1. V) Development of Evolution
Lamarck (1744-1829): acquired characteristics
Hutton (1726-97): geological change happens gradually over long periods of time.
Cuvier (1769-1832): fossils in deeper layers were most different from modern species.
Lyell (1797-1875): geological processes occurred at the same rate in the present as they did in the past.
Wallace (1823-1912): theory of natural selection and wrote to Darwin to discuss it.
Darwin (1809-82): theory of evolution based on observations in South America. Reluctant to publish work because of political and religious upheaval. The Origin of the Species – species were not created in their modern form and natural selection was the mechanism of change.
Social and Political influences:
- Earth believed to be 6000 years old and each species created in present form by God
- Evolution reduced humans to same level as other organisms
- Threatened power of religious influences
- 1925 John Scopes was prosecuted for teaching evolution as a fact – the trial brought issue to public attention
- 1968 US Supreme Court ruled ban on teaching evolution was unconstitutional
2. Gregor Mendel and Inheritance
2. a ) Gregor Mendel’s Experiments
In 1860s Gregor Mendel formulated principles of genetics by careful and methodical experimentation with garden peas.
- Bred plants for over two years for true breeding (homozygous) - height, seed shape
- Crossed pure breeding with alternative form of each trait - monohybrid cross, F1 Gen.
- F1: t x s = t - did not blend, suggested one factor was dominant for each trait
- Allowed first generation to self-pollinate or cross-pollinate with each other
- F2: both short and tall - short was recessive – ratio 3 tall to 1 short for all characteristics (monohybrid ratio)
Conclusion:
- Characteristics weren’t blended but were separate units
- Each characteristic controlled by pair of factors
- Factors separated from one another when sex cells formed
- At fertilisation the offspring received one factor from each parent randomly
- Characteristics either recessive or dominant
2. b) Reasons for Success
- Kept accurate records
- Studied traits that were easy to distinguish
- Controlled pollination process carefully
- Peas – easy to grow, produced offspring rapidly, could self-pollinate
- Used mathematical and statistical analysis
- Used large number of peas plants and repeated many times
- Luck: factors were all on different chromosomes and weren’t genetically linked
2. c) Monohybrid Crosses
Monohybrid crosses involve one factor only – alternate homozygous crossing. Mendel explained the first generation trait as the dominant factor.
2. d) Homozygous and Heterozygous
Same alleles are homozygous, ie. TT and tt.
Different alleles are heterozygous, ie. Tt.
In the heterozygous condition the factor that is fully expressed is dominant and the factor that has no noticeable effect is recessive.
2. e) Genes and Alleles
Genes: inherited instruction on chromosome that encodes a specific protein product. Receive half genes from one parent and half from the other i.e. codes for hair
Alleles: one of several different forms of a particular gene. Every gene has at least 2 alleles. i.e. brown, black, blonde
2. f) Alleles and Phenotype
Dominant alleles are expressed in the phenotype whether they are heterozygous or homozygous. Recessives alleles are only expressed in the phenotype if no dominant allele is present.
2. g) Mendel’s Work was Unrecognised
1866 - Mendel’s ideas published but remained unrecognised for 34 years. Only in 1900 when three other people came up with the same results was the significance Mendel’s experiments recognised.
- Ideas of dominant/recessive inheritance went against accepted idea of blending characteristics
- No knowledge about chromosomes or genes at the time
- He was an unknown monk rather than a leading scientist
- Did not push his ideas and only presented his paper to a few people at local scientific meetings.
- I) Pedigrees
Heterozygous individuals will be affected
Two affected parents can produce an unaffected child
Heterozygous parents will be unaffected
Two affected parents will always have an affected child.
2. II) Monohybrid Crosses
2. III) Hybridisation
Hybridisation is the crossing of the same genus or same species to create new and better combinations of characters.
Santa Gertrudis Cattle developed in 1910 by crossing a Braham bull and a Shorthorn cow. High degree of heat and tick resistance, (Brahams) and little evidence of a hump and improved quality of meat, (Shorthorns).
Triticale is a result of crossing wheat and rye. It is high yielding, drought tolerant, grown in marginal wheat areas, disease resistant.
Plant hybridisation in food crops results in the reduction of biological diversity – disease and pest invasion are more devastating.
3. Chromosomal Structure
3. a) Sutton and Boveri
The Sutton-Boveri hypothesis is that genes are located on chromosomes. Both scientists stained cells to shows changes in the nucleus occurring for mitosis and meiosis.
Boveri (German): connection between chromosome and heredity - studied inheritance patterns in sea urchins. Chromosomes were not all the same and a full set were needed for sea urchins to develop normally.
Sutton (American): studied meiosis in grasshopper sperm. Each chromosome was unique and has a distinct structure that remains through division and fertilisation of sex cells. He explained not all factors followed Mendel’s laws because they were located on the same chromosome and were not independently assorted during meiosis. He concluded that chromosomes carried genes.
3. b) Chromosomes and Genes Chemicals
Chromosomes are thread like structures found in the nucleus of cells - 60% protein (histone) and 40% DNA. Genes are small length DNA linked together to make up a chromosome.
3. c) Structure of DNA
DNA is a nucleic acid that has double stranded molecules twisted into a double helix. The sides are deoxyribose sugar and phosphate. The rungs have complementary nitrogen bases: adenine and thymine, guanine and cytosine.
Nucleotide: sugar, phosphate and a base.
3. d) Chromosomes During Meiosis
Meiosis occurs during the production of sex cells, producing cells with half the normal chromosome number, haploid (n) instead of diploid (2n). Before meiosis chromosomes in the nucleus make copies called chromatids, linked by a centromere. Genes are located on chromosomes so they are copied when chromosomes are copied and separate during second division of meiosis.
First Stage: homologous chromosomes line up. Crossing over occurs - two chromosomes swap chromatid parts (recombinants). Genes located on same chromosome are linked – inherited together but crossing over breaks links meaning some genes change position during meiosis.
Second Stage: chromatids separate and four cells are formed with haploid number of chromosomes – gametes.
3. e) Variability of Offspring
Sexual reproduction leads to greater variability because it combines genetic information.
- Crossing over during meiosis produces new combinations of genes
- Chromosomes move randomly into one of the four gametes created during meiosis resulting in random segregation of genes
- Sex cells come together randomly at fertilisation
3. f) Sex Linkage and Codominance
Co-dominance: both alleles of homozygous parents are expressed in phenotype of heterozygous offspring in unblended form. Roan cattle show mixture of red and white hairs of homozygous parents. Human blood.
Sex Linkage: sex is genetically determined. Female - two X chromosomes. Male - X and Y. Sex linked genes are located on the sex chromosome, usually X. Sex-linked traits are usually recessive and more common in males.
Colour-blind person has recessive allele of no pigment. Y carries no allele for trait, so a single recessive on X is effective in males. Recessive alleles on both X chromosomes are needed for trait to show in females.
3. g) Morgan and Sex Linkage
Thomas Morgan studied inheritance of characteristics in fruit flies. He was fortunate because flies are easy to keep, breed rapidly and have only four chromosomes that are easy to see with a light microscope.
He bred a white-eye male with red-eye female. F1: all red eyes – based on Mendel he suggested red eyes were dominant. For F2 he expected 3:1 ratio of r:w but all females had red eyes and so did 50% of males.
He suggested the gene for eye colour is located on X and there is no corresponding gene on Y; therefore sex-linked.
3. h) Genotypes in Codominance
In a heterozygous trait the dominant allele is expressed in the phenotype. In codominance both alleles are fully expressed.
Shorthorn cattle have an allele for red and white hair, which are neither dominant.
3. i) Environment Affects Gene Expression
An organisms genotype only gives it genetic potential. The potential phenotype may or may not be reached depending on the environment experienced by the organism.
Hydrangeas: soil pH<5 = blue flowers, pH>7 = pink flowers.
Siamese Cats: grow darker fur on areas of the bodies that have a lower temperature.
Identical twins: same genetic material so differences in outward appearance are a result of environmental conditions.
3. I) Meiosis
3. II) Codominance and Sex Linkage
problems using punnet squares
3. III) Effect of Environment
1. One group of wheat plants grown under light
2. Another without light
3. Water, nutrients and other factors constant.
Results: plants without light were significantly shorter.
4. Changes in DNA
4. a) DNA Replication
Replication is the process where an exact copy of DNA is made during meiosis and mitosis.
- bond between bases break and helix unzips
- nucleotides are added to each side to produce identical strands of DNA
- the resulting DNA are the chromatids that separate during cell division
In sexual reproduction half the genetic info from each parent is passed onto the offspring. Information for the structure and chemistry of an organism is within this sequence of genes.
4. b) DNA Controls Polypeptide Production
Polypeptide synthesis involves a nucleic acid, RNA (messenger and transfer), a single strand of nucleotide bases containing ribose sugar, and thymine is replaced by uracil.
In nucleus double stranded DNA molecules unzip and DNA code is transcribed into the single stranded mRNA molecule. mRNA moves out of nucleus into cytoplasm and attaches to a ribosome. mRNA is translated into amino acids. At ribosome mRNA lines up forming a template. A group of three bases (codon) codes for a specific amino acid. Certain codes start and stop the chain formation. tRNA has an anticodon (non-amino acid forming) on one end and an amino acid on other. A polypeptide is formed as each amino acid is added from tRNA to a chain following the sequence on mRNA.
4. c) Proteins and Polypeptides
A polypeptide is made of a chain of amino acids. Polypeptides join to make proteins.
4. d) Mutations as a Source of New Alleles
Mutations occur when DNA deletes, duplicates, substitutes, or through inversion (breaks and joins the wrong way). This results in a new allele which produces a variation in offspring when passed on through sexual reproduction.
Sickle-cell anemia – a single mutation in hemoglobin molecule leads to production of valine instead of glutamic acid.
4. e) Mutagenic Nature of Radiation
Mutations are caused by mutagens, which are natural or human-made agents (physical or chemical) that can alter the structure or sequence of DNA. Mutagens can be carcinogens or teratogens. Only mutations in sex cells are passed onto the next generation. Radiation was the first mutagenic agent known.
- X-rays were first thought to be harmless and were a great novelty. Marie Curie, who discovered radioactivity, and her daughter both died of leukaemia.
- 1926 Hermann Muller showed genes exposed to X-rays were able to mutate cells
- Beadle and Tatum’s ‘one gene-one enzyme’ experiment showed X-rays cause mutations in cells
- Atomic bombs dropped on Hiroshima and Nagasaki (1945) increased the number of birth defects
4. f) Variations Supports Darwin’s Theory
Variation in a population comes from:
- Random pairing of gametes in sexual reproduction
- Crossing over during meiosis
- Random assortment of chromosome pairs in meiosis
- Mutations of chromosomes and genes
Sexual reproduction ensures individuals in any population differ. Selective pressures determine the individuals that survive and pass on their genes. Sources of new genes increase variety, which natural selection works with to lead to evolution, which supports Darwin’s theory of evolution.
4. g) Punctuated Equilibrium
Darwin theorised evolution occurs gradually over time but the fossil record shows groups of organisms that changed little for long periods. Punctuated equilibrium suggests evolution is a sudden random process not a continual gradual process. Long periods of equilibrium are interrupted by sudden environmental changes that lead to rapid evolutionary changes – 10000 years or less.
4. I) Polypeptide Synthesis
1. Nucleus → Transcription → DNA unwinds and one side is copied to mRNA
2. Cytoplasm → mRNA attaches to ribosome
3. Translation → mRNA into amino acid
4. tRNA line up amino acids on the ribosome
5. Amino acids join together to form a specific protein
4. II) Beadle and Tatum
Beadle and Tatum subjected bread mould (Neurospora crassa) spores X-rays in order to cause mutations. They found some of the mutated spores could not grow on normal culture medium (sugar, salts, vitamin biotin) unless provided specific amino acid which later was found to be arginine. They showed genes control biochemical processes. They hypothesised that the gene coded for the enzyme to make arginine has been destroyed by the X-rays which lead to the ‘one gene-one enzyme’ hypothesis. Later it was altered to ‘one gene-one protein’ because enzymes are proteins yet not all proteins are enzymes. Again this was changed to ‘one gene-one polypeptide’ because it was more correct as many proteins are made of more than one polypeptide and each gene codes for a polypeptide.
4. III) DNA Sequences and Cell Activity
Changes in DNA sequencing could lead to the production of dysfunctional protein. A substitution for a single base pair on a strand of DNA will result in a different amino acid codon forming a different polypeptide.
Cell activity is controlled by enzymes. Enzymes are formed from chains of polypeptides. If the amino acid chain forming the polypeptide is not in the right sequence, then the enzyme will not be functional.
4. IV) Natural Selection
Development of insecticide resistance in insect pests or antibiotic resistance in bacteria are modern examples of natural selection.
Mosquito and DDT: Variations occur naturally in the initial population. Thus when DDT sprays were introduced, some of the mosquitoes survived even though many have been wiped out. These survivors who possess the desirable characteristics or adaptations breed and reproduced offspring who will also be resistant to DDT sprays.
4. V) DNA
James Watson - with Crick suggested the double helix structure and pairing of bases; pointing out its significance as a copying mechanism for genetic material.
Francis Crick - studied genetic code after they worked out the structure of DNA.
Rosalind Franklin - produced the picture that showed the double helix of DNA by X-ray diffraction analysis.
Maurice Wilkins - supplied X-ray diffraction patterns that made it possible for Watson and Crick to construct the double helix model. He took them from Franklin without permission.
5. Reproductive Technologies & Genetic Engineering
5. a) Reproductive Technologies
Artificial insemination: fusion of sperm and ova to produce a desirable trait. IVF – fertilisation occurs outside body and the embryo is the implanted in the uterus.
Artificial pollination: pollen from stamens of one plant is dusted onto the stigmas of another to produce the desired genetic composition.
Cloning: reproduction of the same set of genes to produce genetically identical individuals.
5. b) Producing Transgenic Species
Transgenic species are produced by placing a gene from one species into another.
Manipulating DNA:
- Restricting enzymes by cutting DNA at specific locations
- Ligases strengthen bonds between newly reformed DNA
- Multiple copies of the recombinant DNA are created using polymerase chain reaction
Methods:
- Microinjection: DNA is inserted into a cell from another species using a glass needle
- Biolistics: gene gun fires small metal particles coated with DNA into another cell nucleus
- Electroporation: cells are exposed to short electrical impulses so small pores form in the membrane so genes can be inserted
Reasons:
- increasing crop yield
- resistance to pests and diseases
- ability to survive in extreme conditions
- producing drugs and vaccines
- better quality produce
- new products
- tissue and organ donation
5. c) Impact on Genetic Diversity
Reproductive technologies reduce natural variation by producing identical organisms.
Crops: more consistent growth rates and harvesting times but if change in environment occurs then whole lot could be wiped out.
Transgenic species: initially increases genetic diversity because a gene from a species is placed in another, making that gene pool larger.
- Bt cotton: if used exclusively then natural cotton may become extinct and having a monoculture means it is more susceptible to environmental changes and can lead to Bt resistance in insects.
-
Salmon: genetically modified to grow bigger than normal. If fish become released into the wild then the wild females would mate with the larger males and the transgene would spread rapidly in the natural population.
5. I) Cloning
Tissue culture propagation - tissue from roots is taken and root cells separated. Cells are grown in nutrient-rich medium where they become unspecialised. After treatment with plant hormones, calluses develop into seedlings that grow into mature plants genetically identical to the original ‘parent’ plant. Wollemi Pine.
Nuclear transfer is a methodology used in cloning. An egg cells is evacuated and the nucleus from another cell is taken out and inserted into this egg cell. When the egg cells is allowed to develop, it will be a genetically identical clone.
5. II) Transgenic Species
Transgenic species genetic material originating from another organism.
See 5. c
