Question

Explain the various ways new genes have been originated or entered into the genome during evolution of complex unicellular eukaryotes. what are the sources ? why are they important? and what are the mechanisms that these new genes are retained?

looking for a in depth explanation.

Answer 1

There are two ways in which new genes could be acquired by a genome:

• By duplicating some or all of the existing genes in the genome
• By acquiring genes from other species

Both events have been important in genome evolution, as we will see in the next two sections.

Acquisition of new genes by gene duplication

The duplication of existing genes is almost certainly the most important process for the generation of new genes during genome evolution. There are several ways in which it could occur:

• By duplication of the entire genome;
• By duplication of a single chromosome or part of a chromosome;
• By duplication of a single gene or group of genes.       The second of these possibilities can probably be discounted as a major cause of gene number expansions based on our knowledge of the effects of chromosome duplications in modern organisms. Duplication of individual human chromosomes, resulting in a cell that contains three copies of one chromosome and two copies of all the others (the condition called trisomy), is either lethal or results in a genetic disease such as Down syndrome, and similar effects have been observed in artificially generated trisomic mutants of Drosophila. Probably, the resulting increase in copy numbers for some genes leads to an imbalance of the gene products and disruption of the cellular biochemistry (Ohno, 1970). The other two ways of generating new genes - whole-genome duplication and duplication of a single or small number of genes - have probably been much more important.

• Acquisition of new genes from other species

  The second possible way in which a genome can acquire new genes is to obtain them from another species. Comparisons of bacterial and archaeal genome sequences suggest that lateral gene transfer has been a major event in the evolution of prokaryotic genomes. The genomes of most bacteria and archaea contain at least a few hundred kb of DNA, representing tens of genes, that appears to have been acquired from a second prokaryote.

  There are several mechanisms by which genes can be transferred between prokaryotes but it is difficult to be sure how important these various processes have been in shaping the genomes of these organisms. Conjugation , for example, enables plasmids to move between bacteria and frequently results in the acquisition of new gene functions by the recipients. On a day-to-day basis, plasmid transfer is important because it is the means by which genes for resistance to antibiotics such as chloramphenicol, kanamycin and streptomycin spread through bacterial populations and across species barriers, but its evolutionary relevance is questionable. It is true that the genes transferred by conjugation can become integrated into the recipient bacterium's genome, but usually the genes are carried by composite transposons, which means that the integration is reversible and so might not result in a permanent change to the genome. A second process for DNA transfer between prokaryotes.

• There are several mechanisms by which these gene duplications could have occurred:

• Unequal crossing-over is a recombination event initiated by similar nucleotide sequences that are not at identical places in a pair of homologous chromosomes. As , the result of unequal crossing-over can be duplication of a segment of DNA in one of the recombination products.
• Unequal sister chromatid exchange occurs by the same mechanism as unequal crossing-over, but involves a pair of chromatids from a single chromosome.
• DNA amplification is sometimes used in this context to describe gene duplication in bacteria and other haploid organisms (Romero and Palacios, 1997), in which duplications can arise by unequal recombination between the two daughter DNA molecules in a replication bubble.
• Replication slippage could result in gene duplication if the genes are relatively short, although this process is more commonly associated with the duplication of very short sequences such as the repeat units in microsatellites.

• Genome evolution also involves rearrangement of existing genes

  As well as the generation of new genes by duplication followed by mutation, novel protein functions can also be produced by rearranging existing genes. This is possible because most proteins are made up of structural domains , each comprising a segment of the polypeptide chain and hence encoded by a contiguous series of nucleotides. There are two ways in which rearrangement of domain-encoding gene segments can result in novel protein functions.

• Domain duplication occurs when the gene segment coding for a structural domain is duplicated by unequal crossing-over, replication slippage or one of the other methods that we have considered for duplication of DNA sequences. Duplication results in the structural domain being repeated in the protein, which might itself be advantageous, for example by making the protein product more stable. The duplicated domain might also change over time as its coding sequence becomes mutated, leading to a modified structure that might provide the protein with a new activity. Note that domain duplication causes the gene to become longer. Gene elongation appears to be a general consequence of genome evolution, the genes of higher eukaryotes being longer, on average, than those of lower organisms.
• Domain shuffling occurs when segments coding for structural domains from completely different genes are joined together to form a new coding sequence that specifies a hybrid or mosaic protein, one that would have a novel combination of structural features and might provide the cell with an entirely new biochemical function