Солнечная система и ее тайны

Планеты Созвездия НЛО

Genetics and Genetic Material

Genetics - the storage and transfer of hereditary information - is essential to life. Without it there could be no evolution, and no building of new organisms from the old. The genetic molecules are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). They work together to store and make copies of a genetic code. This code specifies the order of the amino acids to be linked together to create a new protein.

How does this work? DNA stores the genetic information; RNA uses that information to create specific new proteins. The process begins by "unzipping" the double helix of the DNA right down the middle allowing the now-separate halves to be rebuilt with the same order of nucleic-acid base pairing as in the original. This creates two identical DNAs, out of the single DNA we started with, always replenishing the DNA.

The genetic code has redundant base-pairing rules, meaning that if we know one base of a pair, we always know what the other half will be. In DNA there are four bases with the rule that adenine only pairs with thymine, and guanine only pairs with cytosine. Those bases can be represented as letters (A, T, G, C) and the letters can then "spell" three-letter words, called codons. These codons specify the amino acid sequence to build a particular protein (see figure below). Even slightly different codon sequences build completely different proteins.

There are 64 possible codons (four bases arranged in every three-letter sequence possible) in the genetic code, which is more than enough codons to specify the 20 individual amino acids that are used in life on Earth, and where those codes start and stop. The length of DNA that carries enough codons to determine a specific protein is called a gene. The full DNA sequence of a living organism is what we call its genome

How is this information used to synthesize a new molecule? It happens in stages. First an enzyme unzips the DNA helix at the starting codon of the gene in question, and links nearby nucleic acid bases into an RNA strand. This new strand is called a messenger RNA. This initial stage when information is copied from DNA to messenger RNA is called the transcription stage.

The messenger RNA now has all the necessary information to make the new molecule. Stage two is when the messenger RNA finds a nearby ribosome. Ribosomes are molecular complexes that are readily available in any cell; they consist of RNA and protein parts. They are the sites where the messenger RNA meets two other types of RNA: transfer RNA and ribosomal RNA. These molecules work together to gather the correct amino acids and string them together in the correct sequence, dictated by the codons. This new polymer molecule (protein) was created with the information that was encoded in the original DNA.

DNA replication is more involved than this process as it has to be less error prone than the synthesis of proteins; it employs enzymes that have evolved over the history of life on Earth. This is in essence a "chicken-and-egg" problem in the origins of life: DNA replication requires enzymes that are encoded within DNA. We will attempt to resolve this later in this course.

Солнечная система и ее тайны