- Use available evidence to analyse, using a named example, how advances in technology have changed scientific thinking about evolutionary relationships:
- New technologies, especially in the field of biochemistry, have increased knowledge about the relationships between species.
- Techniques such as DNA hybridisation, amino acid sequencing and analysis of the antibody-antigen reaction between different species have shown the degree of similarity and evolutionary pathways of organisms.
DNA Hybridisation:
- DNA hybridisation is a process by which the DNA of different species can be compared
- The process uses heat to separate the 2 strands of the double helix, from 2 different species
- The single strands of the different species are then mixed, and cooled
- On cooling, the hydrogen bonds re-form in varying degrees
- The greater the number of bonds between the strands, the greater the degree of genetic similarity between the two species
- Gregor Mendel’s experiments helped advance our knowledge of the inheritance of characteristics:
- Outline the experiments carried out by Gregor Mendel:
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Genetics is the study of heredity.
- Heredity is the transfer of characteristics from one generation to the next.
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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 (Pisum Sativum).
Mendel’s Method:
- Mendel first chose 7 pairs of characteristics that he wanted to study.
These were:
- Round/Wrinkled seed.
- Yellow/Green seed.
- Smooth/Constricted seed pods.
- Green/Yellow pods.
- Violet/White flowers.
- Tall/Short stem.
- Terminal/Axillary flowers.
- Before he began his experiment, he selectively bred plants for each characteristic for 2 years to produce ONLY pure breeding offspring.
- Firstly crossed two pure breeding plants.
- Then crossed their off – spring.
- Mendel’s conclusions about organisms that he made his results are summed up in his
Law of Segregation:
- Law of segregation states that factors for the same characteristic occur in pairs in an individual. These pairs separate at gamete formation, so that a gamete contains only one of each factor.
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Mendel’s second law, The Law of Independent Assortment states that:
- Either factor of a pair can combine with either factor of another pair.
Terms:
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Monohybrid cross – A breeding experiment that looks at the inheritance of only one characteristic.
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Alleles - are genes for the same characteristic.
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Genotype - is the genetic composition of an organism.
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Homozygous – have the same allele for a characteristic. Also called pure – breeding. They are represented by the same letter twice e.g. TT.
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Heterozygous – have different alleles for a characteristic. E.g. Tt.
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Phenotype – is the physical appearance.
- Describe the aspects of the experimental techniques used by Mendel that led to his success:
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He studied a large number of Characteristics.
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He performed a large number of Crosses; i.e. he repeated many times.
- He made exact counts of the characteristics, kept careful records and counted the results he obtained.
- He studied one character at a time.
- He chose a plant (the garden pea plant) that shows easily identifiable, alternative forms.
- Describe the outcomes of monohybrid crosses involving simple dominance using Mendel’s explanations:
- Each characteristic is coded for by at least a pair of genes.
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A Monohybrid cross is a breeding experiment that looks at the inheritance of only one characteristic.
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The Genotype is the genetic composition of an organism.
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The Phenotype is the appearance of an organism.
Mendel’s Monohybrid Crosses:
- Mendel only studied one pair of characteristics at a time (e.g. stem height)
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This type of breeding experiment is called a monohybrid cross
- Mendel first bred one variety of pure-breeding plant (e.g. tall plants) with another variety, also pure-breeding (e.g. short plants).
- But they were ALWAYS different varieties of the same characteristic.
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F1 is known as the first generation, F2 the second generation, and so on.
- The parents were cross-pollinated, and all the off-spring was tall.
- Parents: tall plants x short plants
F1: all tall plants
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Mendel then took these tall offspring and self-pollinated them: He transferred the pollen by hand from the stamens onto the stigmas.
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F1: tall plants x tall plants
F2: approximately - 3 tall plants: 1 short plant
- Mendel repeated this experiment many times, and with different characteristics such as seed colour, but the same ratio kept occurring.
- This ratio 3:1 is called the monohybrid ratio
- Distinguish between homozygous and heterozygous genotypes in monohybrid crosses:
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Homozygous genotypes have the same allele for a characteristic. Also called pure-breeding. They are represented by the same letter twice. E.g. TT
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Heterozygous genotypes have different alleles. E.g. Tt
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Alleles are genes for the same characteristic.
- Distinguish between the terms allele and gene, using examples:
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A chromosome is a long strand of DNA located in the nucleus.
- Chromosomes always come in pairs, one from the mother (maternal) and one from the father (paternal).
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The pair of chromosomes are called Homologous chromosomes.
- A gene is a unit of inheritance, a sequence of DNA.
- An allele is an expression of a particular gene, e.g. in the gene for blood type, the alleles are A, B and O.
- Explain the relationship between dominant and recessive alleles and phenotype using examples:
Dominant and Recessive alleles:
- For every characteristic, there are 2 alleles.
- They are ALWAYS present in pairs in body cells
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If the two genes are the same allele, then the organism is said to be homozygous for that characteristic.
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If the two alleles are different, then the organism is heterozygous for this characteristic.
- In simple genetics, one of the alleles is DOMINANT, and one of them is RECESSIVE
- Taking a characteristic, e.g. Pea Plant height. We represent it’s genotype with 2 letters, each letter representing a gene. T is the dominant tall allele, t is the recessive, short allele
- A tall pea plant can be either TT or Tt, as the dominant gene is always expressed
- A short plant is always tt.
- Outline the reasons why the importance of Mendel’s work was not recognised until some time after it was published:
- He only presented his paper to a small group of scientists
- His work was radically different to previous ideas – possibly not understood
- Significance was possibly not realised at the time
- He had no outstanding reputation as a scientist – possible ignored by scientific community.
- Perform an investigation to construct pedigrees or family trees, trace the inheritance of selected characteristics and discuss their current use:
- Pedigrees are family trees. They show the inheritance of a trait over many generations.
Patterns to recognise characteristics are inherited:
- If two non-affected parents have an affected child, then the trait is a recessive one.
- If two affected parents, have a non-affected child, then the trait is dominant
- If there is a large bias towards males being affected, and sometimes generations are skipped, than the trait is recessive sex-linked
- Process information from secondary sources to describe an example of hybridisation within a species and explain the purpose of this hybridisation:
- In Kenya, hybridisation has been carried out by means of artificial insemination between Jersey cattle and the African Sahiwal cattle. The purpose of the hybridisation is to increase milk yield and quality. The milk yield of the Jersey-Sahiwal crossbred cows is generally higher than that of either the Jerseys or the Sahiwals.
- Chromosomal structure provides the key to inheritance:
- Outline the roles of Sutton and Boveri in identifying the importance of chromosomes:
Walter Sutton:
- An American geneticist.
- 1902 proposed the chromosomal theory of inheritance.
- Suggested Mendel’s inheritance “factors” (genes) are carried on chromosomes.
- Sutton formulated his theory after observing meiosis in grasshopper testicles.
He noted the following features:
- During meiosis, chromosomes line up in pairs, same size and shape.
- Homologous pairs segregate so that each gamete receives one chromosome from each pair.
- After fertilisation, the resulting zygote had a full set of chromosomes.
Theodor Boveri:
- A German zoologist.
- Showed that chromosomes were transferred from one generation to the next in cell division.
- Suggested that chromosomes might be the means of inheritance.
- Together with Sutton noticed that organisms have many more characteristics than they have chromosomes and hypothesised that each chromosome contains many “factors”.
- Argued that chromosomes could exchange factors with each other during meiosis (crossing over).
- Describe the chemical nature of chromosomes and genes:
- 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.
- Genes that occur on the same chromosome are said to be linked. This is because they are usually inherited together.
- Identify that DNA is a double-stranded molecule twisted into a helix with each strand comprised of a sugar-phosphate backbone and attached bases – Adenine (A), thymine (T), cytosine (C) and guanine (G) – connected to a complementary strand by pairing the bases, A-T and G-C:
DNA (deoxyribonucleic acid):
- A double stranded helix
- Made up of sub-units called nucleotides
- Each nucleotide is made up of a phosphate, a sugar and a nitrogenous base
- The sugar is deoxyribose
- The four different bases are adenine, thymine, guanine, and cytosine
- Adenine pairs with thymine (A-T) and guanine with cytosine (G-C)
- A single DNA strand is made up of a chain of nucleotides (a polynucleotide) where the phosphate and sugar alternate as the backbone of the strand
- The bases attach to the sugar
- The other strand of DNA attaches to the strand by complementary pairing of the nitrogenous bases.
- No other pairing is possible because the bases are complementary; their chemical structure makes any other pairing impossible.
- Explain the relationship between the structure and behaviour of chromosomes during meiosis and the inheritance of genes:
The stages of meiosis that lead to the creation of gametes and the inheritance of genes are:
- The chromosomes duplicate. The single stranded chromosomes become double stranded, linked at the centre by a centromere
- In the first meiotic division, the homologous chromosomes separate, but the double-strands of the chromosomes are still joined.
- In the second division, the chromatids of the chromosomes separate and form 4 gametes altogether.
- Explain the role of gamete formation and sexual reproduction in variability of offspring:
The events that create variation in sexual reproduction are:
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RANDOM SEGREGATION: During meiosis, genes on different chromosomes sort independently. They can line up in the middle of the cell in many different ways. This produces many gene combinations, which are different from the parents
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CROSSING OVER: Crossing over of genetic material during meiosis results in the exchange of genes between chromosome pairs. The combinations of alleles of the gametes will vary across cells and differ from the parent
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RANDOM FERTILISATION: When the male and the female mate, the two different gametes randomly fuse. Many different combinations are possible, and this causes variation.
- Describe the inheritance of sex – linked genes, and alleles that exhibit co – dominance and explain why these do not produce simple Mendelian results:
Co – Dominance:
- Sometimes in a monohybrid breeding experiment, the dominant and recessive inheritance does not apply.
- This is because not all genes have dominant and recessive alleles.
- If both alleles of homozygous parents are expressed in the heterozygous genotype, a third phenotype is seen.
An example is roan coloured cattle:
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If a type of cattle has the gene for red, and white, it would not make a pink cow, but the hairs on the cow would be both red AND white, making an interesting roan colour.
- Looking at the cross in the form of a Punnet square, we can see that a cross concerning a codominant trait does not give the simple Mendelian ratio of 3:1
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The cross between the two roan cows of the F1 generation does not give the 3:1 ratio because a heterozygous animal does not give the dominant trait, as would happen in simple dominant-recessive cases. A “heterozygous” animal gives the roan colour, which results in the 1:2:1 ratio.
Incomplete Dominance:
- The alleles in this case do not show simple dominance either
- In this case, if the both alleles are present, a blending of phenotype will occur
- For example if a snapdragon (a flower) has a red a white gene, it will be pink.
Sex-linked Characteristics:
- SEX is a genetically determined characteristic
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The sex of an individual is determined by a pair of chromosomes called the sex chromosomes; in humans, it is the 23rd pair
- For females, both the sex chromosomes are the same. This combination is called XX. Females have two X chromosomes, and the chromosomes are homozygous
- For males, the sex chromosomes are different. The combination is XY. The Y chromosome is shorter than the X chromosome.
- Because the Y chromosome is much shorter than the X chromosome, some characteristic are only coded for by the X chromosome
- This is a special case of inheritance of characteristic.
- Most sex-linked characteristics are recessive
- Take, eg, haemophilia. ‘H’ is the dominant, normal allele; ‘h’ is the recessive, haemophiliac allele.
Females:
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A normal female’s genotype – XHXH
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A carrier female has the genotype - XHXh
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A haemophiliac female has the genotype – XhXh
Males:
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A normal male - XHY
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A haemophiliac male - XhY
- As you can see for the example, males only have to inherit a single gene to have the characteristic.
- A single recessive gene has the same phenotypic effect as a single dominant gene
- This is why some sex-linked characteristics are much more common in males than females.
- Describe the work of Morgan that led to the understanding of sex linkage:
- Morgan studied the breeding of the fruit fly (drosophila)
- Drosophila was an ideal species to study as it only had four pairs of chromosomes, and as it was a fly, it bred quickly.
- As he was breeding the flies, he noticed one white-eyed male fly among the offspring of red-eyed parents
- This was strange as the normal eye colour was red
- He further bred this white male with other flies.
- His results showed that all the white-eyed flies were male
- He hypothesised that the characteristic was sex-limited, and that it was on the X chromosome.
- Explain the relationship between homozygous and heterozygous genotypes and the resulting phenotypes in examples of co – dominance:
- In simple dominance cases, if an organism is homozygous dominant, the phenotype is obviously that of the dominant allele. If it was homozygous recessive, then the phenotype would be that of the recessive allele.
- If the organism was heterozygous, then the dominant allele would be the phenotype of the organism, as the dominant allele would preside over the recessive one.
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HOWEVER, if it was a case of co - dominance, heterozygous organisms would have both phenotypes expressed at the same time, as no allele is totally dominant over the other. Eg, red and white – roan cattle.
- Outline ways in which the environment may affect the expression of a gene in an individual:
- Genes are not the only factor that influences phenotype.
- The environment may affect the expression of a gene in an individual.
- GENES + ENVIRONMENT= PHENOTYPE.
- The environment can control to what extent a genotype is expressed.
- Identical inherited characteristics do not always result in identical organisms because of the effect of the environment.
Example:
- Two people with the same genetic inheritance for tallness might grow to different heights because of differences in nutrition or health.
Hydrangeas:
- Hydrangeas are plants that have different flower colour (pink or blue) depending on the pH of the soil they are grown in. In acid soils (less than pH 5) Hydrangeas are blue. In soils that have a pH greater than 7 Hydrangeas are pink. The pH has an effect on the availability of other ions in the soil and it is these ions that are responsible for the colour change.
- The structure of DNA can be changed and such changes may be reflected in the phenotype of the affected organism:
- Describe the process of DNA replication and explain its significance:
- The significance of the ability of DNA to replicate is that identical copies of the genes can be made.
- DNA replication is made possible because the molecule is a double helix, and because the nitrogenous bases only pair complementarily.
The steps for DNA replication:
- The parent DNA molecule unwinds into two separate strands, at one end.
- The two strands become exposed, free nucleotides floating in the nucleoplasm attach to the exposed bases, A with T and C with G. This ensures that the replication is exact.
- The joining of nucleotides together is catalysed by DNA polymerase.
- Outline, using a simple model, the process by which DNA controls the production of polypeptides:
- 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:
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DNA: a gene contains a sequence of bases to code for a protein.
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RNA: RNA is similar to DNA except that instead of deoxyribose as the sugar, it has ribose. It is single stranded, and instead of Thymine (T), there is Uracil (U).
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mRNA: This is a type of ribonucleic acid. It carries the information from the DNA in the nucleus to the ribosomes in the cytoplasm.
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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.
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Ribosomal RNA: Ribosomes are made up of protein and RNA
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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.
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Enzymes: The enzyme that controls the formation of mRNA is RNA polymerase. There are, of course, many other enzymes that control the process.
Protein Synthesis:
Stage one – Transcription:
- Transcription is the first step of Protein Synthesis.
- It is the process in which the following steps occur:
- DNA unzips.
- mRNA is formed complimentary to the DNA strand. NOTE: A – U, C – G.
- mRNA leaves the nucleus via the nuclear pores into the cytoplasm.
- DNA strand reforms.
Stage two – Translation:
- Translation occurs in the cytoplasm.
- It is the process in which the mRNA translates different Amino acids.
The following steps occur:
- mRNA enters the ribosome.
- tRNA joins complimentary to the mRNA – i.e. the codons of the mRNA join in a complimentary way with the anticodons of the tRNA. E.g.: UAC – AUG.
- After the tRNA join to the mRNA, they break off leaving the amino acids behind.
- Peptide bonds form between the amino acids
- This forms polypeptide chains – which therefore form proteins.
DNA —>mRNA—>tRNA—>Amino acids—>Polypeptide chains—>proteins
- Explain the Relationship between proteins and polypeptides:
- A polypeptide is made up of amino acids linked by peptide bonds
- A protein is made up of one or more polypeptide chains, folded to fit a specific function, often into a globular shape.
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information to a flow chart that shows that changes in DNA sequences can result in changes in cell activity:
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If there is a simple substitution for a single base pair on a strand of DNA such as a G-C replaced by A-T, then this will result in a different amino acid codon forming a different polypeptide. If one base pair is lost from the sequence there will be a shift along the DNA molecule producing different polypeptides.The flow chart below shows the reaction if thymine is lost from the start of a DNA sequence.
- Cell activity is controlled by enzymes. Enzymes are formed from chains of polypeptides. If the chain of amino acids forming the polypeptide is not in the right sequence, then the enzyme formed will not be functional. In this case, there is a premature stop.
- Analyse information to outline evidence that lead to Beadle and Tatum’s ‘one gene – one protein’ hypothesis and explain why this was changed to ‘one gene – one polypeptide’ hypothesis:
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In 1941, Beadle and Tatum published the results of their experiments with a bread mould, Neurpspora Crassa, that provided evidence of a link between genes and proteins. They used x-rays to produce millions of mutated strains of the mould. Each strain lacked the ability to produce one of the essential nutrients that would be needed to make enzyme. By growing different strains with different combinations of nutrients, Beadle and Tatum were able to establish which enzyme was lacking in each mutant strain. They also found that each genetic mutation was at a specific site on the moulds chromosomes. They concluded that different sites were associated with each enzyme. This lead to the famous ‘one gene – one enzyme’ hypothesis. Later this chnaged to ‘one gene – one polypeptide’ theory for two reasons:
- Genes code for many proteins that are not enzymes.
- Many proteins are made up of more than one polypeptide and each gene codes for a polypeptide.
- Explain how mutations in DNA can lead to the generation of new alleles:
- A mutation is a change in the DNA information on a chromosome
- Gene mutation produces NEW ALLELES of genes in species and so creates new genetic variation.
Three things can happen as a result of a mutation:
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Most mutations are lethal and kill the cell the mutation takes place in
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In some cases, the mutation is not advantageous or lethal to the organism. It is a neutral mutation
- VERY RARELY, a mutation will give an organism a phenotypic advantage. These individuals with the new allele will be at a selective advantage, and be better suited to their environment
- A mutation in a body cell is called a somatic mutation. This mutation cannot be passed on to offspring.
- If the mutation occurs in the sex organs, then the mutation will be passed on to offspring
- A mutation in the DNA material affects cell activity, because a change in the base sequences alters protein production.
Types of Mutations:
A Change in Chromosome Number:
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ANEUPLOIDY: Is the case of an abnormal number of chromosomes. Can sometimes occur because chromosomes fail do separate during meiosis. On fertilisation, there will be an abnormal chromosome number; either too little or too much. E.g., Down’s syndrome.
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POLYPLOIDY: This is the case where a cell, or organism, has one or more extra, complete, sets of chromosomes. If the zygote is polyploid, then the whole organism will be. If it is in a body cell, then it will not spread much. Occurs because chromosomes fail to move to each pole in mitosis. Polyploidy is encouraged in horticulture, as plants with this feature grow larger and stronger than normal.
A Change in DNA Sequence:
- Most mutations are a change in the DNA sequence
- The smallest type is a point mutation, where only one base changes
- Large changes can alter the shape of the chromosome:
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DELETION: Some of the DNA is lost from a chromosome
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DUPLICATION: A section of the chromosome is copied on the same chromosome; that is, the same section of DNA appears twice
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INVERSION: A section breaks off and is reattached the wrong way
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TRANSLOCATION: A piece of DNA from one chromosome breaks off and attaches to a neighbouring chromosome
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AMPLIFICATION: A section of DNA is repeated many times; it is a form of duplication, except many more copies
- As changes in the DNA or chromosomes creates new proteins, this can in turn create new alleles of genes
- This increases variation.
- Discuss evidence for the mutagenic nature of radiation:
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Mutagens are environmental factors that induce mutation
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A mutagen is a natural or human-made agent (physical or chemical) which can alter the structure or sequence of DNA. Mutagens can be carcinogens (cancer causing) or teratogens (birth defects causing).
Effect of radiation on DNA strands:
- E.g. UV light, X-rays, radioactive materials, chemicals.
- Can cause bases to be deleted, totally removed from strand
- Can cause thymine bases to link together
- This causes a disruption in the normal functions of DNA
- High-energy radiation levels can actually break up the whole chromosome
Evidence for the mutagenic nature of radiation:
- UV radiation has been recorded to increase the incidence of skin cancers in humans. Some regard it as the sole cause of skin cancer
- First generation radiotherapists, who did not now the dangers of radiation, often died young. Scientists like Marie Curie would carry uranium around in their pockets, and developed cancers very quickly
- People who live in areas which have been affected by high-level radiation, such as Hiroshima, or Chernobyl, still show high incidences of cancers and other mutations in their offspring.
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and information from secondary sources to a modern example of ‘natural’ selection:
- Some organisms, such as bacteria and insects, produce large numbers of offspring.
Amongst large numbers of bacteria offspring, some individuals may carry genes that give them resistance to antibiotics. These individuals are then able to survive and reproduce with reduced competition from other members of the same species. Each generation will produce a higher percentage of individuals containing the resistant genes. This has been the story for antibiotics since they were first used. The initial use of an antibiotic results in good protection from bacteria. Over time the chemicals become less and less effective. A case study provides a good example of how natural selection occurs. A similar situation occurs in the resistance of insects to insecticides.
Selecting those individuals that are able to survive and reproduce increases the frequencies of those genes in the population. This is “survival of the fittest” where the fittest are those that have a natural resistance to a selecting factor, which in the case of bacteria described above, is antibiotics.
- Explain how an understanding of the source of variation in organisms has provided support for Darwin’s theory of evolution by natural selection:
- We know Darwin’s theory requires variation to be present within an organism
Our knowledge of genetics tells us where this variation comes from:
- The random segregation of chromosome pairs during meiosis
- Crossing over of genetic material during meiosis
- Random fertilisation of sex cells
- Random mutation of the genetic material
- The phenotypes that are variable are “chosen” by the environment
- Over time, some genotypes become more prevalent than others
- Process information from secondary sources to describe and analyse the relative importance of the work of:
- James Watson
- Francis Crick
- Rosalind Franklin
- Maurice Wilkins
- In the early 1950’s in England, two young scientists, Rosalind Franklin and Maurice Wilkins, decided to make a crystal of the DNA molecule to study its structure. They succeeded in getting DNA to crystallise, and by using X – ray crystallography they obtained an X – ray diffraction pattern. This pattern gave the first clue to the shape of the DNA molecule, which led to an understanding of how DNA functions. In 1953, James Watson and Francis Crick, also working in England, put together a model of DNA. They had enough information to make an accurate model after they examined Franklin and Wilkins X – ray patter and the results of American scientist Erwin Chargaffs research during the 1940’s.
Watson and Cricks model showed two chains, twisted into spirals (the ‘double helix’), of alternating sugar and phosphate units linked by pairs of the four bases. These four bases form the basic structure of the DNA of all organisms, although different species have a different number and arrangement of the bases.
In 1962 Watson, Crick, and Wilkins received the Nobel Prize in Chemistry for their discoveries. Sadly, Rosalind Franklin had died, and never received the recognition she deserved for her part in the unravelling of the structure of DNA. Their discoveries represented a significant advance in research and formed the basis of experimental work in genetic engineering in the 1970’s.
- Describe the concept of punctuated equilibrium in evolution and how it differs from the gradual process proposed by Darwin:
Darwin’s Gradualism:
- 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 in fact shows periods of stability followed by mass extinctions and rapid change
Punctuated Equilibrium:
- 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; they called it punctuated equilibrium.
- Punctuated equilibrium proposes that, instead of gradual change, there have been periods of rapid evolution followed by long periods of stability, or equilibrium.
- Current reproductive technologies and genetic engineering have the potential to alter the path of evolution:
- Identify how the following current reproductive technologies may alter the genetic composition of a population:
- Artificial insemination:
- Artificial pollination:
- Cloning:
- 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
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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.
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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
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ADVANTAGES: Particularly useful and easy way of breeding new varieties of plants. No expensive equipment required
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DISADVANTAGES: Genetic variation reduced.
Cloning:
- 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
- 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
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ADVANTAGES: In agriculture, cloned plants have identical requirements and grow in similar ways to produce similar yields at the same time. In plants and animals identical copies of desirable varieties can be produced
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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.
- Outline the processes used to produce transgenic species and include examples of this process and reasons for its use:
-
Transgenic species are organisms which have had genetic material from a different species transferred into their chromosomes
- That is, genes from one species have been taken and transferred into another
- The introduced gene instructs the transgenic organism to produce the desired trait or products
- This trait may be passed onto future generations
Processes Used to Produce Transgenic Species:
- The steps in producing a transgenic species is usually like this
- A useful gene, and the chromosome it is on, is identified
- The gene is ‘isolated’ or cut-out of its DNA strand
- Separate 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 used to do this.
- A vector is a carrier of a substance from one species to another
Techniques Used to Produce Transgenic Species:
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Isolating Genes: Once a useful gene is identified, it has to be isolated by ‘cutting’ it out of its DNA strand. Special enzymes, called restriction enzymes are used. More than 800 types are known. They cut DNA by breaking the hydrogen bonds in a triplet – the ends are called “sticky ends”
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Making Recombinant DNA: The DNA strands 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.
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Making Trangenes: An isolated gene cannot function if it is transferred 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 These Processes:
- These processes enable scientists to combine the qualities of different organisms
Transgenic species are being developed to:
- Increase the resistance of plants or animals to diseases, pests or extreme environmental conditions
- For medicines and vaccines and to study human diseases
- To improve productivity of crops, pastures and animals
- To improve the quality of food and efficiency of food processing
Examples of the Use of Transgenic Species:
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BT CROPS: BT is a bacterium that naturally produces chemicals 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 produce their own BT chemicals, and no longer need to be sprayed
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COLD STRAWBERRIES: A gene from a type of salmon that allows it to survive cold temperatures has been isolated, and inserted into a strain of strawberry. This strawberry can survive and grow in cold temperatures.
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BACTERIAL INSULIN: Diabetics previously obtained their insulin from animals, esp. pigs. The gene for insulin production, taken from the human pancreas, was placed in to the DNA of a bacterium. This now provides mass production of insulin.
Ethical Issues of Transgenesis:
- These technologies help treat diseases and increase food production
- Should we be tampering with nature in 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.
- Discuss the potential impact of the use of reproductive technologies on the genetic diversity of species using a named plant and animal example that has been genetically altered:
- 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
- E.g. cotton plants. The main crop being grown all over the world is BT cotton.
As more and more farmers shift from natural cotton to BT cotton, there are many disadvantages:
- Many natural varieties of cotton will be lost
- The species itself becomes vulnerable to extinction. If all cotton grown all over the world is BT, and a disease appears, that kills specifically BT cotton, than there is a risk of cotton becoming an extinct organism
- In another case, 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.