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Chapter 9

BIL 255 Chapter Notes - Chapter 9: Ciona, Alternative Splicing, Phylogenetic Tree

Course Code
BIL 255

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The ability to compare the genomes of a wide-ranging of organisms has provided striking conformation
of Darwin’s explanations for the diversity of life on earth
Evolution is more a thinker than an inventor, it uses raw materials the DNA sequences that each
organism inherits from its ancestors
There is no natural mechanism for making long stretches of entirely novel nucleotide sequences, thus no
gene or genome is entirely new.
Diversity is all the result of variations on preexisting themes
As genetic change piles up over millions of generations, they can produce radical change.
If a gene got changed or mutated and it is the only gene present in the cell, the cell will not survive
especially if it is an important gene that codes for something vital to the cell
the mechanism that generates genetic changes
are the same for all living things.
Generating Genetic Variation :
Mutation within genes: is the changes,
deletion, or duplication of one or more
nucleotides. The mutation can alter the
splicing interaction its encoded protein
or RNA product
Mutation within regulatory DNA: Is
when and where a gene is expressed can
be affected by a mutation in the stretches
of DNA sequences that regulate the
gene’s activity. They are difficult to spot
because they don’t affect protein
sequences and they can be located far
from the coding sequence
Gene Duplication: Is the duplication
and the creation of a closely similar gene
of a set, large DNA segment, or even a
whole genome. The original DNA
sequence and its duplicate can obtain
additional mutations.
Exon shuffling: 2 or more existing
genes can be broken and rejoined within
the long intron sequence to make a
hybrid gene.
Mobile genetic elements: is a specialized DNA sequences that can move from one
chromosomal location to another, and can alter the activity or regulation of a gene. They also can
promote gene duplication, exon shuffling, and other gene rearrangement.
Horizontal gene transfer: is when a piece of DNA can be transferred from the genome of one
cell to that of another (even another species). Is very common among bacteria, but can be seen in
EUK. This process differ from the
VERTICAL GENE TRANSFER that transfer genetic information from parent to progeny
In sexually reproducing organisms, only changes to the germ line are passed on to progeny:
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For bacteria and unicellular organism that reproduce is asexual
They duplicate their genome and pass one copy to each daughter cell when the individual divide
They have a simple branching diagram cell division that links each individual to its progeny an ancestors
For a multicellular organism that reproduce sexually:
the origin of life was 3.5 billion years ago
Has a germ line cells: a specialized reproductive cell that carries a copy of its genome to the next
generation. It contains only half the number of chromosomes than do the other cells in the body and has
the X, Y chromosomes. They form a zygote (when an egg and a sperm come together) a fertilized egg
Has multiple Somatic Cells: compose all the other cells in the body, they are doomed to die without
leaving evolutionary descendants of their own
The family connection is more complex.
Any mutation that occurs in the somatic cell will not be transmitted to the offspring
Any mutation that occurs in the germ line cell will be transmitted to the offspring
It is trough a series of germ-line cell divisions that sexually reproducing organisms trace their descent
back to their ancestors.
Only changes to the germ line are passed along to
A mutation in the somatic line affect ONLY the progeny of that cell
Vs. A mutation in germ line affects the cell as well as is passed to the progeny.
Point mutations are caused by failures of the normal mechanism for copying and repairing
Point mutation:
Is the change that affects a single nucleotide
They arise from rare errors in DNA replication or repair.
It has been determine in the experiment on E.coli and the manufacture of the amino acid Histidine.
It can destroy a gene’s activity or very rarely improve it. More often they o neither of these.
Can be Neutral mutation:
this type of mutation has no effect on the organisms appearance, viability, or ability to reproduce
they fall in regions of the gene where the DNA sequence is unimportant such as interons.
They also can change the 3rd position (WOBBLE POSITION) of a codon such that the amino acid
specifies is uncharged or is so similar that the proteins function is unaffected.
Point mutation in regulatory DNA:
have a profound effect both on the protein and organism.
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EXAMPLE: some people are malaria resistant because the
binding site where the virus bind is mutated due to the
point mutation, thus making them immune to malaria
Also have a role in our ability to digest Lactose. People
who are lactose tolerant have a point mutation in the
regulatory DNA of the lactase gene, which codes for the
enzyme that breaks down lactose. Hence the people who
are Lactose intolerant do not have this mutation. This
mutation has appeared in about 10,000 years ago.
o Cause: mutation converts GC to AT, thus
making a premature stop codon UGA
o Experiment In the presence of Histidine the bacteria
lived, and in the absence of Histidine bacteria dies.
o Observation: that a rare bacteria survivors that contained “REVERSION” mutation in which the
AT is changed back to GC. these bacteria lived even In the ABCENCE of Histidine. These
rare bacteria made an enzyme that allow them to survive in the absence of Histidine
o Conclusion: such mutation rarely happens, and if it does it is by pure chance. This experiment
showed that bacteria has the ability to survive when deprived, they multiply in higher and faster
rate and adapt to their medium quickly. Their ability to replicate at such high rate gives them the
chance to either find a way to survive trough mutation that may occur along their replication thus
finding a mutation that will allow them to become immune to their medium and rise. This is how
some viruses overcome antibiotics, such as H.I.V, etc….
DNA duplication five rise to the families related genes:
Gene duplication:
Most important mechanism for generating
new genes from old ones.
Each of the copies is free to accumulate
mutations that might allow it to preform
slightly different function as long as the
original activity of the gene is not lost.
Occurs gradually. This gave rise to
Gene duplication and divergence:
Process over many million years, one gene can rise to a whole family of genes, each with a
specialized function within a single genome.
Gene duplication is a common evolutionary mechanism to generate diversity
If a crossover occurs, one chromosome will end up with an extra copy of the gene and the other
will lose it
Gene Families:
Half the genes have 1 or more obvious relative elsewhere such as the Globin family genes.
They arose from a single primordial gene
Many gene duplications are believed be generated by homologous recombination
This function can cause the cell to make mistakes when combining them giving rise to uneven
combination and a mutated gene
Homologous genes: t
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