ASTR-3040:Astrobiology

Day 15

The Nature of Life on Earth

Chapter 5

Molecular Components

• Carbohydrates – energy sources– Sugars and starches

• Lipids – energy stores, barriers– Fats

• Nucleic Acids – DNA and RNA

Molecular Components• Proteins – structure & chemistry– Structural materials– Enzymes – catalysts for reactions– Amino Acids (20 used in most life forms)– Left-handed in biological entities.

Major Groupings

• Leeuwenhoek – 1674 – microscope• 5 Kingdoms– Plants– Animals– Protista–Monera– Fungi

• Is there a biochemical classification?

Microscopic Life

• Plants and Animals are different • Cell walls (plants)• Cell Nucleus– Prokaryotes– Eukaryotes

All prokaryotes aresingle celled. They lack a cell nucleus.Eukaryotes – single or multi-celled.

Domains of Life

• Based on biochemistry and genetics:• Bacteria• Archaea• EukaryaTree of life.

Kingdom – Phylum – Class – Order – Family – Genus – Species

The Tree of Life

• Extremely valuable organizational tool. • 3 key ideas– 1. Domains based on biochemistry & genetics is

fundamentally different from the older KPCOFGS based on structural & physiological differences.

– 2. New and fundamentally different view of microbes - lipid structure of cell membranes.

– 3. True diversity almost entirely in microbes.

Metabolism: Chemistry of Life

• Metabolic needs:– 1. Source of raw materials.– 2. Source of energy.

• ATP: adenosine triphosphate – Used by every living cell for internal energy.– Outside energy needed only to produce ATP.

• Carbon sources – form basic materials.• Energy sources – run the metabolism.

Classification by Metabolism

• To obtain carbon:• 1. Heterotrophs: Consume preexisting organic

compounds (i.e. eating).• 2. Autotrophs: Direct uptake from the environment

(self-feeding, plants)

Classification by Metabolism

• To obtain energy:• 1. Photosynthesis: sunlight• 2. chemosynthesis: chemical energy– Ingested food– Inorganic reactions

4 Metabolic Classifications

• Based on energy and carbon acquisition.• 1. Photoautotrophs• 2. Chemoautotrophs• 3. Photoheterotrophs• 4. Chemoheterotrophs

Liquid Water on Earth

• All living cells on Earth depend on liquid water because metabolism requires:

• 1. Organic chemicals are readily available for reactions.

• 2. A means of transporting chemicals to and within cells; waste products away.

• 3. Water plays a role in many of the reactions.

DNA and Heredity

• 3 key features of life on Earth– 1. structural units – cells

– 2. metabolic processes – energy production

– 3. owner's manual – data storage for how a cell is constructed, how it works, and how to build another one, and pass it on to the next generation - heredity.

DNA Structure

• Double helix• DNA bases – 4– Adenine– Guanine– Thymine– Cytosine

• Pair as AT & CG

Replication

• Unzip & each strand acts as a template.• Use RNA in the rebuild process.

Genome• Humans have ~3 X 10^9 bases• 46 chromosomes (23 pairs)• Sequences for individual instructions are called

genes.• Most of DNA is “noncoding” (junk 95%)• Complete sequence – genome.

Some bacteria – 10^5 bases, few 10^2 genesHumans – 20,000 – 25,000 genesRice – 37,000 genes (smaller DNA sequence)

Genetic Code• Words consist of 3 DNA bases.• Sequence has a start and stop word.• Used to build proteins.• Redundancy (64 codes, 20 amino acids)• Most amino acids are coded with the first 2 base

letters only (16). Does this give a clue to evolution of complex organisms?

• Nearly all living organisms have mostly similar genetic code.

RNA• Uses ACG & Uracil• mRNA – transcription (carries the instr.)• tRNA – transfer (gathers the AAs)• rRNA – backbone of building structure.• Mutations - <1:10^9, but can have dramatic

consequences.• Provide the basis for evolution.• Genetic Engineering.• RNA was probable precursor.

Extreme Life - Extremophiles• Thermophiles – heat lovers• Hyperthermophiles– Black smokers (350˚C)– Hot springs

Extreme Life - Extremophiles• Psychrophiles – cold lovers– Antarctica ice pack (-20˚C)– Also very dry

Radiation hardened

Extreme Life - Extremophiles• Endoliths – within rocks– Up to several km below ground

• Endospores (resting) – found on moon– Could these survive interplanetary journeys?