Bio 160: 6 - How Genes and Mutations Work

Gene

a sequence of DNA that codes how to make a specific protein and will be passed on to offspring

Allele

variants of the same gene

Alleles that don’t function

null or loss of function alleles

Beadle and Tatum proposed discovering what genes do by ___

making them defective

Results from Beadle and Tatum led to the _____

one-gene, one-enzyme hypothesis

one-gene, one-enzyme hypothesis

each gene contains information to make an enzyme

Srb and Horowitz further tested the one gene one enzyme hypothesis by studying the ____

three-step metabolic pathway that produces arginine:

• They used Neurospora (bread mold) and created mutations.

• They found mutants that couldn't make arginine.

• They gave the mutants different intermediate compounds in the arginine pathway.

• Some mutants could continue the pathway when given a specific intermediate, showing that one step/enzyme was defective.

• They concluded that a mutation in one gene → loss of one specific enzyme → blocked one step in a metabolic pathway.

The Genetic Code Hypothesis

• Francis Crick proposed that the sequence of bases in DNA acted as a code:

  • DNA is an information storage molecule

  • Different combinations of bases specify the 20 amino acids

• A particular stretch of D N A (a gene) specifies the amino acid sequence of one protein

• Information in sequence of D N A is not directly translated into amino acid sequence of proteins

Gene → ? → Protein

RNA as the Intermediary between Genes and Proteins

• Jacob and Monod suggested that RNA links genes in nucleus to protein synthesis in cytoplasm

• Messenger RNA (mRNA):

  • Found to carry information from DNA to site of protein synthesis

• Enzyme RNA polymerase synthesizes R N A:

  • Uses DNA strand as template

  • Copies code by matching complementary nucleotides

Gene → mRNA → Protein

transcription

process of using DNA template to make complementary RNA (one sequence of DNA can make multiple different mRNA)

Translation

process of using information in mRNA to synthesize proteins (turns nucleotide language into amino acids)

genotype

Determined by sequence of bases in DNA

phenotype

• Observable (or testable) physical traits

• Determined by the proteins it produces

DNA (genotype) → m RNA → ______ → Phenotype

proteins

genetic code

code specifies how a sequence of nucleotides codes for a sequence of amino acids

Codon

group of three bases responsible for specific amino acid

One start codon that codes for methionine

AUG

Three stop codons

UGA, U AA, and UAG

Genetic code is:

• Redundant - All but two amino acids are encoded by more than one codon

• Unambiguous - One codon never codes for more than one amino acid

• Non-overlapping - Codons are read one at a time

• Nearly universal - All codons specify the same amino acids in all organisms

• Conservative - If several codons specify the same amino acid, the first two bases are usually identical

Knowing the genetic code and the central dogma allows biologists to do 2 things:

1. Predict the amino acid sequence from a particular DNA sequence

2. Determine an mRNA and DNA sequence that could code for a particular sequence of amino acids

Exceptions to the Central Dogma

• RNA as end-products

  • Ribozymes

  • The RNA template of telomerase

  • Other small RNA molecules that play functional or regulatory roles

• Retroviruses

  • These viruses have RNA genomes

  • Use reverse transcriptase to copy their RNA into DNA once in a host cell

  • This DNA is then inserted within a host cell chromosome for expression

  • viralRNA → DNA → mRNA → Protein

Mutation

any permanent change in an organism’s DNA

point mutation

result from one or a small number of base changes

chromosome level mutation

larger in scale

Types of point mutations

• missense

• silent

• frameshift

• nonsense

Missense Mutation

change an amino acid in protein

silent mutation

do not change amino acid sequence due to redundancy in the code

frameshift mutation

shift reading frame, altering meaning of all subsequent codons

Nonsense mutations

change codon that specifies an amino acid into stop codon

Mutations have different impact on fitness

1. Beneficial mutations increase fitness of an organism

​2. Neutral mutations do not affect an organism’s fitness

​3. Deleterious mutations decrease fitness of organism

Most point mutations are

neutral or deleterious

Some mutations are not in coding regions but can still affect _____ by affecting gene expression

phenotype

Mutations affecting fitness can drive ______

evolution