Question

Briefly discuss the following:(a) spontaneous mutation   (b) miss-sense mutation   (c) non-sense mutation  (d) reversion (e) semi-conservative replcation  (f) gain of function mutation    

Answer 1

(a) spontaneous mutation

• They can be characterized by the specific change: Tautomerism — A base is changed by the repositioning of a hydrogen atom, altering the hydrogen bonding pattern of that base, resulting in incorrect base pairing during replication.
• A common cause of spontaneous point mutations is the deamination of cytosine to uracil in the DNA double helix. ... Another cause of spontaneous mutations is copying errors during DNA replication. Although replication generally is carried out with high fidelity, errors occasionally occur.
• Point mutations may arise from spontaneous mutations that occur during DNA replication. The rate of mutation may be increased by mutagens. Mutagens can be physical, such as radiation from UV rays, X-rays or extreme heat, or chemical (molecules that misplace base pairs or disrupt the helical shape of DNA).
• Sickle cell anemia is an example of a spontaneous mutation that stuck around to future generations because it is occasionally beneficial. The other type of mutation is induced mutation. ... Point mutations happen when one half of a base DNA pair is changed. Deletion, where part of a chromosome is removed, might also occur.

(b) miss-sense mutation

A missense mutation is a mistake in the DNA which results in the wrong amino acid being incorporated into a protein because of change, that single DNA sequence change, results in a different amino acid codon which the ribosome recognizes. Changes in amino acid can be very important in the function of a protein. But sometimes they make no difference at all, or very little difference. Sometimes missense mutations cause amino acids to be incorporated, which make the protein more effective in doing its job. More frequently, it causes the protein to be less effective in doing its job. But this is really the grist of evolution, when missense mutations happen, and therefore small changes, frequently small changes in proteins, happen, and it happens to be that it improves the function of a protein. That will sometimes give the organism that has it a competitive advantage over its colleagues and be maintained in the population.

A missense mutation is when the change of a single base pair causes the substitution of a different amino acid in the resulting protein. This amino acid substitution may have no effect, or it may render the protein nonfunctional.A common and well-known example of a missense mutation is sickle-cell anemia, a blood disease. People with sickle-cell anemia have a missense mutation at a single point in the DNA. This missense mutation calls for a different amino acid, and affects the overall shape of the protein produced.

(c) non-sense mutation

A nonsense mutation is a genetic mutation in a DNA sequence that results in a shorter, unfinished protein product. DNA is a chain of many smaller molecules called nucleotides. During protein formation, DNA (or RNA) nucleotide sequences are read three nucleotides at a time in units called codons, and each codon corresponds to a specific amino acid or stop signal (stop codon). Stop codons are also called nonsense codons because they do not code for an amino acid and instead signal the end of protein synthesis. Thus, nonsense mutations occur when a premature nonsense or stop codon is introduced in the DNA sequence. When the mutated sequence is translated into a protein, the resulting protein is incomplete and shorter than normal. Consequently, most nonsense mutations result in nonfunctional proteins.

Direct damage to DNA or errors in the processes that generate messenger RNA (mRNA) from the DNA template can introduce mutations, with potentially harmful consequences. ... Nonsense mutations introduce a stop codon 'upstream' of the correct signal so that translation is stopped early and a truncated protein is made.

Examples of diseases in which point-nonsense mutations are known to be among the causes include: Cystic fibrosis (caused by the G542X mutation in the [[cystic fibrosis transmembrane conductance regulatory. Beta thalassaemia (β-globin) Hurler syndrome. Dravet Syndrome.

nonsense mutation. A nonsense mutation is a genetic mutation in a DNA sequence that results in a shorter, unfinished protein product. DNA is a chain of many smaller molecules called nucleotides. ... Thus, nonsense mutations occur when a premature nonsense or stop codon is introduced in the DNA sequence.

(d) reversion

Testing for the reversion of a mutation can tell us something about the nature of the mutation or the action of a mutagen. For example, if a mutation cannot be reverted by action of the mutagen that induced it, then the mutagen must have some relatively specific unidirectional action. In a mutation induced by hydroxylamine (HA), for instance, it would be reasonable to expect that the original mutation is GC → AT, which cannot be reverted by another specific GC → AT event. Similarly, mutations that can be reverted by proflavin are in all likelihood frameshift mutations; thus mutations induced by nitrous acid (NA), which are transitions, should not be revertible by proflavin.

Transversions cannot be induced by the aforementioned agents, but they are definitely known to be common among spontaneous mutations, as shown by studies of DNA and protein sequencing. Thus, in the reversion test, if a mutation reverts spontaneously but does not revert in response to a transition mutagen or a frameshift mutagen, then, by elimination, it is probably a transversion.

(e) semi-conservative replication

Semi-conservative replication means that during DNA replication, the two strands of nucleotides separate. Both strands then form the template for free nucleotides to bind to to create the two identical daughter strands. Hence each daughter strand has half of the DNA from the original strand and half newly-formed DNA.

In the semiconservative hypothesis, proposed by Watson and Crick, the two strands of a DNA molecule separate during replication. Each strand then acts as a template for synthesis of a new strand.

DNA replication is a semi-conservative process, because when a new double-stranded DNA molecule is formed: One strand will be from the original template molecule. One strand will be newly synthesised.

The importance of the semi conservative model is that it makes sure that you have copies of the DNA that are identical to each other. Otherwise you wouldn't be able to make an exact copy of the DNA. This type of replication works thanks to DNA base pairing.

DNA replication occurs in the cytoplasm of prokaryotes and in the nucleus of eukaryotes. Regardless of where DNA replication occurs, the basic process is the same

DNA replication steps

There are three main steps to DNA replication: initiation, elongation, and termination.

In order to fit within a cell’s nucleus, DNA is packed into tightly coiled structures called chromatin, which loosens prior to replication, allowing the cell replication machinery to access the DNA strands.

Before DNA replication can begin, the double helix structure of the DNA molecules has to be ‘unzipped.’ Helicase, an enzyme, is integral to this process, breaking the hydrogen bonds that hold the complementary bases of DNA together (A with T and C with G). The separation creates a ‘Y’ shape called a replication fork and the two single strands of DNA now act as templates for making new strands of DNA.

Next, the Single-Stranded DNA Binding Protein (SSB Protein) binds to the now single-stranded DNA, preventing the separating strands from joining again.

The two strands of the double-helix DNA are joined together by cross-bars, twisted around. For this to work, each DNA strand runs in opposite direction.

One of the strands is oriented in the 3’ to 5’ direction (towards the replication fork), this is the leading strand. The other strand is oriented in the 5’ to 3’ direction (away from the replication fork), this is the lagging strand.

Because the enzyme that carries out the replication, DNA polymerase, only functions in the 5′ to 3′ direction, this means that the daughter strands synthesize through different methods, one adding nucleotides one by one in the direction of the replication fork, the other able to add nucleotides only in chunks. The first strand, which replicates nucleotides one by one is the leading strand; the other strand, which replicates in chunks, is the lagging strand.

The notations 5′ and 3′ mean “five prime” and “three prime,” which indicate the carbon numbers in the DNA’s sugar backbone. These numbers indicate end-to-end chemical orientation, with the numbers 5 and 3 representing the fifth and third carbon atom of the sugar ring respectively. The 5′ carbon has a phosphate group attached to it and the 3′ carbon a hydroxyl (-OH) group. It’s this asymmetry that gives a DNA strand a “direction,” allowing for easy binding between nucleotides of the opposite strands.

It’s important to note that the two sides are replicated through two different processes in order to accommodate the directional difference.

After the formation of both the continuous and discontinuous strands, an enzyme called exonuclease removes all RNA primers from the original strands. The gaps where the primer(s) had been are then filled by yet more complementary nucleotides.

Another enzyme “proofreads” the newly formed strands in order to make sure there are no errors.

The enzyme DNA ligase then joins Okazaki fragments together, forming a single unified strand.

A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA. Telomeres are regions of repetitive nucleotide sequences at each end of a chromatid, which protect the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Think of shoelace caps. Telomeres are also a biomarker of aging, with telomeres shortening with each cellular division or, in other words, as you advance in age. As a cell’s telomeres shorten, it loses its ability to function normally. Basically, shorter telomeres make you more susceptible to a number of diseases, such as cancer or cardiovascular disease.

Finally, the parent strand and its complementary DNA strand coils into the familiar double helix shape. The result is two DNA molecules consisting of one new and one old chain of nucleotides.  Each of these two daughter helices is a nearly exact copy of the parental helix (it is not 100% the same due to mutations).

The human genome — meaning the complete set of genes present in a cell’s nucleus — is comprised of 3 billion base pairs. Remarkably, it takes very little time for our biological machinery to copy something this exceedingly long. Every cell completes the entire process in just one hour!

(f) gain of function mutation

Many mutations are of the loss-of-function type, and in such cases, the mutated trait is commonly inherited recessively. However, the mutated trait is inherited as a dominant trait in some comparatively rare cases. This is observed when the mutation results in a protein having another abnormal function (gain-of-function mutation). The gained function may be inherited as a dominant trait . Loss-of-function mutations may also be inherited as dominant traits. Proteins with intracellular activity are hypothesized to cause disease when they exist at less than 70% of their normal amount. In F1 offspring with the Aa genotype, who are born from parents with AA and aa genotypes, a disease occurs when the wild-type and loss-of-function-type proteins are produced at a ratio of 1:1 because the amount of the wild-type protein produced is insufficient, being only 50%. In other words, the a genotype for the loss-of-function mutation appears to have been inherited as a dominant trait. This is called haploinsufficiency . In such cases, if the amount of the wild-type protein does not change even when the a gene with mutation is introduced into an AA cell and overexpressed, then no disease occurs. However, if the a gene with loss-of-function mutation is introduced from outside into the AA cell and overexpressed, an abnormality may occur.

(Gain-of-function and Phenotypes)

(Haploinsufficiency)