Short interspersed nuclear elements sines 2 nd most

Short interspersed nuclear elements (SINEs) – 2 nd most abundant
o SINEs are usually less than 400 bp long, do not encode proteins
o The reverse transcriptase required for SINE transposition is provided by a
LINE-type element
The genetic + evolutionary significance of transposable elements
o Transposable genetic elements are powerful mutagens + play major roles in
the evolution of living organisms
o Crossing over may occur between homologous transposons located at
different positions on the same/or different chromosomes
o These are referred to as ectopic intra/inter-chromosomal exchanges
Beneficial uses of transposable elements in molecular genetics
o Transposons are used in genetic research to induce mutations – “sleeping
beauty” mutation
o Transposons are used as vector to move DNA within genomes
o Crossing over between paired transposons can create chromosomal
rearrangements
o Intra-chromosomal recombination between transposons in the same
orientation produces a deletion
o Unequal crossing over between transposons on sister chromatids produces a
gene duplication
Chapter 18: Gene regulation in Prokaryotes
o Transcription is the most important step of gene regulation in prokaryotes
o A bacterium often finds itself in an ever changing environment
o Genetic regulation in bacteria is primarily focused on adaptation to this ever
changing environment they are in
o Genes whose products are not required are in general not expressed – unless
environmental conditions change in a way that makes their expression useful
Constitutively expressed genes:
o Products of certain genes (tRNAs, rRNAs, ribosomal proteins, subunits of
RNA polymerase, other enzymes involved in housekeeping functions are
essential components of all living cells
o Gene products are required at all times – always on

o These genes are continuously expressed in most cells = constitutive
expression
Inducible + repressible genes:
o Some gene products are needed for cell growth only under certain
environmental conditions
o Regulatory mechanisms allow the expression of these gene products only
when they are needed = inducible/repressible genes
Induction of expression of genes for lactose utilization
o Glucose is a preferred carbon source for many bacteria, but can use other
carbon sources if glucose is absent
o For utilization of lactose, gene expression is induced when lactose is present
and glucose is absent
o Induction occurs at the level of transcription + alters the rate of translation of
lactose-utilization enzymes
o Enzymes involved in catabolic (breakdown) pathways are often inducible
Repression of genes for tryptophan biosynthesis
o Genes are expressed (or de-repressed) in the absense of tryptophan and
turned off (repressed) when tryptophan is available
o Repression occurs at the level of transcription of genes involved in
tryptophann biosynthesis
o Enzymes involved in anabolic (biosynthesis) pathways are often repressible
Regulation of inducible systems * all depends on binding complex