Transcript Microbial Genetics: Chapter 8

Genetics: Chapter 7
What is genetics?
• The science of heredity; includes the study
of genes, how they carry information, how
they are replicated, how they are expressed
What do you know about DNA?
• Chromosomes made of DNA contain an
organism’s entire genome
• DNA codes for genes….genes code for
proteins
• Chemical composition is nucleotides
• It exists in most cells as a double stranded
structure
DNA is used for many things
DNA Structure
DNA Replication
Enzymes necessary for DNA
replication
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Primase
DNA Polymerase
DNA gyrase
DNA ligase
Helicase
Nucleotides are added to the 3’
position (OH group)
DNA replication…a closer look
DNA replication…closer look
Gene Expression…why is it
important?
• Transcription
• Translation
What do you know about gene
expression?
• Work with a partner to make a list about
what you already know about gene
expression
Transcription: RNA is
transcribed from DNA
Transcription: Promoter orients
direction of transcription
Transcription: DNA to RNA
• Requires an enzyme…..
• RNA nucleotides
• Base pairing rules for building RNA from a
DNA template
• Process proceeds in the direction 5’--->3’
• Process begins at the promoter region and
ends at the terminator sequence
Transcription: RNA synthesis
What are the possible products
from transcription?
• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Ribosomal RNA (rRNA)
Quick check…..
• Do we have a protein yet?
• What have we made?
• What is next?
Translation: RNA to protein
• What is needed for the process?
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mRNA
Ribosomes
Amino acids
tRNA
Translation: RNA to protein
• What is needed for the process?
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mRNA
Ribosomes
Amino acids
tRNA
Translation: reading frame
determines the protein
The Genetic code
Translation
Translation
Both processes occur at the same
time in bacteria…why??
Eukaryotic cells differ in
transcription and translation
• Ribosomes are different size
• 5’ end of mRNA has cap (methylated
guanine)
• 3’ end of mRNA has poly A tail
• Introns are excised, exons spliced together
• Translation is monocystronic
Is it important to regulate protein
synthesis?
• Yes!
• Three types of protein regulation
– Enyme inhibition (feedback inhibition)
– Repression (tryptophan operon)
– Induction (lactose operon)
Are all genes under regulation?
• No!
• Genes to produce enzymes for glucose
metabolism are constitutive (always made)
• Other genes are induced…only made when
needed
• Other genes are repressed…turned off
when not needed
Models for transcriptional
regulation with repressors
Transcriptional regulation by
activators
Lactose Operon as a model
• Used to understand control of gene
expression in bacteria
• Operon consists of three genes needed to
degrade lactose
• Repressor gene(codes for repressor protein)
outside of operon coding region inhibits
transcription unless something else bind to
the repressor protein
Lactose Operon
Diauxic growth curve of E. coli
What conditions are needed for the lactose
operon to be turned “on”?
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No glucose
Lactose present
Increasing levels of cAMP
cAMP binds to CAP, then complex binds
next to lactose operon promoter at the
activator region
• RNA polymerase binds to promoter
How do organisms adapt to other
changes in their environment?
• Some organisms turn genes on/off as
needed
• Some organisms alter gene expression
Gene regulation systems in bacteria
• Signal transduction
– Two component
regulatory system
Gene regulation systems in bacteria
• Signal transduction
- Quorum sensing
Gene expression is influenced by
natural selection
• Random changes enhance survival of some
cells in population
• Antigenic variation of pathogens
• Phase variation
– Switching on/off of certain genes