BIOB11H3 Lecture Notes - Lecture 2: Fluorescent Tag, Avidin, Centromere
Genome evolution and polymorphism
1. Random mutations
SIngle based changes that are just a consequence of DNA being in the same environmental and lots of
chemical reactions get changed periodically> that is relatively a slow process. Sometimes what happens
is that mutations are useful to the organism they are selected for. That particular organism has a better
chance of surviving. It goes to out reproduce some of its competitors.
2. Unequal crossing over
It takes place in meiosis. That leads to two different types of events: duplications and deletions. These
events can be selected for two and the organism can get perhaps a duplication gene that has more
genetic material to be able to work with. To be able to put into good use in order to compete or
outcompete some of its other organisms, just like random mutations, there is a divergence of function
driven by mutation/selection of those gene sets.
3. Large scale changes
You can think of as chromosomal inversions, indels (insertion and deletion), translocations (takes place
in part of the chromosome that breaks off becomes attached to a different chromosome)
There are activity in transposable elements, which are pieces of DNA that can jump from one place to
the other in the genome.
Unequal crossing over:
Generally occurs during meiosis (formation of gametes in mammals wherein sperm and egg are
produced). The homologous chromosomes from the mother and the father come together within the
cell.
Normally, alignment of homologous chromosomes (one pair from father, one from mother) occurs
precisely, so that reciprocal genetic exchange generates complete sets of (potentially recombinant)
chromosomes.
You brothers and sister do not look like you because meiosis from the mom and dad generate the
recombinant chromosome sets that were donated to you through the sperm and the egg.
The chromosomes that are blue and orange have a slight miss alignment of these two chromosomes
sets. Unequal crossing over within that leads to one of the chromosome having a duplicated segments
(C) and the other chromosome that lacks the C segment.
Fig. 10-22
Dotted X depicts the cross over event in which crossing over takes place. After cell division, you end up
with half the cells. The one on the left had gene 1 and its missing a gene 2 that was resulted from
deletion. The one on the right have both copies of gene 2 that was resulted from duplication. Of course,
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this can happen in later generation. The one on the right have 3 copies of gene 2. The implication of this
is that unequal crossing over lead to the increase in gene number. That is referred to as duplication:
create gene family. After the event, they are identical to one another. It can diverge over time through
independent mutations. Also may result in loss of genes for other parts of the gametes (deletion: often
lethal). If you have some gene that is absolutely critical for the functioning of the cell and all of the
sudden it is gone, one of the cell can no longer survive.
The best known example for unequal crossing over and the production of genes in families and the
evolution of gene families is the mammalian globin genes.
Fig 10-23
Ancestral globin gene on the left and there are series of changes that takes place. There is duplication
events, divergence by mutations, genes eventually become separated into two different chromosomes.
On the right, you can see what the current status is.
First of all, you all probably know what is globin and the context of hemoglobin. (Globin= protein of
hemoglobin an O2 carrier in blood). The first thing that happened from the ancestor to the modern gene
is that exon fusion takes in place. Exon: part of a gene that eodes potei Epessed. Intron: part
that is splied out Iteupts. Whe ‘NA spliig takes plae I a keep itos ad iteupts
exon expressed together). Image you have unequal crossing taking place, now you got duplication, two
copies of the same gene. Where you see in panel number four, you will alpha and beta. Over time, both
of those have been accumulated in some mutations and has started to change the encoded sequence of
polypeptide. They are still capable of acting light globin sequences but maybe they have slightly
different structures. In no. 5, you can see the separation of genes. So imagine that all of these genes
resided on chromosome 11. On the top you see the beta gene family on chromosome 11. Suppose that
one of those two genes alpha broke off rom that chromosome and moves to a different chromosome
no. 16. After that, on both chromosomes there were additional duplication events and there were
additional mutations took place in history of the organisms. Divergence occurred such that now
chromosomal 11 we have beta globin gene family that consists of several genes that do include
functional globin polypeptides. On chromosome 16 we have the alpha globin family and same story is
true there. On both cases we can see pseudogene (pseudo=false). These were genes that were obviously
derived from a single common ancestral gene such as globin gene, but they suffered so many mutations
that they are no longer functional. They are false genes or pseudogenes.
Globin gene expression during development
This graph shows you the time and from conception to birth to going into adult hood on one axis. Ther
percent polypeptide chains present on the other. Very early in development, only the ebsolone gene is
on in the embryo. By about 4-5 weeks of development, the epsilon used to be repressed. This
expression is turned down a bit. But alpha, gamma , delta and beta begin to come on. By about the time
you are born, you have about 50% of alpha or so and 25% of gamma and beta. In the adult, what we
have coursing in our veins is hemoglobin consists of approximately of 50% alpha chains and 50% beta
chains. The key point is this, over evolutionary time gene duplication event occurred. But mutations in
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the promoters encoding regions result in different expression patterns of family members with some
sight differences in O2 carrying capacity.
Structural Variation in chromosomes
Fig. 10-30
Chromosome has been banded with certain type of dye. The one on the left depicts normal version,
where you have a big dark spot towards the top. There is a same chromosome on the right where
inversion is taken place to change the location of that. As you can see on the left, you have different
colored boxes. Perhaps those different genes of A though I in the case of inversion, the segments that
are C, D and E think of it as being cut on both ends 180 degrees and stitched back together with original
chromosome. So, the gene order has changed. Now you would think that this might be a problem, but n
fact it is in some cases. The inversion of a chromosome segment can bring a normal cell cycling of a gene
under a control of a strong promoter such that gene always express. That tells the cell to continuously
start new cell divisions. Essentially that is the basis of cancer is that cell cycle gets out of control.
Insertion: foreign DNA (viral/transposon) is integrated
Easiest thing to think about is a virus. Some types of viruses DNA viruses may infect you. They insert
randomly to your chromosomes. That may be problematic. Transposon may be integrated and jump
from spot to spot. That is also usually random as well.
Eaple of a isetioal utatio: Medels ikled peas
Wildtpe: doiat ‘oud seeds
Mutant: wrinkled
He found that the round was dominant over wrinkled. It is now known to be due to mutation in a gene
encoding a starch branching enzyme. The wrinkled gene is due to the insertion of a transposable
element into a gene encoding starch branching enzyme. Starch is a big component in seeds. There are
linear versions and branched versions. Normally, that enzyme is made and it will result in a seed that is
nice and full. But in the case of the wrinkled mutation, you can see the insertion has occured in the
gene. There was a discovery made in England that there was transposon about 800 base pair long went
ito the iddle of this gee that disupted its futio. You dot see stah ahig ezes a
oe. It is liea, the seed doest fill popel ad thus appeas shunken or wrinkled.
Mobile Genetic Elements: Transposons
Barbara McClintock called the transposon/transposable element as controlling elements. She was
studying in maize (corn). She did some controlled crosses and observed some things about the structure
of chromosomes and some traits that seem to follow the changes in structure. For her work she was
awarded a Nobel prize in 1983. Her experiment showed the movement of elements within the genome.
MClitoks esults o the C gee hih otols aize keel color
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Document Summary
Single based changes that are just a consequence of dna being in the same environmental and lots of chemical reactions get changed periodically> that is relatively a slow process. Sometimes what happens is that mutations are useful to the organism they are selected for. That particular organism has a better chance of surviving. It goes to out reproduce some of its competitors: unequal crossing over. That leads to two different types of events: duplications and deletions. These events can be selected for two and the organism can get perhaps a duplication gene that has more genetic material to be able to work with. You can think of as chromosomal inversions, indels (insertion and deletion), translocations (takes place in part of the chromosome that breaks off becomes attached to a different chromosome) There are activity in transposable elements, which are pieces of dna that can jump from one place to the other in the genome.
