Eukaryote genome can evolve through several mechanisms. Describe two mechanisms and discuss how genetic novelty may arise and be maintained in ongoing generations.
The overall explaination in this discussion with few more
additional information is:
Genome Evolution:
Processes such as mutations, duplications, exon shuffling,
transposable elements and pseudogenes have contributed to genomic
evolution.
Accumulating Changes Over Time:
The evolution of the genome is characterized by the accumulation of
changes. The analaysis of genomes and their changes in sequence or
size over time involves various fields. There are various
mechanisms that have contributed to genome evolution and these
include gene and genome duplications, polyploidy, mutation rates,
transposable elements, pseudogenes, exon shuffling and genomic
reduction and gene loss. The concepts of gene and whole-genome
duplication are discussed as their own independent concepts, thus,
the focus will be on other mechanisms.
Mutation Rates:
Mutation rates differ between species and even between different
regions of the genome of a single species. Spontaneous mutations
often occur which can cause various changes in the genome.
Mutations can result in the addition or deletion of one or more
nucleotide bases. A change in the code can result in a frameshift
mutation which causes the entire code to be read in the wrong order
and thus often results in a protein becoming non-functional. A
mutation in a promoter region, enhancer region or a region coding
for transcription factors can also result in either a loss of
function or and upregulation or downregulation in transcription of
that gene. Mutations are constantly occurring in an organism’s
genome and can cause either a negative effect, positive effect or
no effect at all.
Transposable Elements:
Transposable elements are regions of DNA that can be inserted into
the genetic code through one of two mechanisms. These mechanisms
work similarly to “cut-and-paste” and “copy-and-paste”
functionalities in word processing programs. The “cut-and-paste”
mechanism works by excising DNA from one place in the genome and
inserting itself into another location in the code. The
“copy-and-paste” mechanism works by making a genetic copy or copies
of a specific region of DNA and inserting these copies elsewhere in
the code. The most common transposable element in the human genome
is the Alu sequence, which is present in the genome over one
million times.
Pseudogenes:
Often a result of spontaneous mutation, pseudogenes are
dysfunctional genes derived from previously functional gene
relatives. There are many mechanisms by which a functional gene can
become a pseudogene including the deletion or insertion of one or
multiple nucleotides. This can result in a shift of reading frame,
causing the gene to longer code for the expected protein, a
premature stop codon or a mutation in the promoter region. Often
cited examples of pseudogenes within the human genome include the
once functional olfactory gene families. Over time, many olfactory
genes in the human genome became pseudogenes and were no longer
able to produce functional proteins, explaining the poor sense of
smell humans possess in comparison to their mammalian
relatives.
Exon Shuffling:
Exon shuffling is a mechanism by which new genes are created. This
can occur when two or more exons from different genes are combined
together or when exons are duplicated. Exon shuffling results in
new genes by altering the current intron-exon structure. This can
occur by any of the following processes: transposon mediated
shuffling, sexual recombination or illegitimate recombination. Exon
shuffling may introduce new genes into the genome that can be
either selected against and deleted or selectively favored and
conserved.
Genome Reduction and Gene Loss:
Many species exhibit genome reduction when subsets of their genes
are not needed anymore. This typically happens when organisms adapt
to a parasitic life style, e.g. when their nutrients are supplied
by a host. As a consequence, they lose the genes need to produce
these nutrients. In many cases, there are both free living and
parasitic species that can be compared and their lost genes
identified. Good examples are the genomes of Mycobacterium
tuberculosis and Mycobacterium leprae, the latter of which has a
dramatically reduced genome. Another beautiful example are
endosymbiont species. For instance, Polynucleobacter necessarius
was first described as a cytoplasmic endosymbiont of the ciliate
Euplotes aediculatus. The latter species dies soon after being
cured of the endosymbiont. In the few cases in which P. necessarius
is not present, a different and rarer bacterium apparently supplies
the same function. No attempt to grow symbiotic P. necessarius
outside their hosts has yet been successful, strongly suggesting
that the relationship is obligate for both partners. Yet, closely
related free-living relatives of P. necessarius have been
identified. The endosymbionts have a significantly reduced genome
when compared to their free-living relatives.
Eukaryote genome can evolve through several mechanisms. Describe two mechanisms and discuss how genetic novelty may...
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