The Cause of Mutations
Mutations are a random occurrence during DNA replication and the rate of mutation is influenced by external factors such as UV radiation. There are different types of mutation:
1. Deletion where a nucleotide base is deleted. AGTCA becomes AGCA.
2. Substitution where a nucleotide base is replaced by another. AGTCA becomes AGTCG.
3. Insertion where a nucleotide base is added as extra. AGTCA becomes ATGTCA.
The Effect of Mutations
Since the genetic code is degenerate, it’s possible that a mutation won’t have any effect whatsoever! This represents silent mutations. If 2 different triplet codes translate into the same amino acid, the polypeptide chain will remain unchanged. This of course only applies to substitutions.
Another scenario where a mutation may cause no effect is if it arises in an intron. Since these are removed before mRNA is translated, no mutations would be carried along.
What happens if a base is deleted or added? The genetic code is non-overlapping, so the error cannot simply be overlooked and the following triplets read correctly. The entire subsequent code will be shifted. This is called a frameshift.
Deletion: AGT GGC TTA… –> lose the first G –> ATG GCT TA…
Insertion: AGT GGC TTA… –> insert an A after the first A –> AAG TGG CTT A…
The code is affected significantly!!! In fact, it may be totally ruined. One way this can happen is by a nonsense mutation which by a frameshift causes the code to arrive at a stop codon earlier than it’s supposed to. This will result in a shorter polypeptide and therefore truncated protein which may malfunction.
A missense mutation is when a substitution changes the amino acid encoded. This does not necessarily impact the overall protein, but it may result in a protein with an altered binding site and therefore affect its activity.
Mutations and Cancer
Cell division is kept in check by two kinds of genes: proto-oncogenes which trigger division, and tumour suppressor genes which inhibit division. Cancer is caused by mutated genes involved in cell division. Mutated proto-oncogenes, called oncogenes, trigger cell division at a far greater rate than normal, thus allowing cells to divide out of control.
Mutated tumour suppressor genes fail to inhibit division any longer, therefore contributing to the growth of cancerous tissue by indefinite division.
Meiosis is a type of cell division which results in 4 cells that are genetically non-identical from one parent cell. In order for once cell to divide to result in 4 cells, how many divisions must take place?
Two. 1 cell becomes 2, then 2 become 4:
The first division is called meiosis I, and the second is called meiosis II.
…so far so easy? (it should be!)
Cells resulting from meiosis are gametes such as egg cells and sperm cells, hence meiosis only occurs in sexually reproducing organisms. There are 2 key points about this:
1. Gametes are genetically unlike one another – while cells in other tissues such as muscle or blood must be genetically identical to one another (clones), the very basis of sexual reproduction is genetic diversity. So somewhere in the process of division, something takes place which creates genetic diversity (we’ll come to that shortly).
2. If gametes are to fuse and result in a new organism, it is essential that the number of chromosomes should stay constant. Humans have 46 chromosomes in each cell (of course, apart from cells without DNA in them, and “spoiler alert!”, gametes) – if each gamete had 46 chromosomes, then fusing 2 together would result in a zygote with 92 chromosomes, whose offspring would have 184 chromosomes, and before you know it something terrible would have happened.
The above picture illustrates how the number of chromosomes is halved in the final 4 cells. The terms diploid and haploid refer to the number of sets of chromosomes. In humans, somatic cells (i.e. cells other than gametes) are diploid because there are two sets of chromosomes. Gametes are haploid because they have only one set of chromosomes.
A “set” is made up of all chromosomes which are unique, i.e. are not paired with any homologous chromosomes.
X x X X x X x x <———- haploid = 1 set
XX xx XX XX xx XX xx xx <———- diploid = 2 sets
In the first XX, X and X are homologous chromosomes because they occupy the same space and contain DNA with similar purpose/function. Essentially, they are more or less copies of each other. So when 2 gametes fuse, they form a diploid cell with the complete number of chromosomes.
Wikipedia does us the honour with this epic picture:
On to the very important bit now…
How does meiosis achieve genetic diversity without which you would actually look *just* like your siblings?
10 words: Independent Assortment of Homologous Chromosomes, &
Genetic Recombination by Crossing Over
What an unnecessary mouthful. You still have to learn them though.
Independent assortment of homologous chromosomes means that in meiosis I, when the original diploid line-up a.k.a. XX xx XX XX xx XX xx xx becomes X x X X x X x x in 2 resulting cells, which big X’s and which small x’s end up with each other in each cell is random. Pretty simple concept.
If you split the homologous chromosomes, you get Xx in 2 cells. The idea is that there is no rule saying that black must go with black, and red must go with red. You can end up with Xx and Xx, or Xx and Xx with an equal probability. What can I say, genetics likes being a bit random.
Not assigning the expected chromosome or chromatid during meiosis is called chromosome non-disjunction and results in a cell with a different number of chromosomes.
Genetic recombination by crossing over is a lot more interesting. It’s like a bowl of spaghetti. Homologous chromosomes snuggle each other and exchange parts in the process:
Did I mention how important it is to use accurate scientific terminology in the exams? The process is called synapsis, during which mutual exchange of genetic information occurs.
As a finishing touch, I read of this mnemonic to remember the purpose of meiosis.
It is so cringe-worthy, I would rather memorise meiosis off by heart.