The Common Ancestry Thread (page 12)

cofty

frankie - you would enjoy "Life Ascending" by Nick Lane. He survey's the latest thinking in abiogeneis and the origins of replicators.

I will write up a summary if I get time.

I think maybe as our processing power gets greater we will be able to see some evidence either way by collecting as much dna and processing it all, this may give us some surprising information. I'm thinking there was a tremendous amount of sharing going on back and forth in the early stages of DNA structuring.

SunnyDays

The explanation regarding the larygeal nerve was my tipping point from fantasy to reality. The next one I watched was the Richard Dawkins explanation of the development of the eye and then my de-conversion was complete.

Amelia, that was an amazing piece of the puzzle for me, too.

Really enjoying this thread.

:)

Sunny

cofty

TRANSPOSONS

My last couple of contributions to this thread concerned comparisons of exons - the 1.5% of our genome that code for proteins. Specifically we looked at one example - the ubiquitous protein Cytochrome C which is made up of a sequence of approximately 100 amino acids.

Using the 20 amino acids that are found in living things there are more ways to assemble a molecule of Cytochrome C than there are atoms in the known universe and yet humans and chimps have identical sequences.

The pattern of differences in Cytochrome C across the living world exactly reflects the relationships predicted by evolution.

Looking deeper at the underlying DNA that codes for the amino acid sequences we saw that there is even more impressive evidence for evolution. The 4 letters of our DNA - A, C, G and T are read in groups of three which gives us 64 codons, each of which code for amino acids. With 64 codons and only 20 amino acids there is a level of functional redundancy within our DNA code. In other words there are countless ways of coding for exactly the same sequence of amino acids in the Cytochrome C molecule. A comparison of the human and chimp gene shows a difference of just 4 nucleotides.

The family tree of living things predicted by evolution is further confirmed by comparing the degree of similarity of the nucleotide sequences of the same gene in other species.

The exons in our genome - the bits that code for proteins - are split into many parts by non-coding sections known as introns. Imagine reading an instruction manual that contains a few lines of directions then a few paragraphs of gibberish followed by another few words that make sense, followed by more random letters and so on. This is roughly what our genes look like. For example the alpha-collagen gene has 50 exons that range from 45-249 bases but the gene is about 40,000 bases long. The majority of the gene is made up of non-coding introns.

This post is about the way some introns originate as transposons and how they provide powerful evidence for human evolution.

Transposons are mobile pieces of DNA, chunks of text that are able to move randomly around the genome. They come in two basic types, retrotransposons and DNA transposons.

We have explained that the text of the exons in our DNA is read off in groups of three called codons and that these code for amino acids which join together to form protein molecules. DNA is found in the nucleus of cells but the machinery for making proteins is outside the nucleus. So the cell first transcribes the DNA code into a similar code called RNA which is passed on to organelles called ribosomes. Think about it like going to a library to read a very valuable book that is kept in a vault. The librarian makes a photocopy of the text for you and brings it to you to read.

Even though they contain no useful code for producing amino acids, retrotransposons also get transcribed into RNA but then they do something odd, they reverse-transcribe into DNA and insert themselves back into the genome in a new random position.

DNA transposons are slightly different. They skip the whole transcription-reverse transcription process and simply get cut out of the genome and reinserted at a new loci.

The enzymes that perform this trick on DNA transposons can be thought of as being like a "cut and paste" function. Although randomly using cut-and-paste in a document will mess up the text it won't increase or decrease the length of the document.

On the other hand the enzymes that transcribe and reverse transcribe retrotransposons can be thought of as a "copy and paste" function. Imagine copying a piece of text in a document, clicking randomly at a new place in the text and then holding down the "CTL+V" keys.

Retrotransposons come in two basic types called "long interspersed" elements or LINEs and "short interspersed elements" or SINEs. Between them they make up more than 30% of the 3 billion letters of code in the human genome.

Transposons provide powerful evidence of the relationships between all living things. Imagine you were marking essays from undergraduate students. The similarity of a number of submissions makes you suspicious that there has been some cheating going on. It's possible that two or more students would produce very similar essays simply because they used the same sources and attended the same lectures. The smoking gun would not be so much the intelligent text within the essays but any common errors. Let's say you find a random mistake in one of the essays. A student has used cut-and-past to move a sentence to a different part of their essay but they accidently missed off a couple of letters. Now when you look at the other suspicious essays you find exactly the same mistake at the identical place.

What if you found many examples like this, some of which were in many of the essays and some in just a few of them? With a bit of work you could construct a family tree of cheats and prove whose essay was the original and where the various mistakes got copied into the text.

Transposons are used by forensic scientists in a similar way to provide evidence of paternity and to identify criminals from DNA left at a crime scene. The random way transposons insert into the genome creates unique patterns that are sometimes referred to as a genetic fingerprint. Similarly transposons provide proof of the common ancestry between living species.

To give just one of countless possible examples. One type of SINE that is common to all mammals is called the ALU element. It is only 300 nucleotides long but it has been copied-and-pasted so many times it now makes up 10% of the human genome. Looking at just one gene, the alpha-globin cluster, seven separate ALU elements have been discovered. All seven of them are found in precisely the same location in the corresponding chimpanzee gene.

The pattern of common transposons in all living things that have been examined so far reveal a family tree that exactly reflects the common ancestry predicted by evolution.

Anybody who rejects this evidence for human evolution ought to be consistent and refuse to serve on a jury where forensic evidence will be presented.

I would like to post some more about pseudogenes next...

The Common Ancestry Thread

cofty

frankiespeakin

SunnyDays

cofty

jgnat

cofty

cantleave

cofty

jgnat

EntirelyPossible

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