Chapter 16- How Genes Work

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

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

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.==

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.==