16.1 What Do Genes Do?

• Alleles that do not function at all are called null alleles, or loss-oιfunction alleles.

• One-gene, one-enzyme hypothesis: Beadle and Tatum proposed that each of the mutants could not make a particular compound because it lacked an enzyme required to synthesize the compound.

• A genetic screen is any technique for picking particular types of mutants out of many randomly generated mutants

16.2 The Central Dogma of Molecular Biology

• Single stranded molecules of RNA were called messenger RNA, or mRNA

• RNA polymerase polymerizes ribonucleotides into strands of RNA.

• The central dogma summarizes the flow of information from DNA to proteins. It states that DNA codes for RNA, which codes for proteins.

• Transcription is the process of using a DNA template to make an RNA molecule that has a base sequence complementary to the DNA. DNA is transcribed to RNA by RNA polymerase.

• Translation is the process of using the information in the base sequence of mRNA to synthesize proteins. Information in the messenger RNA is translated into proteins by ribosomes.

• A viral enzyme called reverse transcriptase synthesizes a DNA version of the RNA genes

16.3 The Genetic Code

• Genetic code is the rules that specify the relationship between a sequence of nucleo des in DNA or RNA and the sequence of amino acids in a protein.

• A three-base code, known as a triplet code, is the shortest genetic word to code for at least 20 amino acids.

• A group of three bases that specifies a particular amino acid is called a codon.

• A single addition or deletion throws the sequence of codons, or the reading frame, out of register.

• Stop codons do not code for any amino acid but signal the end of the reading frame and therefore, the end of the polypeptide.

• Start codons set the reading frame of the message, locking in which set of three-base triplets constitute “words.

• Once biologists had cracked the genetic code, they saw a set of important properties:

  • The code is redundant

  • The code is unambiguous

  • The code is non-overlapping

  • The code is (nearly) universal

  • The code is conservative

• If a change in DNA sequence leads to a change in the third position of a codon, it is less likely to alter the amino acid in the protein.

16.4 What Are the Types and Consequences of Mutation?

• A mutation is any permanent change in an organism’s DNA.

• A mutation that alters the sequence of one or a small number of base pairs is called a point mutation

• Point mutations that change the identity of an amino acid in a protein are called missense mutations.

• A point mutation that does not change the amino acid sequence of the gene product is called a silent mutation.

• Mutations that shift the reading frame and are aptly called frameshift mutations. These almost always destroy the function of the protein.

• Nonsense mutations occur when a codon that specifies an amino acid is changed by mutation to one that specifies a stop codon.

• Biologists divide mutation into three categories:

  • Beneficial

  • Neutral

  • Deleterious

• A broken segment of a chromosome can be lost, causing a deletion

• Segments of a broken chromosome may be flipped and rejoined, creating a chromosome inversion

• Errors in crossing over or in DNA synthesis can lead to the presence of one or more additional copies of a segment which is a duplication

• A broken piece of a chromosome can become attached to a different chromosome, an event called chromosome translocation.

• Point mutations and chromosome mutations are random changes in DNA that can produce new genes, alleles, and traits.