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Cellular Mutations - How They Occur - Mutations in The DNA Molecule
Mutations within cells
Quick Explanation of a DNA molecule
There are ‘four’ different nucleotides within a molecule of DNA called Thymine, Adenine, Cytosine and Guanine. A nucleotide is sort of like a small block in the assembly of a DNA molecule. A DNA molecule has a pattern and a few rules in which each nucleotide will correctly pair with each other called ‘base pairing’. Thymine and Adenine pair together and Cytosine and Guanine pair together. The order in which these base pairs are within the DNA molecule is known as a DNA sequence. A DNA sequence is read in blocks of three base pairs known as a codon therefore as DNA is being read three nucleotides are being read at a time. Mutations occur when a nucleotide is changed/deleted etc.
In simple terms a protein is made up of Amino Acids. When a DNA molecule is read each codon will translate to an Amino acid therefore it is crucial that the order of nucleotides is correct. To make it a little more complicated an amino acid can have more than one codon. A change in any the DNA sequence may change which amino acid is made and therefore a completely different protein which may not function properly, leading to disease.
For example AGG would code for the protein Arginine. Figure one is a table to explain the different codons and to which amino acid they translate to.
A = Adenine
T = Thymine
C = Cytosine
G = Guanine
·Example DNA sequence
·This would be read as
· An example codon would be AGG
What is a mutation?
- Permanent change in DNA sequence
- Function change in gene product (malfunctioning protein)
- Gene product is usually a protein
The consequences of Mutations!
Mutations occurring in the germ cells (gametes i.e. Sperm and Egg cells) are heritable, and may lead to a genetic disease in the offspring!
For example Hypertrichosis
Mutations occurring in somatic body cells are not inherited by the offspring because the mutations occur in body cells and not in germ cells. Mutations interfere with normal cell functions such as the cell cycle.
Defence against Tumour Formation
- Cell Death - Accumulation of mutations contributed to cell senescence or death (apoptosis). A mutation in a single gene may be sufficient to induce cell death (apoptosis). Mutation is not passed on.
- DNA repair systems detect and repair the damage to that cell: mutation is not passed on to daughter cells
- The cell cannot function and dies: mutation is not passed on
The facts that you may have to read a few times because it’s that surprising!
1 in 30 million nucleotides!
According to the human genome project there are 3164.7million Base Pairs per human genome
Therefore: 105 base changes per cell division!
Now imagine how many cells are within the human body and how much change that is!
Although!! <2% of genome codes for protein (approx. 63 million nucleotides)
Therefore: approx 2 coding base changes per cell division!
It can be said that every time a cell divides mutations occur. Defences against mutations and the severity of the change is what may or may not cause disease because silent polymorphisms can occur (silent polymorphisms explained in another article).
Many of these changes may be silent! (Silent mutations will be explained in another article)
Mutation during gametogensis (Sperm and Egg cells)
To produce an ovum (Egg)
The amount of divisions that occur in Egg cells is very small, only 24 divisions therefore 105 base changes per cell division multiplied by 24 divisions = 2520 random base changes
Only 2 coding changes per cell division due to only <2% human genome codes for proteins.
THIS ALL MEANS! Only 48 random protein coding changes in an Egg cell
To produce a sperm
A sperm cell is constantly dividing unlike an egg cell and therefore the amount of mutations that can occurs is much larger.
A 28 year old male would have had around 334 divisions. Therefore 105 x 334 = 35,070 random base changes!
2 x 334 = 668 RANDOM CODING CHANGES! In a Sperm cell
A 35 year old male would have around 495 divisions therefore 105 x495 = 51,975 random base changes!
2 x 495 = 990 RANDOM CODING CHANGES! In a Sperm cell
The male gamete sperm has a much higher rate of mutation compared to the egg cell and the older a male becomes the higher the chance of mutation.
This final image is just to show what I have been talking about.
DNA is transcribed into mRNA and mRNA is translated to Protein. A change in the DNA sequence will change the final outcome, a protein. Any change that occurs to a protein may result in a disease as proteins are essential for almost everything in the body.
A very brief introduction on how mutations occur! Only part one of many! Keep posted!
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