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RELEVANT LEARNING OUTCOME:
Define the term 'mutation'.
State how mutations can arise
State the 2 types of mutations.
💡Recall!
💡Recall!
A DNA molecule is a double helix, which consists of 2 strands of DNA, each strand is synthesised by joining many deoxyribonucleotides, which eventually give rise to a specific sequence of bases.
The DNA double helix, with the assistance from various proteins is then packed and condensed into structures called chromosomes.
Each species has its unique chromosome number.
For example, humans have 23 pairs of chromosome, so a typical human diploid cell has 23 x 2 = 46 chromosomes.
Packing of DNA
Human karyotype - notice that there are 23 pairs of chromosomes, giving a total of 46 chromosome (in a diploid cell)
💡Definition
💡Definition
Mutation is a change in the amount, arrangement, or structure of the DNA of an organism.
Note that change can be increase or decrease.
By referring to the pictures above,Â
change in amount can refer to changes in amount of base pairs (A, T, G, C) or number of chromosome.
change in arrangement can refer to the sequence of base pairs, or arrangement of genes on chromosomes.
change in structure can refer to sequence of base pairs, or structure of chromosomes.
💡Effect of Mutations
💡Effect of Mutations
Recall the central dogma - DNA contain genes, which are transcribed to mRNA, which are then translated into proteins.
Therefore, when mutations occur, changes at the DNA level will have a downstream impact on the mRNA synthesised and eventually the protein.
The protein expressed will determine the appearance of a characteristic of an individual, termed as the phenotype.
As such, mutations can alter the phenotype of an organism.
Do all mutations result in a change in phenotype?
No! Recall that the genetic code is degenerate, changes to the third base of a codon will not result in a change in the amino acid coded. Thus there will be no change in the protein synthesised.
This is termed as a silent mutation, which will be covered under gene mutation.
Mutations can also occur in non-coding regions, for example introns. These regions do not code for proteins, as such, it is also unlikely to cause a change in the organism's phenotype.
Central Dogma of molecular biology
💡Causes of mutations
💡Causes of mutations
Randomly and spontaneously
Recall that during DNA replication, DNA polymerase III can make a mistake and place a wrong nucleotide. Despite having proof-reading function to correct any mismatch, there is still a random chance that an error will occur and not be corrected. (about 1 in 10 000 000 bases)
Random error can also occur during mitosis/meiosis that results in uneven distribution of chromosomes in daughter cells
Apart from random events, mutations is largely caused by external factors that are able to damage DNA:
Effect of high energy electromagnetic radiation (e.g. ultra-violet light, X-rays, and -rays.)
High energy particles, such as and particles, neutrons, and cosmic radiation
these high energy radiation and particles are able toÂ
break the covalent phosphodiester bond in DNA, causing single / double strand breaks
modify the base found on the nucleotide
Chemical substance
certain chemical substances are able to bind to DNA and modify the structure
this can alter the bases or cause structural damage to the DNA
💡Types of mutations
💡Types of mutations
There are two types of mutations, which will be covered in this topic:
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