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  • A gene can simply be defined as a stretch of DNA that is transcribed into RNA,

  • and most RNA is then translated into protein.

  • Each protein is a unique contiguous chain of amino acids evolved to perform some task in the cell.

  • Some proteins act as structural support.

  • Some proteins copy the DNA.

  • Some proteins help metabolize sugars.

  • And some proteins are motors that move cargo around inside of the cell.

  • But where did this diverse array of proteins, and in turn genes, come from?

  • The human genome contains some 25,000 genes,

  • but bacteria only contain between 1000 and 6000 genes.

  • If the theory of evolution is correct, if humans share a common ancestor with bacteria,

  • then over the past 2 billion years we must have gained at least some 20,000 genes.

  • Of course if you do the math that's only 1 new gene every 100,000 years.

  • That would be only 1 new gene since modern humans first evolved,

  • and we already know of many novel genes that have evolved

  • since humans and chimps diverges roughly 6 million years ago.

  • But where did these genes come from? What is the origin of genes?

  • Now you may have heard the argument that genes cannot form by chance.

  • Take for example a protein that is 200 amino acids long.

  • Since there are 20 different amino acids,

  • the chance of such a protein forming by chance is 20 to the 200th power,

  • or roughly 1 in 1 followed by 260 zeros.

  • say that such an occurrence is unlikely is unlikely is putting it mildly.

  • It is more likely for two people to randomly pick the same subatomic particle

  • in the entire visible universe, than have such a protein form by chance.

  • So how do genes form? Well not by chance, that's for sure.

  • In two previous videos, The Origin of Life - Abiogenesis, and the Origin of the Genetic Code,

  • I covered how the first genes, the ancestral genes, originally formed.

  • But this doesn't explain were the 20,000 some genes required to make a human

  • from a bacterium came from.

  • How does nature, through blind unintelligent processes, form something so improbable?

  • Well the first answer is actually quite easy.

  • Occasionally, small stretches of an organism's genome are copied,

  • and any genes contained within are duplicated.

  • Most often mutations occur causing one copy of the gene to become nonfunctional.

  • Such a mutation would be lethal to the organism if it weren't for the backup copy.

  • But every now and then a mutation happens

  • that changes the function of the protein the gene codes for.

  • Maybe the structural protein now forms branched chains instead of straight ones.

  • Maybe the protein now metabolizes a slightly different sugar.

  • Maybe the motor protein moves in a different direction.

  • If this small change is beneficial to the organism, it will be selected for,

  • and overtime the new gene will spread throughout the population, increasing the count by one.

  • And we can see evidence for such gene duplications in every genome sequenced to date.

  • Occasionally even whole genomes are duplicated.

  • So how else can new genes form?

  • Proteins themselves are modular units.

  • While a protein itself is a single chain of amino acids,

  • that chain may contain a number of independently functional units, called domains.

  • And it's the combination of domains in a single protein that can lead to novel functions.

  • Wally Gilbert first proposed more than 30 years ago the idea that new proteins

  • may evolve through the occasional accidental reshuffling of these domains.

  • So while some new genes may be the result of duplication and mutation,

  • others are the result of novel protein domain arrangements.

  • But are these only ways new genes can form, from pre-existing ones?

  • For the longest time the answer was thought to be yes.

  • The first genes were thought to have formed before DNA and proteins were used by organisms,

  • and from then on it was simply duplication, mutation, and rearrangement.

  • That is, until a study came along in 2006 by Yuuki Hayashi and colleagues.

  • In this remarkable study this group of scientists showed

  • what many biologists thought was so improbable it was essentially impossible,

  • the evolution of entirely new genes from completely random DNA sequences.

  • The exact thing creationists have been arguing is mathematically impossible.

  • In this study the researchers replaced a 139 amino acid domain of a gene

  • that makes up the coat of a virus with a random DNA sequence.

  • Without this protein domain the virus is extremely inefficient at infecting cells.

  • And if creationist math is to be believed,

  • the chances of these 139 amino acids randomly mutating back is so unlikely,

  • the virus will be doomed forever.

  • However, in only 7 generations the group saw a 240 fold increase in infectivity of the virus.

  • After many generations the virus had increased its infection rate by a factor of 17,000.

  • Where once there was just random DNA, now existed a fully functional viral coat gene,

  • but not the original gene, an entirely new gene had evolved to fill the role.

  • What this group showed was that not only could entirely different proteins perform the same job,

  • but these could evolve rapidly starting from completely random stretches of DNA.

  • If a random stretch of DNA through a fortuitous mutation

  • happens to be transcribed by RNA polymerase,

  • the resulting protein will most likely do extremely little.

  • But, if the little it does do happens to help the cell, even just a bit, that new gene will be selected for,

  • and over many generations, through gradual mutations, that originally random stretch of DNA

  • will evolve into a fully functioning and completely novel gene.

  • The creationist argument simply shows that a single specific amino acids sequence

  • will not spontaneously form by chance.

  • This however does not mean new genes don't evolve.

  • Novel genes originate through a variety of mechanisms,

  • and are refined through random mutation and natural selection.

  • All the while increasing the complexity of the organism, and adding information to the genome.

  • More often than not organisms don't reinvent the wheel

  • and use preexisting genes or domains to form new genes.

  • But every now and then, a completely new gene forms from nothing more than random DNA,

  • adding more raw fuel to the engine of life we call evolution.

  • Think about it.

A gene can simply be defined as a stretch of DNA that is transcribed into RNA,

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B2 中高級 美國腔

基因的起源 (The Origin of Genes)

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    江仁深 發佈於 2021 年 01 月 14 日
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