Nucleic Acids and the Origin of Life

• Chapter 4 covers Nucleic Acids and the Origin of Life.

Nucleic Acids

• Nucleic acids are informational macromolecules.
• A nucleotide is the monomer that makes up nucleic acids.
• A nucleotide contains a pentose sugar, a phosphate group, and a nitrogen-containing base.
• Nucleic acids are specialized for storage, transmission, and the use of genetic information.
• RNA is ribonucleic acid.
• DNA is deoxyribonucleic acid.

Distinguishing RNA from DNA

• RNA: Provides a readable genetic language
• RNA enables genes to be expressed as proteins.
• DNA provides long-term, genetic information storage.
• DNA can be easily copied and passed on to future generations.
• Genes are segments of DNA.
• RNA has the sugar ribose, it is a "normal" sugar with one oxygen attached to each carbon atom.
• DNA has the sugar deoxyribose, a modified sugar that missing one oxygen.
• The difference of one oxygen makes the sugars easily distinguishable by enzymes in biological systems.
• The bases of nucleic acids are one of two chemical forms.
• Pyrimidine: a six-membered single-ring. Cytosine (C), thymine (T) and uracil (U). The pneumonic "CUT the Py" can distinguish these chemicals.
• Purine: A fused double-ring structure. Includes adenine (A) and guanine (G); the mnemonic phrase "Pure As Gold" distinguishes these two chemicals.
• Complementary base pairing: purines pair with pyrimidines by hydrogen bonds.
• T (pyrimidine) pairs with A (purine)
• C (pyrimidine) pairs with G (purine)
• Nucleotides are linked by phosphodiester bonds which are covalent bonds.
• Phosphate groups link the 3' carbon in one sugar to the 5' carbon in another sugar.
• Nucleic acids grow in the 5'-to-3' direction.
• RNA is single-stranded, but base pairing can occur between different regions of the molecule which result in a 3D structure.
• Complementary base pairing can also take place between RNA and DNA.
• DNA occurs as two strands form a double helix.
• All DNA molecules have the same basic structure, but the sequence of base pairs differs.
• Genetic information is carried in the sequence of base pairs of a DNA molecule.
• Replication is the process by which a DNA molecule is copied.
• Transcription is the synthesis of RNA using a DNA template; genes are the sequences of DNA that are transcribed into RNA.
• Translation is the synthesis of a polypeptide using the genetic information encoded in an mRNA molecule; there is a change of "language" from nucleotides to amino acids.
• Transcription + translation = gene expression.
• DNA replication and transcription depend on the base pairing properties of nucleic acids.
• Genetic information sits in the sequence of base pairs of a DNA molecule.
• DNA carries hereditary information from one generation to the next.
• Genome is the complete set of DNA in an organism.
• Closely related living species have more similar base sequences than do species that are more distantly related.
• DNA sequencing is now used extensively to trace evolutionary relationships.

Origin of Life

• The small molecules of life originated on primitive Earth.
• People once believed that living things could come from non-living sources or decaying matter by spontaneous generation.
• Frogs can develop from falling drops of rain (the Nile River).
• Flies arose from decaying meat (open markets before refrigeration).
• Francesco Redi first disproved this in 1668.
• Louis Pasteur showed that microorganisms can arise only from other microorganisms, not by spontaneous generation.
• Pasteur's experiments did not prove that spontaneous generation had never occurred.
• The large molecules of life originated from small molecules.
• The two main ideas for how did life first originate: chemical evolution and life came from outside Earth.
• Chemical evolution is the concept that conditions on primitive Earth led to the formation of simple molecules which eventually led to life forms.
• Life from outside Earth: meteorites can contain purines, pyrimidines and amino acids.
• The more popular idea is that of chemical evolution: How did polymers form?
• Catalysts would have been important. Most are proteins called enzymes.
• Proteins are synthesized from information in nucleic acids.
• Which appeared first, nucleic acids or protein catalysts?
• RNA may have been the first catalyst since it could have acted as a catalyst for its own replication and for the synthesis of proteins, and DNA may have eventually evolved from RNA.
• Cells originated from their molecular building blocks and are separated from their environment by a membrane.
• A membrane allows cells to maintain a chemical composition that is different from the external environment.
• Experiments suggest how cells may have formed.
• In water, fatty acids form a lipid bilayer around a liquid compartment.
• Small molecules (sugars and nucleotides) can pass through this lipid bilayer.
• Short nucleic acid strands capable of self-replication may have been present.
• Protocells may be a reasonable model for the evolution of cells; they are organized systems of interacting parts, and they have an interior that is distinct from the exterior environment.
• Protocells can self-replicate.

This section covers Nucleic acids, informational macromolecules that are made of nucleotide monomers. It also touches on the differences between RNA and DNA, including their function related to genetic information, sugars, and structure.