Nucleic Acids and the Origin of Life

Nucleic Acids and the Origin of Life

• Nucleic acids are polymers specialized for storing, transmitting, and using genetic information
• DNA = deoxyribonucleic acid
• RNA = ribonucleic acid
• Monomers of nucleic acids are nucleotides, composed of a pentose sugar, a nitrogenous base, and a phosphate group
• DNA contains deoxyribose, while RNA contains ribose
• Nucleosides consist of a pentose sugar and a nitrogenous base
• Nitrogenous bases are divided into pyrimidines (single ring) and purines (fused double ring)
• Pyrimidines include cytosine (C), thymine (T), and uracil (U)
• Purines include adenine (A) and guanine (G)
• Nucleotides are linked by phosphodiester bonds
• Phosphate groups connect the 3' carbon of one sugar to the 5' carbon of another sugar
• Nucleic acids grow in the 5' to 3' direction
• DNA bases are: adenine (A), guanine (G), cytosine (C), and thymine (T)
• The bases pair in a complementary manner: purines pair with pyrimidines through hydrogen bonds
• RNA: single-stranded but can form 3-D structures through base pairing between different regions
• RNA has uracil (U) instead of thymine (T)
• RNA and DNA can also pair with each other in a complementary manner.
• DNA forms a double helix
• DNA sequences are copied into RNA (transcription)
• RNA sequences specify the amino acid sequence in a polypeptide (translation)
• Transcription + translation = gene expression
• DNA replication and transcription rely on base-pairing properties
• The complete DNA set in an organism is its genome
• Genes are sequences of DNA transcribed into RNA
• DNA carries hereditary information from one generation to the next
• Closely related species have more similar base sequences
• DNA sequencing tracks evolutionary relationships
• Nucleotides have other roles (e.g ATP, GTP, cAMP) related to energy transfer, protein synthesis, and hormone action.
• Early life forms were thought to arise from spontaneous generation
• Francesco Redi disproved spontaneous generation in 1668
• Louis Pasteur's experiments showed that microorganisms arise only from other microorganisms
• Chemical evolution theory proposes that conditions on early Earth led to the formation of simple molecules (e.g. amino acids) which eventually led to life
• Miller-Urey experiment simulated early Earth conditions to produce amino acids
• Volcanoes can also contribute other gases such as CO2, N2, H2S and SO2 to the atmosphere
• Some meteorites contain organic molecules like purines, pyrimidines and amino acids. This suggests life may have originated elsewhere and reached Earth
• The Mars Viking landers sought evidence of life on Mars, but results were inconclusive
• Current and planned missions to observe and sample Mars and its moons will look for signs of life on Mars.
• RNA was likely the first catalyst, because some RNA molecules (ribozymes) have catalytic properties similar to enzymes.
• Ribozymes can catalyze their own replication and protein synthesis.

Investigating LIFE: Can We Find Evidence of Life on Mars?

• A primary goal of NASA is finding evidence for current or past life on Mars
• The Curiosity rover has found complex organic compounds on Mars.
• Other rovers are planned by the European Space Agency and Russia.
• A Mars Sample Return mission will collect Martian soil samples for analysis on Earth.
• Future missions will investigate Jupiter's Europa and Saturn's Enceladus and Titan.
• Another mission will bring samples of the asteroid Bennu.

The Large Molecules of Life Originated from Small Molecules

• The next stage of chemical evolution is explaining how polymers formed.
• Models for this include powdered clays, hydrothermal vents, and hot pools.

Cells Originated from Their Molecular Building Blocks

• Compounds involved in the origin of life needed to be concentrated.
• Today, living cells are separated from their environment by a membrane
• Protocells can concentrate certain compounds and allow some molecules to pass through
• Short nucleic acid strands that are capable of self-replication could be placed inside protocells
• Nucleotides can pass through the protocell membrane. Newly formed nucleotides can be incorporated into polynucleotide chains
• Protocells may have been a model for early cells due to their organized structure and ability to maintain a distinct interior environment.
• Evidence of 3.5 billion-year-old cyanobacteria (blue-green bacteria) in Australian rocks shows their ability to perform photosynthesis using CO2 and yielding a specific carbon isotope ratio.
• Several other bacteria may have been present.