Astrobiology

Replicators

NASA’s Astrobiology Page

• http://astrobiology.nasa.gov/about-astrobiology/

The Legos of Life

• This is review:– What are the Legos?

– How did the Legos make it to Earth (as well as planets throughout the Universe)?

The Legos of Life (Continued)

• What are the Legos (molecules) that life needs (think back to your biology)?

• Amino acids: “NASA and University Researchers Find a Clue to How Life Turned Left”– http://

www.nasa.gov/topics/solarsystem/features/life-turned-left.html

• Amino acids are found in a lot of celestial bodies (including meteoroids and comets).

The Legos of Life (Continued)

• Life on Earth uses 20 amino acids to build proteins.

• Common amino acids include: glycine, alanine, and glutamic acid. – Other amino acids that Earth’s life does not use

have also been found (e.g. isovaline and pseudoleucine).

The Legos of Life (Continued)

• Glycoaldehyde – a two carbon monosaccharide that has been detected in the interstellar medium.– It is made from the amino acid glycine (which has

been found on meteorites, asteroids, and comets.• What very important molecule is a sugar? – Deoxyribose and ribose are also monosaccharides

Analysis of Meteorite

Compound class[11] Concentration (ppm)Amino acids 17-60

Aliphatic hydrocarbons >35Aromatic hydrocarbons 3319

Fullerenes >100Carboxylic acids >300

Hydrocarboxylic acids 15Purines and pyrimidines 1.3

Alcohols 11Sulphonic acids 68

Phosphonic acids 2

Compound class[11] Concentration (ppm)Amino acids 17-60

Aliphatic hydrocarbons >35Aromatic hydrocarbons 3319

Fullerenes >100Carboxylic acids >300

Hydrocarboxylic acids 15Purines and pyrimidines 1.3

Alcohols 11Sulphonic acids 68

Phosphonic acids 2

What are Purines and Pyrimidines?• Nitrogenous bases used in DNA and RNA to code for

proteins. They are the building blocks of genes!

The Legos of Life (Continued)

• One more really cool thing. What are these?

Assembling the Legos

• Questions:– Is evolution, the natural force, confined to only

biological life? – So, what is evolution (what are the requirements

for evolution to take place)? • Answers:– No, it is not confined to living organisms.– In simplest terms: Descent with Modification

The Requirements of Evolution

• What are the necessary constituents of evolution (what factors need to be present for evolution to occur)?– You need replicators capable of autocatalysis. • What kinds of replicators do you know?

– You need some type of inheritance (characteristics passed along from one generation to another).

– You need the occasional mistake in replication.• What is this known as?

One Final Requirement

• There is one final thing that is necessary for descent with modification to take place, what is it?– A differential selection process. What is this

known as?• Natural Selection – more replicators are made each

generation than can survive. The best adapted, to their environment, replicators are typically those that survive.

The Force Known as Evolution

• Evolution is an observable phenomenon and behaves the same way wherever you happen to find yourself.

• It is comparable to gravity – it happened yesterday, it is happening right now (today), and it will happen tomorrow the same way.

• Astrobiology is evolution writ large!

How were the Legos Put Together

• This is the question we are trying to answer.• We have all of the Legos but we are “looking

for the instructions.”– What is a good candidate for first replicator?• Not DNA, which is very complex, but simpler RNA.

– What do you know about RNA? How many kinds of RNA are there? • In biology class we learn only about three. What are

they?

RNA Compared w/ DNA

Type Abbr. Function Distribution

Messenger RNA

mRNA Codes for protein All organisms

Ribosomal RNA

rRNA Translation All organisms

Signal recognition particle RNA

7SL RNA or SRP RNA

Membrane integration All organisms

Transfer RNA tRNA Translation All organisms

Transfer-messenger RNA

tmRNARescuing stalled ribosomes

Bacteria

RNAs involved in protein synthesis

Type Abbr. Function Distribution

Small nuclear RNA snRNA Splicing and other functions Eukaryotes and archaea

Small nucleolar RNA snoRNA Nucleotide modification of RNAs Eukaryotes and archaea

SmY RNA SmY mRNA trans-splicing Nematodes

Small Cajal body-specific RNA scaRNA Type of snoRNA; Nucleotide modification of RNAs

Guide RNA gRNA mRNA nucleotide modification Kinetoplastid mitochondria

Ribonuclease P RNase P tRNA maturation All organisms

Ribonuclease MRP RNase MRP rRNA maturation, DNA replication Eukaryotes

Y RNA RNA processing, DNA replication Animals

Telomerase RNA Telomere synthesis Most eukaryotes

Spliced Leader RNA

RNAs involved in post-transcriptional modification or DNA replication

Type Abbr. Function Distribution

Antisense RNA aRNATranscriptional attenuation / mRNA degradation / mRNA stabilisation / Translation block

All organisms

Cis-natural antisense transcript Gene regulation

CRISPR RNA crRNA Resistance to parasites, probably by targeting their DNA Bacteria and archaea

Long noncoding RNA Long ncRNA Various Eukaryotes

MicroRNA miRNA Gene regulation Most eukaryotes

Piwi-interacting RNA piRNA Transposon defense, maybe other functions Most animals

Small interfering RNA siRNA Gene regulation Most eukaryotes

Trans-acting siRNA tasiRNA Gene regulation Land plants

Repeat associated siRNA rasiRNA Type of piRNA; transposon defense Drosophila

7SK RNA 7SK negatively regulating CDK9/cyclin T complex

Regulatory RNAs

Type Function Distribution

Retrotransposon Self-propagating Eukaryotes and some bacteria

Viral genome Information carrier

Double-stranded RNA viruses, positive-sense RNA viruses, negative-sense RNA viruses, many satellite virusesand reverse transcribing virusesViroid Self-propagating Infected plants

Satellite RNA Self-propagating Infected cells

Parasitic RNAs

RNA World Hypothesis

Strengths of RNA World Hypothesis

• RNA can act as:– Hereditary material– Coding material– And can have catalytic (including autocatalytic)

activity.• RNA can act as its own enzyme.

Weakness of RNA World Hypothesis

• We still have not found a clear pathway from the “RNA World” to a world that includes RNA and proteins. – A strength of the RNA World hypothesis is also a

weakness. • RNA polymers are fragile (though in a vesicle

they would be protected).

Important Questions Being Studied

• How did RNA polymerases emerge?– RNA polymerase is necessary to create RNA chains

(this is an important question with respect to autocatalysis).

• How were RNA molecules incorporated into membranes?

RNA World Hypothesis