12-4 MUTATIONS
• Mutations – changes in the DNA sequence that affect genetic
information
• Gene mutations result from changes in a single gene. Chromosomal
mutations involve changes in whole chromosomes.
• Gene Mutations
• Some gene mutations involve several nucleotides, but the majority
involve just one.
• Point mutations – mutations that affect one nucleotide
• Point mutations occur at a single point in the DNA sequence
When a point mutations involves the insertion or deletion of a
nucleotide, much bigger changes happen. The genetic code is read in
groups of three, called a codon. If a nucleotide is deleted, the base is
still read in groups of three, but now the groupings are shifted for every
codon that follows. Inserting an extra nucleotide has a similar affect.
This is called frameshift mutations because they shift the genetic
message.
Example of frameshift mutation:
THE FAT CAT ATE THE RAT
Delete the h
TEF ATC ATA TET HER AT
Chromosomal Mutations
• A chromosomal mutation involves changes in the number or structure
of chromosomes. Chromosomal mutations may change the locations
of genes on chromosomes and even the number of copies of some
genes.
12-5 GENE REGULATION
• An expressed gene is a gene that is transcribed into RNA. Only a
fraction of the genes in a cell are expressed at any given time.
Gene Regulation: An Example
Operon – A group of genes that operate together
Example: E. Coli
• The protein-encoding genes in this bacterium include a cluster of
three genes that are turned on or off together. Because these genes
must be expressed in order for the bacterium to be able to use the
sugar lactose as a food, they are called the lac operon.
• The lac genes are turned off by repressors and turned on by the
presence of lactose
Eukaryotic Gene Regulation
• Most eukaryotic genes are controlled individually and have regulatory
sequences that are much complex than those of the lac operon.
Development and Differentiation
• Regulation of gene expression is especially important in shaping the
way a complex organism develops. Each of the specialized cell types
found in the adult develops from the same fertilized egg cell. So, cells
don’t just grow and divide during embryonic development; they also
undergo differentiation.
Differentiation – cells become specialized in structure and function
Hox genes – control the differentiation of cells and tissues in the
embryo
• A mutation in a hox genes can completely change the organs that
develop in specific parts of the body.
• (Mutations affecting the hox genes in a fruit fly can replace the fly’s
antennae with legs growing on its head)