- changes in physical conditions in the environment
- changes in chemical conditions in the environment
- competition for resources
An environment is any living (biotic) or non-living (abiotic) factors in the surroundings of an organism.
Changes in physical conditions in the environment.
- Physical conditions in the environment include temperature, wind and the amount of rainfall or moisture available.
- These physical conditions can change daily or over long periods of time.
- For example, a decrease in the amount of rainfall over a long period of time has caused, the drying of the Australian landmass as it has drifted north over time has caused a change in vegetation from rainforest to dry sclerophyll forests and grasslands over much of the continent.
Changes in chemical conditions in the environment.
- Changes in the chemical conditions of the environment are possible.
- For example, there has been a gradual increase in the salinity of Australian soils. Some plants have a tolerance for soils that contain high levels of salts but most do not and die. This increase in salinity has made it possible for saltbush to tolerate these conditions and thus reproduce successfully. However, some are unable to extract water from the soil. In high salt environments, selection favours species, such as saltbush, that can tolerate these conditions.
Changes in competition from resources.
- Resources are not unlimited in an environment.
- The number of offspring produced is greater than can be supported by the environment. This causes competition for survival from within species and between different species.
- If a new species is introduced into an area it must compete with the organisms already there.
- Some species introduced to Australia have outcompeted the local species, leading to extinctions of the natural population.
- For example, the European rabbit has outcompeted the bilby in most areas as the rabbit can access the resource better than the bilby and breed faster.
- palaeontology, including
- transitional forms
- comparative embryology
- comparative anatomy
- Palaeontology is the study of fossils.
- Fossils are the remains of, or evidence of past life usually found preserved in rocks. They may be the actual preserved remains, moulds, casts or impressions in rock, or may be mineralised or opalised replacements of the remains.
- The fossil record shows the history of life on earth. E.g. oldest fossil of cyanobacteria at 3.5 mya, earliest reptile 350 mya.
- Fossil record is incomplete.
- When fossils are studied the following trends emerge:
- The progression from single celled to multi cellular organisms
- Simple organ system to complex organ system
- From only aquatic to many terrestrial organisms
Transitional forms: fossils that have features that place them between different groups of organisms. They are thought have been intermediate between groups.
- Supports the theory that birds evolved from reptiles
- Example: Archaeopteryx
- Reptilian features: teeth, long tail, solid bones and claws
- Bird features: feathers and attachments for flight muscles to sternum.
- Study of the geographical distribution of organisms.
- Separation of landmasses created physical barriers, such as mountains, seas and rivers were formed.
- Physical barriers prevented organisms from different areas from interbreeding thus limiting the gene pool.
- With barriers in place, new species could be developed.
- Thus, these changes in geographical isolation and isolated populations give support for the theory of evolution.
- Example, in Australia, Africa and Madagascar, exists different species of Boabab tree. It is thought that the three species are descended from a common Gondwanan ancestor, and through isolation, they evolved after Gondwana broke up. Due to different conditions in environments, they are evolved, and thus there are differences between the three species.
- Comparison of embryos of different species at early stages of development and provides support for the theory of evolution. Embryos of vertebrates – fish, amphibians, reptiles, birds and mammals have structures that appear to be gill slits and tails (at embryo stage).
- Gill slits will develop into gills for fish and in humans they develop into part of the Eustachian tubes in the ear.
- (diagram of structure of embryos)
Comparative anatomy (Perform a first-hand investigation or gather information from secondary sources (including photographs/diagrams/models) to observe, analyse and compare the structure of a range of vertebrate forelimbs)
- Some organisms have similar anatomy or structures which can be seen as evidence of evolution from a common ancestor.
- Structures that share common features are called homologous structures. They have basic similarity regardless of function.
- Example: The pentadactyl (five fingered) limbs of vertebrates e.g. bats and humans.
Organisms that share a common ancestry also share the same basic chemical building blocks e.g. DNA< RNA, proteins and haemoglobins.[/wpsm_accordion_section] [wpsm_accordion_section title=”1.3. Explain how Darwin/Wallace’s theory of evolution by natural selection and isolation accounts for divergent evolution and convergent evolution.”]
- Divergent evolution:
- Occurs when closely related species experience quite different environments and as a result vastly different characteristics will be selected.
- The species, over time, will evolve differently and will eventually appear quite different.
- Convergent evolution:
- occurs when two relatively unrelated species develop similar structures, physiology or behaviours in response to similar selective pressures from similar environments.
- For example, dolphins (mammals) and sharks (cartilaginous fish) have evolved a streamlined body shape and fins that enable them to move efficiently through their aquatic environment, yet they are only remotely related as vertebrates.
- Adaptive radiation refers to the variety of different species that evolve from an ancestral line as a result of migration and isolation.
- The Darwin/Wallace theory of natural selection and isolation provides a mechanism for adaptive radiation.
- If groups of a population become isolated, the chances are high that they will encounter differing selective pressure as each environment evolves independently.
- Eventually, if the differences are great enough, the groups may not be able to interbreed to produce fertile offspring, i.e. new species have developed.
Thus, adaptive radiation (natural selection and isolation) can lead to changes that can cause either divergence from a common ancestor or convergence so that different species have superficialsimilarities.
[/wpsm_accordion_section] [wpsm_accordion_section title=”1.5 Analyse information from secondary sources to prepare a case study to show how an environmental change can lead to changes in a species.”]
Case Study: Peppered Moths
Forests on outskirts of England –
- Forests of England had light-coloured bark
- There was variation within the moth species; black and white.
- Pre-industrial revolution:
- The forests were pale in colour. Hence, the white moths were camouflaged and the black moths were easily seen by predators.
- Increase in white moth population and decrease in black moth population
After Industrial revolution:
- Selecting agent: pollution
- Forests became dark due to the depositing of soot on trees:
- Black moths hard to see which increased in population
- White moths more easily seen which decreased in population
- Technology: DNA-DNA hybridisation
- Recent technological advances have allowed comparison of chemicals in organisms. Species that share a recent common ancestor are more chemically similar than other organisms.
- 98% similar in DNA between chimpanzees and humans
- 91% similar in DNA between Rhesus and humans.
- Evolutionary relationships have also been determined through:
- Amino acid sequences
- Evidence shown through these studies support traditional classification schemes and also found relationships not previously known.
- Before chemical analysis, gorillas were thought to be the closest related primates to humans
- However, biochemical studies show that chimpanzees are more closely related to humans, and humans are more closely related to chimpanzees than any other species.
- These studies have caused a change in the classification of primates.
- DNA-DNA hybridisation studies have also changed classification of the bear family. New evidence places the giant panda in the bear family and the lesser panda is now palced in the racoon family.
- genetics is the study of heredity
- heredity is the transfer of characteristics from one generation to the next
- the founder of the modern study of genetics was an Austrian monk, Gregor Mendel who lived in the 19th century
- he studied the genetics of the garden pea plant – pasum sativum
- Mendel first chose 7 pairs of characteristics that he wanted to study
- these were
- stem length – short or tall
- the colour of the seed – yellow or green
- the colour of the seed coat – white or grey
- the shape of the seed – round or wrinkled
- the colour of the unripe pod – yellow or green
- flower position – terminal (at the top) or auxiliary (off the sides)
- pod shape – inflated or constricted
before he began his experiment, he selectively bred plants for each characteristic for 2 years to produce only pure bred offspring. The reason he chose pea plants is because they were ideally suited as they can be easily grown and cross-bred.
- firstly, he crossed two pure bred plants
- then he crossed their offspring
Mendel’s conclusions about organisms that he made his about results are summed up in his law of segregation
- an organism’s characteristics are determined by factors – we can call them genes – that occur in pairs
- in a sex cell –gamete – only one factor is present
- during fertilisation, the factors pair up again, they don’t blend, but match up with each other
- Mendel also observed that one factor is dominant over the other, they don’t blend. His result can be explained through the use of Punnett squares
Gene: a code or instruction on our chromosomes
Allele: 2 copies of each gene (one from the mother and one from the father
Homozygous: 2 copies of the same allele e.g. RR
Heterozygous: 2 different copies e.g. Rr
Diploid: full complement of chromosomes – 23 pairs every cell except sperm and eggs
Haploid: half the number of chromosomes – sperm and egg
Punnet square: used to carry out a monohybrid cross
Chromosome: part of your DNA, contains the instructional code[/wpsm_accordion_section] [wpsm_accordion_section title=”2.2 Describe the aspects of the experimental techniques used by Mendel that led to his success “]
Mendel’s experiments were well controlled as he only tested one variable at a time and the first hand data that he gathered was quantitative. His techniques were
- Valid and reliable: Mendel changes only one variable at a time and controlled all others. He used large sample sizes and repeated his experiments for different traits. He analysed the results mathematically to identify patterns and trends and then applied appropriate formulae to draw valid conclusions
- Accurate: he reduced the possibility of experimental error – all experiments were conducted in a controlled environment (green house). The crosses that relied on self-fertilisation (establish pure breeding lines) were conducted by keeping the plants isolated form any others, ensuring that no accidental cross-pollination would occur
- In plants that require cross pollination, mendel removed stigma of some of the anthers of others and then manually transferred pollen from the anthers of one plants to the stigma of another preventing errors arising from accidental cross pollination
Monohybrid: a monohybrid is an individual that has contrasting factors for one characteristic (mono = one, hybrid = mixed or contrasting)
Monohybrid inheritance: is the inheritance of a single pair of contrasting characteristics
Mendel’s laws of domination and segregation
- The characteristics of an organisms are determined by factors that occur in pairs but only one member of the pair of factors can be represented. Offspring inherit one factor from each parent
- When two hybrids bred it Is expected that the ratio will be three to one dominant to recessive traits
- Mendel called the traits passed on factors today we call them genes and we call contrasting forms of the same genes alleles e.g. tall and short are alleles of the gene of height
Mendel’s model of inheritance is based on the following conditions
- Each characteristic/trait in an individual is controlled by a pair of inherited factors
- Mendel’s factors pass as unmodified units to successive generations according to set ratios
- Each individual has two factors for each characteristic. These factors could be the same (pure breeding individuals) or different (hybrid individuals)
- The trait that is expressed in hybrid individuals is the dominant trait, where as the one that is hidden is the recessive trait
- During gamete formation, the pair of factors for a trait segregate and each gamete receives only one for that trait
When the inheritance of more than one trait is studied, the pair of factors for each trait separate independently of the other pairs of factors (law of independent assortment)[/wpsm_accordion_section] [wpsm_accordion_section title=”2.4 Distinguish between homozygous and heterozygous genotypes in monohybrid crosses “]
pure breeding is known as homozygous and hybrid breeding is known as heterozygous. The terms are used to describe the combination of alleles present in a cells
if both copies of the gene in a cell are the same for this trait they are said to be homozygous. Meaning the organism has identical alleles for a particular genetic trait
if the alleles are contrasting or different for that trait are said to be heterozygous
zygote meaning a fertilised egg
homo meaning the same
hetero meaning different[/wpsm_accordion_section] [wpsm_accordion_section title=”2.5 Distinguish between the terms allele and genes using examples “]
- We know that cells contain units of heredity known as genes on chromosomes and different genes influence different characteristics
- Each cell contains two copies of every autosomal gene, one inherited from each parent
- Different variations of the same gene are known as alleles of that gene
- The variation of these genes are found in identical positions on a pair of similar chromosomes within cells
- Diploid individuals have two alleles for each gene an haploid cells (gamete) have only one allele of each gene
- The genetic makeup or genotype of organisms (homozygous or heterozygous) determines the physical appearance or the phenotype
- That is the genotype is the cause of the way it looks and the phenotype is the effect, the actual physical appearance
- When a pair of alleles occurs in an individual and only one of the alleles is expressed, the allele is known as the dominant allele. The allele that is not expressed is known as the recessive allele. These alleles are different variations of the same gene
- The phenotype of an organisms is determined by the dominant genes
- The recessive alleles in humans are often carried for generations without appearing in the phenotype(appearance) but may appear in later generations
- This later appearance of recessive traits only reappears if it occurs in the homozygous recessive form (tt)
- In current genetic studies, phenotype is recognised as not only the physical appearance of an organisms but may also include its physiology (functioning’s) and aspects of its behaviour
- Phenotype is generally determined by genotype but may also be influenced or modified due to interacton with the environment
- g. the height of a human comes down to their genotype and their nutrition
his work was published in 1886, but was not recognised until his work was rediscovered 35 years later in 1900. The value was recognised independently by three different cytologists.
- Mendels work was to progressive – it appeared to be based on very little known background and at the time very little was known about cells, chromosomes, mitosis and meiosis and the study of genetics did not exist
- He presented his papers to a very small group of scientist (40) at two meetings of the national science society of Britain – a fairly low profile gathering of scientist in the province of Moravia
- His work differed quite radically from previous research and the scientist to whom he presented may not have understood it and didn’t recognise it significance. The accepted belief at the time was ‘blending’ of characteristics in the offspring of contrasting pure-breeding parents. His use of mathematical and statistics to analyse results and make predictions in biology was also different to the norm of that time and may not have been understood
- He had no established reputation or recognition in the broader scientific world because he had done no prior significant research and had not interaction with other well-known scientists. As a result, his standing as a scientist may have been doubted
- Boveri worked on sea urchins and showed that their chromosomes were not all the same and that a full complement was required for the normal development of an organism.
- Sutton worked on grasshoppers and showed that their chromosomes were distinct entities. He said even though they duplicate and divide they remain as a distinct structure. He associated the behaviour of chromosomes with Mendel’s work on the inheritance of factors and concluded that chromosomes were the carriers of hereditary units.
- Each chromosome is made up of about 60% protein and 40% DNA
- The DNA is coiled tightly around a protein core (histone proteins)
- A gene is a section of DNA on a chromosome
- It is made up of a particular sequence of bases
- Different genes are different lengths
- Watson and crick discovered that DNA was a double stranded helix or twisted ladder made up of two strand of monomers or sub units known as nucleotides
- Each nucleotide consists of three parts a phosphate a sugar and a nitrogenous base
- There are four types of bases, each nucleotide being named after the base that is carries – adenine, thymine, guanine, cytosine
- These bases are arranged in a sequence along each strand of DNA and so each DNA molecule is thousands of bases long
- The stands are held together by weak hydrogen bonds in the centre and the two strands have an anti-parallel arrangement – meaning they run in opposite directions
- The vertical sides of the ladder are made up of alternating sugar and phosphate molecules while the ‘rungs’ of the ladder are made up of pairs of nitrogenous (attached to sugar)
- A locus is the position of a gene on a chromosome
- During meiosis, genetic variation arises as a result of the behaviour of chromosomes at two stages
- During crossing over
- When chromosomes randomly segregate and paternal and maternal chromosomes assort independently of each other
- During meiosis I
- Chromosomes line up in homologous pairs (one for maternal and one for parental) during prophase 1
- Crossing over occurs – arms of homologous chromosomes exchange genetic material – this introduces genetic variation as genes that occur on the same chromosomes are said to be linked
- Crossing over (synapsis) ensures that linked genes on a chromosome can be inherited independently of each other
- The exchanging of genetic material of homologous chromosomes during crossing over causes the mixing of parental and maternal genes and as a result any number of combinations of genes may be transmitted by the gamete to the offspring
- The chromosomes in each pair of chromosomes separate (during anaphase 1) – so that one entire chromosome of each pair moves into a daughter cell
- Random segregation – this also ensure that the number of chromosomes in the gamete cell will be half that of the original cell
- The matter in which the chromosomes separate is termed independent assortment – and paternal and maternal chromosomes sort themselves independently of each other
- So the maternal chromosomes do not all move into one gamete and the paternal other it is completely random which pair of chromosomes ends up in a gamete an is therefore determined completely independently of the separation of any other gene pair
- During meiosis II
- Two daughter cells that result from meiosis I each undergo meiosis II, which is similar to mitosis and the behaviour of chromosomes in the second meiotic division does not further effect genetic variation
Variability in genetics relates to the different forms of a gene within a population (meaning the total of all alleles present in the gene pool of a population. Both variation and variability may have one of three origins
- A combination of both genes and the environment
Both variation and variability are of evolutionary advantage if they have a genetic basis
- Crossing over – homologous chromosomes exchange genes and so the resulting combinations of alleles on chromatids differ from those originally on the parent chromosomes
- Random segregation and independent assortment – genes on different chromosomes sort independently of each other, giving different gene combinations in gametes from those of the parents.
- Gamete that arise from genetically dissimilar parent (cross-fertilisation) and likely to differ from each other more than those produced by self-fertilisation. Cross fertilisation produced a great number of gamete and therefore increased variability
- Random fertilisation – each egg is different from the other and each sperm is different from the others what is random is which sperm fertilises which egg
- Mutation – this is the only one that applies to asexual reproduction
- Two example that do not show mendels ratios are sex linked inheritance and co-dominance
- Every cell contains 23 chromosomes – 22 autosomes (chromosomes that code for genetic traits within the body) and 1 pair of sex chromosomes
- Sex chromosomes carry genes that determine the sexual characteristics of a person
- Sex chromosomes in individuals may differ from each other in size and shape
- Females genotype is XX (homogametic) and males is XY (heterogametic)
- The offspring of most animals have an equal 50% chance of being male or female
- This is determined by the following mechanisms during the life cycle
- The segregation of sex chromosomes during meiosis
- The transfer of one sex chromosome to each gamete
- The fusion of gametes during fertilisation
Sec linked genes
- The larger sex chromosome (X in humans) may also carry a few genes that code for non-sexual body characteristics. These are terms sex linked genes as they are physically linked to the sex chromosomes and are inherited together with the sexual traits
- Sex linked genes in female and males tend to differ in their inheritance patterns, since males lack one X chromosome and therefore only have one allele for each sex linked gene rather than a pair – present in females
- Since males only have one X, the occurrence of X linked traits occur more frequently in males than Mendel’s ratios would predict – in females you can rely on the other X chromosome to avoid that trait
- A situation where two different alleles for a particular gene are present and both alleles will be expressed
Sex linkage and codominance do not produce Mendel’s ratios because
- Mendelian ratio – 3dominant: 1 recessive
- There are traits found on the X chromosome and not found on the Y and vise versa
- Similarly, in codominance the hybrid showing a different phenotype means there are three possible phenotypes and Mendel’s 3:1 ratio does not account for the third phenotype
Looked to solve the question of: if chromosomes are the basis of inheritance, why do the number of traits that separate during meiosis exceed the number of chromosomes?
- Sutton and Boveri had suggested that more than one trait was present on each chromosome, but this had not yet been demonstrated
- He experimented on the fruit fly (drosophila melanogaster) because
- They were small flies and required little space
- They bred easily in captivity as females lay 200 eggs just two weeks after mating
- The two sexes can be easily distinguished
- They have a small number of chromosomes (8) so they can be readily examined and identified
- His experiments showed
- The gene for eye colour in fruit flies was on the X chromosome
- Hereditary factors can be exchanged between X chromosomes of an individual
- He began with a series of crosses in the typical mendelian sequence to see if the gene for white eyes would show a mendelian pattern
- Cross 1: he cross-bred pure-breeding parents to obtain F1 hybrid offspring. He crossed a white eyed male and a pure-bred (homozygous) red eyed female
- Cross 2: he then crossed the F1 hybrid offspring to obtain the F2
- Morgan was expecting the 3:1 ratio but instead more than 80% of flies had red eyes and less than 20% had white eyes – most flies with white eyes were male – he considered that female flies could not have white eyes
- Cross 3: he performed a typical ‘test cross’ to investigate this hypothesis. He crossed white eyed male with a hybrid red eyed female (F1). His results showed in the F2 that both females and males could have white eyes
- He arrived at the conclusion that the white eye characteristic was sex limited and carried out on the chromosome
Codominance is an exception to Mendel’s 3:1 ratio
- In a heterozygote where two different alleles of the same gene are present and both alleles are expressed (dominant) as separate, unblended phenotypes and therefore termed codominant
- g. pure bred red and white cattle – Hybrid cows have one red allele and one white allele and therefore have a roan appearance – both red and white hairs are present, not in patches but interspersed
- g. ABO blood groups – A and B are codominant as if they are both present they will both produce the surface antigens
Some variations in organisms are genetically determined, whereas others are influenced by the environment. However, many variations arise as a result of an interaction between the two
The effect of the environment on gene expression
- a variety of studies on both plants and animals show that the effect of a gene can be enhanced or masked by variation in the environment
- a variety of studies have also been done on identical twin has they have the same genetic material (genotypes) and therefore any difference in phenotype would be an impact of the environment
- g. the Himalayan rabbit – these animals can alter their coat colour with the changes in temperature. In low temperatures their fur is black and In high temperatures their coat is white
- g. pea plants – tallness (T) is dominant over the gene dwarfness (t). if a pea plant with the tall gene (TT and Tt) is growing in nutrient-deficient soil, it may only rouw to the size of a swarf plant. Thus the expression of the tall gene (T) has been influenced by the physical environment in which the plant grew
- environment on phenotype
Aim: To investigate how altering the environment affects the phenotype of seedlings.
- Collect two pots of chilly plants with 8
- Record the number of leaves for both A and B
- Record the height up to
- Place one pot of chilly plant in a spot with sufficient sunlight everyday.
- Place the other pot of chilly plant in a spot with no sunlight at all.
The plants growing in the sunlight grew taller, the number of leaves increased, the colour remained green. However the plants growing the dark did not grow much taller, decreased in the number of leaves and turned yellow.
The plants growing in the dark, were exposed to no sunlight and had limited amount of oxygen, therefore it affected their phenotype. Thus, their growth and colour. However, the plants that were not growing in the dark were exposed to sunlight and so their phenotype was not affected.
Therefore, the environment of an organism does affect the phenotype of the organism.[/wpsm_accordion_section] [wpsm_accordion_section title=”4.1. Describe the process of DNA replication and explain its significance”]
- DNA replication is the production of two identical double stranded molecules of DNA from one original double stranded helix molecule
- It is termed semi-conservative as the original double helix is split to give rise to a new complementary stand
- This ensures that the genetic material is copied exactly
- Watson and crick noted immediately that there a possible copying mechanisms as if the strands separated each could act as a template for the synthesis of a new complementary strand
The process of DNA replication
- DNA double helix strand unwinds – an enzyme called helicase causes the DNA that causes the double helix to progressively unwind
- DNA unzips (separate) – the weak hydrogen bonds break
- Nucleotides are added to each single strand – each strand of the existing DNA molecule acts as a template for the production of a new strand. Nucleotides are picked up by the enzyme DNA polymerase and sotted in opposite their complementary partner. These nucleotides are picked up from a pool of nucleotides called the nuclear sap
- The direction in which nucleotide insertion occurs in antiparallel on the two opposite strands
- The base pairing is checked by another enzyme; this ensures accuracy as incorrect base pairing results in a change in DNA sequencing called a mutation
The significance of DNA replicating
- DNA has two main functions
- Heredity – this relies on DNA replication
- Gene expression – this relies on protein synthesis
- The genetic material of a cell must be transmitted from
- One cell to another – during meiosis allowing for growth and repair
- One generation to another during meiosis e.g. when gametes are formed for sexual reproduction
- Replication of DNA ensures that the genetic code of a cell is passed onto each new daughter cell. An exact replica or copy of the DNA must be produced so the new cells have the same distinct message the original cell had
- If DNA replication goes wrong it has a direct effect on the phenotype of the individual
- DNA holds the information for creating proteins in cells
- As we know, a protein is made up of one or more chains of polypeptides, and each polypeptide is made up amino acids and peptide bonds
- The way DNA codes for proteins:
- A set of 3 bases is called a triplet code, or a codon.
- Every codon codes for one amino acid
- There are 20 different amino acids
- However, with sets of 3 bases, and 4 different bases, there are 64 combinations possible
- This means that for one amino acid, there can be more than one triplet code.
- For example, TCT, TCC, TCA or TCG on the DNA strand in the nucleus codes for the amino acid “serine”
- The structures involved in polypeptide synthesis are:
- DNA: A gene contains a sequence of bases to code for a protein
- RNA is similar to DNA except that instead of deoxyribose as the sugar it has ribose. It is single stranded and instead of thymine there is uracil.
- There are three forms involved in polypeptide synthesis
- mRNA: Messenger RNA carries the genetic code outside the nucleus, into the cytoplasm, where it can be read by ribosomes
- tRNA: Transfer RNA carries the amino acids to the ribosomes to link and form a polypeptide chain. tRNA are shaped like clover leaves; there is a different type for every amino acid. At the bottom of every tRNA molecule is an anti-codon that binds to the codon on the mRNA strand. That is how the amino acid is linked to the codon
- Ribosomal RNA: Ribosomes are made up of protein and RNA
- Ribosomes: the ribosome is the active site for protein synthesis. It is made up of protein and RNA molecules. It can accommodate 2 tRNA at a time
- Enzymes: the enzyme that control the formation of mRNA is RNA polymerase. there are of course many other enzymes that control the process
The steps involved in protein synthesis
- An enzyme, RNA polymerase binds to a part of the DNA called the promoter and the DNA unzips – just a short length in that part of the DNA that has the gene to be used
- Only one strand of DNA contains the genetic information needed to make a protein called non-coding strand or sense strand. The other stand is called the coding strand or anti-sense strand
- Transcription of the gene occurs, controlled by the enzymes RNA polymerase. The sense strand of DNA acts as a template and the RNA nucleotides are assembled forming a complementary single stranded mRNA molecule. It is the same except instead of a T there is a U
- The mRNA moves out of the nucleus and into the cytoplasm, where is encounters millions of ribosomes in the cell
- Translation: the ribosomes move along the mRNA molecule and as they do so they attach tRNA molecules by temporary pairing with the bases of tRNA anticodons with their complementary triplets of bases
- The amino acids are linked together by another anzyme to form a polypeptide chain. The amino acids are then sliced off their tRNA carriers
- The tRNA moves away from the mRNA leaving the growing chain of amino acids and moves back into the cytoplasm where they can pick up another amino acid and be reused
- The polypeptide chain may be joined by one or more polypeptides. They are further processed and folded into their correct shape forming a protein
- The mRNA is broken down into its individual nucleotides which can then be reused
- Proteins are large, complex macromolecules made up of one or more long chains called polypeptides
- Each polypeptide chain consists of a linear sequence of many amino acids joined by peptide bonds
- One or more polypeptides can be twisted together into a particular shape, resulting in the overall structure of a protein
- The sequence and arrangement of amino acids determined the configuration of the protein.
- Any change in the amino acid sequence that results in a change in the shape of the protein molecule could affect the ability of the protein to carry out its function in the cell
- Mutations alter genes by changing the nucleotide sequence in DNA and as a result one or more genes may be affected. This creates an allele
- These changes could result in the production of new proteins. Usually have little to no effect on the organism but some will lead to genetic disorders and inherited diseases
Inheritable mutations pass onto future generations
- Changes in somatic cells are not passed onto offspring – it may cause effect on the individual but not affect future generations
- However, mutations in germ-lines cells (gametic mutations) produce alleles that an be inherited
Effect of mutations at the gene and chromosome level
- Changes to genetic material arise during replication and they may result in a change to a single gene (gene mutations). Other mutations involve the rearrangement of a block of genes or whole chromosome (chromosome mutations)
- Gene mutations: on a molecular level, mutations may involve
- Base substitution (point mutation): one pair of nucleotides (C-G) is substituted for another pair (A-T)
- Frame shift (macro mutation): extra bases are added or deleted from a strand of DNA, changing the whole sequence of nucleotides
- A sequence within a gene may be duplicated or translocated
- Chromosomal mutations
- On a molecular level if whole chromosomes become rearranged (eleted, duplicated, translocated and attached to another) a change in chromosome number may arise
- This usually occurs as a result of chromosomes not separating out correctly during meiosis and the resulting cells have one less chromosome than normal (or one extra)
- The effect can result in disorders such as down syndrome, where individuals have three copies of chromosome 21. This has numerous phenotypic effects
most gene mutations produce recessive alleles because they prevent the gene from producing a functional protein[/wpsm_accordion_section] [wpsm_accordion_section title=”4.5. Discuss evidence for the mutagenic nature of radiation”]
- The link between exposure to ionising radiation and an increase in the occurrence of certain illnesses such as leukaemia and other cancers was identified in the 1900’s, but further evidence was needed to accept that radiation was directly causing the cancers
- Between 1925 and 1940 experimental research provided evidence of the mutagenic nature of radiation. Advances in cell studies provided further report
- First generation radiotherapists, who did not know the dangers of radiation often died young. Scientists like Marie curie would carry uranium around in their pockets and develop cancers very quickly
- People who live in areas that have been affected by high level radiation such as Hiroshima and Chernobyl still show high incidences of cancer and other mutations to offspring
- Effect of radiation of DNA strands
- UV light, X rays and radioactive materials can cause bases to be deleted, totally removed from stands. Can cause thymine bases to link together causing disruption in the normal functions of DNA
- Mutation is the basic source of all variation
- It supports Darwin’s theory of evolution because it provides a mechanisms to explain how heritable variation arises
New alleles led to change in phenotype
- A mutation that results in change in phenotype may be negligible in its effect or may confer some advantage or disadvantage. Mutations therefore rovide the diversity of genetic materal that results in variation in phenotype.
- If mutations can be inherited they provide variation on which natural selection acts for evolution to occur
- For evolutionary purposes, a mutation can be defined as a heritable change in the genetic material
- He proposed that populations change slowly and gradually over time
- However, the fossil record only shows rare occasions where this happens
- If an environment remains stable for many years, we would expect there to be no change in the organisms living there
- It is only when the environment changes that natural selection occurs
- The fossil record shows periods of stability followed by mass extinctions and rapid change
- The fossil record suggests that organisms evolve suddenly and remain stable for millions of years
- In 1972, 2 scientists, Gould and Eldridge, put forward a theory to explain this an they called it the punctuated equilibrium
- The punctuated equilibrium proposed that instead of gradual change, there have been periods of rapid evolution followed by long periods of stability or equilibrium
- in 1941, Beadle and Tatum published the results of their experiments on the bread mould, Neurospora crassa.
- These results showed that genes control biochemical processes.
- They exposed the sport of the mould to x-rays in order to cause mutations and found that some of the mutated spores could not grow on the normal culture medium unless they added specific amino acids or vitamins.
- From this, they hypothesised that the x-rays had destroyed the gene that coded for the enzyme to make arginine. They called this the ‘one gene –protein’
- The hypothesis was changed as it became clear that not all proteins are enzymes. It was then changed to the ‘one gene – one polypeptide’ Whilst this may be true for some enzymes it is not true for all. (N.B: All enzymes are proteins, but not all proteins are enzymes.
- This was later changed to ‘one gene-one polypeptide’ because genes codes for many proteins that are not enzymes. Many proteins are made up of more than one polypeptide, and a gene only codes for one polypeptide
Antibiotics and natural selection –
- When antibiotics were first introduced they were very effective in killing bacteria.
- However, over a few generations many bacteria developed resistance to antibiotics and the effectiveness of treatments decreased.
- Populations of bacteria are large and produce large numbers of offspring rapidly. Then a population is subjected to antibiotics, most individuals are killed. However, some may have a gene that gives them resistance to the antibiotic. Individuals that have natural resistance survive and reproduce: they have no competition from other members of their species, either for reproduction or other available resources.
- This means that the genes that allow them to survive are passed onto the next generation in a higher proportion than would normally be the case. Each subsequent generation will have a higher percentage of resistant individuals and will pre-adapt to the treatment so the antibiotic will no longer be effective.
Selective breeding can be thought of as a form of artificial selection imposed by humans, when they conduct specific crosses of living organisms to obtain a combination of desirable characteristics.
- Humans have selectively bred plants and animals for centuries
- However, it had always been for the benefit of humans; we breed animals and crops to be bigger, grow-faster, tastier, etc.
- Selective breeding is the deliberate crossing or mating of individuals of the same species with the characteristics wanted; over time, these characteristics become dominant.
- However, the overall genetic variation of populations tends to be reduced
- Artificial Insemination: Refers to animals
- It is the injection of male semen into a female
- Commonly used with species of large mammals, eg cows, sheep, horses, etc
- The sperm is collected from a male with desirable characteristics
- ADVANTAGES: Can be used to inseminate many females from one male. Transport of semen is much easier than transporting a whole animal. Semen can be stored for a period of time.
- DISADVANTAGES: Reduced the genetic variations found in populations, making them susceptible to changes in the environment (e.g. new disease)
- Artificial Pollination:
- Plant breeders carry out artificial pollination to breed plants with specific characteristics (like Mendel did).
- Pollen from the male anther is collected. It is then dusted onto the female stigma of another plant. The pollinated flower is covered to prevent pollination from other flowers
- ADVANTAGES: Particularly useful and easy way of breeding new varieties of plants. No expensive equipment required
- DISADVANTAGES: Genetic variation reduced. –
- Cloning is a method of producing genetically identical organisms
- A clone is a collection of genetically identical copies
- PLANT CLONING:
- The most commonly used method, and the oldest, is cutting and grafting. A stem of short section of another plant is cut off, dipped in root-growth hormones, and planted into soil. The plant that grows is a clone of the original plant
- Tissue culture technology has allowed mass cloning of plants. Firstly, a section of a plant, eg, a root tip, is pulverised using a blender to release the individual plant cells. The cells are grown on a nutrient medium, and incubated under controlled conditions. Genetically identical plants are produced.
- ANIMAL CLONING:
- Much more difficult than plant cloning
- First animal to be cloned was Dolly (named after Dolly Parton…LoL)
- Technique used is called “nuclear transfer technology‟ :
- Adult sheep tissue cell removed from sheep and cultured in lab
- Nucleus removed from one of these cells and placed in an enucleated egg cell (egg cell with genetic info removed)
- Gentle electric pulse causes nucleus to fuse with egg cell
- A second electric pulse starts cell division and embryo formation
- This new cell is implanted into a female sheep where it grows into a new organism
- ADVANTAGES: In agriculture, cloned plants have identical requirements and grow in similar ways to produce similar yields at the same time. In plants and animal’s identical copies of desirable varieties can be produced
- DISADVANTAGES: In crops – all plants susceptible to the same diseases. Cloning is expensive with limited advantages over reproductive techniques. Cloning of animals has raised ethical questions about the cloning of humans. The health/life expectancy of cloned animals is questionable, with the death of Dolly the sheep being earlier than expected.
- Transgenic species are organisms which have had a genetic material from a different species transferred into their chromosomes
- That is, genes from one species have been taken and transferred to another
- The introduced gene instructs the transgenic organisms to produce the desired trait or products
- This trait may be passed onto future generations
the process used to produce transgenic species
- A useful gene and the chromosome it is on is identified
- The gene is isolated or cut out of its DNA strand
- Separated DNA sequences for regulation may have to be added to ensure the gene will work
- The gene is inserted into the cell of another organism, sometimes a vector is use to do this
- a vector is a carrier of a substance from one species into another
techniques used to produce transgenic species
- isolating genes: ones a useful gene is identified, isolated and cut out. Special enzymes called restriction enzymes are used. More than 800 types are known. They cut the DNA by breaking the hydrogen bonds in a triplet (they care called sticky ends)
- making recombinant DNA: the DNA strand from 2 organisms are cut using the same enzyme, the sticky ends will match. When they are mixed the new gene will match with the DNA strands and link up. This is called ANNEALING. DNA ligases are added to strengthen the bonds
- making transgenes: an isolated gene cannot function if it is alone. It has to be transferred with a promoter sequence attached to ensure it works
inserting genes into bacteria
- most bacteria contain small, circular pieces of DNA called plasmids
- plasmids can be used as vectors or carriers to transfer transgenes into bacteria
reasons for using this process
- these process enable scientists to combine the qualities of different organisms
- transgenic species are being developed to
- increase the resistance of plants or animals to disease, pets or extreme environmental conditions
- for medicines and vaccines and to study human disease
- to improve the productivity of crops, pastures and animals
- to improve the quality of food and efficiency of food processing
examples of the use of transgenic species
- BT crops: BT is a bacterium that naturally produced chemical that kills many insects. The chemicals are specific to many pests and do not kill other insects. Genetically modified crops have had the gene of BT pesticide inserted into them. They produced their own BT chemicals and no longer need to be sprayed
- Cold strawberries: a gene from a type of salmon that allows it to survive old temperatures has been isolated and inserted into a strain of strawberry. This strawberry can now survive and grow in cold temperatures
- Bacterial insulin: diabetics previously obtains their insulin from animals, especially pigs. The gene for insulin production, taken from the human pancreas, was placed into the DNA of bacterium. This now provides mass production of insulin
Ethical issues of Transgenesis
- These technologies help treat disease and increase food production
- Should we be tampering with nature this way?
- Is it right to change living organisms for commercial gain?
- Transgenesis disrupts evolutionary relationships between organisms
- If a transgenic species was released into the natural environment, it could out compete the natural organisms
- Health risks and side effects with eating GM foods
- The main fear behind the use of genetic and reproductive breeding techniques on organisms is that the natural diversity and variation within populations is decreased
- g. cotton plants. The main crop being grown all over the world is BT cotton. As more farmers shift to BT cotton there are many disadvantages
- Many natural varieties of cotton will be lost
- The species itself becomes vulnerable to extinctions. If al cotton grown all over the world is BT, and a disease appears that specifically kills BT cotton than there is a risk of all cotton becoming extinct
- In another case, if a population of cattle that have all been fathered by the same bull, through artificial insemination techniques is at risk of environmental changes. A lack of variation is a major risk factor in extinction of a species