HSC Biology – Blueprint of Life
4. The structure of DNA can be changed and such changes may be reflected in the phenotype of the affected organism.
The phenotype of an organism is its total appearance determined during development by an interaction between its genetic make-up (genotype) and the environment.
A genome is all of the genetic material (DNA) within a cell and is specific to each organism. Genomes influence nearly all the traits or phenotypes. The phenotypic appearance is therefore directly affected by gene expression. The extent of phenotypic differences depends on how different the DNA sequences are in individuals, but may also be influenced by the environment.
· Outline evidence that led to Beadle and Tatum’s ‘one gene-one protein’ hypothesis and explain why this was altered to the ‘one gene – one polypeptide’ hypothesis
Beadle and Tatum carried out experiments with red bread mould. The normal variety of mould can manufacture certain substances that it needs for living, including vitamin B1, B2, B4 and B12. The normal moult possesses specific enzymes that catalyse the different reactions that produce these vitamins.
Beadle and Tatum produced several varieties of the bread mould, each of which had a change in one of its genes. They tested these varieties and found that some had lost their ability to make vitamin B2 while others could no longer make vitamin B4 and so on.
The results obtained by Beadle and Tatum showed that a change in various genes of the bread mould resulted in the loss of different enzymes and the failure of specific products to appear. These results indicated that one gene controlled vitamin B2 production and a different gene controlled vitamin B4 production.
CHANGED GENE -> NO ENZYME -> NO REACTION -> NO VITAMIN
Beadle and Tatum concluded that different genes are involved in making different enzymes that catalyse different reactions in a cell. They stated that there was a ‘one to one relationship between a gene and specific enzymic reaction.
ONE SPECIFIC GENE -> ONE PROTEIN (ENZYME) THAT CATALYSES REACTION
Many proteins consist of a single polypeptide that is built of amino-acid subunits. However some proteins consist of several polypeptides that become associated to form an active protein. For example, haemoglobin consists of four polypeptides. Recognition that proteins may consist of one or more polypeptides means that Beadle and Tatum’s conclusion is now expressed more accurately as:
One particular gene -> one polypeptide chain => one cellular activity
Extra:
The proposal that a gene is responsible for the production of a specific protein was first put forward in 1909. This idea, together with work on the inheritance of eye colour in Drosophila, led biologists to investigate the importance of genes in enzyme production. George Beadle and Edward Tatum worked on mutants of the fungus Neurospora crassa (a mould), leading to their groundbreaking discovery that genes provide the instructions for making proteins.
They put forward the hypothesis that one gene controls the production of one enzyme. This was based on observations made in Beadle’s earlier experiments on fruit flies—he found that if fruit flies with normal eye colour were exposed to X-rays, their offspring would show a change in eye colour. He hypothesized that this occurred as a result of a defective enzyme for the eye pigment. Since evidence was needed to support this hypothesis, Beadle and Tatum designed and carried out an experiment to attempt to mutate genes of the mould Neurospora crassa, the enzyme functioning of which they could test fairly simply. Experimental evidence led them to propose a hypothesis that genes affect enzyme production—the details of this were later modified as the understanding of the relationship between genes and proteins advanced.
Beadle and Tatum’s experiment
1. They irradiated the bread mould Neurospora crassa with X-rays to induce mutations. The resulting mould forms they called mutants.
2. Further experimentation showed that some of the mutants could no longer produce an essential amino acid (implying that a particular enzyme was no longer functioning).
3. To test that whether this loss of function had a genetic basis, they then crossed these mutant moulds with the normal moulds and found that some of their offspring shared the mutant phenotype, proving that the inability to produce the amino acid could be inherited—that is, it was due to a genetic defect or mutation.
4. With further analysis it was found that different enzymes in different mutants had been altered or were missing, proving that a gene determines the structure of a specific enzyme. This led them to propose their fi rst hypothesis: the ‘one gene—one enzyme’ hypothesis.
A changed hypothesis
Beadle and Tatum’s ‘one gene—one enzyme’ hypothesis changed to the ‘one gene—one protein’ hypothesis, once it was demonstrated that there are other proteins besides enzymes that are encoded by genes. After the discovery by biologists that one gene is not necessarily responsible for the structure of an entire protein, but for each polypeptide chain making up that protein, the current one gene—one polypeptide hypothesis was adopted. This is the currently accepted theory and has stood up to rigorous testing, holding true for all predictions so far.
· Describe the process of DNA replication and explain its significance
The active functioning of DNA in the everyday activities of cells takes two forms:
1. DNA replication: In its hereditary role, DNA must be able to make an exact copy of itself so that when a cell divides, the resulting daughter cells each have a full complement of DNA.
2. Protein synthesis: Genes are expressed in terms of the protein products they produce. Many of these proteins are enzymes, which control the chemical functioning of cells. Other proteins produced nay form a structural part of the cell (e.g. the protein in cell membranes, pigment in skin and eyes, and silk in insect cocoons) and some proteins form essential chemicals such as hormones (e.g. insulin).
Process of DNA Replication
The process of DNA replication is termed semi-conservative, as the two strands of the original DNA molecule separate and each gives rise to a new complementary strand. This mechanism ensures that the genetic material is copied exactly.
DNA replication begins when a region of double-stranded DNA unwinds to form two short lengths of single-stranded DNA. An enzyme called helicase causes the DNA helix to progressively unwind.
Each of the single strands acts as a template for building a new complementary strand. Nucleotide building blocks are the raw material for DNA replication. The nucleotides come into place – where G is the template strand, a C-containing molecule is brought into place. Where there is a T in the template strand, an A-containing molecule is brought into place. Where there is a T in the template strand, an A-containing molecule is brought into place, and so on. In this way, the base sequence of a double-stranded molecule of DNA controls the order of the nucleotides in two new single strands of DNA. This process is catalysed by the enzyme, DNA polymerase. Each nucleotide joins to its neighbour in the chain by a strong sugar phosphate bond. The direction in which the nucleotide insertion occurs is antiparallel on the opposite strands – on one strand it begins at the replication fork and goes towards the end of the strand whereas on the other, it begins at the end single strand and goes towards the replication fork. The DNA polymerase is essential for editing any incorrect additions to ensure accuracy. Incorrect base pairing will result in a change in the DNA base sequence, known as a mutation.
DNA replication results in the formation of two double-stranded molecules of DNA, each of which is identical to the original double stranded DNA molecule. The two DNA molecules that are produced contain one old strand from the original molecule and one new synthesised strand and the genetic instructions they carry are precisely copied.
Where does DNA replication occur?
DNA replication must occur before mitosis takes place in tissues such as the germinal layer of the skin.
DNA replication is part of a process of cell reproduction (mitosis) that is responsible for growth of organisms and for replacement of cells and tissues.
Growth involves many cycles of cell reproduction and during each cycle, the DNA of the chromosomes is replicated. Likewise, tissue replacement involves cell reproduction such the replacement of skin tissue and blood cells. The precision of DNA self-replication means that the new body cells produced by an organism during growth or replacement have exact copies of the double set of genetic instructions present in the fertilised egg from which that organism developed.
The Significance of DNA Replication
DNA has two main functions:
1. Heredity – this relies on DNA replication.
2. Gene Expression – this relies on protein synthesis
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.
In each of these situations, it is necessary for the DNA to exactly replicate. This replication ensured that the genetic code of a cell is passed onto each new daughter cell that arises from it. An exact replica must be produced so that the new cells have the same, distinctive message that the original cell had. If DNA replication goes wrong, this has a direct effect on the phenotype of the individual.
· Explain the relationship between proteins and polypeptides
Proteins are large, complex macromolecules made up one or more long chains called polypeptides. Each polypeptide chain consists of a linear sequence of many amino acids joined by a peptide bond. 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 determines 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.
· Outline, using a simple model, the process by which DNA controls the production of polypeptides
Multicellular organisms are made up of a variety of different cells; e.g. humans have over 200 different types of cells including skin cells, muscle cells, blood cells and many others. Despite differing in structure, every cell that has a nucleus has a full copy of the same coded genetic information in its DNA. This encoded information directs the production of cell products such as polypeptides which form proteins, the key to cell specialisation and differentiation. In specialised cells, coded instructions for the production of a particular protein are ‘switched on’. This ensures that the cell develops a particular structure, in keeping with the type of tissue to which it belongs.
The process of protein synthesis
DNA never leaves the nucleus. In order for a cell to make proteins, only the relevant instructions for those proteins are accessed in the DNA nucleotide sequence. Since the DNA instructions must remain in the nucleus, an intermediate molecule – messenger RNA (mRNA) – is created; this 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 translated the message carried by the mRNA into a cell product such as protein.
The sequence of information transfer necessary for DNA to direct the production of proteins is summarised below, in a framework known in genetics as the central dogma:
DNA => RNA => Protein
The chemicals involved in protein synthesis
DNA:
DNA consists of long chain of nucleotides wound into a double helix. The sequence of nucleotide bases determines the meaning of the message – because it codes for the sequence of RNA nucleotides and ultimately the sequence of amino acids that form the polypeptide chain.
RNA:
Like DNA, RNA is a nucleic acid made from a chain of nucleotides, but it differs from DNA in the following ways:
- Most RNA is single-stranded
- The sugar in RNA is ribose sugar
- RNA has a nitrogenous base uracil (U) instead of thymine.
There are three types of RNA:
- messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA):
- mRNA is single-stranded and is not twisted into a helix. mRNA molecules are a few thousand bases long, mush shorter than DNA. They are found in both the nucleus and cytoplasm. They function as an intermediate molecule, carrying information from DNA in the nucleus to the ribosomes in the cytoplasm.
- tRNA molecules occur in the cytoplasm. Each one is 75 nucleotides long and twisted into the shape of a clover leaf. On one end of the tRNA there are three unpaired bases called the anticodon, which attach the tRNA to its complementary bases on the mRNA strand. The other end of the tRNA is able to bind with an amino-acid temporarily. Each tRNA molecule will only attach to one particular amino acid. The specific sequence of these three bases at the anticodon end determines which amino acid will be carried by the tRNA.
- rRNA forms a structural part of the ribosomes
The Steps involved in Protein Synthesis
1) An enzyme, RNA polymerase, binds to a part of the DNA called the promoter and the DNA ‘unzips’ – that is, the DNA unspirals, hydrogen bonds break between the two strands and the strands separate over a short length, just in that part of the DNA that holds the gene to be used. Only one strand of DNA contains the genetic information to make a protein; rather confusingly, it is called the non-coding strand or sense strand. The other strand is called the coding strand, or anti-sense strand.
2) Transcription of the gene occurs, controlled by the enzyme RNA polymerase: the sense strand of the DNA acts as a template and the RNA nucleotides are assembled forming a complementary single stranded mRNA molecule. The sequence of nucleotide bases on the mRNA molecule is the same as the DNA coding strand, except that is has U instead of T.
3) The mRNA moves out of the nucleus and into the cytoplasm where it encounters a ribosome. In eucaryotes, ‘editing’ of the mRNA may take place at this point.