between the effects of the factors
Any conclusions drawn are usually tentative (not clear) and should be treated with
caution
Discontinuous Variation:
Only due to genetic factors (there are few distinct groups e.g. blood group A, B, AB,
O)
Usually controlled by a single gene (little or no environmental influence)
Continuous Variation:
Due to genetic & environmental factors (characteristics merge together, forming a
continuum)
Controlled by many genes (polygenic), environmental factors have a large influence

36. Selective Breeding
It involves humans selecting certain plants or animals that have the
desired characteristics (e.g. high-yield)
This reduced the genetic diversity in some populations
Once an organism has the desired characteristics, only that type of
organism will be produced
So nearly all the population have similar alleles
So they are more susceptible to diseases

37. Interpreting Normal Distribution
Mean:
Measurement of the maximum height of the curve
The mean of a sample provides an average useful when comparing
No information about the range though
Standard Deviation:
Measure of the width of the curve
Gives an indication of the range of values either side of the mean
Distance from mean to point of inflexion (where curve changes from convex to
concave)
SD better than Range:
Spread around the mean instead of the highest lowest values
Not influences by single outlier
Allows statistical tests to be carried out