For my September 2014 diary, go here.

Diary — October 2014

October 1, 2014

The Picture of Dorian Gray

...the face appeared to him to be a little changed. The expression looked different. One would have said that there was a touch of cruelty in the mouth. It was certainly strange. — Oscar Wilde
Sadly, I haven't read this story yet — I've just seen a movie version. And I've read some quotes. Since Wilde was a master of the epigram, you can enjoy these like popping down peanuts until you get sick to your stomach:
Nowadays people know the price of everything and the value of nothing.

He was always late on principle, his principle being that punctuality is the thief of time.

I never talk during music — at least, during good music. If one hears bad music, it is one's duty to drown it in conversation.

The basis of optimism is sheer terror.

The only way to get rid of temptation is to yield to it. Resist it, and your soul grows sick with longing for the things it has forbidden to itself.

Some people cause happiness wherever they go. Some people cause happiness whenever they go.

... and so on. You can like them without agreeing with them. If you agreed with them all, Wilde would probably be disappointed.

The picture here is by an artist named Gerwell.

October 28, 2014

Your cells contain mitochondria, little factories that help convert food into useful chemical energy, using oxygen. These guys were once bacteria in their own right! About 1.5 billion years ago they joined forces with cells that couldn't handle oxygen. Now you need them and they need you.

But they still have their own DNA, separate from the rest of the DNA in your cells. Mitochondria are passed down only from mother to child, via the egg cell. So, your mitochondrial DNA gives information about you, your mother, your grandmother, and so on.

Why does mitochondrial DNA come only from the mother? For starters, an egg cell contains 100,000 to 1,000,000 molecules of mitochondrial DNA, while a sperm has only 100 to 1000. On top of that, most mitochondria in a sperm cell stay in the tail, and sometimes the tail is lost during fertilization. But on top of that, in mammals it seems the egg actively destroys any mitochondria that happen to get in from the sperm.

Puzzle 1: Why?

But in biology everything is complicated. Biologists argue about how likely it is for people to inherit mitochondrial DNA from their father. In a test of 172 sheep, three were found to inherit mitochondrial DNA from their father! But in humans, there is so far just one recorded case of it happening.

Your mitochondrial DNA has just 37 genes. It's made of about 16,600 base pairs: molecules called A, T, C and G, just like your ordinary DNA. The information gets copied to RNA when the genes are used to make proteins, and the T gets copied to U, while the rest stay the same.

The chart above shows the mitochondrion's genetic code. More precisely: each codon, or triple of base pairs U, G, A, and C, is translated into an amino acid. Amino acids, the building blocks of proteins, have fun names like phenylalanine, leucine, isoleucine and so on - but in this chart they're abbreviated to Phe, Leu, Ile, etc.

The mitochondrial genetic code is a bit different than the genetic code used elsewhere in your cells! The differences are marked in red:

The same code is used for mitochondria in all vertebrates, as far as I know. Some invertebrates have slightly different mitochondrial genetic codes.

Puzzle 2: Why is the mitochondrial genetic code different in the above ways?

This is an extremely hard puzzle, and I doubt anyone knows the answer for sure, since it could simply be due to random events that happened billions of years ago. But I bet people have thought about it, and I'd love to know any good ideas they've had.

Here's a clue: when the mitochondrial genetic code differs from the 'usual' one, it tends to be simpler! All the mitochondrial genetic code consists of blocks of 2 or 4 codons that do the same thing. Most of the usual code is this way - but AUA and UGA break that rule.

For more, try the comments on my Google+ post, and also

For my November 2014 diary, go here.


© 2014 John Baez
baez@math.removethis.ucr.andthis.edu

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