1. Blueprint of life Miss Heretakis
Past HSC questions :
Miss Heretakis

2. DNA functioning - changes in DNA structure are reflected in phenotype
4. The structure of DNA can be changed and such changes may be reflected in the phenotype of the affected organism
A genome is all of the genetic material (DNA) within a cell and is specific to each organism.
The extent of phenotypic differences depends on how different the DNA sequences are in individuals, but may also be influenced by the environment.

3. Beadle and Tatum: The ‘one gene - one polypeptide’ hypothesis
analyse information from secondary sources to outline the evidence that led to Beadle and Tatum’s ‘one gene - one protein’ hypothesis and to explain why this was altered to the ‘one gene—one polypeptide’ hypothesis
George Beadle and Edward Tatum (1941) hypothesised that one gene controls the production of one enzyme.
They experimented on the bread mould Neurospora crassa:
They irradiated the bread mould with X-rays to induce mutations. The resulting mould forms they called mutants.
Further experimentation showed that some of the mutants could no longer produce an essential amino acid, because it lacked a necessary enzyme.
6/3/17

4. Beadle and Tatum: The ‘one gene - one polypeptide’ hypothesis
Growth of different mutant strains with different combinations of nutrients helped establish which enzyme was lacking in each strain.
Normal mould grew on complete and minimal medium
Mutant spores grew on complete medium, but could not grow on the minimal medium - mutant moulds required additional nutrients
6/3/17

5. Beadle and Tatum: The ‘one gene - one polypeptide’ hypothesis
To test whether this loss of function had a genetic basis, they crossed these mutant moulds with the normal moulds. They found that some of their offspring shared the mutant phenotype, proving that the inability to produce the amino acid could be inherited.
With further analysis it was found that different enzymes in different mutants had been altered or were missing, causing a block at one step in the metabolic pathway. This led them to propose their first hypothesis: the ‘one gene—one enzyme’ hypothesis.

6. Beadle and Tatum: The ‘one gene - one polypeptide’ hypothesis
The one gene-one enzyme hypothesis was later changed to the one gene-one protein hypothesis once it was discovered that there are other proteins, besides those that make up enzymes that are encoded by genes.
This changed again to the currently accepted theory, the one gene-one polypeptide hypothesis, once it was discovered that one gene is not necessarily responsible for the structure of an entire protein, but for each polypeptide chain making up that protein.

7. DNA functioning - replication and protein synthesis
DNA has two main functions:
Heredity - relies on DNA replication
Gene expression - relies on protein synthesis
The genes found on DNA code for the production of proteins which:
control the chemical functioning of cells
form a structural part of the cell (e.g. the protein in cell membranes, pigment in skin and eyes)
form essential chemicals such as hormones (e.g. insulin)
form defence proteins (e.g. antibodies)
form transport proteins (e.g. haemoglobin)

8. DNA functioning - replication and protein synthesis
Incorrect base pairing will result in a change in the DNA base sequence – known as a mutation
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 helix molecule.
The process of DNA replication:
Nucleotides are added to each single strand
Nucleotides are picked up by the enzyme DNA polymerase and are added to each single strand, with complementary bases added to each original base.
The base pairing is checked by another DNA polymerase enzyme which ‘edits’ any incorrect additions.
DNA unzips—that is, the two strands separate.
The weak hydrogen bonds between the base pairs break, exposing the nucleotide bases.
Two single strands of DNA are formed.
The direction in which nucleotide insertion occurs is antiparallel on the two opposite strands—on one strand it begins at the replication fork and goes towards the end of the strand, whereas on the other strand it begins at the end of the single strand and goes towards the replication fork.
The DNA double helix unwinds.
An enzyme called helicase causes the DNA helix to progressively unwind.

9. DNA functioning - replication and protein synthesis
Result of DNA replication:
Each resulting DNA molecule contains one strand of the existing DNA
molecule and a newly-synthesised strand.
The replicated DNA molecules rewind into the double helix conformation, like the original molecule. The end result is that there
are two molecules of DNA, each a double-stranded helix, and they are identical to each other and to the original molecule from which they formed.
Colouring DNA replication -

10. DNA functioning - replication and protein synthesis
The significance of DNA replication:
HEREDITY AND THE NEED FOR REPLICATION
The genetic material of a cell must be transmitted from:
one cell to another during mitosis, allowing for growth, repair and maintenance of an organism
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 on to each new daughter cell that arises from it. If DNA replication goes wrong (mutation occurs), this has a direct effect on the phenotype of the individual.
Colouring DNA replication -

11. Protein synthesis - making protein from a DNA message
explain the relationship between proteins and polypeptides
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.
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.

12. The significance of protein synthesis
outline, using a simple model, the process by which DNA controls the production of polypeptides

13. The significance of protein synthesis
outline, using a simple model, the process by which DNA controls the production of polypeptides

14. The significance of protein synthesis
outline, using a simple model, the process by which DNA controls the production of polypeptides
In order for a cell to make particular proteins, only the relevant instructions for those proteins are accessed in the DNA nucleotide sequence.
DNA never leaves the nucleus – it holds the original copy of all instructions.
An intermediate molecule, called messenger RNA (mRNA), is created and carries a transcribed copy of the relevant instructions from the nucleus to the ribosomes in the cytoplasm.
The ribosomes can be considered as the ‘machinery’ that translates the message carried by the mRNA into a cell product such as protein.

15. The significance of protein synthesis
The sequence of information transfer necessary for DNA to direct the production of proteins is summarised in a framework known as the central dogma:
DNA → RNA → protein

16. The significance of protein synthesis
DNA → RNA → protein
DNA consists of long chains of nucleotides wound into a double helix.
The sequence of nucleotide bases codes for the sequence of RNA nucleotides and ultimately the sequence of amino acids that form the polypeptide chain.

17. The significance of protein synthesis
DNA → RNA → protein
Like DNA, RNA is a nucleic acid made from a chain of nucleotides, but it differs from DNA in the following ways:
RNA is single stranded
The sugar in RNA is ribose sugar (not deoxyribose sugar as in DNA).
RNA has the nitrogenous base uracil (U) instead of thymine (T).

18. The significance of protein synthesis
There are three types of RNA:
mRNA (messenger RNA):
Are single stranded molecules that carry the information from DNA to the ribosomes in the cytoplasm of the cell.

19. The significance of protein synthesis
tRNA (transfer RNA):
Are found in the cytoplasm. They are a twisted clover-leaf structures which carry amino acids to the location of protein synthesis. On one end of the tRNA there are three unpaired bases (called an anticodon) which attach the tRNA to its complementary bases on the mRNA strand. The other end of the tRNA is temporarily bonded with an amino acid. The specific sequence of three bases at the anticodon end determines which amino acid will be carried by that tRNA.

20. The significance of protein synthesis
rRNA (ribosomal RNA):
Is a structural component of ribosomes, the protein synthetic factors in the cell.
Ribosomes are where RNA is translated into protein. This process is called protein synthesis. Protein synthesis is very important to cells, therefore large numbers of ribosomes are found in cells. Ribosomes float freely in the cytoplasm, and are also bound to the endoplasmic reticulum (ER)

21. The significance of protein synthesis
DNA → RNA → protein
See pages for steps involved in protein synthesis

22. Developing a model of protein synthesis
perform a first-hand investigation or process information from secondary sources to develop a simple model for polypeptide synthesis

23. DNA functioning gone wrong - mutations
A mutation is a change in the genetic material of a cell—that is, a change in the sequence of nucleotides of DNA.
All mutations do not arise in the same manner—some mutations arise spontaneously whereas others are induced.
Mutations that are heritable are the direct source of all new alleles.

24. DNA functioning gone wrong - mutations
Environmental agents that cause mutations are termed mutagens.
Exposure to these substances over a long period of time increases their harmful effects.
The process of inducing a mutation is termed mutagenesis.
There are many mutagens known, including:
Chemical mutagens
Biological mutagens
Mutagenic radiation
Ultraviolet radiation

25. The mutagenic nature of radiation
discuss evidence for the mutagenic nature of radiation
During the late 1800s and early 1900s, many scientists were involved in studying radiation. At this time, the harmful effects of radiation were still unknown.
Scientists who were exposed to large amounts of radiation over prolonged periods of time developed various illnesses.
Marie Curie worked with ionising radiation for most of her career and died from leukaemia due to overexposure to radioactive emissions.

26. The mutagenic nature of radiation
Survivors of the 1945 bombing of Hiroshima suffered physical mutations as a result of radioactive output from the nuclear explosion.
More recently, victims of the nuclear meltdown in Chernobyl have suffered high levels of infertility and genetic mutations, as well as non-cancerous side effects including cardiovascular and respiratory conditions.

27. 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 early 1900s.
Between 1925 and 1940, experimental research provided evidence of the mutagenic nature of radiation. Advances in cell studies provided further support.

28. Mutations may lead to new alleles
explain how mutations in DNA may lead to the generation of new alleles
Mutations alter genes by changing the nucleotide sequence in DNA.
If a gene is altered from its original form the two variations of the gene are termed alleles of that gene.
These changes may result in the production of new proteins. Most new proteins have little effect on the organism, but a few will lead to genetic disorders and inherited diseases (e.g. a changed allele for the haemoglobin gene, results in the disease sickle cell anaemia).

29. Mutations may lead to new alleles
INHERITABLE MUTATIONS PASS TO FUTURE GENERATIONS
Mutations in somatic cells are not passed on to offspring— the new allele may cause a defect in an individual, but will not affect future generations.
Mutations in germ-line cells (gametic mutations) produce alleles that can be inherited and may therefore have significant effects on populations and so are important in evolution.

30. Mutations may lead to new alleles
EFFECTS OF MUTATIONS AT THE GENE AND CHROMOSOME LEVEL
Changes to genetic material arise during replication and may result in a change to a single gene—termed gene mutations.
Most gene mutations produce recessive alleles because they prevent the gene from producing a functional protein. If these alleles occur in the homozygous form, the mutation can affect the phenotype of the individual.
This phenotypic change may be of advantage to the organism (e.g. pesticide resistance in an insect), or it may be harmful (e.g. may cause a disease such as haemophilia).
Mutations may involve:
base substitution of one pair of nucleotides (point mutation)
frame shift where extra bases are added or deleted from a strand of DNA (macromutation)
A sequence within a gene may be duplicated or translocated

31. Mutations may lead to new alleles
Other mutations involve the rearrangement of a block of genes or whole chromosomes— termed chromosome mutations.
If whole chromosomes become rearranged, a change in chromosome number may arise. This usually occurs as a result of chromosomes not separating out correctly during meiosis. The resulting cells may have one chromosome less or more than normal.
e.g. individuals with Down Syndrome have 3 copies of chromosome 21 – this has numerous phenotypic effects including slanting eyes, flat back of the head, short, growth failure and mental retardation.

32. Flow chart to show that changes in DNA activity result in changes in cell activity
process information to construct a flow chart that shows that changes in DNA sequences can result in changes in cell activity
Changes in DNA sequence will alter the codons on the mRNA, which may change the amino acid sequence as different tRNA molecules will bring different amino acids. The omission or addition of one nitrogenous base will cause a shift in the entire DNA molecule, causing a different polypeptide to be produced.
For example, if thymine were omitted:
A change in amino acid formation will cause different polypeptide to be formed, which will fold differently to form a different protein or enzyme.
As enzymes control cell activity, modifying the DNA, which changes the enzyme, will cause changes in cell activity.
Changes in DNA —> change the the messenger RNA —> changes the transfer RNA —> changes the code reproduction —> changes the amino acid formation —> changes the protein.

33. Darwin revisited A mutation is the basic source of all variation.
explain how an understanding of the source of variation in organisms has provided support for Darwin’s theory of evolution by natural selection
A mutation is the basic source of all variation.
The understanding that mutations affect the base sequence of DNA provides support for Darwin’s theory of evolution because it provides a mechanism to explain how heritable variation arises.
A mutation will result in a variation in phenotype that may be negligible in its effect, or it may confer some advantage or disadvantage to the organism.

34. Drug resistant bacteria multiply and thrive
Darwin revisited
If mutations can be inherited, they provide the variation on which natural selection acts, for evolution to occur. Organisms with more favourable characteristics for a particular environment are more likely to survive and reproduce, passing on these favourable variations.
Mutation in DNA
Drug resistant bacteria multiply and thrive