17.1: Introduction
Introduction:
Our genetic information is coded within the macromolecule known as deoxyribonucleic acid (DNA). DNA belongs to a class of organic molecules called nucleic acids . The building block, or monomer, of all nucleic acids is a structure called a nucleotide. A nucleotide has three parts: phosphate , deoxyribose sugar , and a nitrogen base .
There are four different nucleotides that make up a DNA molecule, each differing only in the type of nitrogenous base. These include adenine (A), thymine (T), cytosine (C), and guanine (G), often indicated by their first letters only.
James Watson and Francis Crick discovered the three-dimensional shape of DNA in the early 1950s. The shape, which they described as a double helix, has the shape of a twisted ladder.
The Genetic Code:
Think of the four nucleotides that make up DNA as the letters of an alphabet. To spell out a word (in this case an amino acid) three “letters” from our alphabet are required. Since only about 20 amino acids make up all the proteins, having a four-letter alphabet is more than sufficient to spell out the 20 “words” (see the calculations that follow). The genetic code is universal (almost) for all living things. What this means is that the triplet code spells the same amino acid in different organisms, from dolphins to plants to bacteria!
| Sequence of Nucleotides | # Amino Acids Coded |
|---|---|
| one | 4 1 = 4 (not enough) |
| two | 4 2 =16 (not enough) |
| three | 4 3 =64 (more than enough) |
The Gene Concept:
Think of a gene as a segment of DNA on a chromosome that codes for a series of amino acids that when linked together makes up what is known as a polypeptide . Polypeptides are then folded into complex three-dimensional shapes that become functional proteins.
The Central Dogma:
All organisms use the same fundamental mechanism for gene expression.
DNA → RNA → Polypeptide → Protein
Protein Synthesis:
Protein synthesis is a two-step process.
DNA —(transcription)→ RNA —(translation)→ Polypeptide
Transcription happens when the information from the DNA template is transcribed onto another form of nucleic acid known as ribonucleic acid or RNA (actually messenger RNA).
Translation happens when the information from the language of nucleic acid is translated into the language of proteins.
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- Biology Labs. Authored by: Wendy Riggs. Provided by: College of the Redwoods. Located at: http://www.redwoods.edu . License: CC BY: Attribution
CC licensed content, Shared previously
- 0322 DNA Nucleotides. Provided by: OpenStax. Located at: http://cnx.org/content/col11496/1.6 . License: CC BY: Attribution
- Central Dogma of Molecular Biochemistry with Enzymes. Authored by: Dhorspool. Located at: https://commons.wikimedia.org/wiki/File:Central_Dogma_of_Molecular_Biochemistry_with_Enzymes.jpg . License: CC BY-SA: Attribution-ShareAlike
Public domain content
- The completed chart of the genetic code. Provided by: National Institutes of Health. Located at: http://history.nih.gov/exhibits/nirenberg/HS5_cracked.htm . License: Public Domain: No Known Copyright