The Origin of Genes


Uploaded by cdk007 on 14.10.2009

Transcript:
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.