Chapter 4.4 + 4.5 Causes of Mutations: Errors in DNA Replication and Cell Division + Mutagens ✓ (double check) Flashcards
Where do spontaneous mutations occur?
The can occur during the S phase (synthesis) of the cell cycle, when the DNA is exposed during replication and is venerable to damage.
Occasionally, how does DNA replication make mistakes?
When DNA polymerase inserts the wrong nucleotide.
Define mutation rates
The number of changes per gene copy in a population over a period of time
What are the three main ideas emerged by scientists about mutations?
-Mutations arise spontaneously and are not directed by the environment. Environmental influences can greatly affect the mutation rate but cannot induce a particular mutation to occur
-Mutations are persistent. They tend to be transmitted through many cell divisions without further change, although there is always the possibility that another mutation may occur, either producing a new feature or a return to the original condition.
-Majority of mutations present disadvantages on the organisms that inherit them. The premature death of organisms with harmful mutations prevents harmful mutations accumulating in populations. The occurrence of a successful mutation is an extremely rare event
During meiosis, how do mutations occur?
Mutation occur when unequal crossing over occurs which results in one gamete gain an extra nucleotide (leading to an insertion mutation) and one gamete losing some nucleotides (leading to deletion mutation)
Define insertion mutation
A mutation in which one or more nucleotide pairs have been added to a segment of DNA
Define deletion mutation
A mutation in which one or more nucleotide pairs have been lost from a segment of DNA
What is the purpose of DNA replication?
The purpose of DNA replication is to create an identical copy of the DNA so that genetic information can be passed on to a daughter cell during cell division
If a spontaneous mutation occurs during DNA replication, will it alway be passed on during cell division?
Not necessarily because…
DNA repair mechanisms
-Many replication errors are corrected by proofreading and repair enzymes before cell division.
Cell type considerations
-In somatic cells, a mutation will only be passed on to daughter cells
-In germline cells, a mutation has the potential to be inherited by offspring
Mutation outcome
-If the error escapes repair and the cell divides, the mutation will be passed on.
-If the mutation is lethal (meaning that if its lethal it prevents the cell from surviving) or fixed before division, it wont be transmitted
What are examples of physical mutagens?
-UV light
-X-rays
-Nuclear radiation
What effect does UV light have on DNA?
Simply, it distorts the structure by cross-linking neighbouring nucleotides
Extended, UV light fuses adjacent thymines and cytosines in the DNA sequence
What effect does X-ray irradiation have on DNA?
Simply, it causes genes and chromosomes aberrations (abnormality)
Extended, it causes a loss of adenine and guanine bases, although the DNA backbone remains intact, creating gaps in the double helix. These aberrations disrupt complementary base pairing and ultimately, incorrect bases may be inserted in their place during DNA replication.
Simply, what does nuclear radiation cause to the DNA structure?
It causes breaks in the DNA strands
Define double-strand breaks
A mutation involving breaks in the sugar-phosphate backbones of both DNA strands at the same nucleotide pair, resulting in the complete breakage of chromosomes
What are some chemical mutagens
-Mustard gas (sulfur mustard)
-2-aminopurine,5-bromouracil
-Colchicine
-Nitric acid
What effect does mustard gas have on DNA?
Mustard gas affects the base guanine, causing a substitution method
What effect does 2-aminopurine, 5-bromouracil have on DNA?
Simply, causes nucleotide substitution
Extended, 2-aminopurine, 5-bromouracil resembles thymine and can be incorporated in place of it during replication.
Unlike thymine, it forms hydrogen bonds with either adenine and guanine.
What effect does colchicine have on DNA?
It prevents spindle fibre formation in mitosis so it doubles chromosome number
What effect does nitric acid have on DNA?
Adenine in DNA is deaminated so it behaves like guanine
Define horizontal gene transfer
The process by which genetic material from one organisms becomes incorporated into the genome of another organism
Define cloning vector
In cloning, the vector is the DNA molecule that is used to carry the cloned piece of DNA
What are the three types of mutagens?
-Physical
-Chemical
-Biological
What are examples of biological mutagens?
-Bacteria
-Viruses
-Fungi
Simply, define physical, biological and chemical mutagens
Physical: Damages DNA by breaking bonds of bases e.g. UV light
Chemical: Adds or removes bases e.g. mustard gas
Biological: Pathogens disrupts cell function by putting its DNA in the host cell e.g. Crown Gall
