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Part A: Chromosomes, genes, and DNA

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DNA is found in the nucleus of a cell, in the chromosomes
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Genes determine features by instructing cells to produce proteins which develop the
feature
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The structure of DNA:
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Made of two strands of nucleotides (polynucleotide)
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Double-helix shape
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Only one DNA strand codes for the manufacture of proteins in a cell: the sense strand
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Proteins manufactured: intracellular enzymes (enzymes that control internal cell processes),
extracellular enzymes (enzymes that are secreted to affect the outside of the cell), structural
proteins, and hormones
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It is the bases that code for the amino acid since the sugar and phosphate are the same in all
strands
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DNA Replication:
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3
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The DNA strands separate
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DNA polymerase assembles into two new strands, according to the base-pairing rule
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Gene Mutations:
*Mutation: a change in the DNA of the cell, resulting in a gene mutation
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Duplication: the nucleotide is inserted twice
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Substitution: a different nucleotide is used
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The changing of
the amino acid can alter the functioning of a protein
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Mutations that occur in body cells with only affect that particular cell, and if harmful, they
usually die off without replicating
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Only mutations in the sex cells or cells that divide to become sex cells can be inherited
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Mutagens that affect genetic mutations: ionizing radiation (UV light, X-rays, gamma rays),
chemicals (mustard gas, nitrous oxide)
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Each chromosome contains a different DNA molecule with different genes
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X and Y chromosomes are the sex chromosomes
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Genes and Alleles:
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Different forms of a gene are called alleles
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Chromosome Mutations:
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Cells do not always divide properly: bits of chromosome can break off and attach to another
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Mutated sex cells may contain less or more chromosomes than normal
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Heart and lung
defects in the children are common
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The cell must copy each chromosome before it divides
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All somatic cells are formed by mitosis from a zygote
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The cell cycle—
 Prophase: two chromatids, nuclear membrane breaks down
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 Anaphase: spindle fibres shorten and chromatids are pulled to poles
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Meiosis:
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The cell must copy each chromosome so that there is enough genetic material to be shared
between the four daughter cells
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Genetic variation
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During the second division, the chromosome separates into two parts, one going in each daughter
cell
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Exception: identical twins (monozygotic twins)—when the zygote divides, the two genetically
identical cells formed do not stay together, but rather separate and behave as though each is an
individual cell
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Asexual Reproduction and Cloning:
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No fusion of sex cells
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Common in plants and some animals
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Most features of the pea plants Mendel studied only had two alternate forms and there were no
intermediate forms
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He then
collected all the seeds produced by the P generation and allowed them to self-fertilize after noting
all their features
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The offspring of these self-fertilized plants formed
the F2 generation of pea plants
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Modern-day explanation of Mendel’s results:
o Each feature is controlled by a gene in a chromosome
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o Sex cells only have one copy of each chromosome
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o One allele is dominant over the other
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Codominance:
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Complete dominance: one allele completely masks the effect of the recessive allele
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 Snapdragon plants have red, white, or pink flowers
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Most genes do not show complete dominance
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The gender of a zygote essentially depends on if the sperm cell carries an X chromosome or a Y
chromosome, since all egg cells carry X chromosomes
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The overall ratio of male and female births is 1:1 but this ratio is not always met
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Natural selection is the mechanism by which new species arise, by enabling the most adapted
organisms for certain environments to survive and reproduce
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He was
influenced by geologist Lyell’s studies because he noticed gradual changes in the fossils in the
layers of rock
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Darwin’s Observations:
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Organisms tend to produce more offspring than needed to replace them
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Members of the same species are not identical
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If some offspring survive whilst others die, those organisms best suited to their environment will
reproduce
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Changing conditions (selection pressure) favors one form of species, which has the selective
advantage
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The frequency of the less adapted species decreases under this selection pressure
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The Peppered Moth:
*Peppered moths are food for birds, and with increasing industrialization and increased deaths of many
lichens growing on trees, darker moths became more prevalent
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In the country, there is less soot on the trees; in the cities, there is more
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Camouflaged moths are less likely to be eaten by birds
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Gradually, the peppered moth population would increase in the country, and in the city the dark
moth population would increase
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The use of antibiotics has increased
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Since
antibiotics are widely used, this resistant bacterium has an advantage over the normal bacteria
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Super bugs (bacteria that are resistant to many antibiotics) can be caused by jumping genes
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Natural Selection and Sickle-Cell Anaemia:
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Sickle-cell anaemia is caused by a mutant allele that affects the formation of haemoglobin in red
blood cells
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Heterozygous carriers of the allele usually show no symptoms, but they do benefit:
 When the malaria parasites enter the body, the fragile red blood cells of the carriers often
burst before the parasite has time to develop, so the parasite dies
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The Course of Evolution:
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Life began in water as a result of chemical reactions
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These unicells became more complex as more organelles were gained
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Multicellular organisms became more complex
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From time to time, events caused mass extinctions
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*Niche: a habitat of any organism and the role of the organism in that habitat
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Human choice rather than environmental factors provide the selection pressure
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Brassica is a genus of cabbage-like plants that was selectively bred to produce cabbage,
cauliflower, Brussel sprouts, kale, broccoli, and kohlrabi
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Crops are bred to produce strains that:
 Give higher yields
 Are resistant to diseases
 Are resistant to pest damage
 Are hardier
 Have better nutrients

Selective Breeding in Animals:
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Artificial insemination (widely available, and used particularly with cows) involves taking
organisms with desired characteristics and keeping them until their semen is obtained
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Modern sheep are domesticated wild sheep; dogs are domesticated wolves; pigs are domesticated
wild boars
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4
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Dip the side shoot into rooting powder to encourage root formation
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Place under glass to speed up growth and reduce water loss (heat and humidity under greenhouse
conditions)
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Large numbers of genetically identical plants can be produced rapidly
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Plants can be produced at any time of the year
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Genetic modifications can be introduced to large numbers of plants quickly

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