Summary
Prokaryotic Gene Structure Summary
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Summary of the prokaryotic gene structure module covered in the MCB2020F course
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Prokaryotic Gene Structure• All cells share 4 key components —> plasma membrane
—> cytoplasm
—> DNA
—> ribosomes
• BUT!! Prokaryotes = smaller, unicellular
• No membrane bound organelles
• Smaller simpler genomes
• DNA not arranged in chromosomes —> no histones
• Smaller ribosomes
• Cells wall = di erent to plants, fungi/protists
• Murein and pseudomurein present!
• Di er in mechanisms of genetic replication, expression and recombination
• External structures ie. Pili, agella, mbriae
• All DNA replication and transcription occurs in cytoplasm!
• No RNA modi cation (mRNA produced is mature and does not need processing before
translation)
BACTERIA OVERVIEW
• Grow/divide via binary ssion
• Symmetric vs asymmetric
• Can sense and respond to environmental stimuli
• Metabolically active
• Able to synthesise biomolecules
• Anabolic and catabolic pathways + can assimilate nutrients
• Pneumococcal niche adaptation
• Bacteria can enter physiological states that enable antibiotic tolerance
• Some possess capability for dormancy/sporulation
• Cell can shut down into a vegetative state as a result of environmental change
• Can move ( agella)
DNA AS GENETIC MATERIAL
• Hershey Chase experiment
• Labelling of bacteriophage with radio labelled protein/DNA extracts
• Bacterial Transformation experiment
• Streptococcus pneumonia — used a rough (harmless) and smooth (pathogenic) strain
• Why did the mice in experiment 3 die??
• Genetic exchange has occurred!
• DNA encoding for the capsule surrounding the smooths train is absorbed by the rough
strain which become pseudo-smooth strain and therefore function as the virulent S-strain
• TRANSFORMATION = a change in genotype and phenotype due to assimilation of
external DNA by a cell
• Horizontal gene transfer from pathogenic bacterial forms to less pathogenic strains!!! —>
demonstrates similar outcome to mouse experiment
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, Gri th’s Experiment
• Radioactive Tracing Experiment
• Tested whether DNA or protein component of T2 phages infected bacterial cells to
reprogram them and produce more phages
• Radioactive sulphur + phosphorus sued to trace protein and DNA in E. Coli gut infection
• To infect a cell, virus must take over a cell’s metabolic machinery
RESULTS = when investigating proteins, they were found to exist outside of the bacteria cell but
when DNA was labelled, radioactivity was found inside the cell. Therefore, DNA can be regarded
as the genetic material of the T2 phage
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, Hershey-Chase Experiment
CENTRAL DOGMA
• DNA —> RNA —> protein
• DNA = deoxyribonucleic acid = repository of genetic information
• RNA = ribonucleic acid = mRNA, rRNA, tRNA
• mRNA = record information from DNA
• rRNA = makes up about 60% of ribosomal structure
• tRNA = delivers amino acid to elongate polypeptide chain within ribosomes
• Can have siRNA, miRNA, snRNA, snoRNA, sRNA
• Seen in eukaryotic gene expression + post translational modi cation
• Horizontal gene transfer = sharing of genetic information cross bacterial strains
• Transcription = synthesis of mRNA from DNA template
• Translation = synthesis of a protein using the code of the mRNA
• Gene expression = process by which genetic information used to synthesize gene products is
regulated
• Genetic Code = nucleotide triplets of DNA and RNA that carry genetic information in living cells
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, GENES
• Basic unit of genetic information
• Polynucleotide sequence that codes for a functional product
• Polypeptide, tRNA, rRNA
• Linear sequence of nucleotides that have a xed start and end point
• Protein coding gene results in mRNA that can be “read”
• Reading frame
• mRNA is read in discrete sets of 3 nucleotides —> codon
• Each codon codes for a speci c amino acid
• But! An amino acid can be coded for by multiple codons (redundancy in code)
• Reading frame = result of organisation of gene sequence
• Leads to single protein product
• Viruses can have overlapping genes —> overlapping reading frames resulting in >1 protein
product
• Allows for compression of more genetic information in smaller space
• Bacterial + viral coding information = continuous
• NO INTRONS
• Architecture of a gene =
• Promotor
• Leader sequence
• Gene of interest
• Trailer
• Terminator
• In above diagram —> red bottom strand = template strand (read 3’ to 5 ‘)
• UTR = untranslated regions
• Promotor + operator + leader
• Trailer + terminator
• Promotor = -35 and -10 region of DNA (located at the start of a gene)
• Not transcribed or translated
• Operator = sequence of DNA recognised by binding proteins speci c for that sequence
• May lie at various places relative to promotor
• Could overlap or lie downstream/upstream of promotor
• Additional level of gene regulation eg. Lac operon
• -35 is NB for recognition
• -10 is NB for binding of RNA polymerase (TATA box)
• Lot of A/T pairs
• NB! That these have 2 hydrogen bonds, easier to melt DNA and form replication bubble!
• Leader sequence and trailer = transcribed BUT NOT TRANSLATED
• Shine-Dalgarno sequence = NB for recognition and initiation of translation of an mRNA strand
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