Download Chapter 7: Microbial Genetics

Chapter 7: Microbial Genetics
Microbial Genetics
Most prokaryotes have a single, circular chromosome
When stretched out, this single DNA molecule is about 1mm long (~1000 times longer than cell)
This immense molecule fits compactly into the cell nucleoid by twisting around itself (supercoiling)
Supercoiled DNA
DNA can be supercoiled in either a positive or negative direction
Topoisomerases
Supercoiling of DNA in prokaryotes is typically brought about in a much different manner than in
eukaryotes
In bacteria and most archaea, DNA gyrase (topoisomerase II) introduces negative supercoils
Chromosome: The DNA molecule must contain genetic information essential for the continuous survival of
the organism
Plasmids contain only genetic information that could be helpful to organisms, but that they could survive
without
Gene - The basic unit of heredity; a linear sequence of nucleotides of DNA that form a functional unit of a
chromosome or a plasmid
Genes with different information at the same locus are called alleles
Because prokaryotes have a single chromosome, they generally have only one version or allele
of each gene
Protein Synthesis—Prokaryotes vs. Eukaryotes
In Prokaryotes, both transcription and translation occur in the cytoplasm
In Eukaryotes: transcription takes place in the nucleus; translation occurs in the cytoplasm
Eukaryotes and Achaeans have regions of genes that code for proteins (exons) and noncoding regions
called introns
In RNA, uracil (U) pairs with adenine (A) rather than thymine (T)
RNA transcribed in the 5’ to 3’ direction
Coding sequences of DNA called exons (E) and non-coding sequences called introns (I)
After being transcribed into RNA, introns removed and exons spliced together by spliceosomes
The resulting mRNA is ready to direct protein synthesis and to leave the nucleus
Three types of RNA participate in protein synthesis:
Ribosomal RNA (rRNA)
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Each is synthesized by transcription using single strand of DNA as a template
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Prokaryotic Ribosomal Structure
Does not contain a genetic message, instead it serves as site for protein synthesis
Tightly binds to specific proteins to produce two ribosomal subunits (large and small), which combine
together to form a ribosome
The region of protein synthesis is at the juncture of the three components
mRNA
Carries genetic information from the gene (DNA) out of the nucleus, into the cytoplasm of the cell
where it is translated to produce a protein
Contains base triplets called codons that constitute the genetic code
Attaches to one or more ribosomes
The Genetic Code
Standard three letter abbreviation for amino acids known as codons
UAA, UAG and UGA do not code for any amino acids, instead all three code for Stop designating
terminator codons
AUG codes for Start and the amino acid, methionine (met)
Therefore, protein synthesis always begins with met
Transfer RNA Structure
Function is to transfer amino acids from the cytoplasm to the ribosomes for placement into a protein
molecule
Each tRNA molecule consist of 75 to 80 nucleotides folded back on itself to form several loops that
are stabilized by complimentary base pairing
Each tRNA has a three base anti-codon region complimentary to a particular mRNA codon
Each tRNA also contains an amino acid binding site, specified by the mRNA codon
Amino acid attachment to specific tRNA molecules dictated through action of amino acid activating
enzymes and ATP
There is a specific tRNA for each of the 20 different amino acids
Transcription is the synthesis of mRNA from a DNA template
Translation is the process where ribosomes synthesize proteins using the mature mRNA transcript produced
during transcription
Several ribosomes can be attached at different points along a mRNA molecule to form a polyribosome
(polysome)
Polyribosome
The presence of many ribosomes all “riding” simultaneously along one piece of mRNA
Allows for concurrent transcription and translation in prokaryotes
Protein Synthesis Flow Chart:
DNA → Polymerase → Transcription → mRNA → Ribosome (protein+rRNA) →
Translation → tRNA +Amino acid → Peptide bond → Polypeptide → Protein
Mutation – Change in the genetic sequence of DNA in a cell which may or may not cause a change in the
amino acid sequence coded from that section of DNA.
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Legend:
Process whereby DNA encodes for the production of amino acids and proteins.
This process can be divided into two parts:
1. Transcription
Before the synthesis of a protein begins, the corresponding RNA molecule is produced by RNA transcription.
One strand of the DNA double helix is used as a template by the RNA polymerase to synthesize a messenger
RNA (mRNA). This mRNA migrates from the nucleus to the cytoplasm. During this step, mRNA goes
through different types of maturation including one called splicing when the non-coding sequences are
eliminated. The coding mRNA sequence can be described as a unit of three nucleotides called a codon.
2. Translation
The ribosome binds to the mRNA at the start codon (AUG) that is recognized only by the initiator tRNA. The
ribosome proceeds to the elongation phase of protein synthesis. During this stage, complexes, composed of an
amino acid linked to tRNA, sequentially bind to the appropriate codon in mRNA by forming complementary
base pairs with the tRNA anticodon. The ribosome moves from codon to codon along the mRNA. Amino
acids are added one by one, translated into polypeptidic sequences dictated by DNA and represented by
mRNA. At the end, a release factor binds to the stop codon, terminating translation and releasing the complete
polypeptide from the ribosome.
One specific amino acid can correspond to more than one codon. The genetic code is said to be degenerate
http://www.accessexcellence.org/RC/VL/GG/protein_synthesis.html
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