Structural Properties of The DNA

BIOLOGY NOTES
SCASA ELABORATION #1
 the structural properties of the DNA molecule, including nucleotide
composition and pairing and the hydrogen bonds between strands
of DNA, allow for replication.
STRUCTURE/ PROPERTIES OF DNA
 DNA stands for Deoxyribonucliecacid.
 It is made up of molecules called nucleotides.
 DNA is a molecule that contains instructions an organism needs to
develop, live and reproduce. Production of Proteins.
 These instructions are found everywhere, and are passed from
parents to children.
 DNA consists of two strands in the shape of double helix/ ladder.
Attached to it are nucleotides to form two long strands that spiral
creating a double helix.
NUCLEOTIDE COMPOSITION
 Nucleotides are made up of sugar (deoxyribose), a phosphate group
and nitrogenous base.
 Nitrogenous bases are Adenine, Thymine, Guanine and Cytosine.
 Adenine pairs with Thymine and Guanine pairs with Cytosine.
 The phosphate and sugar molecules are sides of double helix and
the rungs are made up of nitrogenous bases.
 Amount of guanine is the same for cytosine and the same amount
for adenine and thymine.
 Called the complementary pairs – guanine always hydrogen bond
with cytosine and adenine always hydrogen bond with thymine.
These pairing help produce 3D helical structure of DNA.
ANTIPARALLEL
 The two strands run in opposite directions to each other and are
called antiparallel and twisted into a double helix.
 Nucleotides on opposite strands pair.
SUGAR PHOSPHATE BACKBONE
 Each phosphate group is attached to two sugar molecules by ester
bonds and is called a phosphodiester bond. The five carbon atoms in
sugar molecules which form a ring are numbered 1 to 5.
 One ester bond is formed from 3 carbon and another from 5 carbon
of the next sugar ring. The chain of alternating sugar molecules and
phosphate groups is called sugar phosphate backbone.
 DNA synthesis occurs in 5 to 3 directions.
HYDROGEN BONDS

 The base pairs on the rungs of the ladder held together by hydrogen
bonds.
 The hydrogen bonds break during DNA replication.
 Adenine with Thymine shares two hydrogen bonds while Guanine
with Cytosine shares three hydrogen bonds.
FEATURES OF DNA
 DNA is able to encode large amount of information (function)
 DNA is chemically stable
 It is able to accurately replicate itself
 it controls and directs protein synthesis (function)
 Occasional mutations occur.
DNA REPLICATION
 Is the process by which DNA makes a copy of itself during cell
division.
 The purpose is to duplicate the code it carries. The code can then be
passed to daughter cells. For preparation of mitosis and meiosis
processes.
 The antiparallel nature of DNA and the direction that DNA
polymerase functions (they add free nucleotides in a 5'-3' direction)
influences how the leading and lagging strands are replicated.
 Occurs in the S phase of Interphase during Cell Cycle
SEMI - CONSERVATIVE
 Consists of one parental strand and one new strand. It is one of two
strands conserved or retained from generation to next while the
other is a new strand.
DNA PROCESS
 The first step is to unwind (unzip) the double helix structure of the
DNA molecule.
 Each of two strands is copied, acting as a template, becoming half
of the new DNA molecule.
 New strand is complementary to original strand. A - T AND C - G.
 It starts with an enzyme called DNA helicase unzipping the long
molecule of double stranded DNA by breaking the weak bonds
between nucleotides, exposing nucleotide bases.
 Hydrogen bonds holding two strands of DNA together are weak and
the enzyme is able to separate them easily.
 This separation creates a ‘Y’ shape called a replication fork.
+Replication FORK is the junction between unwounded single strand
and intact double helix. It moves along parental DNA strands, so
that it can unwind the parental strands.
 One of the strands is oriented in the 3’ to 5’ direction (towards the
replication fork), this is the leading strand. The other strand is
oriented in the 5’ to 3’ direction (away from the replication fork),

this is the lagging strand. As a result of their different orientations,
the two strands are replicated differently:
 Once all of the bases are matched up (A with T, C with G), an
enzyme called exonuclease strips away the primer(s). The gaps
where the primer(s) where are then filled by yet more
complementary nucleotides.
 The new strand is proofread to make sure there are no mistakes in
the new DNA sequence.
 Finally, an enzyme called DNA ligase? seals up the sequence of DNA
into two continuous double strands.
 The result of DNA replication is two DNA molecules consisting of one
new and one old chain of nucleotides. This is why DNA replication is
described as semi-conservative, half of the chain is part of the
original DNA molecule, half is brand new.
 Following replication the new DNA automatically winds up into a
double helix.
 A short piece of RNA ?called a primer? (produced by an enzyme
called primase) comes along and binds to the end of the leading
strand. The primer acts as the starting point for DNA synthesis.
 DNA polymerase? binds to the leading strand and then ‘walks’
along it, adding new complementary? nucleotide?bases (A, C, G
and T) to the strand of DNA in the 5’ to 3’ direction.
 This sort of replication is called continuous.
4. Lagging strand:
5. Numerous RNA primers are made by the primase enzyme and bind
at various points along the lagging strand.

6. Chunks of DNA, called Okazaki fragments, are then added to the
lagging strand also in the 5’ to 3’ direction.
7. This type of replication is called discontinuous as the Okazaki
fragments will need to be joined up later.
SCSA Elaboration
 the genetic code is a base triplet code; genes include ‘coding’ and
‘non-coding’ DNA, and many genes contain information for protein
production
 Proteins, including enzymes and structural proteins, are essential to
cell structure and functioning
Protein Synthesis
Genome: it is a complete set of genetic instructions for an organism.
The genome of an organism is composed of coding and non-
coding DNA.
Describe the function of each.
 NON CODING DNA
→ they do not code for amino acids
→ most of them lies between genes on the chromosome
→ plays in the role of gene regulation.
→ does not provide instruction for the production of proteins
 CODING DNA
G
Relationship between Genetic Code and Protein
 A gene is a basic unit of heredity in a living organisms that resides
in the long stranded DNA called chromosomes
Functions of Proteins
 It helps repair and build your body's tissues, allows metabolic
reactions to take place and coordinates bodily functions. In addition
to providing your body with a structural framework, proteins also
maintain proper pH and fluid balance.
→ growth and maintenance

→ causes biochemical reaction
Types of Mutation: Gene Mutation
Gene Mutation: is where one gene's mutates could possibly affect the proteins produced.
Point Mutation: is a type of gene mutation that changes the base sequence on one gene and
can form a new allele if not a silent mutation.
Single Nucleotide Polymorphism: a single nucleotide difference that occurs at a given
position in the genomes of two or more individuals.
3 types of substitution mutation
 Silent: a mutation in which the DNA codon for one amino acids becomes another
DNA codon for the same amino acids.
 Missense: a gene mutation that results in one amino acid being replaced by another
amino acid in the encoded protein.
 Nonsense: a mutation in which a codon for an amino acid is changed to one that
codes for a stop codon, terminating translation.
Deletion and Insertion
BIOLOGY - MUTATION
SCSA ELABORATION
 Mutations in genes and chromosomes can result from errors in DNA replication or
cell division, or from damage by physical or chemical factors in the environment
Mutation: a permanent change in the DNA sequence of a gene, an only source of new
alleles in a population’s gene pool; the process of generating a mutation.
Distinguish between Somatic and Germ - Line Cells
SOMATIC CELLS GERM LINE CELLS
 Are any cells that are not involved in the
production of gametes.
 Cells that create reproductive
cells or gametes.
 Arranged into different types of tissues in
body of multicellular organisms,
 Produce male and female
gametes to participate in sexual

performing specific functions reproduction.
 Undergoes Mitosis  Undergoes Meiosis
 Mutation may not pass through the
generations and have no effect on
evolution
 Mutation passes through the
generation having an effect on
evolution.
How can mutation be passed onto offspring?
 A mutation can change the genetic sequence. Some mutations are hereditary
because they are passed down to offspring from a parent carrying a mutation through
the germ line - an egg or sperm carrying mutations. Mutation can affect sex cells
called gametes and can be inherited and incorporated into every cell of offspring.
BIOTECHNOLOGY
 Describes the use of living things to make new products
E.g. Golden Rice, GM Cotton
 Genetic Engineering: refers to the process in changing the genetic
sequence of an organism using modern biotechnological

techniques.
 Because scientists in biotechnology use living things to create new products,
these new products are referred to as a genetically modified organism or
transgenic organism.
 Transgenic refers to DNA sequences that contain foreign DNA artificially
introduced.
 Transgenic Organism (genetically modified): is an organism that has been
altered through recombinant DNA technology which involves either the
combining of DNA from different genomes or the insertion of foreign DNA into
a genome.
WHY DO THEY DO GENETIC ENGINEERING?
 In biotechnology, Genetic Engineering is used to overcome a problem.
E.g. an insect grazing that affects crop production (output) or deficiency of
essential nutrients in a population.

 Used to enhance or modify the characteristics of an individual
organism.
Gene of Interest/ Target Sequence
 If scientists identify genes of interest from one species that we want to
introduce into the DNA sequence of another species, we use Restriction
Enzymes to cut the gene of interest/ target sequence.
Restriction Enzymes
 Enzymes are Proteins - speeds up the rate of all the chemical reactions that
take place in the cells.
 Restriction enzymes cleave or cut the gene of interest.
 Originally, it was sourced from bacteria but now it can be made in a lab.
 Restriction enzymes to cut/ cleave DNA molecules on Recognition sites.
 Recognition sites are DNA sequences (4-8 base pairs)
 There are different restriction enzymes - at least 400 different types
 Each enzyme is specific to a recognition sites
HOW DO RESTRICTION ENZYME CLEAVE/ CUT?

There are two ways in which restriction enzymes cuts/ cleaves into DNA sequence at
recognition sites to form
 Blunt ends
 Sticky ends - staggered ends to produce overhang or sticky ends.
What happens after the genes of interest are
isolated?

 The gene of interest (now that has been isolated) can be recombined into a
DNA sequence of the vector

 This vector is another species, different to where the genes(s) was isolated
from.
 Bacterial plasmids are commonly used vectors.
Vectors and Plasmid Vectors
 A vector is a tool/ vehicle that carries the gene of interest into the target
organism.
 In transgenic organisms (e.g. creating golden rice), the plasmids sourced from
bacteria is the vector that carries the gene of interest - the beta - carotene
precursor of Vitamin A.
 There are other sources of vectors other than the plasmids.
Vector - Viral Vectors
 Viruses are obligate intracellular parasites
 Viruses need a host to replicate their viral proteins. They lack the protein
making machinery.
 They have the ability to insert their genetic information into the host.
 By using recombinant DNA techniques, it is possible to insert desired genes
into viral DNA or RNA.
 Then use the virus to insert this new gene to target cells.
Vectors - Liposome Vectors
 Liposome vectors are small spheres surrounded by a membrane composed of
a phospholipid bilayer.
 •The liposomes can be artificially made to
 •carry the gene of interest along with
 •specific molecules attached to its surface.
 Inserted into foreign organism
 These molecules can be recognised by the target cell and the liposome fuses
into the membrane of the target cell – containing gene of interest
 This technique is used extensively to insert foreign DNA into cells cultured
in petri dishes.

Process of DNA ligation - annealing of gene of interest
•The vector has been cleave/cut by the same restriction enzyme.
•We insert/anneal the gene(s) of interest/target sequence in the Vector ligate
into a DNA sequence, DNA ligase (another type of enzyme) is used.

•Once it is recombined into another DNA sequence of a different species,
Recombinant plasmid DNA is produced.
DNA SEQUENCING (SANGER METHOD)
 It refers to the methods and technologies used to determine the
order of the nucleotides bases in DNA molecules : adenine,
guanine, cytosine and thymine. The genome (the entire DNA
sequence of an organism) can be determined.
 The method and technologies used are PCR and Gel
Electrophoresis
 Scientists cut DNA into fragments to sequence one section at a
time. The entire set can then be put together to create a whole
genome.
 The genome of thousands of species have been sequenced,
allowing genomes and genes to be compared.
 Knowing the sequences can help scientists determine the genetic
code for particular phenotypes. There may be survival benefits in
identifying, for example genes that increase drought resistance or
salt tolerance.
 Sequencing genes of different species has assisted scientists in
determining genetic relatedness and evolutionary links.
PCR
 Thermal cycler
 Free nucleotides
 Primers (radioactive
 DNA polymerase
DNA SEQUENCE
 Thermal cycle
 Free Nucleotide
 Primers
 DNA Polymerase
 Terminator - modified nucleotides - dideoxynucleotides.
 Short fragment - start - travels further because it is light weight.
DNA PROFILING (SEQUENCING)

 Summary
1. Identify and Isolate DNA : short tandem repeats (STRs)
2. PCR - once STR’s are isolated - (amplify) make multiple copies of
STR.
3. Gel Electrophoresis: load amplified sample and separate
fragments.
4. Analyse and draw conclusions.
STR - Short Tandem Repeat
 They are non-coding DNA (don’t code for proteins)
 Highly variable between individuals of the same species.
 There are multiple loci regions of STR’s on the human genome.
 DNA sequence 3-5 base pairs (bp) in length
 They are repeated.
Process of DNA profiling
1. DNA samples are collected, processed and DNA is extracted from
individuals.
2. Primers for each location are added.
- Primers are flanked to the DNA to be amplified to allow DNA
polymerase to initiate elongation.
-> Multiple locus is used, therefore multiple primers would be added - this
increases in the accuracy of DNA profiling.
E.g. if four loci are used, there would be 8 different primers.
3. The DNA and fluorescent primers are run through the polymerase chain
reaction (PCR) to amplify the targeted STR region on the DNA.
4. The amplified DNA in a sample is separated by electrophoresis in a
genetic analyzer
5. DNA fragments (negative charge) move through the gel tube by size,
smallest first
6. Results are analysed
GEL ELECTROPHORESIS
 The main purpose of a gel electrophoresis is to separate a large sample of
DNA fragments of various sizes
 Various components of a gel electrophoresis include
 DNA sample (previously underwent PCR)
 Agarose Gel (Made of agar jelly)
 Positive and negative terminal on either side of the Gel electrophoresis
 Wells to place DNA sample
 One well is used for “markers” that contain fragments of known size so it is
possible to det

STEPS
 First a gel is prepared. E.g. Agarose gel similar to hard jelly.
 This consistency offers resistance to the pieces of DNA as they try to
move through the gel.
 The gel is prepared with wells at one end so that DNA samples can be
loaded into the gel wells.
 DNA is negatively charged due to the phosphate structure. The wells
are one side with a negative electrode, and at the other end is a positive
electrode.
 As the piece of DNA moves through the gel, they will meet with
resistance.
 Thus, larger fragments will move slower than smaller fragments. This
allows the separation of the different size of DNA fragments.
Application to DNA profiling
 Animal Identification
 Animal breeders to accurately identify parentage of offspring.
 Captive Breeding Programs
 •If a particular species is endangered in the wild and aim is to
increase genetic variation in the population.
 •Zoos undertake captive breeding programs
 •They select suitable individuals. Individuals selected are
genetically different and come from different parentages.
 •They breed and offspring eventually reintroduced back into
the wild
 Forensics
 Victim identification in mass disasters.
 Crime Scenes
 Evolutionary Studies
 Determining the relatedness of species
 Similarity in branding pattern
 Parentage
 Identify who the parents are through DNA sequencing.
Ethics with Biotechnology

Artificial Selection: (Selective Breeding) - requires human intervention
Certain desirable traits are selected (by humans) - selective breeding
 Once selected the parental stock are mate
 Desirable traits are often passed on to the offspring.
 The offsprings are continually mated
 Over the time this changes the allele frequency
Comparing Artificial Selection ( Selective Breeding) with Genetically
modified (transgenics) organisms.
Similarities
 Both involve human intervention
 Both achieve to have desirable traits in the organisms
 Both processes changes the allele frequency over time (how common
an allele is in a gene pool)
Differences
 Artificial selection involves one species, whilst transgenic organisms
contain genes from more than one species.
 Transgenic organisms create new traits/ phenotypes from the foreign
9from other species) genes that were inserted in host organisms,
compared with Artificial selection selects for genes (traits) already
present in a species.
 Artificial selection involves crossing ideal candidates in the hope to
produce the ideal phenotype in offspring, this takes time and success
won’t occur until a few generations and with each generation, will
involve reproduction, gestation (animals) and growth of offspring to see
whether the ideal phenotype was a success. This is a longer process.
Whilst with creating a transgenic organism gene intervention leads onto
the ideal phenotypes this is a much shorter process compared to
artificial selection.
Applications of Genetically Modified Organisms/ Genetic Engineering -
Farming
Farming could be:
 Genetically engineering salmon to increase in size - food security -
more food, more output
 Genetically modifying wheat plants to produce more wheat per plant -
food security

 Genetically modifying rice - golden rice (beta - carotene precursor for
Vitamin A) health benefits
 Genetically modifying agricultural plants to increase drought tolerance
(water scarcity)
 Genetically modifying plants to be produce a protein that is herbicide
resistant to a common insect ( using less chemicals)
FOR AND AGAINST BIOTECHNOLOGY
 For: Genetic Engineering has existed for many years with farmers
selectively breeding (artificial selection)
COMPARING AMINO ACID SEQUENCES
1a. What is cytochrome c?
Cytochrome C is a respiratory protein found in all the aerobic species and it codes is part of
mtDNA. It consists of a chain of around 100 amino acids with slight difference with
substituted amino acid at some places.
Heme - protein attached with inner mitochondrial membrane. They are highly water soluble,
redox and electron transfer in complex.
1b. Which Organisms have cytochrome c?
Mitochondria Inner Membrane have cytochrome c - aerobic organisms - respiration
1c. Why is cytochrome c useful to show evolution?
As the protein is highly conserved in its protein structure in different species, they are
compared to work out the relationship between different species. The degree of similarity in
the DNA sequence of cytochrome c of two organisms show the closeness of their
relationship, hence is indication of the time distance to a common ancestor.
Eg. Organisms with high degree of similarity would have a recent common ancestor, while
organisms with many differences would have had a more distant common ancestor.
 Cytochrome C is a protein common in all aerobic species and therefore, it is used in
evolutionary relationships. Difference is amino acid sequences of cytochrome C, can
be used to establish relationships. Least differences / similar amino acid sequence
closely related or common ancestor is recent.
 Less closely related and their common ancestor is distanced.
2. Use Table 79.1 to compare the relationship between humans, the pig and the tuna.
 If there is less difference, more closely related and common ancestor recent
 There are 10 difference with pig amino acids. than with human and tuna having 21
amino acids.
3. Describe DNA-DNA hybridization?

DNA-DNA hybridisation is a technique where double stranded DNA is heated so that
nitrogenous base pairs in DNA are broken to produce single strands of DNA. When these
single strands cool, they reassociate into double strands. If single strands from different
species are mixed, they will join to form a hybrid double stranded DNA. In this hybrid DNA,
there will be mismatches in the base pairs. The more closely related the species, the fewer
the mismatches.
CHAPTER 8 EVOLUTIONS - BIOLOGY
 Evolution is the mechanism that drives species to change/ evolve over time. The
theory of evolutions of life takes millions of years ago (mya)
Evidence for theory of evolutions includes:
Fossils are important evidence for evolution because they show that life on earth was once
different from life found on earth today. ... Paleontologists can determine the age of fossils
using methods like radiometric dating and categorize them to determine the evolutionary
relationships between organisms.
 Fossils tell us when organisms lived, as well as provide evidence for the progression
and evolution of life on earth over millions of years.
 Fossils are the preserved remains or traces of animals, plants, and other organisms
from the past.
 fossil record: All discovered and undiscovered fossils and their placement in rock
formations and sedimentary layers.
 The development of radiometric dating techniques in the early 20th century allowed
geologists to determine the numerical or “absolute” age of various strata and their
included fossils.
Evidence for Evolution
Fossils provide solid evidence that organisms from the past are not the same as those found
today; fossils show a progression of evolution. Fossils, along with the comparative anatomy
of present-day organisms, constitute the morphological, or anatomical, record. By
comparing the anatomies of both modern and extinct species, paleontologists can infer the
lineages of those species. This approach is most successful for organisms that had hard body
parts, such as shells, bones or teeth. The resulting fossil record tells the story of the past and
shows the evolution of form over millions of years.
 Determining the ages of fossils is an important step in mapping out how life
evolved across geologic time.
 Scientists use carbon dating when determining the age of fossils that are less than
60,000 years old, and that are composed of organic materials such as wood or
leather.

The principle of radiocarbon dating is simple: the rates at which various radioactive
elements decay are known, and the ratio of the radioactive element to its decay products
shows how long the radioactive element has existed in the rock. This rate is represented by
the half-life, which is the time it takes for half of a sample to decay.
The half-life of carbon-14 is 5,730 years, so carbon dating is only relevant for dating fossils
less than 60,000 years old. Radioactive elements are common only in rocks with a volcanic
origin, so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash
layers. Carbon dating uses the decay of carbon-14 to estimate the age of organic materials,
such as wood and leather.
 Homology is a relationship defined between structures or DNA derived
from a common ancestor and illustrates descent from a common
ancestor.
 Analogous structures are physically (but not genetically) similar
structures that were not present the last common ancestor.
 Homology can also be partial; new structures can evolve through the
combination or parts of developmental pathways.
 Analogy may also be referred to as homoplasy, which is further divided
into parallelism, reversal, and convergence.

EVOLUTION
 Gene pool: the sum of alleles in a population in a given place and time.
 Allele frequency - how common/ frequent as allele is in gene pool.
For the gene pool to change over time through natural selection the following prepositions
occur:
1. Variation - high genetic variation in a population due to mutations (new alleles in a
gene pool) and meiosis (independent assortment and crossing over)
2. Over-population/ over-reproduction
 High numbers than an ecosystem can support (carrying capacity)
 → natural selection is happening
3. Competition for resources - individuals that have the selective advantage/
advantages allele survive or natural selection confers for the advantages allele
 The effect of natural selection
4. Reproduction and heritability - those who surviving individuals mate and pass on the
advantageous allele to offspring
YEAR 12 BIOLOGY ATAR: UNIT 3
TASK 5 - EXTENDED RESPONSE - CHAPTER 8 AND 9
Question 2
Glossary
 Mutation: a permanent change in the DNA sequence of a gene; a source of new alleles in a
population’s gene pool.

 Gene Pool: A collection of all the alleles for all the genes in the reproducing members of a
population at a given time; it is the genetic reservoir from which a population can obtain it’s
traits. (A gene pool represents the sum total of alleles for all genes present in a sexually
reproducing population)
 Gene Flow: is the movement of genes between different populations. As a result of
immigration or emigration.
 Allele Frequency: it refers to how common an allele is in a population.
 Allopatric Speciation: speciation that occurs to due physical or geographical isolation.
 Genetic Drift: A change in the gene pool of a population as a result of chance; it usually
occurs more noticeably in a small population.
 Population: a group of individuals of the same species that live in the same geographic area
and interbreed, producing fertile offspring.
 Speciation: is the evolutionary process by which populations evolve to become distinct
species.
1. Explain how new alleles in a gene pool can occur?
 New alleles arise in a gene pool through mutation, which are a key source of genetic
variation that introduces alleles into a population.
 Introducing new alleles which means new proteins, which means potentially new traits.
 The sum total of all alleles present in a population is called the gene pool.
 The more variation a population has, the better its ability to adapt to changes in its
environment through natural selection. If there is more variation, the odds are better that
there will be some alleles already present that allow organisms to survive and reproduce
effectively under the new conditions.
2. Explain how genetic variation exists in a gene pool in a population?
 Genetic variation is a natural occurring genetic difference among individuals of the same
species.
 Genetic variation exists in a gene pool in a population through:
 Mutations: is a change in the nucleotide sequence of a section of DNA coding for a specific
trait. Beneficial mutation change the sequence to create new trait (missense mutations)
 Gene Flow: the movement of alleles in a population due to immigration or emigration.
 Meiosis
 Independent Assortment: when homologous chromosomes line up in
metaphase 1, their orientation towards the opposite poles is radom.
 Crossing Over: the exchange of segments of DNA between homologous
chromosomes during prophase 1.

 Random Fertilisation: the fusion of two haploid gametes results in formation of a diploid
zygote.
3. Explain why overpopulation and mutation is important for natural selection?
Mutation is important for natural selection because they are the changes in the structure that make
up genes. Mutations can be harmful, neutral or sometimes helpful, resulting in new advantageous
traits. If the environment changes rapidly, some species may not be able to adapt fast through
natural selections. Through studying the fossil record, we know that many of the organisms that
once lived on Earth are now extinct. Dinosaurs are one example. An invasive species, a disease
organism, a catastrophic environmental change, or a highly successful predator can all contribute to
the extinction of species.
Overpopulation: One of the factors that must be present for Natural Selection is the ability of a
population to overproduce offsprings. Overproduction by definition, in biology, means that each
generation has more offspring than can be supported by the environment. Because of this,
competition takes place for limited resources. Individuals have traits that are passed down to
offspring. Some of these traits give individuals an advantage when it comes to surviving to
reproduce. The organisms with these traits are more likely to live and have offspring who will inherit
the helpful traits. Overproduction of offspring leads to competition for survival.
a stable population will inevitably outgrow its resource base, leading to competition for survival
 When there is an abundance of resources, a population will grow according to its biotic
potential (exponential J-curve)
 With more offspring, there are less resources available to other members of the population
(environmental resistance)
 This will lead to a struggle for survival and an increase in the mortality rate (causing
population growth to slow and plateau)
SPECIES CHANGE OVER TIME AND SPACE - SURVIVAL OF THE FITTEST
4. Describe the process of natural selection?
Natural Selection: is the process through which populations of living organisms adapt and change.
→ Natural selection can only occur if there is variation among members of the same species
→ Mutation, meiosis and sexual reproduction causes variation between individuals in a species
 Variation: individuals in a population are naturally variable, therefore having variation
means that some individuals are better suited to the environment than

others.
 Individuals with selective adaptive traits are more likely to survive and reproduce, resulting
in a high rate of population growth.
 Inheritance: Overtime these adaptive traits are passed onto offspring resulting in
advantageous traits being more common in the populations. Such traits are heritable,
whereas other traits are strongly influenced by environmental conditions and show weak
heritability.
 Individuals possessing traits that are well suited for the struggle for local resources will
contribute more offspring in the next generations.
 Individuals with advantageous traits adapt to the condition they are presented with,
resulting in mating and production of offspring.
VISTA
 Inherited Variation – There is genetic variation within a population which can be inherited
 Competition – There is a struggle for survival (species tend to produce more offspring than
the environment can support)
 Selection – Environmental pressures lead to differential reproduction within a population
 Adaptations – Individuals with beneficial traits will be more likely to survive and pass these
traits on to their offspring
 Evolution – Over time, there is a change in allele frequency within the population gene pool
Natural Selection also leads to speciations, where one species gives rise to new distinctly different
species. It is one of the processes that drives evolution and diversity of earth.
5. Explain how natural selection can change allele frequency of a gene pool over many
generations?
 Natural Selection can change the allele frequency of a gene pool
 Allele frequency change under directional selection favoring.

6. Describe the relationship between alleles and phenotypes in the concept of natural
selection?
 Phenotype: The central precept of natural selection is that variation exists in a population
and that because certain individuals (with certain traits) are better at staying alive, they have
a better chance of surviving and are more efficient at passing on their genes. The traits that
an individual expresses, its phenotype, are what will give that individual an advantage or
disadvantage in the struggle to survive and reproduce. Natural selection operates on
phenotype. Unequal mortality or reduced reproductive success across populations are
associated with variation in phenotypes.
GENE POOL
 A large gene pool indicates high amounts of genetic diversity, increasing the chances of
biological fitness and survival.
 A small gene pool indicates low amounts of genetic diversity, reducing biological fitness and
increasing chances of extinction
Artificial Selection: is the identification by humans of desirable traits in plants and animals, the step
taken to enhance and perpetuate those traits in the future. - Human Interference.
 They are long used methods in agriculture to produce animals and crops with desirable
traits.
 This artificial selection is more appealing to Humans, than natural selection because artificial
selection is a faster process in which it allows humans to mold organisms to their needs.
Humans have adapted the process by extracting plasmids from bacteria and using them as a vector
to deliver genes of interest
 This has allowed humans to transfer genes not only within generations, but between
separate species
 This form of evolutionary modification is a type of artificial selection as it occurs as a direct
result of human intervention

COMPARE NATURAL SELECTION WITH ARTIFICIAL SELECTION

Differences
Natural Selection:
→ natural selection fittest organism is selected naturally, able to cope and adaptable to cope with
different situations like genetic drift, weather, temperature.
→ chance of surviving increased
→ slower and longer process
→ controlled by nature
→ performed on all types of organism
Artificial Selection:
→ organisms with the desired traits are been selected and further, they are been genetically
modified with the advancing technologies evolving in biology.
→ selection of species done artificially hence chance of survival of new breeding at risk
→ faster process
→ controlled by humans
→ only processed on some selective organisms of human desires.

 Unlike natural selection, artificial selection doesn’t result in evolution or speciation.
→ Selection pressures are external agents which affect an organism’s ability to survive in a
given environment
 Selection pressures can be negative (decreases the occurrence of a trait) or positive
(increases the proportion of a trait)
 Selection pressures may not remain constant, leading to changes in what constitutes a
beneficial adaptation
Types of selection pressures include:
 Resource availability – Presence of sufficient food, habitat (shelter / territory) and mates
 Environmental conditions – Temperature, weather conditions or geographical access
 Biological factors – Predators and pathogens (diseases)

Selection pressures can be density-dependent (affected by population size) or density-independent
(unaffected by population)
Population Bottlenecks
Population bottlenecks occur when an event reduces population size by an order of magnitude (~
>50%)
 These bottlenecks may result from natural occurrences (e.g. fires, floods, etc.) or be human
induced (e.g. overhunting)
 The surviving population has less genetic variability than before and will be subject to a
higher level of genetic drift
 As the surviving members begin to repopulate, the newly developing gene pool will be
divergent to the original
 Example: Northern elephant seals have reduced genetic diversity compared with southern
seals due to overhunting
Founder Effect
The founder effect occurs when a small group breaks away from a larger population to colonise a
new territory
 As this population subset does not have the same degree of diversity as a larger population,
it is subject to more genetic drift
 Consequently, as this new colony increases in size, its gene pool will no longer bAe
representative of the original gene pool
 The founder effect differs from population bottlenecks in that the original population
remains largely intact
 Example: Certain Amish communities have a higher incidence of polydactyly because of
inter-marriage within the community
 Population bottlenecks and the founder effect will exacerbate genetic differences between
geographically isolated populations

 Respiratory protein are similar
Comparative genomics has found that they are all related and diverged from the common ancestor
via adaptive radiation. Adaptive radiation is the divergence of lineages, where a large number of
descendants arose from one common ancestor because a variety of niches became available.
Year 12 Biology
ALLOPATRIC SPECIATION leads to DIVERGENT EVOLUTION.
STEPS TOWARDS
SPECIATION (DIVERGENT
EVOLUTION)
DESCRIPTION OF STAGES
OVERPOPULATION AND
COMPETITION
· The original parent population existed in a region (millions of
years ago).
· As these were sexually reproducing organisms, there was
already genetic variation in the population –
o as a result of mutations arising from physical and
chemical mutagens that affect their germline cells
(gametes via meiosis).
· Over time population increases.
· There is a struggle for existence (increase competition) as
Resources (e.g. food, space) become limited.
· Populations gradually move to different regions.

GEOGRAPHICAL BARRIER
(Geographical Isolation)
· Over time geological movement occurs resulting in a
geographical barrier
· For example – Ocean, mountain barriers.
· This prevents gene flow (migration of individuals between the
two populations)
· Gene flow is when individuals who migrate between
populations and reproduce and pass on their alleles to
offspring.
NATURAL SELECTION · DIFFERENT Selection pressures (in this example climate), act
on the populations of horses.
o A mutation may also occur in one of the populations.
· For example, a drier climate will act against those individual
horses who are not adapted to a drier climate.
· Individuals that can survive periods of limited water access
(have the advantageous allele/selective advantage), survive
and reproduce and pass on their genes/alleles to their
offspring
· This changes the allele frequency in each gene pool of over
many generations
SPECIATION AND
REPRODUCTIVE ISOLATION
· The gene pool changes over time (millions of years).
· Speciation occurs- where the parent population evolves and
becomes two new species.
· Species A can only reproduce with individuals of their own
kind- they are said to be reproductively isolated.
· This means that if those two species interact they cannot
breed with each other
o *In cases where two species can mate and produce a
hybrid- e.g. horse and donkey produce a mule- the
mule is INFERTILE and cannot reproduce.
Things to note:
· Speciation is the formation of a new species
· Allopatric speciation formation of a new species based on the concept of a geographical
barrier (oceans, mountains) that prevents gene flow.
· Divergent Evolution – is when TWO or more SPECIES speciate from a recent common
ancestor – as shown in this example
· Natural selection can only work if there is genetic variation in a population and an
overpopulation
· Selection Pressures or Environmental Pressures are mechanisms/factors that give rise to
natural selection. Examples of selection pressures include.
o Climate influencing drought and availability of water and food resource
o a bird feeding on a particular phenotype that is easily seen

o Antibiotic resistance in bacteria. How some antibiotics become useless (antibiotic
resistant), as a mutation occurs in a bacteria and is able to pass that gene to
nearby bacteria.
o food resource accessibility – why giraffes have longer necks (from textbook).
o Founder effect. When a small population of a species, migrates from the original population (due
to overpopulation). The gene pool of this small population does not represent (random) the original
population. If there is no movement (or gene flow) between populations, the small population will
experience different (and random) selection pressures and over time (millions of years), evolve to a
new species.
Genetic Drift, Founder Effect and Genetic Bottleneck
Explain how changes to the gene pool can lead to evolution of a or many species.
- The concepts of Genetic drift, Founder Effect and Population Bottleneck are examples in which its
effect can lead to the evolution of one or many species (as the gene pool changes over time).
- When any of these are occurring, the changes are random.
- When these occur there must not be any gene flow (migration between populations and
reproducing with individuals).
- The effect of these changes are more obvious in small populations compared to large
populations
Founder effect. When a small population of a species, migrates from the original population (due to
overpopulation). The gene pool of this small population does not represent (random) the original
population. If there is no movement (or gene flow) between populations, the small population will
experience different (and random) selection pressures and over time (millions of years), evolve to a
new species.
- Genetic Bottlenecks. A natural disaster e.g. a Flood, Bushfire acts randomly on all individuals in the
population, regardless if they possess an advantageous allele or not. The surviving individuals (and
their alleles) are small in population and does not represent the gene pool of the original
population. Over time, this gene pool will experience different selection pressures and can lead to
the evolution of a new species.
- Genetic drift does not involve a disaster or a small population migrating to a new area. This is
simply the mechanism in which drives evolution. This occurs also randomly.
§ What is genetic drift? Genetic drift is change in allele frequencies in a population from generation
to generation that occurs due to chance events.
§ Over time the gene pool changes due to selection pressures that act on individuals in a population
over time.
§ The effect of genetic drift is more obvious in small populations than large populations.
J§ Genetic drift is the main driver/mechanism that leads to evolution.